CONTAINMENT SUMP NEUTRALIZATION USING TRISODIUM PHOSPHATE - PARAMETRIC ANALYSIS

摘要

For post-LOCA conditions, the pH of the aqueous solution collected in the containment sump after completion of injection of containment spray and ECC water, and all additives for reactivity control, fission product removal, and other purposes, should be maintained at a level sufficiently high to provide assurance that significant long-term iodine re-evolution does not occur. Long-term iodine retention may be assumed only when the equilibrium sump solution pH is above 7. This pH value should be achieved by the onset of the spray recirculation mode. A trisodium phosphate (TSP)-based, passive system can be used to achieve this pH value. This is a proven technology that is already in use in nuclear power plants. This system consists of several wire mesh baskets, filled with TSP and strategically located in the sump in order to insure timely dissolution of TSP and rapid pH rise under LOCA conditions. Accurate determination of the total quantity of TSP required to raise the pH of borated water in the sump to within the acceptable range is the key element to a proper design of this system. However, this type of analysis is quite involved and highly iterative, which requires the use of a computer program. This paper describes the basis for a computer program that determines the required quantity of TSP as a function of the quantity of borated water in the sump, the boron concentration, the sump temperature, and the specified pH value. The equilibrium quantities of boric acid species are calculated iteratively based on its molal equilibrium quotients. The equilibrium quantities of phosphoric acid species are calculated iteratively based on its dissociation constants. The charge balance error (CBE) is the sum of ionic charges for all species and ions in the solution, including sodium. All species are in equilibrium when the CBE reduces to zero. The paper also presents the results of a parametric analysis that is performed using this computer program. Ranges of borated water quantity, boron concentration, and sump temperature are considered. For these borated water quantities and sump conditions, TSP quantities required to achieve sump pH values that range from 7.0 to 8.0 are determined. The results show significant dependence on temperature. In addition, the paper provides recommendations regarding TSP basket sizing and locations within the sump, as well as pH surveillance test requirements.