Protactinium-231 – New burnable neutron absorber

Abstract

Burnable neutron absorbers such as gadolinium and erbium are used for compensating excess reactivity in nuclear reactors. Their daughter nuclides resulting from neutron absorption by erbium and gadolinium do not play important role from the viewpoint of neutronics processes occurring in the reactor core. Selection of such burnable neutron absorber, daughter nuclides of which would favorably impact fission chain reaction, is of significant interest.The aim of the present study is to investigate neutronics properties of 231Pa – the new burnable neutron absorber – and possibilities of its producing in significant quantities. The chain of isotopic transformations starting from 237Np is the analogue of the chain of isotopic transformations started from 231Pa. However, improvement of neutron-multiplication properties in the 237Np-chain can only be achieved in fast neutron spectra while in the case of 231Pa-chain positive neutron balance can be achieved both in fast and thermal neutron spectra. From this viewpoint the chain starting from 231Pa is unique. In addition, 237Np can be produced in nuclear reactors as the result of neutron radiative capture by 235U while significant amounts of 231Pa can only be produced through the threshold (n,2n) and (n,3n)-reactions on 232Th under its irradiation with super high-energy neutrons. Such neutrons with super high energies are practically absent even in fast spectrum reactors, but, however, these neutrons are available in fusion reactors. Breeding of 231Pa in fusion reactors and further use of 231Pa in nuclear power reactors can make it possible to realize potential capabilities of fusion facilities for radical increase of nuclear reactor fuel burn-up. Thus, 231Pa isotope is the new and unique burnable neutron absorber never suggested for the purpose before.Evaluated nuclear data libraries JENDL-4.0 and ENDF/B-V, as well as computer software system SCALE-4.3 were used in the implementation of the present study.The following results were obtained.(1) In contrast to conventional burnable neutron absorbers on the basis of gadolinium and erbium, the protactinium isotope suggested in this paper appears to be more attractive because it allows us not only compensating initial excess reactivity, but, also, ensuring high fuel burn-up due to good multiplication properties of its daughter nuclides.(2) Significant quantities of protactinium could be produced in hybrid fusion–fission reactors acting as sources of neutrons (not sources of energy) with parameters already achieved by the present moment by experimental facilities in USA, Japan, UK.