Reactivity feedback and control margins in natural uranium fueled and heavy water moderated nuclear research reactors

Three-dimensional neutronics analyses were performed to find the reactivity feedback coefficients and control margins under normal and presumed accidental conditions for the novel proliferation resistant, safer and economical natural uranium fueled and heavy water moderated nuclear research reactor cores [Annals of Nuclear Energy 31 (2004) 1331–1356 & 32 (2005) 612–620, Progress in Nuclear Energy (2006a,b) in press]. The results were compared with the reference core similar in design to National Experimental Reactor (NRX) and Canadian Indian Reactor (CIR). Standard reactor physics simulation codes WIMS-D/4 and CITATION were employed for this study. It was found that as opposed to the reference design, the new proposed cores are inherently stable as far as temperature reactivity feedback is concerned. Voids formation (loss of coolant) adds positive reactivity in all the cores. However, in the case of proposed cores, the reactivity added would be about 10% of that added in the reference core for loss of a given fraction of coolant. Moreover, as opposed to the NRX/CIR core, the shut-off rods, under the one stuck rod criterion, can shutdown and maintain the proposed cores subcritical in the accidental conditions.