直接测量法测量核电厂气态流出物^(14)C的准确度分析

Analysis of the accuracy of 14C in gaseous effluent of nuclear power plant by direct measurement method

以某核电厂气态流出物^(14)C样品为研究对象,对直接法测量核电厂气态流出物^(14)C准确度的影响因素进行实验分析。开展了样品和闪烁液最佳配比实验、淬灭影响实验、夹带氚和氚对^(14)C分析结果的影响水平实验等。实验结果表明:采用HAGUE 7000型^(14)C取样器采样时,气态流出物^(14)C样品会夹带氚,夹带氚对^(14)C计数的贡献约为氚活度的1%~2%;样品和闪烁液的最佳配比为6 mL和14 mL;样品淬灭的差异会引起探测效率的偏差。影响直接法测量气态流出物^(14)C准确度的主要因素是样品和闪烁液配比、夹带氚和探测效率准确性。开展实际样品测量时应选用最佳样品和闪烁液配比,保证探测效率的准确性,可通过解谱卡道、采用悬浮分析法或改变采样方式减少夹带氚的影响,提高测量准确度。

[Background]There is no national standard specifically for the analysis of gaseous effluents ^(14)C from nuclear power plants in China,only one group standard available for reference.The group standard is technical specification for ^(14)C analysis of gaseous effluents for nuclear power plants(TBSRS 005―2019).Most nuclear power plants in China refer to this group standard and use direct measurement method to analyze ^(14)C in gaseous effluent.[Purpose]This study aims to investigate the accuracy of ^(14)C in gaseous effluent of nuclear power plant by the direct measurement method.[Methods]A series of experiments were carried out,including optimal ratio of sample and scintillation cocktail,quenching effect,entrainment of tritium and influent factors of tritium on the analysis of ^(14)C.The optimal ratio of sample and scintillation cocktail was selected after measuring different volume ratios of sample and scintillation liquid.The experiment of quenching effect was to measure the samples with different quenching degrees and to observe the influence of different quenching degrees on the measurement efficiency.Entrainment of tritium and the influence level experiment of tritium was to determine the existence of entrainment of tritium through the comparison of samples spectrogram,and to determine the influence level of tritium on the counting of ^(14)C by added different proportions of tritium to ^(14)C sample.Finally,direct measurement results of sixty-seven ^(14)C samples of gaseous effluent from a nuclear power plant were statistically analyzed.[Results]The experimental results show that tritium is entrained in the gaseous effluent ^(14)C sample when the HAGUE 7000^(14)C sampler is used,and 1%~2%of the entrainment of tritium is contributed to the count of ^(14)C.The optimal ratio of sample and scintillation cocktail is 6 mLvs.14 mL.The difference of sample quenching causes the deviation of detection efficiency.The direct measurement results of sixty-seven ^(14)C samples show that the maximum efficiency difference of 4.6%is introduced for different quenching degrees of samples.[Conclusion]The main factors affecting the accuracy of the direct measurement method are the ratio of sample and scintillation cocktail,entrainment of tritium and accuracy of detection efficiency.The influence of entrainment of tritium can be reduced by resolving spectral channel,using suspension analysis method or changing sampling method.