Small Thorium Fission Chambers for Recording Fast Neutrons in Reactor Installations

The results of development work on experimental samples of small fission ionization chambers for recording fast neutrons (En ~ 1–20 MeV) in a reactor core are presented. A technological solution for obtaining a radiator based on ThO2 as a result of nonstationary low-temperature consolidation of an oxide coating from a solution of thorium carboxylate was developed and realized. Samples of ThO2-based radiators on the surfaces of multichannel electrodes of diameter 3 and 5 mm and coatings based on the isotope 238U (in the form U3O8 based on uranium with 235U content 0.00002%) were fabricated and small fission chambers based on them were assembled. Sample radiators were bench tested for stability during thermal cycling (15 cycles, heating to 750°C) and under vibration loads (to 200 Hz). Physico-mechanical and reactor tests of the fission chambers were conducted and the discrimination characteristics of samples and their sensitivity to heat and fast-neutron fluxes were determined. A comparative analysis of the conditions of effective application of small 238U- and 232Th-based fission chambers in long-time operation as well as in single neutron measurements is made.