Апсе ENVIRONMENTAL PROTECTION 2014

3.What peculiarities of MOX-fuel should be taken into account for its loading into power light-water reactors?

4.What rates can be reached by power light-water reactors in energy utilization of reactor-grade plutonium?

5.What reasons can limit the number of MOX-fuel recycles in power light-water reactors?

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CHAPTER 6. URANIUM AND PLUTONIUM DENATURING AS A WAY TOWARDS NUCLEAR WEAPONS NONPROLIFERATION

This chapter is devoted to the problems related with protection of the existing and advanced nuclear fuels against uncontrolled proliferation by introducing some additives into fuel compositions. These additives must create certain almost insuperable internal barriers any possible use of nuclear fuel for NED manufacturing, and, at the same time, these additives must preserve (or even upgrade) energy potential of nuclear fuels for their peaceful utilization at civilian NPP. Such an approach to the proliferation protection of nuclear materials is often called as an isotope denaturing. The term “isotope denaturing” m eans any changes in natural isotope compositions of chemical elements with ultimate goal to give them some new and desirable properties.

6.1. NM proliferation protection in NFC

As is known, natural uranium and thorium are basic materials of contemporary nuclear energy systems. Natural uranium contains only 0.71% 235U, the sole natural fissile isotope, and the self-sustaining chain fission reaction (CFR) on thermal neutrons can be initiated in large volumes of natural uranium, graphite or heavy water. As for natural thorium, this chemical element does not contain at all any fissile isotope that could be regarded as an analogue of 235U. Therefore, manufacturing of small-sized, uranium-based NED requires applying some difficult operations on uranium isotope enrichment in order to produce significant quantity of highly enriched (or weapon-grade) uranium (WG-U) containing up to 95% 235U. The other way towards NED manufacturing is the use of artificial fissile materials (plutonium and 233U) but their wide-scale production could be organized in the dedicated nuclear reactors, by neutron irradiation of natural uranium and thorium.

Nuclear reactors of contemporary civilian NPP are mainly loaded with low-enriched uranium fuel. So, from the standpoint of nuclear nonproliferation, fresh nuclear fuel takes up an intermediate position between natural uranium and WG-U. Very sophisticated and very expensive technologies of isotope separation should be applied to increase

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uranium enrichment (relative content of 235U) up to the weapon-usable level and, thereby, convert reactor-grade uranium into WG-U.

Another weapon-usable material – plutonium – is acc umulated in nuclear fuel in the process of nuclear reactor operation. As is known,

weapon-grade plutonium (WG-Pu) is composed of the following components: about 93% 239Pu, up to 7% 240Pu and very small fraction of the

heavier plutonium isotopes. WG-Pu can be built-up in the dedicated (plutonium-producing) nuclear reactors with rather short irradiation cycle. At the same time, nuclear power reactors with substantially longer irradiation cycle and, as a consequence, substantially higher values of fuel burn-up can build-up plutonium with significantly larger contents of heavy plutonium isotopes. This plutonium is usually called as reac- tor-grade plutonium (RG-Pu). Lengthy neutron irradiation of uraniumbased fuel can change considerably uranium isotope composition by appearance of isotopes 236U and 232U. For example, if uranium fuel with initial enrichment of 4.4% 235U was irradiated in LWR to fuel burn-up of 4% HM, then its isotope composition would be substantially changed (see Table 6.1).

Table 6.1

Uranium and plutonium isotope compositions in fresh and spent LWR fuel

So, uranium fuel of nuclear power reactors can be used, in principle, to produce weapon-usable materials but their extraction requires applying some additional and very complicated operations.

Neutron irradiation of natural thorium in nuclear power reactors can build-up well-fissile and weapon-usable isotope 233U. However, the

process of 233U build-up is accompanied by accumulation of other uranium isotopes 234U, 235U, 236U and the lightest uranium isotope 232U

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which is able to complicate handling with uranium extracted from spent thorium-based fuel.

As it was already mentioned above, SNF reprocessing with removal of radioactive fission products, whose ionizing radiation created a protective barrier against uncontrolled NM proliferation, can be regarded as the operation which can completely remove or substantially weaken the radiation protective barrier. SNF partitioning into separated chemical elements or groups of elements is able to facilitate NM diversion from peaceful to military applications. Appropriate countermeasures on NM physical protection, control and accountability must be undertaken to eliminate or, at least, mitigate this threat. In addition to some organizational actions related with the security forces and strict control of nuclear technological processes, the following barriers can be established to prevent diversion of fissile weapon-usable materials:

1.Radiation barrier can be formed by radioactive fission products. This barrier can be naturally set up in the process of long-term fuel irradiation in nuclear power reactors or by a special short-term irradiation of fresh fuel. Additional inherent component of the radiation barrier is a natural radiation background produced by fissile isotopes.

2.Incomplete removal of radioactive fission products from spent fuel that keeps high-intensity radiation fields at further operations with regenerated fuel.

3.Isotope dilution of 235U, i.e. the use of low-enriched uranium.

4.Incomplete separation of uranium-plutonium mixture, or even full exclusion of any technological operations able to separate plutonium from uranium. Co-extraction of uranium and plutonium makes it impossible to use such nuclear materials as a charge of NED.

5.The use of automatic distant technologies for manufacturing of nuclear fuel, fuel rods and fuel assemblies that can complicate access to fissile nuclear materials.

6.Dilution (denaturing) of fissile isotopes by other isotopes of the same chemical element. It is preferable to apply those isotopes whose proper-

ties can complicate using the denatured materials in NED. The wellknown dilution of fissile isotope 235U by fertile isotope 238U in. lowenriched uranium is a particular case of isotope denaturing. One else example of isotope denaturing is the spent mixed thorium-uranium fuel

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where fissile isotope 233U can be also diluted (denatured) by fertile isotope 238U.

As for 239Pu, the heavier plutonium isotopes (240Pu, in the first turn) can perform the denaturing mission. A particular place is taken up by the lightest plutonium isotope 238Pu, intense source of decay heat (T1/2 = 87.7 years) and spontaneous fission neutrons. These properties of 238Pu can substantially complicate (or even make it unfeasible) using RG-Pu with large 238Pu content as a charge of NED.

Similar role can be played by the lightest uranium isotope 232U (T1/2 = 68.9 years) in thorium-uranium fuel cycle.

In practice, the countermeasures listed above are already used in some combinations. For example, the once-through NFC of power LWR can perform the following proliferation-proof functions:

∙Isotope uranium dilution (low-enriched uranium as a fuel).

∙High radiation barrier created mainly by radioactive fission products.

∙SNF contains uranium-plutonium mixture that is very difficult to separate.

Just the combined proliferation-protective barriers constituted a basis for the Spent Fuel Standard adopted in the USA.

Thus, uranium-based fuel can be proliferation protected by the lightest uranium isotope 232U. So, it seems reasonable here to consider main nuclear properties of 232U and its nearest neutron predecessor 231Pa in more details.

6.2. Nuclear properties of 232U and 231Pa

Some relevant nuclear properties of main uranium isotopes are presented in Table 6.2. As is seen, unique properties of 232U make its a valuable material for protection of uranium-based fuel against uncontrolled proliferation.

232U is a starting isotope for rather long chain of radioactive decays.

Some daughter products of 232U can emit high-energy γ-rays (mainly, 208Tl and 212Bi with Eγ = 2,6 MeV and 1,8 MeV, respectively). These γ-

rays can improve detectability of uranium-bearing materials and complicate any unauthorized actions. Main parameters of 232U decay products are gathered in Table 6.3.

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Similarly to 238U, isotope 231Pa is a fertile isotope that can not be fissioned by thermal neutrons but it can breed fissile materials. Energy

dependencies of neutron radiative capture cross-sections are shown in Fig. 6.1 for 238U and 231Pa.

Table 6.3

232U decay products (a-emitters)

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Fig. 6.1. Energy dependencies of neutron radiative capture crosssections for 238U and 231Pa.

The following two important aspects should be noted here. Firstly, within thermal energy range, 231Pa is a superior neutron absorber as compared with 238U. For example, radiative capture cross sections of

thermal neutrons (En = 0.0253 eV) for these two isotopes are equal to: σc(231Pa) = 227 barns, σc(238U) = 3 barns. So, the presence of 231Pa in

fuel composition can promote effective generation of fissile isotopes

232U and 233U. Secondly, there is a rather large energy distance between capture resonances of 238U and 231Pa. Capture resonances of 231Pa be-

long to relatively low energies, below 100 eV (Fig. 6.1). This means the presence of 231Pa in fuel composition can depress thermal region in energy spectrum of neutrons (Fig. 6.2).

It can be seen that, although neutron energy spectrum in VVER-1000 contains a certain fraction of thermal neutrons, introduction of 231Pa into fuel composition can remove the thermal fraction completely. That is why stainless steel may be used here as a structural material. Indeed, the absence of thermal fraction in neutron spectrum does not result in additional neutron loss but fuel rods can keep their ability for working up to the higher values of fuel burn-up than those with zirconium-based alloys.

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ЭнергияNeutron нейтронаenergy, eV, эВ

Fig. 6.2. Effect of 231Pa on neutron spectrum

Now let us consider nuclear properties of 232U, product of neutron capture by 231Pa and rapid β-decay of 232Pa (T1/2 (232Pa) = 1,3 days). Like 235U, isotope 232U is a fissile nuclide. Dependencies of fission crosssections on neutron energy are presented in Fig. 6.3 for 235U and 232U.

It can be seen that, within thermal energy range, fission crosssections of 232U are substantially lower than those for 235U while radiative capture cross-sections of these isotopes are comparable each other.

For example, radiative capture cross sections of thermal neutrons (En = 0.0253 eV) for these two isotopes are equal to: σc(232U) = 73 barns,

σc(235U) = 99 barns. So, neutron-multiplying properties of 232U are inferior to those of 235U within thermal energy range.

This conclusion can be confirmed by Fig. 6.4 which demonstrates energy dependency of (νef - 1), i.e. the number of excess fission neutrons per one absorbed neutron. 235U looks superior to 232U within thermal energy range but quite another situation takes place in resonance range. So, it may be expected that introduction of 231Pa into uraniumbased fuel composition with aim to increase fuel burn-up will be more efficient just in resonance neutron spectrum.

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Fig. 6.3. Dependency of fission cross-sections on neutron energy for isotopes 235U and 232U

ЭнергияNeutronнейтронаenergy, eV, эВ

Fig. 6.4. Energy dependency of the number of excess fission neutrons per one absorbed neutron for isotopes 235U and 232U

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