What is Plutonium

Plutonium is a transuranic chemical element with atomic number 94. The chemical symbol for plutonium is Pu. Plutonium is mostly produced in nuclear reactors. Material Properties

Plutonium is a transuranic chemical element with atomic number 94 which means there are 94 protons and 94 electrons in the atomic structure. The chemical symbol for plutonium is Pu. It is a man-made isotope and is created from uranium in nuclear reactors. Therefore we can find this element in irradiated nuclear fuel or as a fissile material in nuclear weapons. In fact, scientists have found trace amounts of naturally-occurring plutonium. Four isotopes (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu and ²⁴⁴Pu) can be found in the nature.

Trace amounts of ²³⁹Pu originate in radiative capture of neutron on ²³⁸U. Free neutrons come from a spontaneous fission reaction of ²³⁸U.
Trace amounts of ²⁴⁴Pu originate in its relatively long half-life of about 80 million years.
The isotope ²⁴⁰Pu is in a decay chain of the isotope ²⁴⁴Pu. These nuclei are present in the unalterable proportions of the radioactive equilibrium between ²⁴⁴Pu and ²⁴⁰Pu.
Extremely small amounts of ²³⁸Pu originate in extremely rare double negative beta decay of naturally-occurring isotope ²³⁸U.

Fissile / Fertile Material Cross-sections. Comparison of total fission cross-sections.

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

Plutonium is mostly produced in nuclear reactors. It is a product of the transmutation and subsequent nuclear decay of fertile isotope ²³⁸U. The transmutation and decay chain is shown below:

Neutron capture may also be used to create fissile ²³⁹Pu from ²³⁸U, which is the dominant constituent of naturally occurring uranium (99.28%). Absorption of a neutron in 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.

Higher isotopes of plutonium (²⁴⁰Pu, ²⁴¹Pu and ²⁴²Pu) are created by also by neutron radiative capture, but in this case an absorber must be the plutonium nucleus. For example, ²⁴⁰Pu which is the second most common isotope, is formed by radiative capture of a neutron by ²³⁹Pu.The transmutation and decay chain is shown below.

Plutonium in Commercial Power Reactors

The nuclear transmutation of ²³⁸U into fissile isotopes of plutonium (the plutonium breeding) in the fuel cycle of all commercial light water reactors plays a significant role. In recent years, the commercial power industry has been emphasizing high-burnup fuels (up to 60 – 70 GWd/tU), which are typically enriched to higher percentages of 235U (up to 5%). As burnup increases, a higher percentage of the total power produced in a reactor is due to the plutonium bred inside the reactor.

At a burnup of 30 GWd/tU (gigawatt-days per metric ton of uranium), about 30% of the total energy released comes from bred plutonium. At 40 GWd/tU, that percentage increases to about forty percent. This corresponds to a breeding ratio for these reactors of about 0.4 to 0.5. That means, about half of the fissile fuel in these reactors is bred there. This effect extends the cycle length for such fuels to sometimes nearly twice what it would be otherwise. MOX fuel has a smaller breeding effect than 235U fuel and is thus more challenging and slightly less economic to use due to a quicker drop off in reactivity through cycle life.

Source of data: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

Isotopes of Plutonium

About twenty plutonium isotopes have been discovered and described. Except 244Pu, all these isotopes are artificial isotopes. The main isotopes, which have to be considered in the fuel cycle of all commercial light water reactors, are:

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.

Half-lifes of Isotopes of Plutonium

Isotope	Half-life / Decay mode	Product
²³⁸Pu	87.7 y / alpha decay	²³⁴U
²³⁹Pu	24 100 y / alpha decay	²³⁵U
²⁴⁰Pu	6 560 y / alpha decay	²³⁶U
²⁴¹Pu	14.3 y / beta decay	²⁴¹Am
²⁴²Pu	373 500 y / alpha decay	²³⁸U
²⁴³Pu	4.96 d / beta decay	²⁴³Am
²⁴⁴Pu	80 000 000 y / alpha decay	²⁴⁰Pu

Half-lifes of isotopes of plutonium.
Source: Java-based Nuclear Data Information Software
Library: The JEFF-3.1.1 Nuclear Data Library

Isotope

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.

See also: Plutonium 239

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.

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.