Deposition-Dominated Cool ant Activity Analysis

Abstract

In an attempt to determine sheath wear rates at Darlington NGS, a model of the behaviour of 95Zr and 95Nb in the Primary Heat Transport System (PHTS) and the Gaseous Fission Product (GFP) monitoring systems was developed. This model uses the coolant activities of 95Zr and g6Nb as measured by the GFP to determine the relative rate of bundle sheath wear. Three different models of Zr/Nb deposition in the PHTS are evaluated, and the one which best reproduces the coolant activity data is selected for use. This model, referred to as the perfect sink deposition model, is applied to coolant activity data from Darlington Unit 1, identifying several periods of high bundle sheath wear. Since the model is used to predict the measured activities of both 95Zr and 95Nb, the ratio of these activities can be compared to the ratio measured in the GFP system to determine whether a large increase in Zr/Nb activity levels is due to release from in the reactor flux or from outside the flux region. This is useful in differentiating between bundle wear excursions and crud bursts. Due to the fact that the Unit 2 GFP system was out of service for extended periods, only Unit 1 GFP data are analysed in this paper.