Study of reactivity effects of fission products and actinides after power transients in PWR NPP

This thesis presents a safety issue concerning NPP’s that restart after a period of a few days consecutive to a shutdown. It concerns reactivity effects due to actinides and fission products during transients in the PWR reactors operated in Belgium by Electrabel. The purpose of the work is firstly to identify the approximations made at present in the standard calculation chain used by Tractebel Engineering S.A. in support of the operation of the reactors on the reactivity created by fission products and actinides after a shutdown. Secondly, we will develop a correlation on basis of a single pin model for the evolution of the reactivity of fission products and actinides after a shutdown that depends on important parameters as burn up, power density, time and enrichment. Finally, we will use this correlation to generalise the effect on reactivity for a whole reactor in order to correct some approximations made by the calculation chain used by TE. The results of calculations show that the reactivity created by fission products and actinides nearly compensate the Sm-149 effect after a shutdown and have an influence on the reactivity balance to bring the reactor critical.