Research Project BUL-13801 'VVER-1000 Coolant Chemistry Improvement by Extended Fuel Cycles'

The worldwide experience of the Pressurized Water Reactors (VVER- 1000, VVER-440 and another PWR's) shows that the concentrations of both boric acid and applied alkalizing reagent (KOH or LiOH) determine the high temperature coolant pHT values. These pHT values have the important impact on the release rate of primary circuit's constructive materials, on the solubility, migration and deposition of corrosion products. At this basis the primary water chemistry will influence also the process of the activity buildup and the increase of dose rate of the circuit's components. The EPRI investigations and the USA PWR operation practice show that Axial Offset Anomaly (AOA) phenomena is connected with boric acid hideout on the fuel assemblies, which occurs as a result of sub-cooled nucleate boiling on higher duty cores. (The corrosion product tendency to deposit in these boiling areas and, in turn, the deposits constitute a matrix for boron hideout). According to one of the existing presumption, the precipitation of lithium metaborate (LiBO2) in corrosion product deposits on the fuel cladding surfaces is the reason for boron hideout. According to another investigators the hideout mechanism is possible to be connected not with precipitation of boric acid salts but with adsorption of boric acid on corrosion products deposited on fuel cladding surfaces. The Callaway NPP (USA) investigations shows that not only lithium metaborate (LiBO2) but also Ni2FeBO5 is possible precipitate. Hideout of coolant species is a complicated process with different mechanisms occurring under different operational conditions of a reactor. Practically by all types of reactors conditions exist possibilities for the hideout phenomenon to manifest itself. In some cases the effects of hideout return are more easily recognizable than those of hideout. This is the reason to be studied the hideout return and hideout effects in primary circuits during transient conditions, during significant changes in the Unit's power (shut down periods) as an information source for the processes connected with the activity buildup in primary circuits surfaces. Scientific Scope of the Project: (i) The main factors causing AOA will be joined interpreted in order to be explained the critical conditions for the formation of higher local H3BO3 concentration of the core bottom part; (ii) Determination of the most important factors impacting on the corrosion rate of primary circuit's constructive materials, on the solubility, migration and deposition of corrosion products; (iii) Interpretation of the data for hideout and hideout return effects in primary circuits as a possible information source for the activity buildup processes on primary circuit surfaces; (iv) Development of such kind of water chemistry regimes, addressed to the different periods of reactor operation cycle, which will limit the corrosion rate and will provide a limitation of the amount of corrosion product deposits on the fuel assemblies. Expected Outputs: The VVER-1000 coolant chemistry improvement will provide possibilities for: (i) limitation of the occurrence of AOA phenomenon; (ii) limitation of the corrosion of the primary circuit surfaces and mitigation of the corrosion product deposition and activity buildup processes.