Nick Connor

The thermal conductivity of uranium dioxide is very low when compared with metal uranium, uranium nitride, uranium carbide and zirconium cladding material. The thermal conductivity is one of parameters, which determine the fuel centerline temperature. This low thermal conductivity can result in localised overheating in the fuel centerline and therefore this overheating must be avoided.  Overheating of the fuel is prevented by maintaining the steady state peak linear heat rate (LHR) or the Heat Flux Hot Channel Factor – F_Q(z) below the level at which fuel centerline melting occurs. Expansion of the fuel pellet upon centerline melting may cause the pellet to stress the cladding to the point of failure.

Although the melting point of UO2 is over 2,800°C,fuel is usually operated at a much lower peak centerline temperatures (less than 1,400°C). This provides enough margin to fuel melting and to loss of fuel integrity. In general, fuel melting must be excluded for condition III and condition IV accidents as well. But the Fukushima Daiichi nuclear disaster in 2011 raises the safety problem of nuclear power plants to a new level in the world. It is difficult to predict these events and all other beyond design-basis accidents and prepare for them due to their extreme rarity. Under these infrequent circumstances the plant may be unable to operate safely. Reduction in the safety margin of a plant can cause a catastrophic failures such as meltdowns

In case of nuclear fuel melting, it is necessary to distinguish in which event the fuel melting temperature is reached. Fuel melting can occur:

• Slow fuel rod overpower. In the event of an increase in fuel overpower that is slow compared to the rate of heat transfer through the fuel, melting occurs only on a local scale.
• Loss of ultimate heat sink. In the event of loss of reactor coolant, the power of the rod decreases, the fuel temperature is only a few tens of degrees Celsius higher than the cladding temperature.
• RIA accidents. In these accidents, the large and rapid deposition of energy in the fuel can result in melting, fragmentation, and dispersal of fuel.

Reactor Core Melt Accident

Reactor core melt accident is an event or sequence of events that result in the melting of part of the fuel in the reactor core. Although this event is very unlikely, it cannot be ruled out. There are many and many barriers that have to be breached. Especially, common (usually 3×100%) failure of the Emergency Core Cooling System (ECCS) must occur after severe loss of coolant accident.

This type of accident is known under term a nuclear meltdown (core meltdown), but this is not officially defined by the International Atomic Energy Agency or by the Nuclear Regulatory Commission. The core melt accident is a severe nuclear reactor accident that results in core damage from overheating. It occurs when the heat generated by a nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point. The heat causing the melting of a reactor may originate from the nuclear chain reaction, but more commonly decay heat of the fission products contained in the fuel rods is the primary heat source.

If the reactor core remains dry for a considerable length of time, the temperature of the fuel rods rises and may locally reach levels that cause significant and irreversible core degradation. The mechanisms of this degradation are both chemical and mechani­cal. Depending on the local temperature levels, degradation may result in more or less severe hydrogen production, fission product (FP) release, and molten corium formation and propagation towards the lower head.

Special reference: Nuclear Power Reactor Core Melt Accidents ISBN: 978-2-7598-1835-8, IRSN 2015.

Corium

Corium, also called fuel-containing material (FCM), is a lava-like material created in the core of a nuclear reactor during a meltdown accident. It consists of:

• mixture of nuclear fuel and oxidized zirconium clad,
• fission products,
• control rods,
• structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam,
• and, in the event that the reactor vessel is breached, molten concrete from the floor of the reactor room.

If the temperature reaches the melting point of UO2, a fuel degrades usually from the center of the core. Due to the formation of the eutectic liquids, the melting temperature may be several hundred degrees below that of the UO2 melting point (3100 K). Zirconium from fuel clad, together with other metals, reacts with water and produces zirconium dioxide and hydrogen. The production of hydrogen is a major danger in reactor accidents. As the eutectic molten mass increases, the corium pool may be formed and expands axially and radially in the core until it reaches either the baffle or the core support plate. At this moment, the corium flows into the lower head. The degradation may ultimately result in very different configurations in the core simultaneously, ranging from intact or barely degraded rods to the formation of a corium pool or a bed of debris.

In all cases, the corium gradually evaporates the water present in the lower head. If there is no additional water supply and the debris con­figuration is such that it cannot be cooled effectively, the materials’ temperature gra­dually rises until it reaches the melting point of the steel structures (plates, tubes, etc.) located in the lower head. In the case of adequate cooling of the corium, it can solidify and the damage is limited to the reactor itself. However, in the absence of adequate cooling, corium may melt through the reactor vessel and flow out or be ejected as a molten stream by the pressure inside the reactor vessel.

However, core reflooding may not be beneficial in all conditions. The following pheomena can occur during reflooding:

• massive steam generation, with hydrogen production and an increase in reactor
• coolant system pressure;
• steam explosion through corium-water interaction;
• continuation of core melt, despite water inflow;
• faster release of fission products.

In the event of reactor-vessel failure during a core melt accident, the corium resulting from this core melt and the melting of internal structures will pour onto the reactor pit basemat. The molten core-concrete interaction (MCCI) is treated as one of the important phenomena that may lead to the late containment failure by basemat penetration in a hypothetical severe accident of light water reactors (LWRs). The process is driven by the high initial temperature of the molten corium and the decay heat that is generated inside the melt by the radioactive decay of the fission products. Obviously, the progression of MCCI takes paramount importance and plays a key role to threaten the integrity of the containment, the last barrier of fission products.

 

See above:
Material Problems

Neutrons with sufficient energy can disrupt the atomic arrangement or crystalline structure of materials. The influence of structural damage is most significant for metals because of their relative immunity to damage by ionizing radiation. Pressurized-water reactors operate with a higher rate of neutron impacts and their vessels therefore tend to experience a greater degree of embrittlement than boiling-water reactor vessels. Many pressurized-water reactors design their cores to reduce the number of neutrons hitting the vessel wall. This slows the vessel’s embrittlement. The NRC’s regulations address embrittlement in 10 CFR Part 50, Appendix G, “Fracture Toughness Requirements” and Appendix H, “Reactor Vessel Material Surveillance Program Requirements.” Since the reactor pressure vessel is considered irreplaceable, neutron irradiation embrittlement of pressure vessel steels is a key issue in the long term assessment of structural integrity for life attainment and extension programmes.

Radiation damage is produced when neutrons of sufficient energy displace atoms (especially in steels at operating temperatures 260 – 300°C) that result in displacement cascades which produce large numbers of defects, both vacancies and interstitials. Although the inside surface of the RPV is exposed to neutrons of varying energies, the higher energy neutrons, those above about 0.5 MeV, produce the bulk of the damage. In order to minimize such material degradation type and structure of the steel must be appropriately selected. Today it is known that the susceptibility of reactor pressure vessel steels is strongly affected (negatively) by the presence of copper, nickel and phosphorus.

As was written, the distinction between brittleness and ductility isn’t readily apparent, especially because both ductility and brittle behavior are dependent not only on the material in question but also on the temperature (ductile-brittle transition) of the material. The effect of temperature on the nature of the fracture is of considerable importance. Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the ductile–brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It can be determined by some form of drop-weight test (for example, the Charpy or Izod tests).

To minimize neutron fluence:

• Radial neutron reflectors are installed around the reactor core. Neutron reflectors reduce neutron leakage and therefore they reduce the neutron fluence on a reactor pressure vessel.
• Core designers design the low leakage loading patterns, in which fresh fuel assemblies are not situated in the peripheral positions of the reactor core.

If the metal is heated to elevated temperatures after irradiation (a form of annealing), it is found that the strength and ductility return to the same values as before irradiation. This means that radiation damage can be annealed out of a  metal.

See also: Ductile-brittle Transition Temperature

See also: Irradiation Embrittlement

See also: Thermal Annealing

Reactor Vessel Material Surveillance Program

Reactor vessel surveillance programs provide information on the effect of radiation on vessel materials under operating conditions. Reactor vessel surveillance program utilizes capsules located on the vessel wall directly opposite the center of the core. The capsules contain reactor vessel steel specimens obtained during vessel fabrication and are withdrawn periodically from the reactor vessel.  The surveillance capsules must be located near the inside vessel wall in the beltline region so that the material specimens duplicate, to the greatest degree possible, the neutron spectrum, temperature history, and maximum neutron fluence as experienced at the reactor vessel’s inner surface. A specimen capsule containing specimens for use in Charpy V-notch, tensile, and fracture mechanics tests can be removed from the reactor during normal refueling periods.

The Charpy V-notch (CVN) technique is most commonly used. The Charpy V-notch test uses a notched sample of defined cross-section. For these dynamic loading conditions and when a notch is present, we are using notch toughness. Charpy and Izod impact tests are used to measure this parameter, which is important in assessing the ductile-to-brittle transition behavior of a material. Similarly as for tensile toughness, notch toughness is measured in units of joule per cubic metre (J·m−3) in the SI system, but in this case we are measuring the area at the notch position.

There can also be special dosimeters, including pure nickel, copper, iron, aluminum-cobalt or uranium-238, which can be placed in spacers specially drilled to contain the dosimeters.

According to 10 CFR 50  Appendix H, no material surveillance program is required for reactor vessels for which it can be conservatively demonstrated by analytical methods applied to experimental data and tests performed on comparable vessels, making appropriate allowances for all uncertainties in the measurements, that the peak neutron fluence at the end of the design life of the vessel will not exceed 10¹⁷ n/cm² (E>1 MeV).

Special Reference: NUREG-1511, Reactor Pressure Vessel Status Report. U.S. Nuclear Regulatory Commission, Washington, DC, 1994.

See above:
Power Plant Materials

Pressurized water reactors use a reactor pressure vessel (RPV) to contain the nuclear fuel, moderator, control rods and coolant. They are cooled and moderated by high-pressure liquid water (e.g. 16MPa). At this pressure water boils at approximately 350°C (662°F).  Inlet temperature of the water is about 290°C (554°F). The water (coolant) is heated in the reactor core to approximately 325°C (617°F) as the water flows through the core. As it can be seen, the reactor has approximately 25°C subcooled coolant (distance from the saturation).

The reactor pressure vessel is the pressure vessel containing the reactor core and other key reactor internals. It is a cylindrical vessel with a hemispherical bottom head and a flanged and gasketed upper head. The bottom head is welded to the cylindrical shell while the top head is bolted to the cylindrical shell via the flanges. The top head is removable to allow for the refueling of the reactor during planned outages.

The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel (e.g. 304L) in order to minimize corrosion.

Low-carbon steel, also known as mild steel is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Low-carbon steel contains approximately 0.05–0.25% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it has high toughness and it is easy to form. Special requirements for materials of reactor vessel include low activation capability (especially due to Co-60 formation). Examples of high-quality low-alloy carbon steels:

• SA-508 Gr.3 Cl.2 (low-alloy ferritic steel)
• 15Kh2NMFA (low-alloy ferritic steel)

Alloying Agents

Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or chromium for instance) greatly increases its mechanical strength. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties. The four major alloying elements are:

• Chromium. In these steels chromium increases hardness and strength. Generally speaking, the concentration specified for most grades is approximately 2%. This level appears to result in the best balance between hardness and toughness. Chromium plays an important role in the hardening mechanism and is considered irreplaceable. At higher temperatures, chromium contributes increased strength.
• Nickel. Nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase.
• Molybdenum. Molybdenum (about 0.50-8.00%) when added to a steel makes it more resistant to high temperature. Molybdenum increases hardenability and strength, particularly at high temperatures due to the high melting point of molybdenum. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel.

Austenitic stainless steels, which is used as a corrosion-resistant clad, contain between 16 and 25% of chromium and can also contain nitrogen in solution, both of which contribute to their relatively high corrosion resistance. The best known grade is AISI 304 stainless, which contains both chromium (between 15% and 20%) and nickel (between 2% and 10.5%) metals as the main non-iron constituents. 304 stainless steel has excellent resistance to a wide range of atmospheric environments and many corrosive media. These alloys are usually characterized as ductile, weldable, and hardenable by cold forming.

Type 304L stainless steel, which is widely used in nuclear industry, is an extra-low carbon version of the 304 steel alloy. This grade has slightly lower mechanical properties than the standard 304 grade, but is still widely used thanks to its versatility. The lower carbon content in 304L minimizes deleterious or harmful carbide precipitation as a result of welding. 304L can, therefore, be used “as welded” in severe corrosion environments, and it eliminates the need for annealing. Grade 304 has also good oxidation resistance in intermittent service to 870 °C, and in continuous service to 925 °C. Since grade 304L does not require post-weld annealing, it is extensively used in heavy gauge components. Examples of used stainless steels:

• Type 304L stainless steel
• Type 08Kh18N10T stainless steel

The reactor pressure vessels are the highest priority key components in nuclear power plants. The reactor pressure vessel houses the reactor core and because of its function it has direct safety significance. During the operation of a nuclear power plant, the material of the reactor pressure vessel is exposed to neutron radiation (especially to fast neutrons), which results in localized embrittlement of the steel and welds in the area of the reactor core. In order to minimize such material degradation radial neutron reflectors are installed around the reactor core. There are two basic types of neutron reflectors, the core baffle and the heavy reflector. Due to higher atomic number density heavy reflectors reduce neutron leakage (especially of fast neutrons) from the core more efficiently than core baffle. Since the reactor pressure vessel is considered irreplaceable, these ageing effects of the RPV have the potential to be life-limiting conditions for a nuclear power plant.

Radiation Damage to Metals

Materials in nuclear service are subjected to various types of radiation. Some of these can cause significant damage to the crystalline structure of materials. Nuclear radiation focuses large amounts of energy into highly localized areas. Damage is caused by the interaction of this energy with the nuclei and/or orbiting electrons.

As was written, charged particles with high energies can directly ionize atoms or they can cause excitation of surrounding electrons. The ionization and excitation dissipates much of the energy of heavier charged particles and does very little damage. This is because electrons are relatively free to move and are soon replaced. The net effect of beta and gamma radiation on metal is to generate a small amount of heat. Heavier particles, such as protons, alpha-particles, fast neutrons, and fission fragments, will usually transfer sufficient energy through elastic or inelastic collisions to remove nuclei from their lattice (crystalline) positions. This addition of vacancies and interstitial atoms causes property changes in metals.

In general, the effects of greatest interest can be described by the following groupings:

• Vacancies or Knock-ons. Vacancy defects result from a missing atom in a lattice position. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. This can be caused by the direct interaction of a high energy neutron or a fission fragment. If a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for most metals) in a collision with a radiation particle (usually a fast neutron), the nucleus will be displaced from its equilibrium position in the crystal lattice. During a lengthy irradiation (for large values of the neutron fluence), many of the displaced atoms will return to normal (stable) lattice sites (that is, partial annealing occurs spontaneously).
• Interstitials. Interstitial defects result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of being at its lattice position. An interstitial is formed when an atom, which is knocked out from its position, comes to rest at some remote point.
• Ionization. Ionization is caused by the removal of electrons from their electronic shells and has the effect of changing the chemical bonds of molecules. In metal, ionization does not cause dramatic changes in material’s properties. This is due to free electrons, which are typical only for metallic bond.
• Thermal and Displacement Spikes. Thermal and displacement spikes can cause distortion that is frozen as stress in the microscopic area. These spikes can cause a change in the material’s properties. This term identifies localized high temperature domains caused by the deposition of energy from neutrons and fission fragments. A displacement spike occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock-ons). A 1 MeV neutron may affect approximately 5000 atoms, making up one of these spikes. The presence of many displacement spikes changes the properties of the metal being irradiated, such as increasing hardness and decreasing ductility.
• Impurity Atoms. The capture of neutrons and nuclear reactions induced by various radiations has the effect of transmuting an atom into an element which is foreign to the material.
• Radiation Induced Creep. In nuclear reactors, many metal components are subjected simultaneously to radiation fields, elevated temperatures and stress. Metal under stress at elevated temperature exhibits the phenomenon of creep, ie. the gradual increase in strain with time. Creep of metal components at reactor operating temperatures becomes faster when they are exposed to a radiation field.

Neutrons with sufficient energy can disrupt the atomic arrangement or crystalline structure of materials. The influence of structural damage is most significant for metals because of their relative immunity to damage by ionizing radiation. Pressurized-water reactors operate with a higher rate of neutron impacts and their vessels therefore tend to experience a greater degree of embrittlement than boiling-water reactor vessels. Many pressurized-water reactors design their cores to reduce the number of neutrons hitting the vessel wall. This slows the vessel’s embrittlement. The NRC’s regulations address embrittlement in 10 CFR Part 50, Appendix G, “Fracture Toughness Requirements” and Appendix H, “Reactor Vessel Material Surveillance Program Requirements.” Since the reactor pressure vessel is considered irreplaceable, neutron irradiation embrittlement of pressure vessel steels is a key issue in the long term assessment of structural integrity for life attainment and extension programmes.

Radiation damage is produced when neutrons of sufficient energy displace atoms (especially in steels at operating temperatures 260 – 300°C) that result in displacement cascades which produce large numbers of defects, both vacancies and interstitials. Although the inside surface of the RPV is exposed to neutrons of varying energies, the higher energy neutrons, those above about 0.5 MeV, produce the bulk of the damage. In order to minimize such material degradation type and structure of the steel must be appropriately selected. Today it is known that the susceptibility of reactor pressure vessel steels is strongly affected (negatively) by the presence of copper, nickel and phosphorus.

As was written, the distinction between brittleness and ductility isn’t readily apparent, especially because both ductility and brittle behavior are dependent not only on the material in question but also on the temperature (ductile-brittle transition) of the material. The effect of temperature on the nature of the fracture is of considerable importance. Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the ductile–brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It can be determined by some form of drop-weight test (for example, the Charpy or Izod tests).

To minimize neutron fluence:

• Radial neutron reflectors are installed around the reactor core. Neutron reflectors reduce neutron leakage and therefore they reduce the neutron fluence on a reactor pressure vessel.
• Core designers design the low leakage loading patterns, in which fresh fuel assemblies are not situated in the peripheral positions of the reactor core.

If the metal is heated to elevated temperatures after irradiation (a form of annealing), it is found that the strength and ductility return to the same values as before irradiation. This means that radiation damage can be annealed out of a  metal.

See also: Ductile-brittle Transition Temperature

See also: Irradiation Embrittlement

See also: Thermal Annealing

See above:
Power Plant Materials

Materials in nuclear service are subjected to various types of radiation. Some of these can cause significant damage to the crystalline structure of materials. Nuclear radiation focuses large amounts of energy into highly localized areas. Damage is caused by the interaction of this energy with the nuclei and/or orbiting electrons.

As was written, charged particles with high energies can directly ionize atoms or they can cause excitation of surrounding electrons. The ionization and excitation dissipates much of the energy of heavier charged particles and does very little damage. This is because electrons are relatively free to move and are soon replaced. The net effect of beta and gamma radiation on metal is to generate a small amount of heat. Heavier particles, such as protons, alpha-particles, fast neutrons, and fission fragments, will usually transfer sufficient energy through elastic or inelastic collisions to remove nuclei from their lattice (crystalline) positions. This addition of vacancies and interstitial atoms causes property changes in metals.

In general, the effects of greatest interest can be described by the following groupings:

• Vacancies or Knock-ons. Vacancy defects result from a missing atom in a lattice position. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. This can be caused by the direct interaction of a high energy neutron or a fission fragment. If a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for most metals) in a collision with a radiation particle (usually a fast neutron), the nucleus will be displaced from its equilibrium position in the crystal lattice. During a lengthy irradiation (for large values of the neutron fluence), many of the displaced atoms will return to normal (stable) lattice sites (that is, partial annealing occurs spontaneously).
• Interstitials. Interstitial defects result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of being at its lattice position. An interstitial is formed when an atom, which is knocked out from its position, comes to rest at some remote point.
• Ionization. Ionization is caused by the removal of electrons from their electronic shells and has the effect of changing the chemical bonds of molecules. In metal, ionization does not cause dramatic changes in material’s properties. This is due to free electrons, which are typical only for metallic bond.
• Thermal and Displacement Spikes. Thermal and displacement spikes can cause distortion that is frozen as stress in the microscopic area. These spikes can cause a change in the material’s properties. This term identifies localized high temperature domains caused by the deposition of energy from neutrons and fission fragments. A displacement spike occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock-ons). A 1 MeV neutron may affect approximately 5000 atoms, making up one of these spikes. The presence of many displacement spikes changes the properties of the metal being irradiated, such as increasing hardness and decreasing ductility.
• Impurity Atoms. The capture of neutrons and nuclear reactions induced by various radiations has the effect of transmuting an atom into an element which is foreign to the material.
• Radiation Induced Creep. In nuclear reactors, many metal components are subjected simultaneously to radiation fields, elevated temperatures and stress. Metal under stress at elevated temperature exhibits the phenomenon of creep, ie. the gradual increase in strain with time. Creep of metal components at reactor operating temperatures becomes faster when they are exposed to a radiation field.

Neutrons with sufficient energy can disrupt the atomic arrangement or crystalline structure of materials. The influence of structural damage is most significant for metals because of their relative immunity to damage by ionizing radiation. Pressurized-water reactors operate with a higher rate of neutron impacts and their vessels therefore tend to experience a greater degree of embrittlement than boiling-water reactor vessels. Many pressurized-water reactors design their cores to reduce the number of neutrons hitting the vessel wall. This slows the vessel’s embrittlement. The NRC’s regulations address embrittlement in 10 CFR Part 50, Appendix G, “Fracture Toughness Requirements” and Appendix H, “Reactor Vessel Material Surveillance Program Requirements.” Since the reactor pressure vessel is considered irreplaceable, neutron irradiation embrittlement of pressure vessel steels is a key issue in the long term assessment of structural integrity for life attainment and extension programmes.

Radiation damage is produced when neutrons of sufficient energy displace atoms (especially in steels at operating temperatures 260 – 300°C) that result in displacement cascades which produce large numbers of defects, both vacancies and interstitials. Although the inside surface of the RPV is exposed to neutrons of varying energies, the higher energy neutrons, those above about 0.5 MeV, produce the bulk of the damage. In order to minimize such material degradation type and structure of the steel must be appropriately selected. Today it is known that the susceptibility of reactor pressure vessel steels is strongly affected (negatively) by the presence of copper, nickel and phosphorus.

As was written, the distinction between brittleness and ductility isn’t readily apparent, especially because both ductility and brittle behavior are dependent not only on the material in question but also on the temperature (ductile-brittle transition) of the material. The effect of temperature on the nature of the fracture is of considerable importance. Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the ductile–brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It can be determined by some form of drop-weight test (for example, the Charpy or Izod tests).

To minimize neutron fluence:

• Radial neutron reflectors are installed around the reactor core. Neutron reflectors reduce neutron leakage and therefore they reduce the neutron fluence on a reactor pressure vessel.
• Core designers design the low leakage loading patterns, in which fresh fuel assemblies are not situated in the peripheral positions of the reactor core.

If the metal is heated to elevated temperatures after irradiation (a form of annealing), it is found that the strength and ductility return to the same values as before irradiation. This means that radiation damage can be annealed out of a  metal.

See also: Ductile-brittle Transition Temperature

See also: Irradiation Embrittlement

See also: Thermal Annealing

See above:
Defects

Nuclear reactors are significant sources of radiation, especially neutron radiation. At power operation, the fission reaction is responsible for the power generated in a nuclear reactor, and the fission reaction rate is proportional to the neutron flux. When a reactor is shut down, fission essentially ceases, but decay energy is still being produced. The energy produced after shutdown is referred to as decay heat. Nuclear reactors are therefore sources of various types of radiation with neutrons being most important. Each type of radiation interacts in a different way, therefore we must describe interaction of particles (radiation as a flow of these particles) separately. For example, charged particles with high energies can directly ionize atoms. On the other hand electrically neutral particles interacts only indirectly, but can also transfer some or all of their energies to the matter.

This is the key feature of the categorization of radiation sources. They are usually categorized into two general types as follows:

• Charged particles (directly ionizing)
  • Beta particles. Beta particles are fast electrons or positrons emitted in nuclear beta decay, as well as energetic electrons produced by any other process. Beta radiation ionizes matter weaker than alpha radiation. On the other hand the ranges of beta particles are longer and depends strongly on initial kinetic energy of particle.
  • Heavy charged particles. Heavy charged particles are all energetic ions with mass of one atomic mass unit or greater, such as protons, alpha particles (helium nuclei) or fission fragments. The stopping power of most materials is very high for alpha particles and for heavy charged particles. Therefore alpha particles have very short ranges.On the other hand they produce massive ionisation of surrounding matter.
• Neutral particles (indirectly ionizing)
  • Gamma radiation (electromagnetic radiation). Gamma rays ionize matter primarily via indirect ionization. Gamma rays are very penetrating, on the other hand their ionisation is not so intense as for charged particles. Although a large number of possible interactions are known, there are three key interaction mechanisms  with matter.
    • Photoelectric effect
    • Compton scattering
    • Pair production
  • Neutrons. Neutrons can be emitted by nuclear fission or by the decay of some radioactive atoms. Neutrons have no net electric charge, therefore they cannot be affected or stopped by electric forces. Neutrons ionize matter only indirectly, which makes neutrons highly penetrating type of radiation. Neutrons scatter with heavy nuclei very elastically. Heavy nuclei very hard slow down a neutron let alone absorb a fast neutron. An absorption of neutron (one would say shielding) causes initiation of certain nuclear reaction (capture, rearrangement or even fission), which is accompanied by a number of other types of radiation. In short, only neutrons make matter radioactive, therefore with neutrons we have to shield also the other types of radiation.

Radiation induced Crystallographic Defects

Materials in nuclear service are subjected to various types of radiation. Some of these can cause significant damage to the crystalline structure of materials. Nuclear radiation focuses large amounts of energy into highly localized areas. Damage is caused by the interaction of this energy with the nuclei and/or orbiting electrons.

As was written, charged particles with high energies can directly ionize atoms or they can cause excitation of surrounding electrons. The ionization and excitation dissipates much of the energy of heavier charged particles and does very little damage. This is because electrons are relatively free to move and are soon replaced. The net effect of beta and gamma radiation on metal is to generate a small amount of heat. Heavier particles, such as protons, alpha-particles, fast neutrons, and fission fragments, will usually transfer sufficient energy through elastic or inelastic collisions to remove nuclei from their lattice (crystalline) positions. This addition of vacancies and interstitial atoms causes property changes in metals.

In general, the effects of greatest interest can be described by the following groupings:

• Vacancies or Knock-ons. Vacancy defects result from a missing atom in a lattice position. The stability of the surrounding crystal structure guarantees that the neighboring atoms will not simply collapse around the vacancy. This can be caused by the direct interaction of a high energy neutron or a fission fragment. If a target or struck nucleus gains about 25 eV of kinetic energy (25 eV to 30 eV for most metals) in a collision with a radiation particle (usually a fast neutron), the nucleus will be displaced from its equilibrium position in the crystal lattice. During a lengthy irradiation (for large values of the neutron fluence), many of the displaced atoms will return to normal (stable) lattice sites (that is, partial annealing occurs spontaneously).
• Interstitials. Interstitial defects result from an impurity located at an interstitial site or one of the lattice atoms being in an interstitial position instead of being at its lattice position. An interstitial is formed when an atom, which is knocked out from its position, comes to rest at some remote point.
• Ionization. Ionization is caused by the removal of electrons from their electronic shells and has the effect of changing the chemical bonds of molecules. In metal, ionization does not cause dramatic changes in material’s properties. This is due to free electrons, which are typical only for metallic bond.
• Thermal and Displacement Spikes. Thermal and displacement spikes can cause distortion that is frozen as stress in the microscopic area. These spikes can cause a change in the material’s properties. This term identifies localized high temperature domains caused by the deposition of energy from neutrons and fission fragments. A displacement spike occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock-ons). A 1 MeV neutron may affect approximately 5000 atoms, making up one of these spikes. The presence of many displacement spikes changes the properties of the metal being irradiated, such as increasing hardness and decreasing ductility.
• Impurity Atoms. The capture of neutrons and nuclear reactions induced by various radiations has the effect of transmuting an atom into an element which is foreign to the material.
• Radiation Induced Creep. In nuclear reactors, many metal components are subjected simultaneously to radiation fields, elevated temperatures and stress. Metal under stress at elevated temperature exhibits the phenomenon of creep, ie. the gradual increase in strain with time. Creep of metal components at reactor operating temperatures becomes faster when they are exposed to a radiation field.

Neutrons with sufficient energy can disrupt the atomic arrangement or crystalline structure of materials. The influence of structural damage is most significant for metals because of their relative immunity to damage by ionizing radiation. Pressurized-water reactors operate with a higher rate of neutron impacts and their vessels therefore tend to experience a greater degree of embrittlement than boiling-water reactor vessels. Many pressurized-water reactors design their cores to reduce the number of neutrons hitting the vessel wall. This slows the vessel’s embrittlement. The NRC’s regulations address embrittlement in 10 CFR Part 50, Appendix G, “Fracture Toughness Requirements” and Appendix H, “Reactor Vessel Material Surveillance Program Requirements.” Since the reactor pressure vessel is considered irreplaceable, neutron irradiation embrittlement of pressure vessel steels is a key issue in the long term assessment of structural integrity for life attainment and extension programmes.

Radiation damage is produced when neutrons of sufficient energy displace atoms (especially in steels at operating temperatures 260 – 300°C) that result in displacement cascades which produce large numbers of defects, both vacancies and interstitials. Although the inside surface of the RPV is exposed to neutrons of varying energies, the higher energy neutrons, those above about 0.5 MeV, produce the bulk of the damage. In order to minimize such material degradation type and structure of the steel must be appropriately selected. Today it is known that the susceptibility of reactor pressure vessel steels is strongly affected (negatively) by the presence of copper, nickel and phosphorus.

As was written, the distinction between brittleness and ductility isn’t readily apparent, especially because both ductility and brittle behavior are dependent not only on the material in question but also on the temperature (ductile-brittle transition) of the material. The effect of temperature on the nature of the fracture is of considerable importance. Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the ductile–brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements. It can be determined by some form of drop-weight test (for example, the Charpy or Izod tests).

To minimize neutron fluence:

• Radial neutron reflectors are installed around the reactor core. Neutron reflectors reduce neutron leakage and therefore they reduce the neutron fluence on a reactor pressure vessel.
• Core designers design the low leakage loading patterns, in which fresh fuel assemblies are not situated in the peripheral positions of the reactor core.

If the metal is heated to elevated temperatures after irradiation (a form of annealing), it is found that the strength and ductility return to the same values as before irradiation. This means that radiation damage can be annealed out of a  metal.

See also: Ductile-brittle Transition Temperature

See also: Irradiation Embrittlement

See also: Thermal Annealing

Reactor Vessel Material Surveillance Program

Reactor vessel surveillance programs provide information on the effect of radiation on vessel materials under operating conditions. Reactor vessel surveillance program utilizes capsules located on the vessel wall directly opposite the center of the core. The capsules contain reactor vessel steel specimens obtained during vessel fabrication and are withdrawn periodically from the reactor vessel.  The surveillance capsules must be located near the inside vessel wall in the beltline region so that the material specimens duplicate, to the greatest degree possible, the neutron spectrum, temperature history, and maximum neutron fluence as experienced at the reactor vessel’s inner surface. A specimen capsule containing specimens for use in Charpy V-notch, tensile, and fracture mechanics tests can be removed from the reactor during normal refueling periods.

The Charpy V-notch (CVN) technique is most commonly used. The Charpy V-notch test uses a notched sample of defined cross-section. For these dynamic loading conditions and when a notch is present, we are using notch toughness. Charpy and Izod impact tests are used to measure this parameter, which is important in assessing the ductile-to-brittle transition behavior of a material. Similarly as for tensile toughness, notch toughness is measured in units of joule per cubic metre (J·m−3) in the SI system, but in this case we are measuring the area at the notch position.

There can also be special dosimeters, including pure nickel, copper, iron, aluminum-cobalt or uranium-238, which can be placed in spacers specially drilled to contain the dosimeters.

According to 10 CFR 50  Appendix H, no material surveillance program is required for reactor vessels for which it can be conservatively demonstrated by analytical methods applied to experimental data and tests performed on comparable vessels, making appropriate allowances for all uncertainties in the measurements, that the peak neutron fluence at the end of the design life of the vessel will not exceed 10¹⁷ n/cm² (E>1 MeV).

Special Reference: NUREG-1511, Reactor Pressure Vessel Status Report. U.S. Nuclear Regulatory Commission, Washington, DC, 1994.

Reactor Pressure Vessel Annealing

During the operation of a nuclear power plant, the material of the reactor pressure vessel and the material of other reactor internals are exposed to neutron radiation (especially to fast neutrons >0.5MeV), which results in localized embrittlement of the steel and welds in the area of the reactor core. This phenomenon, known as irradiation embrittlment, results in:

• Steadily increase in DBTT. It is not likely that the DBTT will approach the normal operating temperature of the steel. However, there is a possibility that when the reactor is being shut down or during an abnormal cooldown, the temperature may fall below the DBTT value while the internal pressure is still high.
• Drop in the upper shelf fracture energy. Radiation effects are also manifested by a drop in the upper shelf fracture energy and decrease in fracture toughness.

All these effects must be monitored by plant operators. Therefore nuclear regulators require that a reactor vessel material surveillance program be conducted in watercooled power reactors.

Once a material of RPV is degraded by radiation embrittlement (e.g. significant increase in Charpy ductile‐brittle transition temperature or reduction of fracture toughness), thermal annealing of the RPV is the only way to recover the RPV material toughness properties.

According to 10 CFR 50.66 – Requirements for thermal annealing of the reactor pressure vessel:

„For those light water nuclear power reactors where neutron radiation has reduced the fracture toughness of the reactor vessel materials, a thermal annealing may be applied to the reactor vessel to recover the fracture toughness of the material.“

Thermal annealing („dry“ method) of reactor pressure vessel is a method by which the pressure vessel (with all reactor internals removed) is heated up to some temperature (usually between 420 – 460 °C) by use of an external heat source (electrical heaters, hot air), held for a given period (e.g. 100 – 200 hours) and then slowly cooled. The annealing equipment is usually a ring‐shaped furnace with heating elements on its external surface. The power output of installed heaters may reach up to 1 MWe. It was shown that for the specially fabricated materials the upper shelf recovered 100 % after 24 hours annealing and more rapidly than transition temperature. Annealing for 168  hours recovered 90 % of transition temperature shift.

Wet Annealing

There is also a possibility of the so‐called “wet” annealing method which was applied in USA and Belgium. The annealing at that temperature ~340 °C was reached without external heating, but by increasing the coolant temperature achieved by the energy of the circulating pumps of the primary circuit. This type of annealing provides only partial recovery for the material due to the limitation in maximum temperature.

Special Reference: Annealing and re-embrittlement of reactor pressure vessel materials. AMES report N.19; ISSN 1018-5593. European Communities, 2008.

See above:
Power Plant Materials

The NSSS heatup from Cold Shutdown (MODE 5) to Hot Standby (MODE 3) is performed by reactor coolant pumps which are very powerful (they can consume up to 6 MW each) and therefore its work together with a decay heat can be used for heating the primary coolant before a reactor startup. To operate the reactor coolant pumps, reactor coolant system pressure must be increased to satisfy net positive suction head requirements. Reactor coolant pumps are started sequentially. Primary plant heatup rate is limited to about 30°C per hour in order to minimize internal stress in the material of the pressure vessel, primary piping and other components.

Heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Large components such as flanges, the reactor vessel head, and even the reactor vessel itself are the limiting components. Usually the most limiting component will set the heatup and cooldown rates.

See above:
Power Plant Materials

Pressure stresses are stresses induced in vessels containing pressurized materials. The loading is provided by the same force producing the thermodynamics/thermodynamic-properties/what-is-pressure-physics/”>pressure. Thermal stresses exist whenever temperature gradients are present in a material. Different thermodynamics/thermodynamic-properties/what-is-temperature-physics/”>temperatures produce different expansions and subject materials to internal stress. This type of stress is particularly noticeable in mechanisms operating at high temperatures that are cooled by a cold fluid. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together. These stresses, cyclic in nature, can lead to fatigue failure of the materials.

The reactor pressure vessel and piping, by contrast, are subjected to large load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the steel. Thermal sleeves are used in some cases, such as spray nozzles and surge lines, to minimize thermal stresses. Heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Additionally, plant design modifications include for example heating up of the Emergency Core Cooling System (ECCS) water tanks or sumps in order to reduce the temperature difference between injected water and tha material of RPV.

One safety issue that is a long-term problem brought on by the aging of nuclear facilities is pressurized thermal shock (PTS). PTS is the shock experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change.

Special Reference: Reactor Pressure Vessel Status Report, U.S. NRC. NUREG-1511. Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission, Washington, 1994.

Pressurized Thermal Shock – PTS

In general thermal shock is a mechanical load caused by a rapid change of temperature of a certain point. The change in temperature causes stresses on the surface that are in tension, which can encourage crack formation and propagation. Usually ceramics materials are usually susceptible to thermal shock, but under some circumstances also pressurized vessels suffer from thermal shocks. With rapid heating (or cooling) of a thick-walled vessel such as the reactor pressure vessel, one part of the wall may try to expand (or contract) while the adjacent section, which has not yet been exposed to the temperature change, tries to restrain it.

Pressurized Thermal Shock, PTS,  means an event or transient in pressurized water reactors (PWRs) causing severe overcooling (thermal shock) concurrent with or followed by significant pressure in the reactor vessel. In this accident scenario, cold water enters a reactor while the vessel is pressurized. This rapidly cools the vessel and places large thermal stresses on the steel. Severe reactor system overcooling events that could be accompanied by pressurization or repressurization of the reactor vessel can result from a variety of causes. Pressure in the reactor system raises the severity of the thermal shock due to the addition of stress from pressure. Transients, which combine high system pressure and a severe thermal shock, are potentially more dangerous due to the added effect of the tensile stresses on the inside of the reactor vessel wall. PTS related transients include:

• stuck-open valves in the primary system,
• stuck-open valves in the secondary system,
• small-break loss-of-coolant accidents with subsequent injection of emergency core cooling system (ECCS) water,
• main steam line breaks,
• feedwater line breaks.

The NRC created 10 CFR Part 50.61 and 50.61a – the “PTS rule” and “alternate PTS rule” – to ensure the vessel’s steel remains strong enough to protect the vessel’s integrity. These rules require additional evaluations or other actions if embrittlement reaches certain limits.

RT_NDT = RT_NDT(U) + M + ΔRT_NDT

 where:

• RT_NDT means the reference temperature for a reactor vessel material, under any conditions. For the reactor vessel beltline materials, RTNDT must account for the effects of neutron radiation.
• RT_NDT(U) means the reference temperature for a reactor vessel material in the pre-service or unirradiated condition.
• ΔRT_NDT is the increase in RT_NDT caused by irradiation
• M is a margin added to cover uncertainties in the initial properties, copper and nickel contents, fluence, and calculation procedures. The greater the amounts of copper, nickel, and neutron fluence, the greater the increase.

As long as the fracture toughness of the reactor vessel material is relatively high, such events will not threaten RPV integrity. However, the fracture toughness of reactor vessel materials decreases with exposure to fast neutrons during the life of a nuclear power plant. If the fracture toughness of the vessel material has been reduced sufficiently, severe PTS events could cause propagation of small flaws that might exist near the inner surface of the vessel. The assumed initial flaw might propagate into a crack through the vessel wall of sufficient extent to threaten vessel integrity and, therefore, core cooling capability.

While PTS doesn’t affect boiling-water reactors, there are very limited conditions where those vessels could overpressurize at low temperatures.

Special Reference: NUREG-1511, Reactor Pressure Vessel Status Report. U.S. Nuclear Regulatory Commission, Washington, DC, 1994.

Special Reference: DOE FUNDAMENTALS HANDBOOK MATERIAL SCIENCE Volume 2 of 2, DOE-HDBK-1017/2-93, Washington, DC, 1993.

See above:
Power Plant Materials

Pressure stresses are stresses induced in vessels containing pressurized materials. The loading is provided by the same force producing the thermodynamics/thermodynamic-properties/what-is-pressure-physics/”>pressure. Thermal stresses exist whenever temperature gradients are present in a material. Different thermodynamics/thermodynamic-properties/what-is-temperature-physics/”>temperatures produce different expansions and subject materials to internal stress. This type of stress is particularly noticeable in mechanisms operating at high temperatures that are cooled by a cold fluid. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together. These stresses, cyclic in nature, can lead to fatigue failure of the materials.

The reactor pressure vessel and piping, by contrast, are subjected to large load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the steel. Thermal sleeves are used in some cases, such as spray nozzles and surge lines, to minimize thermal stresses. Heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Additionally, plant design modifications include for example heating up of the Emergency Core Cooling System (ECCS) water tanks or sumps in order to reduce the temperature difference between injected water and tha material of RPV.

One safety issue that is a long-term problem brought on by the aging of nuclear facilities is pressurized thermal shock (PTS). PTS is the shock experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change.

Special Reference: Reactor Pressure Vessel Status Report, U.S. NRC. NUREG-1511. Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission, Washington, 1994.

Pressure and Temperature (P/T) Limits

Pressure and Temperature (P/T) Limits are limiting curves defined in plant’s Technical Specification. Each P/T limit curve defines an acceptable region for normal operation. 10 CFR 50, Appendix G, requires the establishment of P/T limits for specific material fracture toughness requirements of the pressure boundary materials. The P-T limits are derived on the basis of linear elastic fracture mechanics (LEFM) analyses. In these analyses, the minimum temperature necessary to ensure adequate margins against RPV failure is determined as a function of pressure.

P/T are based on reactor vessel and head stress limitations and the need to preclude reactor vessel and head brittle fracture. The usual use of the curves is operational guidance during heatup or cooldown maneuvering, when pressure and temperature indications are monitored and compared to the applicable curve to determine that operation is within the allowable region. The curves used by operations also incorporate instrument error to ensure adequate safety margin. Because of the embrittling effects of neutron irradiation, the MPT curve will shift to the right over core life to account for the increased brittleness or decreased ductility.

See above:
Power Plant Materials

Pressure stresses are stresses induced in vessels containing pressurized materials. The loading is provided by the same force producing the thermodynamics/thermodynamic-properties/what-is-pressure-physics/”>pressure. Thermal stresses exist whenever temperature gradients are present in a material. Different thermodynamics/thermodynamic-properties/what-is-temperature-physics/”>temperatures produce different expansions and subject materials to internal stress. This type of stress is particularly noticeable in mechanisms operating at high temperatures that are cooled by a cold fluid. These stresses can be comprised of tensile stress, which is stress arising from forces acting in opposite directions tending to pull a material apart, and compressive stress, which is stress arising from forces acting in opposite directions tending to push a material together. These stresses, cyclic in nature, can lead to fatigue failure of the materials.

The reactor pressure vessel and piping, by contrast, are subjected to large load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the steel. Thermal sleeves are used in some cases, such as spray nozzles and surge lines, to minimize thermal stresses. Heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Additionally, plant design modifications include for example heating up of the Emergency Core Cooling System (ECCS) water tanks or sumps in order to reduce the temperature difference between injected water and tha material of RPV.

One safety issue that is a long-term problem brought on by the aging of nuclear facilities is pressurized thermal shock (PTS). PTS is the shock experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change.

Special Reference: Reactor Pressure Vessel Status Report, U.S. NRC. NUREG-1511. Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission, Washington, 1994.

Pressure and Temperature (P/T) Limits

Pressure and temperature limits (P/T) are limiting curves defined in plant’s Technical Specification. Each P/T limit curve defines an acceptable region for normal operation. 10 CFR 50, Appendix G, requires the establishment of P/T limits for specific material fracture toughness requirements of the pressure boundary materials. The P-T limits are derived on the basis of linear elastic fracture mechanics (LEFM) analyses. In these analyses, the minimum temperature necessary to ensure adequate margins against RPV failure is determined as a function of pressure.

P/T are based on reactor vessel and head stress limitations and the need to preclude reactor vessel and head brittle fracture. The usual use of the curves is operational guidance during heatup or cooldown maneuvering, when pressure and temperature indications are monitored and compared to the applicable curve to determine that operation is within the allowable region. The curves used by operations also incorporate instrument error to ensure adequate safety margin. Because of the embrittling effects of neutron irradiation, the MPT curve will shift to the right over core life to account for the increased brittleness or decreased ductility.

Pressurized Thermal Shock – PTS

In general thermal shock is a mechanical load caused by a rapid change of temperature of a certain point. The change in temperature causes stresses on the surface that are in tension, which can encourage crack formation and propagation. Usually ceramics materials are usually susceptible to thermal shock, but under some circumstances also pressurized vessels suffer from thermal shocks. With rapid heating (or cooling) of a thick-walled vessel such as the reactor pressure vessel, one part of the wall may try to expand (or contract) while the adjacent section, which has not yet been exposed to the temperature change, tries to restrain it.

Pressurized Thermal Shock, PTS,  means an event or transient in pressurized water reactors (PWRs) causing severe overcooling (thermal shock) concurrent with or followed by significant pressure in the reactor vessel. In this accident scenario, cold water enters a reactor while the vessel is pressurized. This rapidly cools the vessel and places large thermal stresses on the steel. Severe reactor system overcooling events that could be accompanied by pressurization or repressurization of the reactor vessel can result from a variety of causes. Pressure in the reactor system raises the severity of the thermal shock due to the addition of stress from pressure. Transients, which combine high system pressure and a severe thermal shock, are potentially more dangerous due to the added effect of the tensile stresses on the inside of the reactor vessel wall. PTS related transients include:

• stuck-open valves in the primary system,
• stuck-open valves in the secondary system,
• small-break loss-of-coolant accidents with subsequent injection of emergency core cooling system (ECCS) water,
• main steam line breaks,
• feedwater line breaks.

The NRC created 10 CFR Part 50.61 and 50.61a – the “PTS rule” and “alternate PTS rule” – to ensure the vessel’s steel remains strong enough to protect the vessel’s integrity. These rules require additional evaluations or other actions if embrittlement reaches certain limits.

RT_NDT = RT_NDT(U) + M + ΔRT_NDT

 where:

• RT_NDT means the reference temperature for a reactor vessel material, under any conditions. For the reactor vessel beltline materials, RTNDT must account for the effects of neutron radiation.
• RT_NDT(U) means the reference temperature for a reactor vessel material in the pre-service or unirradiated condition.
• ΔRT_NDT is the increase in RT_NDT caused by irradiation
• M is a margin added to cover uncertainties in the initial properties, copper and nickel contents, fluence, and calculation procedures. The greater the amounts of copper, nickel, and neutron fluence, the greater the increase.

As long as the fracture toughness of the reactor vessel material is relatively high, such events will not threaten RPV integrity. However, the fracture toughness of reactor vessel materials decreases with exposure to fast neutrons during the life of a nuclear power plant. If the fracture toughness of the vessel material has been reduced sufficiently, severe PTS events could cause propagation of small flaws that might exist near the inner surface of the vessel. The assumed initial flaw might propagate into a crack through the vessel wall of sufficient extent to threaten vessel integrity and, therefore, core cooling capability.

While PTS doesn’t affect boiling-water reactors, there are very limited conditions where those vessels could overpressurize at low temperatures.

Special Reference: NUREG-1511, Reactor Pressure Vessel Status Report. U.S. Nuclear Regulatory Commission, Washington, DC, 1994.

Special Reference: DOE FUNDAMENTALS HANDBOOK MATERIAL SCIENCE Volume 2 of 2, DOE-HDBK-1017/2-93, Washington, DC, 1993.

See above:
Power Plant Materials

Pressurized water reactors use a reactor pressure vessel (RPV) to contain the nuclear fuel, moderator, control rods and coolant. They are cooled and moderated by high-pressure liquid water (e.g. 16MPa). At this pressure water boils at approximately 350°C (662°F).  Inlet temperature of the water is about 290°C (554°F). The water (coolant) is heated in the reactor core to approximately 325°C (617°F) as the water flows through the core. As it can be seen, the reactor has approximately 25°C subcooled coolant (distance from the saturation).

The reactor pressure vessel is the pressure vessel containing the reactor core and other key reactor internals. It is a cylindrical vessel with a hemispherical bottom head and a flanged and gasketed upper head. The bottom head is welded to the cylindrical shell while the top head is bolted to the cylindrical shell via the flanges. The top head is removable to allow for the refueling of the reactor during planned outages.

The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel (e.g. 304L) in order to minimize corrosion.

Low-carbon steel, also known as mild steel is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Low-carbon steel contains approximately 0.05–0.25% carbon making it malleable and ductile. Mild steel has a relatively low tensile strength, but it has high toughness and it is easy to form. Special requirements for materials of reactor vessel include low activation capability (especially due to Co-60 formation). Examples of high-quality low-alloy carbon steels:

• SA-508 Gr.3 Cl.2 (low-alloy ferritic steel)
• 15Kh2NMFA (low-alloy ferritic steel)

Alloying Agents

Pure iron is too soft to be used for the purpose of structure, but the addition of small quantities of other elements (carbon, manganese or chromium for instance) greatly increases its mechanical strength. The synergistic effect of alloying elements and heat treatment produces a tremendous variety of microstructures and properties. The four major alloying elements are:

• Chromium. In these steels chromium increases hardness and strength. Generally speaking, the concentration specified for most grades is approximately 2%. This level appears to result in the best balance between hardness and toughness. Chromium plays an important role in the hardening mechanism and is considered irreplaceable. At higher temperatures, chromium contributes increased strength.
• Nickel. Nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase.
• Molybdenum. Molybdenum (about 0.50-8.00%) when added to a steel makes it more resistant to high temperature. Molybdenum increases hardenability and strength, particularly at high temperatures due to the high melting point of molybdenum. Molybdenum is unique in the extent to which it increases the high-temperature tensile and creep strengths of steel.

Austenitic stainless steels, which is used as a corrosion-resistant clad, contain between 16 and 25% of chromium and can also contain nitrogen in solution, both of which contribute to their relatively high corrosion resistance. The best known grade is AISI 304 stainless, which contains both chromium (between 15% and 20%) and nickel (between 2% and 10.5%) metals as the main non-iron constituents. 304 stainless steel has excellent resistance to a wide range of atmospheric environments and many corrosive media. These alloys are usually characterized as ductile, weldable, and hardenable by cold forming.

Type 304L stainless steel, which is widely used in nuclear industry, is an extra-low carbon version of the 304 steel alloy. This grade has slightly lower mechanical properties than the standard 304 grade, but is still widely used thanks to its versatility. The lower carbon content in 304L minimizes deleterious or harmful carbide precipitation as a result of welding. 304L can, therefore, be used “as welded” in severe corrosion environments, and it eliminates the need for annealing. Grade 304 has also good oxidation resistance in intermittent service to 870 °C, and in continuous service to 925 °C. Since grade 304L does not require post-weld annealing, it is extensively used in heavy gauge components. Examples of used stainless steels:

• Type 304L stainless steel
• Type 08Kh18N10T stainless steel

The reactor pressure vessels are the highest priority key components in nuclear power plants. The reactor pressure vessel houses the reactor core and because of its function it has direct safety significance. During the operation of a nuclear power plant, the material of the reactor pressure vessel is exposed to neutron radiation (especially to fast neutrons), which results in localized embrittlement of the steel and welds in the area of the reactor core. In order to minimize such material degradation radial neutron reflectors are installed around the reactor core. There are two basic types of neutron reflectors, the core baffle and the heavy reflector. Due to higher atomic number density heavy reflectors reduce neutron leakage (especially of fast neutrons) from the core more efficiently than core baffle. Since the reactor pressure vessel is considered irreplaceable, these ageing effects of the RPV have the potential to be life-limiting conditions for a nuclear power plant.

Material Problems and Challenges of Nuclear Reactors

The main problems or rather challenges, that must be taken into account when designing reactors, are:

• Pressure and temperature stresses with associated limits
  • Pressure and Temperature (P/T) Limits
  • Heatup and cooldown rates
  • Low-temperature Overpressure Protection Limits
  • Pressurized Thermal Shock
• Radiation Damage to Reactor Materials
  • Ductile-brittle Transition Temperature
  • Upper-shelf Toughness
• Corrosion

Special Reference: Reactor Pressure Vessel Status Report, U.S. NRC. NUREG-1511. Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission, Washington, 1994.

See above:
Power Plant Materials