Research On Physics Performance Of Very High Burn-ups PWR Core

Average discharge burn-ups of Pressurized Water Reactor have steadily increased withtime as experience has accumulated and as technological developments have progressed.Current discharge burn-ups are approaching double those of30years ago. With increasingdischarge burn-up, fuel cycle cost decreases; increase operational flexibility; the volume offuel discharged is reduced. However, cores containing very high burn-up fuels will havedifferent safety and transient characteristics that will need to be re-assessed.In order to research the performance of very high burn-up core，this paper servesQinshan Ⅱ reactor core as the basic reactor core design. Burn-up calculations ofsingle-batch-loaded cores with a series of enrichments are made, and the single-batch burn-upcorrelation vs. reload fuel enrichment is obtained. Using the linear reactivity model, therelationship among discharge burn-up, enrichment, number of batch and cycle length isestablished. Two approaches by which the discharge burn-up of a fuel cycle can be increasedare analyzed. Characterized by the hydrogen-to-heavy-metal ratio and enrichment, the neutronspectrum effect on very high burn-up core design is investigated. In this paper, a very highburn-up core is designed, very high burn-up levels is achieved with changing enrichment andnumber of batch and cycle length. The CASMO-4/SIMULATE-3code system is used tomodel the high burn-up core and calculate the safety-related core neutronics parameters（e.g.radial power distribution, control rod worth, boron worth, reactivity feedback coefficients,shutdown margin） for the burn-up more than60MWd/kg. This paper analyzes variety ofthose parameters with cycle burn-up, under various running states（e.g. temperature, poison,boron concentration）.The result indicates that the high burn-up approach of using more batches is preferred;the increase of the hydrogen-to-heavy-metal ratio will lead to a decrease in cycle length eventhough the burn-up is increased. It is found that very high burn up levels could be achievedusing higher enrichments and various feed assembly and loading pattern options. Furthermore,specific in-core analyses show that safety margins could be adequately maintained. Theresults show that the hardening of the neutron spectrum leads to more negative moderatortemperature coefficients at high burn-ups irrespective of whether or not there is burnable poison, and the magnitude of the moderator temperature coefficient tends to increase withburn-up during a cycle; the is little variation with fuel temperature coefficient. The shutdownmargin decreases at very high burn-ups due to the combined effect of reduced control rodreactivity worth and smaller soluble boron reactivity worth.