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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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