Plutonium

What is Plutonium

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-sectionsFissile / 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:

Equation - Plutonium 239 breeding from Uranium 238

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.

plutonium breedingSource 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

IsotopeHalf-life / Decay modeProduct
238Pu87.7 y / alpha decay234U
239Pu24 100 y / alpha decay235U
240Pu6 560 y / alpha decay236U
241Pu 14.3 y / beta decay241Am
242Pu 373 500 y / alpha decay238U
243Pu 4.96 d / beta decay243Am
244Pu 80 000 000 y / alpha decay240Pu
Half-life of plutonium isotopes.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
Fissile / Fertile Material Cross-sections
Source: JANIS (Java-based Nuclear Data Information Software)
Library: ENDF/B-VII.1

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:

Equation - Plutonium 239 breeding from Uranium 238

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
Fissile / Fertile Material Cross-sections
Source: JANIS (Java-based Nuclear Data Information Software)
Library: ENDF/B-VII.1

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.

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Uranium

See above:

Nuclear Fuel

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