Nick Connor

Natural boron consists primarily of two stable isotopes, ¹¹B (80.1%) and  ¹⁰B (19.9%). In nuclear industry boron is commonly used as a neutron absorber due to the high neutron cross-section of isotope  ¹⁰B. Its (n,alpha) reaction cross-section for thermal neutrons is about 3840 barns (for 0.025 eV neutron). Isotope  ¹¹B has absorption cross-section for thermal neutrons about 0.005 barns (for 0.025 eV neutron). Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.

Moreover, isotope ¹⁰B has high (n,alpha) reaction cross-section along the entire neutron energy spectrum. The cross-sections of most other elements becomes very small at high energies as in the case of cadmium. The cross-section of ¹⁰B decreases monotonically with energy. For fast neutrons its cross-section is on the order of barns.

Boron as the neutron absorber has another positive property. The reaction products (after a neutron absorption), helium and lithium, are stable isotopes. Therefore there are minimal problems with decay heating of control rods or burnable absorbers used in the reactor core.

On the other hand production of helium may lead to significant increase in pressure (under rod cladding), when used as the absorbing material in control rods. Moreover ¹⁰B is the principal source of radioactive tritium in primary circuit of all PWRs (which use boric acid as a chemical shim), because reactions with neutrons can rarely lead to formation of radioactive tritium via:

 10B(n,2x alpha)3H                             threshold reaction (~1.2 MeV)

and

10B(n,alpha)7Li(n,n+alpha)3H     threshold reaction (~3 MeV).

See also: Tritium

Boron letdown curve (chemical shim) and boron 10 depletion during a 12-month fuel cycle.

At the beginning of specific fuel cycle concentration of boric acid is highest. At the end of this cycle concentration of boric acid is almost zero and a reactor must be refueled, because there is no positive reactivity that can be inserted to compensate negative reactivity of fuel burnup (increase in reactor slagging).

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Glossary

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Cadmium

Natural cadmium consists of eight isotopes, ¹⁰⁶Cd (1.3%),  ¹⁰⁸Cd (0.9%), ¹¹⁰Cd (12.5%), ¹¹¹Cd (12.8%), ¹¹²Cd (24.3%), ¹¹³Cd (12.2%), ¹¹⁴Cd (28.7%) and ¹¹⁶Cd (7.5%). Two of them are radioactive isotopes with very long half-life (¹¹³Cd – 7.7 x 10¹⁵ y and ¹¹⁶Cd – 2.9 x 10¹⁹ y).

Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1 Nuclear Data Library

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Glossary

Thorium 232

Thorium 232, which alone makes up nearly all natural thorium, is the most common isotope of thorium in the nature. This isotope has the longest half-life (1.4 x 10¹⁰ years) of all isotopes with more than 83 protons. In fact, its half-life is considerably longer than the age of earth. Therefore ²³²Th belongs to primordial nuclides.²³²Th decays via alpha decay into ²²⁸Ra . ²³²Th  occasionally decays by spontaneous fission with very low probability of 1.1 x 10^-9 %.

²³²Th is a fertile isotope. ²³²Th is not capable of undergoing fission reaction after absorbing thermal neutron, on the other hand ²³²Th can be fissioned by fast neutron with energy higher than >1MeV. ²³²Th does not meet also alternative requirement to fissile materials. ²³²Th is not capable of sustaining a nuclear fission chain reaction, because too many of neutrons produced by fission of ²³²Th have lower energies than original neutron.Isotope ²³²Th is key material in the thorium fuel cycle. Radiative capture of a neutron leads to the formation of fissile ²³³U. This process is called nuclear fuel breeding. 

Uranium 233 breeding

²³²Th is the predominant isotope of natural thorium. If this fertile material is loaded in the nuclear reactor, the nuclei of ²³²Th absorb a neutron and become nuclei of ²³³Th. The half-life of ²³³Th is approximately 21.8 minutes. ²³³Th decays (negative beta decay) to ²³³Pa (protactinium), whose half-life is 26.97 days. ²³³Pa decays (negative beta decay)  to ²³³U, that is very good fissile material. On the other hand proposed reactor designs must attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.

What is Thorium

Thorium is a naturally-occurring chemical element with atomic number 90 which means there are 90 protons and 90 electrons in the atomic structure. The chemical symbol for thorium is Th. Thorium was discovered in 1828 by norwegian mineralogist Morten Thrane Esmark. Joens Jakob Berzelius, the swedish chemist, named it after Thor, the Norse god of thunder.Thorium  is a naturally-occurring element and it is estimated to be about three times more abundant than uranium. Thorium is commonly found in monazite sands (rare earth metals containing phosphate mineral).Thorium has 6 naturally occurring isotopes. All of these isotopes are unstable (radioactive), but only ²³²Th is relatively stable with half-life of 14 billion years, which is comparable to the age of the Earth (~4.5×10⁹ years). Isotope ²³²Th belongs to primordial nuclides and natural thorium consists primarily of isotope ²³²Th. Other isotopes (²³⁰Th, ²²⁹Th, ²²⁸Th, ²³⁴Th and ²²⁷Th) occur in nature as trace radioisotopes, which originate from decay of ²³²Th, ²³⁵U and ²³⁸U.

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Thorium

What is Thorium

Thorium is a naturally-occurring chemical element with atomic number 90 which means there are 90 protons and 90 electrons in the atomic structure. The chemical symbol for thorium is Th. Thorium was discovered in 1828 by norwegian mineralogist Morten Thrane Esmark. Joens Jakob Berzelius, the swedish chemist, named it after Thor, the Norse god of thunder.

Thorium  is a naturally-occurring element and it is estimated to be about three times more abundant than uranium. Thorium is commonly found in monazite sands (rare earth metals containing phosphate mineral).

Thorium has 6 naturally occurring isotopes. All of these isotopes are unstable (radioactive), but only ²³²Th is relatively stable with half-life of 14 billion years, which is comparable to the age of the Earth (~4.5×10⁹ years). Isotope ²³²Th belongs to primordial nuclides and natural thorium consists primarily of isotope ²³²Th. Other isotopes (²³⁰Th, ²²⁹Th, ²²⁸Th, ²³⁴Th and ²²⁷Th) occur in nature as trace radioisotopes, which originate from decay of ²³²Th, ²³⁵U and ²³⁸U.

Thorium 232

Thorium 232, which alone makes up nearly all natural thorium, is the most common isotope of thorium in the nature. This isotope has the longest half-life (1.4 x 10¹⁰ years) of all isotopes with more than 83 protons. In fact, its half-life is considerably longer than the age of earth. Therefore ²³²Th belongs to primordial nuclides.

²³²Th decays via alpha decay into ²²⁸Ra . ²³²Th  occasionally decays by spontaneous fission with very low probability of 1.1 x 10^-9 %.

²³²Th is a fertile isotope. ²³²Th is not capable of undergoing fission reaction after absorbing thermal neutron, on the other hand 232Th can be fissioned by fast neutron with energy higher than >1MeV. ²³²Th does not meet also alternative requirement to fissile materials. ²³²Th is not capable of sustaining a nuclear fission chain reaction, because too many of neutrons produced by fission of ²³²Th have lower energies than original neutron.

Isotope ²³²Th is key material in the thorium fuel cycle. Radiative capture of a neutron leads to the formation of fissile ²³³U. This process is called nuclear fuel breeding.

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Plutonium

See also:

Nuclear Fuel

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Plutonium 241

²⁴¹Pu is a fissile isotope, which means ²⁴¹Pu is capable of undergoing fission reaction after absorbing thermal neutron. Moreover ²⁴¹Pu meets also alternative requirement that the amount of neutrons produced by fission of 241Pu (~2.94 per one fission by thermal neutron) is sufficient to sustain a nuclear fission chain reaction. Its fission cross-section for thermal neutrons is about 1012 barns (for 0.025 eV neutron). For fast neutrons its fission cross-section is on the order of barns.

Most of absorption reactions result in fission reaction, but a part of reactions result in radiative capture forming ²⁴²Pu. The cross-section for radiative capture for thermal neutrons is about 363 barns (for 0.025 eV neutron). Therefore about 74% of all absorption reactions result in radiative capture of neutron. About 26% of all absorption reactions result in fission.

It is a man-made isotope and can be found in an irradiated uranium fuel or in a spent uranium fuel. Isotope ²⁴¹Pu is formed in a nuclear reactor from fertile isotope ²⁴⁰Pu. Absorption of a resonance or thermal neutron by the ²⁴⁰Pu nucleus yields ²⁴¹Pu.²⁴¹Pu decays via beta decay into ²⁴¹Am with half-life of only 14.3 years. ²⁴¹Am has relatively high cross-section for radiative capture for thermal neutrons (~680 barns – 0.025eV). This two phenomena (decrease in fissile isotope and increase in neutron absorber) cause slight decrease in reactivity of irradiated fuel when stored in a spent fuel pool.

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Plutonium 240

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Plutonium

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Plutonium 240

²⁴⁰Pu is a fertile isotope because its fission cross-section is very low in comparison with fissile isotopes. Radiative capture of a neutron leads to the formation of fissile ²⁴¹Pu similarly to ²³⁸U which radiative capture leads to the formation of fissile ²³⁹Pu.

It is a man-made isotope and can be found in an irradiated uranium fuel or in a spent uranium fuel. Isotope ²⁴⁰Pu is formed in a nuclear reactor from fissile isotope ²³⁹Pu. Absorption of a resonance or thermal neutron by the ²³⁹Pu nucleus yields ²⁴⁰Pu.  Trace amounts can be found in the nature. The isotope ²⁴⁰Pu is in a decay chain of the primordial isotope ²⁴⁴Pu.  These nuclei are present in the unalterable proportions of the radioactive equilibrium between ²⁴⁴Pu and ²⁴⁰Pu.

²⁴⁰Pu has relatively high radiative capture cross-section (about 290 barns for thermal neutrons).²⁴⁰Pu decays via alpha decay into ²³⁶U with half-life of 6560 years.

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Plutonium 239

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Plutonium

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Plutonium 241

Plutonium 239

²³⁹Pu is a fissile isotope, which means ²³⁹Pu is capable of undergoing fission reaction after absorbing thermal neutron. Moreover, ²³⁹Pu meets also alternative requirement that the amount (~2.88 per one fission by thermal neutron) of neutrons produced by fission of ²³⁹Pu is sufficient to sustain a nuclear fission chain reaction. This isotope is the principal fissile isotope of plutonium in use.It is a man-made isotope and can be found in an irradiated uranium fuel or in a spent uranium fuel. Isotope ²³⁹Pu is formed in a nuclear reactor from fertile isotope ²³⁸U, which constitute more than 95% of uranium fuel (e.g.  PWRs and BWRs require 3% – 5% of ²³⁵U). Absorption of a resonance or thermal neutron by the ²³⁸U nucleus yields ²³⁹U. The half-life of ²³⁹U is approximately 23.5 minutes. ²³⁹U decays (negative beta decay) to ²³⁹Np (neptunium), whose half-life is 2.36 days. ²³⁹Np decays (negative beta decay)  to ²³⁹Pu. The transmutation and decay chain is shown below:²³⁹Pu itself decays via alpha decay into ²³⁵U with half-life of 24 100 years. ²³⁹Pu occasionally decays by spontaneous fission with very low rate of 0.00000000031%. On the other hand ²³⁹Pu has very high absorption cross-section for thermal neutrons. When loaded into the reactor core ²³⁹Pu can be easily fissioned by a neutron or can be transformed into the ²⁴⁰Pu via a radiative capture reaction.

Plutonium 239 Fission

Plutonium 239 is a fissile isotope and its fission cross-section for thermal neutrons is about 750 barns (for 0.025 eV neutron). For fast neutrons its fission cross-section is on the order of barns. Most of absorption reactions result in fission reaction, but a part of reactions result in radiative capture forming ²⁴⁰Pu. The cross-section for radiative capture for thermal neutrons is about 270 barns (for 0.025 eV neutron). Therefore about 27% of all absorption reactions result in radiative capture of incident neutron. About 73% of all absorption reactions result in fission.

Typically, when plutonium 239 nucleus undergoes fission, the nucleus splits into two smaller nuclei (triple fission can also rarely occur), along with a few neutrons (the average is 2.89 neutrons per fission by thermal neutron) and release of energy in the form of heat and gamma rays. The average of the fragment atomic mass is about 120, but very few fragments near that average are found. It is much more probable to break up into unequal fragments, and the most probable fragment masses are around mass numbers 103 and 134 (and around atomic numbers 40 – Zirconium and 54 – Xenon).

Most of these fission fragments are highly unstable (radioactive) and undergo further radioactive decays to stabilize itself, therefore part of the released energy is radiated away from the reactor. On the other hand most of the energy released by one fission (~175MeV of total ~207MeV) appears as kinetic energy of these fission fragments. The fission fragments interact strongly with the surrounding atoms or molecules traveling at high speed, causing them to ionize. Creation of ion pairs requires energy, which is lost from the kinetic energy of the charged fission fragment causing it to decelerate. The positive ions and free electrons created by the passage of the charged fission fragment will then reunite, releasing energy in the form of heat (e.g. vibrational energy or rotational energy of atoms). This is the principle how fission fragments heat up fuel in the reactor core.

Source: JANIS (Java-based Nuclear Data Information Software); ENDF/B-VII.1

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Half-life of Plutonium

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Plutonium

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Plutonium 240

Half-lifes of Isotopes of Plutonium

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Plutonium Isotopes

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Plutonium

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Plutonium 239

• ²³⁸Pu. ²³⁸Pu belongs to the group of fertile isotopes. ²³⁸Pu decays via alpha decay to ²³⁴U with half-life of 87.7 years. ²³⁸Pu generates very high decay heat and has very high rate of spontaneous fission.
• ²³⁹Pu. ²³⁹Pu belongs to the group of fissile isotopes. ²³⁹Pu decays via alpha decay to ²³⁵U with half-life of 24100 years. This isotope is the principal fissile isotope in use.
• ²⁴⁰Pu. ²⁴⁰Pu belongs to the group of fertile isotopes. ²⁴⁰Pu decays via alpha decay to ²³⁶U with half-life of 6560 years. ²⁴⁰Pu has very high rate of spontaneous fission and has high radiative capture cross-section for thermal and also for resonance neutrons.
• ²⁴¹Pu. ²⁴¹Pu belongs to the group of fissile isotopes. ²⁴¹Pu decays via negative beta decay to ²⁴¹Am with half-life of 14.3 years. This fissile isotope decays to non-fissile isotope with high radiative capture cross-section for thermal neutrons. An impact on reactivity of the nuclear fuel  is obvious.
• ²⁴²Pu. ²⁴²Pu belongs to the group of non-fissile isotopes. ²⁴²Pu decays via alpha decay to ²³⁸U with half-life of 37300 years. ²⁴²Pu has very high rate of spontaneous fission but its quantity in the irradiated nuclear fuel is relatively low.

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Plutonium and Power Reactors

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Plutonium

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Half-life of Plutonium