Passing of Reactors WWER-1000 of NPP Kozloduy - Bulgaria to Extended Fuel Cycle Operation (Implementation of ~(235)U Higher Enriched Fuel)

摘要

On the base of evaluation of the experience by burnup extensions in PWRs it is concluded that 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 main consequences of these basic factors are the modifications of chemical/electrochemical environments of the nuclear fuel cladding- and reactor coolant system- surfaces.An explicit result of the synergic impact of the boiling- and water radiolysis- processes on the Pressurized Water Reactor fuel cladding surface areas is the change of fuel cladding chemical environment from reducing (hydrogen rich) to oxidizing, which is the most important, with dramatically consequences, change. These circumstances determined the requirement that the nuclear fuel cladding materials in PWRs operating with SNB, should have high corrosion resistance by oxidizing conditions.By the implementation only of dissolved in coolant boric acid as a neutron absorber, the critical boron concentration in coolant increases significantly when passing to burnup extension. The enough boric acid neutralization requires the corresponding increases of the coolant alkalizing reagent concentrations necessary for the keeping of coolant pH_(300) above 6.9. Usually, these concentrations are extremely high and are not allowed. One effective way for the resolution of this problem is the application of solid burnable absorbers as Gd_2O_3 integrated in UO_2 fuel which allows the boric acid coolant concentrations in the required limits to be kept.The above mentioned considerations have been taken into account by the passing to burnup extension of WWER-1000 Units in NPP Kozloduy - Bulgaria. This transition of WWER-1000 Units in Bulgarian Kozloduy NPP has been realized in accordance with a coordinated program between Kozloduy NPP and JSC TVEL (Russia) through step by step implementation of a new type of advanced fuel assemblies (TVSA). Some important features of these advanced fuel assemblies are: burnable absorber (Gd) integrated in the fuel - (UO_2 - Gd_2O_3fuel), increased burnup up to 55 MWd/kg U in assemblies, possibility to realize full 4-year fuel cycle. The step-by-step transition process is foreseen to be completed in the frame of five fuel cycles (from 12-th to 16-th of Unit 5) between years 2005-2010.The objects of these investigations are the water chemistry aspects of the step-by-step process of introduction of burnup extension through TVSA application in Unit5 of Kozloduy NPP.As water chemistry data with an exceptionally importance were evaluated the data for the coolant critical boron concentrations at Beginning of Cycle (BOC) by Hot Full Power state (HFP), and the necessary total alkalizing agent concentrations (mainly potassium hydroxide) which ensure the high temperature coolant pH values: between pH_(300)=7.0 and pH_(300)-7.2. The reason for this selection is the recent NPP Kozloduy water chemistry primary circuit guidelines, which keeping is the guarantee for the minimizing of constructional materials corrosion processes in primary circuit (mainly fuel element claddings and steam generators tubing) and ensures the real limitation of the fuel cladding depositions (to 10 /μm clad corrosion product thickness).The obtained plant data of 12-th, 13-th and 14-th fuel cycles show that the coolant pH_(300) values above 7.0 are ensured by HFP states with dosing in the coolant of potassium hydroxide below 0.45mmol/kg. The activated coolant corrosion products during the start up , all operation period and shutdown are followed.