Critical experiments to determine amount of U-235 in research reactor fuel assemblies

Four different critical core configurations of the IRR1 [Bettan, M., Hirshfeld H., Levine, S.H., 2002. Method to determine the burnup of the IRR1 fuel assemblies. In: Proceedings of the 21st Conference of the Nuclear Societies in Israel, p. 38–41], Cores 1, 2, 3, and 4, have been used to determine experimentally the reactivity changes caused by interchanging different fuel assemblies (FAs) in the same core position [Bettan, M., Hirshfeld H., Levine, S.H., 2002. Method to determine the burnup of the IRR1 fuel assemblies. In: Proceedings of the 21st Conference of the Nuclear Societies in Israel, p. 38–41; Levine, S.H., Kim, S.S., 1985. Development of an asymmetric multiple-position neutron source (AMPNS) method to monitor the criticality of a degraded reactor core. Ann. Nucl. Energy 12, 517; IAEA-TECDOC-633, 1992. Determination of research fuel burnup, January]. The different fuel assemblies varied in their burnup or loss in U-235 causing the core reactivity to change systematically with each FA interchange. The control blade height was altered to compensate for this change in the core reactivity to maintain the core critical. The control blade reactivity worth as a function of its height was measured by period measurements so that the reactivity change due to the interchange of an FA was determined. The WIMS/EXT2 [WIMS-D4: Winfrith Improved Multigroup Scheme (WIMS) code system, 1969. Computer Code Package CCC-576, RSICC/ORNL, Oak Ridge, TN; Fowler, T.B. et al., 1967. EXTERMINATOR-II: A FORTRAN IV Code for solving multigroup diffusion equations in two dimensions, ORNL-4078, ORNL, April] codes are used to evaluate the change in core reactivity as a function of FA U-235 or burnup. By relating the calculation of the reactivity change with the measured reactivity change, the burnup or U-235 mass in each FA is determined. The U-235 mass of the FAs gave good agreement between measurements made of the same FA in different cores and appears to bring better agreement between the keff calculated for the different cores when each are just critical. The results of these measurements and calculations are presented in this paper.