DEVELOPMENT OF A COMPACT PULSED NEUTRON SYSTEM FOR MEASURING THE MASS OF FISSILE NUCLIDES IN SOLID RADIOACTIVE WASTE

A system for measuring the mass of fissile nuclides (FN) in solid radioactive waste (SRW) in containers up to 200 liters has been developed, manufactured and tested. The measuring system includes a pulsed neutron generator (PNG) with a neutron output of 2·108 neutrons/s, a neutron moderating unit with counters of supra-cadmium neutrons and a chamber for containers up to 200 liters, a time pulse analyzer, a personal computer with a software for accumulation and processing of time spectra of pulses from neutron registration. Experimental and computational studies have been carried out for models of containers with solid radioactive waste of 120 and 211 liters with matrices of quartz sand, graphite and paper of various densities with FN content in an amount from 0.01 to 100 g using reference or well-characterized samples with different enrichment in U-235 (from 5 to 90 %) and PuO2 reference sample, with different chemical composition (dioxide, uranyl nitrate solution, oxide-nitrous oxide), different geometrical sizes, placed homogeneously or heterogeneously over the volume of the container. The parameters of the measuring system were determined, including the response values in units of counts per second per gram of FN in the container and per the PNG neutron. The measurement time at an PNG frequency of 20 Hz with a neutron yield of ~107 neutrons/pulse is from 100 to 300 s. The lower limit for U-235 mass measuring for typical TPO matrices is 0.01 g per container. The influence of various factors on the measurement results was estimated: the moisture content of the matrix, the chemical composition and density of the matrices, the mass fraction of U-238 in uranium, the heterogeneity of the FN arrangement in the container, the presence of internal neutron sources. Measurements of the mass of fissile nuclides in containers using the developed system are possible for the case of the presence of internal neutron sources in the container with an intensity of up to 5·107 neutrons/s per container.