RADIOLYTIC HYDROGEN GENERATION INSAVANNAH RIVER SITE (SRS) HIGH LEVEL WASTETANKS COMPARISON OF SRS AND HANFORDMODELING PREDICTIONS

Abstract

In the high level waste tanks at the Savannah River Site (SRS), hydrogen is produced continuously by interaction of the radiation in the tank with water in the waste. Consequently, the vapor spaces of the tanks are purged to prevent the accumulation of H{sub 2} and possible formation of a flammable mixture in a tank. Personnel at SRS have developed an empirical model to predict the rate of H{sub 2} formation in a tank. The basis of this model is the prediction of the G value for H{sub 2} production. This G value is the number of H{sub 2} molecules produced per 100 eV of radiolytic energy absorbed by the waste. Based on experimental studies it was found that the G value for H{sub 2} production from beta radiation and from gamma radiation were essentially equal. The G value for H{sub 2} production from alpha radiation was somewhat higher. Thus, the model has two equations, one for beta/gamma radiation and one for alpha radiation. Experimental studies have also indicated that both G values are decreased by the presence of nitrate and nitrite ions in the waste. These are the main scavengers for the precursors of H{sub 2} in the waste; thus the equations that were developed predict G values for hydrogen production as a function of the concentrations of these two ions in waste. Knowing the beta/gamma and alpha heat loads in the waste allows one to predict the total generation rate for hydrogen in a tank. With this prediction a ventilation rate can be established for each tank to ensure that a flammable mixture is not formed in the vapor space in a tank. Recently personnel at Hanford have developed a slightly different model for predicting hydrogen G values. Their model includes the same precursor for H{sub 2} as the SRS model but also includes an additional precursor not in the SRS model. Including the second precursor for H{sub 2} leads to different empirical equations for predicting the G values for H{sub 2} as a function of the nitrate and nitrite concentrations in the waste. The difference in the two models has led to the questions of how different are the results predicted by the two models and which model predicts the more conservative (larger) G values. More conservative G values would predict higher H{sub 2} generation rates that would require higher ventilation rates in the SRS tanks. This report compares predictions based on the two models at various nitrate and nitrite concentrations in the SRS HLW tanks for both beta/gamma and for alpha radiation. It also compares predicted G values with those determined by actually measuring the H{sub 2} production from four SRS HLW tanks (Tanks 32H, 35H, 39H, and 42H). Lastly, the H{sub 2} generation rates predicted by the two models are compared for the 47 active SRS high level waste tanks using the most recent tank nitrate and nitrite concentrations and the beta/gamma and alpha heat loads for each tank. The predictions of the models for total H{sub 2} generation rates from the 47 active SRS waste were, for the most part, similar. For example, the predictions for both models applied to 25 tanks agreed within {+-}10% of each other. For the remaining 22 tanks, the SRS prediction was more conservative for 9 tanks (maximum 29% higher) and the Hanford prediction was more conservative for 13 tanks (maximum 19% higher). When comparing G values predicted by the equations presuming only alpha radiation or only beta/gamma was present the results were somewhat different. The results of predictions for alpha radiation, at the 47 current nitrate and nitrite concentrations in the SRS tanks indicated that all the SRS predictions were higher (up to 30%) than the Hanford predictions and thus more conservative. For beta/gamma radiation the predictions for both models agreed to {+-}10% for 18 of the combinations, the Hanford model predicted higher values (11 up to 17%) for 25 of the concentrations considered, and the SRS model predicted higher G values for the remaining two combinations (12 and 17%). For the four SRS tanks, where we compared measured G values to those predicted by the two different models, the results for two tanks (Tanks 35 and 39) were in good agreement with predictions from both models. For the other two tanks (Tanks 32 and 42) the predictions of both models were conservative. The predictions were 3 to 4X higher than the measured G values for H{sub 2} production.