Reduction in cobalt ingress from fuelling machine stellite wear products into primary heat transport system: results from modeling assessment of ion exchange clean-up

This paper presents an analysis of the benefit of installing ion exchange capability in the heavy water auxiliary system of a Darlington fuelling machine. An overall mass balance model was developed to estimate the potential reduction in cobalt ingress from the fuelling machine into the primary heat transport system. The analysis considered generation terms from a) on-going ram ball wear and b) dissolution from the surfaces of trapped filter particulates as well as from ram ball surfaces and removal terms arising from a) flow injection into the reactor system, b) filtration and c) ion exchange. Model equations were solved for each activity in the fueling cycle to determine the aggregate behavior over an extended period of operation. Results from model calculations suggest that, without ion exchange provisions, about 16.7 g/year of cobalt enters the reactor system from each fuelling machine. The majority of this cobalt (66%) is in particulate form with the balance being in dissolved form (soluble as well as colloidal). Most of the dissolved cobalt (80 %) is generated from the surfaces of the ram balls, the remainder being from the surfaces of the trapped filter particulates. The potential reduction in cobalt ingress from ion exchange operation was estimated to be 21% or 3.5 g/year per fuelling machine. Based on a postulated growth curve for the Co-60 reactor face fields, estimates were also obtained for the net plant dose savings that would result from ion exchange operation. This was estimated for the period 2010 - 2022 to exceed 480 rem. Based on the analysis performed, installation of ion exchange purification capability in the heavy water auxiliary system is strongly recommended. (author)