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

G. Bezhunov, N. Rykov, B. Ryazanov
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Abstract

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.
用于测量固体放射性废物中可裂变核素质量的紧凑型脉冲中子系统的研制
已经开发、制造和测试了一种测量200升容器中固体放射性废物(SRW)中裂变核素(FN)质量的系统。该测量系统包括一个中子输出为2.108中子/秒的脉冲中子发生器(PNG)、一个带有超镉中子计数器的中子减速装置、一个容量为200升的容器室、一个时间脉冲分析仪、一台装有中子配准脉冲时间谱积累和处理软件的个人计算机。以不同密度的石英砂、石墨和纸为基质,FN含量为0.01 ~ 100 g,采用不同富集度的U-235(5 ~ 90%)对照样品和不同化学成分(二氧化氮、硝酸铀酰溶液、氧化物-氧化亚氮)的PuO2对照样品,对120升和211升放射性固体废物容器模型进行了实验和计算研究。不同的几何尺寸,均匀或不均匀地放置在容器的体积上。确定了测量系统的参数,包括响应值,单位为容器中每克FN每秒计数和每PNG中子。在20 Hz的PNG频率下,中子产率约为107中子/脉冲,测量时间为100 ~ 300秒。典型TPO基质的U-235质量测量下限为0.01 g /容器。估计了各种因素对测量结果的影响:基质的含水量、基质的化学成分和密度、铀中U-238的质量分数、容器中FN排列的不均匀性、内部中子源的存在。在容器中存在内部中子源的情况下,使用所开发的系统可以测量容器中可裂变核素的质量,每个容器的强度高达5·107中子/秒。
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