The term thermal annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. This process alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. In this process, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. Metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking. Typically, annealing is carried out to relieve stresses, increase softness, ductility, and toughness; and/or produce a specific microstructure.
Generally, in plain carbon steels, annealing produces a ferrite-pearlite micro-structure. Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are relatively inexpensive and are produced in large tonnages.
Any annealing cycle consists of three stages:
- heating to the desired temperature,
- holding or “soaking” at that temperature,
- cooling, usually to room temperature.
Time and annealing temperature are important parameters in these procedures. Especially the target temperature defines the annealing thermal cycle.
Reactor Pressure Vessel Annealing
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 in order to minimize corrosion.
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.
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