Investigation of Reactivity Insertion Transients at the BR2 Reactor Using Refined Multi-Channel PARET Models

A preliminary analysis of reactivity insertion transients that were protected by the control rods but had zero reactivity feedback coefficients at the BR2 reactor was presented at RERTR2009 in Beijing. The admissible reactivity insertion rate, which can be tolerated in the BR2 was determined for nominal reactor operation and at manipulation conditions (low reactor power and no flow) with respect to the accepted safety limits in SAR-BR2. The calculations were performed by PARET/ANL V7.5 for a simplified 12-channel model representing the hot assembly in the BR2 core. In the present paper a detailed 48-channel model has been developed which describes the reactor core in four clusters representing the typical BR2 fuel assemblies. The distribution of the power and reactivity feedback in each cluster of the HEU representative core is obtained from a best-estimate MCNPX calculation. The sensitivity of the reactor response to power, temperature and energy distributions is studied for protected and unprotected reactivity insertion transients, with and without reactivity feedback coefficients. The 48-channel model is compared vs. the 12-channel model and vs. a 24-channel model representing the reactor as one hot cluster and the remainder of the core – as an average cluster.