Actions Taken to Significantly Reduce Activity Levels on Primary Loop Surfaces at Loviisa NPP

摘要

At Loviisa NPP high activity levels of antimony (Sb-124 and Sb-122) in the primary coolant have been the major (～40-50%) contributors to the dose rates. The most efficient countermeasure to reduce doses is to remove the most significant source term. Loviisa NPP has for many years had a program for identifying materials having high inactive antimony content in components or in used additives in contact with primary coolant. Chemistry laboratory has analyzed chemical composition of various process chemicals as well as boric acid concentrates in order to identify possible deviations from chemical specifications. In addition, greases, gaskets, seals and all kinds of materials exposed to primary coolant have been examined. Based on the acquired data it is quite evident that the primary coolant pump (PCP) seals might be the major contributors to the measured Sb levels in the primary coolant. In these pumps the seal water flows over the sealing surfaces and antimony is either dissolved or mechanically removed from the surface to the primary coolant and is activated in the core region. Prior to actual changes in sealing material composition, the sealing water process lines were modified in 2011 to enable evaporation of water containing high concentration of inactive antimony instead of re-routing the water back to primary coolant. In addition extensive tests were carried out to verify that the existing seal design with a new, alternative seal composition would be able to handle all typical operating and accident conditions. Finally in 2012 a new carbon impregnated seal was installed to one of the pumps and during the following outage in 2013 six more similar seals replaced antimony impregnated ones. This paper presents results from various impurity measurement campaigns, as well as the effects of process modifications on the activity levels on primary coolant surfaces and on the inactive Sb concentrations in the primary water.