Comparison of correlation relations for determining the relative 242Pu content in plutonium produced in the RT-1 plant at the Industrial Association Mayak

Abstract

The correlation relations for determining the 242 Pu mass fraction which are used in the MGA and FRAM computer codes are compared with the relations proposed in the present article for plutonium stored in the RT-1 plant of the Industrial Association Mayak. The need to determine the 242 Pu mass fraction using the correlations arises in nondestructive analysis of plutonium. The MGA and FRAM codes use correlations which are based on data on the isotopic composition of the plutonium that has accumulated in American reactors so that the possibility of using these codes for Russian plutonium must be checked. This check was based on an analysis of data obtained on more than 6000 isotopic compositions during certification of the RT-1 plutonium. A characteristic of the relation proposed in this article is that it takes account of the change in the isotopic composition of plutonium over time.High-resolution gamma-spectroscopy with ultrapure germanium detectors is used to analyze the isotopic composition of plutonium, and special computer programs, usually MGA and FRAM, are used to calculate the isotopic composition [1,2]. This approach yields information on a sample by a nondestructive method in an acceptable period of time and without using standard samples. One factor affecting the accuracy of the results is the uncertainty of the 242 Pu content. Its mass fraction in plutonium can reach 10%, and it increases with increasing fuel burnup in power reactors [3]. 242 Pu cannot be determined directly from gamma-spectrometric measurements. It can be determined by nondestructive analysis only if a correlation relation between the isotopes is used. Correlations are present because of the common origin of the plutonium isotopes, formed as a result of nuclear reactions and radioactive decay.The correlation relation can be determined by two fundamentally different methods. The first one is theoretical and is based on finding a simple relation by analyzing and simplifying chains of accumulation and decay of nuclides, which reflect the gradual change of the isotopic composition with fuel burnup and incorporate a large number of nuclear characteristics. The correlations obtained in this manner are tied to the type of reactor considered and are not universal. In addition, any theoretical result must be confirmed experimentally. The second one is based on the use of experimental data on the iso-