作物和土壤中有机氚迁移转化规律及模型研究

It has the profound significance to organically bound tritium in environmental assessment for nuclear facilities accidents. The organically bound tritium may be the major contributor to the dose of radiation received following its ingestion by man. The increased interest in OBT study was emphasized in the world, but the present information is insufficient for transfer and transformation of OBT in crop and soil, especially for the night release. The different kinds of crops (the leaf vegetable, grain plant, root vegetable, oil plants, fruit vegetable, and typical C4 crop) near nuclear facilities were selected as experimental materials. The crops grown in pot were exposed to tritiated water (HTO) vapor for simulating atmospheric release of HTO at different growth stages in daytime and nighttime, respectively. During the period between the exposure and harvest，the samples were collected in differen times, and the organically bound tritium (OBT) concentrations of the different parts of crops (root, stem, leaf, and edible parts) were measured. The mechanism of OBT formation at daytime and nighttime and translocation and conversion of OBT from leaves to various parts of the vegetable were studied systematically. The primary purpose of the studies was to extract parameters about production of organically bound tritium （OBT）and translocation and conversion of OBT in crops. The organically bound tritium （OBT）concentration prediction model in crops were builded with parameters from experimental work, at the same time the knowledge of growth simulation model was incorporated into the model. The validity and uncertainty of model were analyzed utilizing the data obtained from experiments. The OBT concentration of different depth soil in the vicinity of Qinshan Nuclear Power Base was surveyed, the transfer and transformation of OBT in soil was studied.The study provides the theory basis for effective prediction of tritium concentration in the plants and accurate evaluation doses under the condition of releases of gaseous HTO.

有机氚（OBT）在核设施环境评价中具有重要的意义，食入受氚污染的食物，OBT可能是人类所受氚辐射剂量的主要贡献因子，近年来OBT的研究越来越受到关注，但是农作物和土壤中有机氚的迁移转化行为系统研究还不够深入，尤其是氚的夜间释放。本研究选取不同种类农作物（叶类、籽粒类、根茎类、油料类、浆果类和典型的C4农作物）作为研究对象，采用盆栽试验在不同生育期于白天和晚上模拟气态氚化水（HTO）释放，分多次采样测量不同部位（根、茎、叶和可食部分）OBT的浓度，系统研究不同种类农作物中OBT在白天和晚上的形成规律及迁移转化行为，获取相关参数，结合植物生长发育模拟的有关理论，建立植物中OBT浓度预测模型，并进行有效性和不确定性分析。对秦山核电基地周围不同深度土壤样品有机氚的浓度进行连续调查，研究土壤中有机氚的迁移转化行为。通过本研究以期为气态氚释放后环境介质中氚浓度的有效预测及准确的剂量评价提供理论依据。

对不同种类农作物于不同生育期分白天和晚上在密闭的暴露室内进行气态HTO事故释放的模拟暴露试验，研究结果表明，（1）晚上试验叶子对HTO的吸收约为白天的1/4—1/3，叶子和可食部分OBT的形成能力约为白天的1/3和1/2；玉米叶子对HTO的吸收、晚上和白天OBT的形成能力的差异均小于其他种类。（2）在试验结束后的24小时内，作物中的氚浓度降低速度最快，随后呈缓慢的降低趋势，作物中TFWT随时间的变化呈一维指数递减模式；晚上试验结束后部分作物中OBT浓度的变化与白天试验相比呈升高的趋势，晚上试验收获时OBT浓度降低幅度往往小于白天试验。（3）易位指数（TLI）与暴露试验时间有关，越靠近收获，TLI值越大，小麦中晚上试验TLI与白天差异不大，玉米后两次晚上试验TLI大于白天，其余晚上试验TLI小于白天。（4）收获时，不同种类农作物可食部分有机氚（OBT）的剂量贡献往往大于TFWT，晚上试验与白天试验差异不大。（5）利用试验结果对模型的验证发现，本研究所用模型误差较大，可能是由于参数选择有误，还需进一步完善和改进。.对秦山核电基地土壤中氚浓度分布的研究发现，（1）土壤中的氚化水（HTO）和有机氚（OBT）浓度具有一定的时空分布，与释放源的距离越近，土壤中HTO的浓度越高，但是OBT浓度的空间分布不如HTO明显。（2）不同深度土壤中HTO和OBT浓度的分布规律性不强，除了最大值外，其余各层浓度差异不显著。（3）土壤中有机氚浓度与土壤颗粒粒径的分布存在一定的相关性。（3）土壤中含氢微生物主要为枯草芽孢杆菌，大多集中于表层土壤，未耕作土壤中的芽孢杆菌数是耕作土壤中的2倍。（4）未耕土壤中HTO和OBT的浓度往往大于耕作土壤，但是差异不大。（5）土壤中氚浓度与含水量、微生物数量以及有机质的含量相关性不大。（6）土壤中HTO浓度与空气中HTO浓度的比值约为0.55，植物中HTO浓度与土壤中HTO的比值约为0.79。（7）非耕作和耕作土壤中OBT/HTO比值的平均值为0.94±0.52和0.92±0.73。本研究可为气态HTO释放后植物和土壤中氚浓度预测模型的研究提供相应的参数和理论依据。