The sensitivity of research reactor based BNCT irradiation facility performance to core conversion from high to low enriched uranium fuels

A critical requirement for boron neutron capture therapy (BNCT) is a strong source of thermal (i.e < 1eV) or epithermal (1 eV - 10 keV) neutrons. Nuclear reactors are the currently available most intense sources of such neutrons, however, the neutron field in nuclear reactor cores is heavily contaminated by gamma radiation. Nuclear reactors also produce fast (i.e. > 10 keV) neutrons which can cause damage to the healthy tissue through H(n,n')H (i.e. proton recoil) reactions and lead to the generation of additional gamma photons through (n,γ) reactions with hydrogen. The effectiveness of reactor based BNCT strongly depends on how well the fast neutron and gamma dose to healthy tissue is reduced through efficient irradiation facility design. Optimal facility design, in turn, is a strong function of the core neutron spectrum. A large number of nuclear reactors in the world which are suitable as neutron sources for BNCT use highly enriched uranium (HEU) fuels (i.e. 93 atom % 235U). In view of the restrictions on the export of HEU from U.S. since 1979 to prevent nuclear weapons proliferation and also the 1986 U.S. Nuclear Regulatory Commission ruling, these reactors are in the process of being converted to use low enriched uranium (LEU) fuels (i.e. < 20 atom % 235U). The conversion process usually leads to the hardening of the core spectrum (i.e a shift to higher neutron energies) due to the need to increase the fuel-to-moderator ratio to compensate for the increased parasitic neutron absorption in 238U. This paper presents the results of a 3-dimensional Monte Carlo study which investigates the potential impacts of such a core spectrum hardening on a hypothetical irradiation facility utilizing the reactor as a thermal neutron source for BNCT