Plutonium-239, fissionable isotope

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Another safety issue to be considered which might be exacerbated in the reprocessing option is that the plutonium generated in power reactors, called reactor-grade plutonium because it is made up of a variety of plutonium isotopes, contains plutonium-241, which is subject to spontaneous fission (8). The mixture of isotopes makes it extremely difficult to build an effective nuclear weapon. However, an explosive device could be built using this mixture if control of detonation is sacrificed (48). 

Since the amount of fissile material in the fuel assemblies is only about 3 percent of the uranium present, it is obvious that there cannot be a large amount of radioactive material in the SNF after fission. The neutron flux produces some newly radioactive material in the form of uranium and plutonium isotopes. The amount of this other newly radioactive material is small compared to the volume of the fuel assembly. These facts prompt some to argue that SNF should be chemically processed and the various components separated into nonradioac-tive material, material that will be radioactive for a long time, and material that could be refabricated into new reactor fuel. Reprocessing the fuel to isolate the plutonium is seen as a reason not to proceed with this technology in the United States. 

Plutonium is the most important transuranium element. Its two isotopes Pu-238 and Pu-239 have the widest applications among all plutonium isotopes. Plutonium-239 is the fuel for nuclear weapons. The detonation power of 1 kg of plutonium-239 is about 20,000 tons of chemical explosive. The critical mass for its fission is only a few pounds for a solid block depending on the shape of the mass and its proximity to neutron absorbing or reflecting substances. This critical mass is much lower for plutonium in aqueous solution. Also, it is used in nuclear power reactors to generate electricity. The energy output of 1 kg of plutonium is about 22 million kilowatt hours. Plutonium-238 has been used to generate power to run seismic and other lunar surface equipment. It also is used in radionuclide batteries for pacemakers and in various thermoelectric devices. 

FUEL. In the conventional sense, a fuel is a material or combination of materials which, when burned with air, produces heal. This heat, in turn, can he used in numerous ways—as in the conversion of water lo steam. The steam, in turn, can be used in many ways—as in a steam turbine to produce electricity, Fuels also are burned to oblain explosive or mechanical energy—as in an internal combustion engine where heal per se is an inevitable, bul undesired byproduct. The term fuel is also used in connection with nuclear reactions—as the material, such as uranium and plutonium isotopes, which undergoes fission and. in so doing, yields heat energy, Fuel also appears in the term fuel cell, in which chemical reactions other than what may be considered as conventional combustion are carried out 10 yield electrical energy. 

Weapons-grade plutonium, dispersed at military accidents such as Thule in 1968 or as non-fissioned weapon particles after detonation of a Pu-bomb can be characterized by high Pu content relative to the other Pu-isotopes, while accidentally dispersed Pu from the previously widely used nuclear-powered satellites are characterized by high Pu content." The ratio of americium-241 to plutonium isotopes (as " Am is formed by the decay of Pu) is proportional to the initial " Pu concentration, thus it can also be used as an indicator to assess the origin of contamination. However, in most cases, as several sources may contribute to the transuranics content in environmental samples, mixing models applying several isotope ratios are required to assess the origin of possible contamination sources. 

The fissionable isotopes are U-233, U-235, Pu-239, and Pu-241. The fertile isotopes U-238 and Th-232 are converted to fissionable isotopes by neutron absorption (U-238 into plutonium isotopes and Th-232 into U-233). Natural uranium contains 0.71% U-235, 99.28% U-238, and 0.006% U-234. Fuel enriched in U-233 and plutonium must be produced from thorium and U-238, respectively (Fig. 1) by neutron capture the neutrons are provided initially by fission of U-235. 

The primary use for plutonium (Pu) is in nuclear power reactors, nuclear weapons, and radioisotopic thermoelectric generators (RTGs). Pu is formed as a by-product in nuclear reactors when uranium nuclei absorb neutrons. Most of this Pu is burned (fissioned) in place, but a significant fraction remains in the spent nuclear fuel. The primary plutonium isotope formed in reactors is the fissile Pu-239, which has a half-life of 24 400 years. In some nuclear programs (in Europe and Japan), Pu is recovered and blended with uranium (U) for reuse as a nuclear fuel. Since Pu and U are in oxide form, this blend is called mixed oxide or MOX fuel. Plutonium used in nuclear weapons ( weapons-grade ) is metallic in form and made up primarily (>92%) of fissile Pu-239. The alpha decay of Pu-238 (half-life = 86 years) provides a heat source in RTGs, which are long-lived batteries used in some spacecraft, cardiac pacemakers, and other applications. 

During its lifetime, a fusion reactor presents little radiation hazard. The internal structure, particularly the vacuum containment vessel and the heat exchanger, will be subject to intense neutron bombardment. The neutrons will convert some of the elements of the structure into long-lived radioactive isotopes. Selecting construction materials that do not easily become activated can minimize radioisotope production. No material is entirely resistant to neutron activation, thus the decommissioning of a fusion reactor will require the handling and disposal of potentially hazardous radioactive isotopes. Because of the lack of uranium, plutonium, and fission products, the total radiation exposure hazard from the decommissioned fusion reactor is 10,000 to 1,000,000 less than from a decommissioned fission reactor. 

The most important plutonium isotope is plutonium-239, for Pu-239 has the property of undergoing nuclear fission when bombarded with slow neutrons. This means that it can be used for nuclear power and nuclear weapons. 

We are now in position to derive equations that will give the degree uf bumup nuclear fuel can experience before it ceases to be critical. First, we must determine how the concentration of each nuclide that affects the neutron balance changes with time. We consider fuel that at time zero contains N s atoms of U per cubic centimeter, atoms of U, and no other uranium isotopes, plutonium, or fission products. This fuel is then exposed to a thermal-neutron flux 0(0, which may be a function of time. The variation in concentration of each nuclide in this fuel with time is obtained as follows. 

Another radiation problem arises from fast neutrons produced in spontaneous fission of the even-mass plutonium isotopes. Half-lives and specific activities for spontaneous fission of the plutonium isotopes are listed in Table 8.17. 

Pu. The isotope Pu is produced by neutron capture in Pu. It is not fissionable by thermal neutrons, but, like all other plutonium isotopes, it fissions with fast neutrons. Pu is converted to a fissionable nuclide by neutron capture. Therefore, like Th and it is a fertile material. It undergoes alpha decay, with a half4ife of 6580 years, to form which then decays to Th, the parent of the 4n decay series discussed in Chaps. 6 and 8. Like the other even-mass plutonium isotopes, Pu produces neutrons by spontaneous fission. It is present in greater concentration in reactor plutonium than any of the other even-mass plutonium isotopes. 

The long-lived plutonium isotope Pu (belonging to the 4/i series see also Fig. 16.1), which dec s through a-emission and spontaneous fission (0.13%) with a total half-life of 8.26 X 10 y, was discovered in rare earth minerals in 1971. If this is a survival of primeval Pu, only 10 % of the original can remain. An alternate possibility is diat this Pu is a contaminant from cosmic dust (e.g. from a supernova explosion in more recent times than the age of the solar system). 

Neutron capture and /3-decay lead to the formation of higher actinides. This is illustrated in Figs. 16.2, 16.3, 19.5 and 19.7. Pu and Pu also fission, contributing significantly to the energy production (Fig. 19.8). Truly, all plutonium isotopes lead to fission, since the n-capture products and daughters, Am, Am, Cm, Cm, and most other actinides are either fertile, fissible or fissile. 

H, Sm, Ag, "Cd, Nb, Sn, Zr, Tc, Te and Sn. Secondly, neutron capture reactions are occurring. One of these results in the formation of fissile Pu from fertile according to Scheme 1. Some of the Pu produced will also undergo fission, but Pu may represent some 0.5% of the actinide content of the fuel discharged from the reactor. Neutron capture reactions also result in the formation of other plutonium isotopes along with some americium and curium. 

Sources of radiation in fresh fuel are plutonium isotopes, products of decay of the plutonium isotopes, and impurities of products of fission in the regenerated plutonium. As a result, the gamma and neutron radiation dose on a surface of fresh fuel bundles generated by fuel from weapon plutonium exceeds by more than an order of magnitude the appropriate dose capacity for FB from uranium fuel. Moreover, capacity of dose on a surface of FB with regenerated plutonium exceeds on an order of magnitude the dose capacity for FB with weapons plutonium. 

It should be noted that as fuel is exposed in the reactor, U-235 densities are burnt down and both plutonium and fission products are produced. This isotopic change can lead directly to some variation in the behaviour of both void and power coefficients. There can also be an important indirect effect when the reactivity is high (e.g. with fresh fuel) and fixed absorbing rods are inserted to compensate, a large thermal reactor tends to behave as a number of small, admittedly linked, reactors with a different balance of capture, leakage and production of neutrons. This indirectly affects the various reactivity coefficients. 

On the earth, the most likely source of macroscopic amounts for technetium and promethium is the reprocessing of irradiated uranium and plutonium from fission reactors. This mechanism produces only a few, and not the longest-lived, isotopes. 

The application of nuclear forensic techniques to samples of purified heavy elements is well developed however, when applied to unseparated spent reactor fuel, the methods become more complicated. The radionuclide content of a spent fuel sample is not controlled solely by radioactive decay, but is strongly influenced by neutron-induced transmutation. Chronometry based on the decay of the light plutonium isotopes cannot be performed due to the initial presence of an overwhelming quantity of uranium. The isotopic distribution of the plutonium isotopes and the concentration of fission products can provide a means by which the degree of transmutation can be estimated, unless the material started out as MOX fuel (where reprocessed plutonium is incorporated into fuel fabricated from uranium with insufficient fissile content to support the reactor application). More study is needed to extend the methodology to unprocessed fuel. 

Inductively coupled plasma sector field mass spectrometry (ICP-SFMS) is used to identify and characterize radiation-producing particles found in soils that received fallout from the Chernobyl accident in 1986 [99]. ICP-SFMS provided sufficiently low detection limits for the analysis of plutonium isotopes found in soils along with high levels of other isotopes produced in nuclear reactions (i.e., fission products). 

The MTR- Phoenix Fuel Experiment Is a unique pure plutonium-fueled depletion experiment supported by a comprehensive set of critical experiments. Phoenix fuel Is a mixture of plutonium Isotopes with a relatively high content of Pu. During irradiation, this fertile Pu is converted to fissionable Pu and partially compensates for the loss of fisslonablie Pu. The MTR experiment is designed to demonstrate the longer reactivity lifetime characteristics of this concept. 

The decay heat power comes mainly from five sources (1) unstable fission products, which decay via a, p-, p+, and y ray emission to stable isotopes (2) unstable actinides that are formed by successive neutron capture reactions in the uranium and plutonium isotopes present in the fuel (3) fissions induced by delayed neutrons (4) reactions induced by spontaneous fission neutrons (5) structural and cladding materials in the reactor that may have become radioactive. Heat production due to delayed neutron-induced fission or spontaneous fission is usually neglected. Activation of light elements in structural materials plays a role only in special cases. 

The most important isotope of plutonium is Pu = 24,200 years). It has a short half-life so only ultra traces of plutonium occur naturally in uranium ores, and most plutonium is artificial, being an abundant byproduct of uranium fission in nuclear power reactors. The nuclear reactions involved include the radiative capture of a thermal neutron by uranium, U( , y) U the uranium-239 produced is a beta-emitter that yields the radionuclide Np, also a beta-emitter that yields Pu. To date, 15 isotopes of plutonium are known, taking into account nuclear isomers. The plutonium isotope Pu is an alpha-emitter with a half-life of 87 years. Therefore, it is well suited for electrical power generation for devices that must function without direct maintenance for time scales approximating a human lifetime. It is therefore used in radioisotope thermoelectric generators such as those powering the Galileo and Cassini space probes. 

Of the a-emitting nuclides, Cm represents more than 95% of the total a activity, with the combined plutonium isotopes 238, 239 and 240 accounting for the remainder. The relative activity proportions of these nuclides in the water as well as those of other insoluble fission products such as Zr, Ce and ° Ru are the same as in the fuel. 

Much waste from nuclear sites will contain significant amounts of transuranic nuclides. While Am can readily be measured by gamma spectrometry, the plutonium isotopes cannot. However, some of the even-mass plutonium isotopes undergo spontaneous fission (at sub-critical levels ) and emit neutrons at measurable rates as they do so. Drum scanning systems can be combined with neutron-detection systems that can be used to estimate... 

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