The Main Changes and Consequences by Burnup Extensions in PWRs: Some Recommendations for Negative Plant Performance Issue Limitations

The higher duty cores are always attended with the onset of sub-cooled nucleate boiling (SNB) on the fuel cladding surfaces and the initial excess reactivity of core. The impacts of these factors on the chemical environments of the nuclear fuel cladding- and reactor coolant system- surfaces are discussed: The SNB at the cladding surfaces strongly influences the behavior of gases dissolved in cladding water layer phase, causing their stripping out which will favor the creation of oxidizing conditions. It is considered that the creation of oxidizing conditions in the nuclear fuel cladding environment is not a direct boiling consequence but it is a result of the synergic impact of the boiling- and water radiolysis- processes on the Pressurized Water Reactor fuel cladding surface areas. This requires new criteria for corrosion resistance properties of fuel cladding materials. These materials must have high corrosion resistance to oxidizing conditions. If only boric acid as neutron absorber is implemented, the critical boron concentration in coolant increases significantly when implementing an extended fuel cycle. This effect can be limited through application of 10B enriched boric acid or by application of such solid burnable absorbers as Gd2O3 integrated in UO2 fuel. Some recommendations to address negative plant performance issues are given in addition to the results of these investigations.