Analysis of MNSR core composition changes using the codes WIMSD-4 and CITATION

The burn-up results have indicated that the core life-time of MNSR is being mainly estimated by Sm149 followed by Gd157 and Cd113. The accumulation of these fission products during 100 continuous operation days caused a reduction of about 4.3 mk for the excess reactivity. This result seems to be in good agreement with the available empirical value of 3.5 mk, which relates to the whole discontinuous operation period of the reactor since its start up to now. The calculation procedure simulates the sporadic operation with an equivalent continuous operation period. This approximation is valid for the long-lived fission products that mainly dictate the core life-time. However, it is an overestimation for the concentration of short-lived radioactive products like Xe135.