First-order numerical optimization of fission-chamber coatings using natural uranium and thorium

Abstract

Presently, fission chamber technology utilizes neutron-sensitive coatings often composed of radioisotopes that are becoming increasingly difficult to obtain. Using only natural uranium and thorium, a maximized stable device lifetime in a constant neutron flux can be obtained by optimizing the regenerative fission chamber coating. A system of coupled differential equations was developed by considering neutron-induced fission, neutron absorption, and radioactive decay of 17 radioisotopes spanning the product family tree from 232Th to 241Pu. By solving the system of coupled differential equations, the interaction rate of the regenerative fission chamber coating can be found as a function of time (or total fluence) in a constant neutron flux. The Kansas State University (KSU) TRIGA Mark II nuclear reactor was used as a basis to study the effectiveness of mixed fission chamber coatings over time. By varying the initial composition of the fission chamber coating, an optimal composition of 232Th and natural uranium is capable of maintaining stable device operation (<5% deviation from initial response) under a constant neutron flux of 2.2 × 1013 n cm−2 s−1 up to a total neutron fluence of 7.65 × 1022 n cm−2, corresponding to over 110 years.