It is widely experienced that operation with a low primary coolant pHT leads to heavy deposition on fuel-pin cladding in pressurized water reactors (PWRs). This is thought to be due principally to solubilities of corrosion products exhibiting negative temperature dependencies at low coolant pHT, leading to precipitation from the solution onto core surfaces. Solubilities also increase at low pHT values and this may be an additional reason for the increased deposition. Particulate deposition may also depend on coolant pHT. Operation at low coolant pHT may thus cause increased corrosion product activity transport, leading to higher dose rates around the primary circuit. The possible correlation between low pHT operation and steam generator channel head dose rates was investigated, using detailed data from nine Westinghouse PWRs. The coolant chemistry was quantified by calculating both the percentage of operating time at low pHT and a numerical “precipitation index” in order to establish the extent of operation below that coolant pHT above which little core crud deposition is expected. Time averaged pH’s were also calculated for each cycle. End-of-cycle dose rates were plotted against these coolant chemistry parameters on a plantby-plant basis and statistical tests were applied following linear regression analysis. Positive correlations were obtained and it was found that, for the limited number of plants in the survey, these correlations were between the categories “significant” and “probably significant” for cycle 1 and cycle 2 data.
