Development of refined MCNPX paret multi channel model for transient analysis in research reactors

Reactivity insertion transients are in general analyzed (RELAP, PARET) using a two channel model, representing the hot assembly with specifi ed power distribution and an average assembly representing the remainder of the core . For the analysis of protected by the reactor safety system transients this approximation proves to give adequate results. Howeve r a more refined multi channel model representing the various assemblies, coupled through the reactivity feedback effects to the whole reactor core is needed for the analysis of unprotected transients with excluded over power and period trips. In the present paper a detailed multi channel PARET model has been developed which describes the reactor core in different clusters representing typical BR2 fuel assemblies . The distribution of power and reactivity feedback in each cluster of the reactor core is obt ained from a best estimate MCNPX calculation using the whole core geometry model of the BR2 reactor. The sensitivity of the reactor response to power, temperature and energy distributions is studied for protected and unprotected reactivity insertion tran sients, with z e r o a n d n o n z e r o reactivity feedback coefficients. The detailed multi channel model is compared vs. simplified few er channel models. The sensitivities of transient characteristics derived from the different models are tested on a few reactivi