Study of the bubble-void distribution effect on reactivity in the MNSR research reactor

In this work, the effect of void on reactivity in the MNSR core has been studied, where void was represented as a localized vapor bubble moving within the core coolant. The effect of void on reactivity, in both axial and radial directions, was determined. An average void coefficient was calculated and found to be 4.57689 mk/%. Since neither measured nor published data are available on void coefficients in MNSR, comparison was not possible. However, the current calculated results seem acceptable and agree with the physical sense. Moreover, space-distribution of thermal neutron flux in all regions of the reactor has been obtained. As a result, a map of this flux distribution has been made. The map gives a clear picture on the neutron flux distribution through out various components of the MNSR. Two neutronics codes have been used in this analysis: WIMSD4 and CITATION. WIMSD4 was used to generate macroscopic cross sections for all materials in the reactor i.e. unit cell calculations, whereas the CITATION code was used for reactor core calculation. Finally, the effect of moderator density reduction on neutron flux spectrum has been calculated too. (Authors)