乏燃料后处理过程中“红油”的形成及分解机理研究

Spent fuel reprocessing, which refers to the process of extracting useful elements such as uranium and plutonium from spent fuel and treating radioactive waste safely and properly, is an important technical step in the advanced nuclear fuel cycle. Evaporation and concentration is the commonly used method for increasing the concentration of feed solution and treating the waste liquid in spent fuel reprocessing plant. The composition of liquid solution processed in evaporation and concentration is complex, including organic phase, aqueous phase, uranium- plutonium nitrate, oil-water mixture, etc. In the process of evaporation and concentration, which is operated at high temperature, the “red oil” is easily formed and it could cause serious safety risk. Several “red oil” explosion accidents have occurred abroad, causing damage to equipment and facilities, and also radioactive contamination to environment. In our country, “red oil” explosion accident has been classified as a design basis accident for the reprocessing plant. However, due to the lack of formation mechanism and explosion data of “red oil”, it is impossible to determine a suitable design basis and safety limit. Therefore, the development of study on the "red oil" explosion accident is urgent.. In this project, quantum chemistry calculation, thermal stability and thermal runaway decomposition experiments of “red oil”, and its behavioral studies under simulating operational condition for evaporation and concentration will be performed to solve some scientific problems of “red oil”, i.e., screening, verification, formation and decomposition mechanism, thermal runaway reaction behavior of the key components of “red oil” in complex system. It is hoped to provide data and theoretical support for the preparation of design basis and safe operation limits of “red oil” explosion accident in spent fuel reprocessing demonstration project.

乏燃料后处理是指提取乏燃料中的铀钚等有用元素并安全妥善处理放射性废物的过程，是构成先进核燃料循环体系的重要技术环节。蒸发浓缩是后处理厂提高料液浓度和处理废液的常用手段。蒸发浓缩的处理对象非常复杂，包含有机相、水相、铀钚硝酸盐、油水混合物等，其在高温操作下易形成“红油”，存在严重安全隐患。国外已发生过数起“红油”爆炸事故，造成设备和设施损坏、环境放射性污染。我国已将“红油”爆炸事故列为后处理厂设计基准事故，但尚缺少形成机理及爆炸数据，无法确定合适的设计基准和安全限值。因此，“红油”爆炸事故相关研究已迫在眉睫。. 本项目针对我国核燃料循环后处理工程设计建设的实际需求，拟采用量子化学理论计算、“红油”热稳定性及热失控分解实验研究、模拟蒸发浓缩操作环境下的“红油”行为研究等手段探讨“红油”关键组分的筛选、形成及分解机制、热失控反应行为等关键科学问题，为核燃料后处理示范工程的红油爆炸事故设计基准及安全运行限值提供数据及理论支持。

红油爆炸是乏燃料后处理蒸发浓缩工艺中的重要安全问题。本项目通过形成爆炸的“红油”关键组分的筛选、形成及分解过程研究，深层次探讨红油爆炸的本质原因及关键影响因素。本项目研制红油关键组分形成机制反应容器1套，发表文章9篇，培养博士研究生三名。主要研究结论是：. (1)通过量子化学计算和实验验证，初步确定了复杂体系中“红油”爆炸关键组分。. 以后处理厂用萃取剂磷酸三丁酯(TBP)、稀释剂煤油或正十二烷，硝酸介质为研究对象。运用密度泛函理论与反应性分子动力学模拟，以正十二烷和正己烷为代表，计算得到正十二烷焓值大于环己烷，正十二烷的硝化产物为“红油”物质关键组分，其成分、反应将更为复杂。采用DSC和ARC绝热量热等测试，研究了TBP、正十二烷、硝酸复杂体系，获得了大量基础数据；结合显著放热和压力骤增等实验特征，初步确定了正丁醇+硝酸、丁酸+硝酸、硝酸戊酯、硝酸戊酯+硝酸等组合是“红油”物质关键组分。.(2)通过对TBP及其分解产物在硝酸“红油”爆炸关键组分的形成及分解机制。. 实验确定TBP分解产物如正丁醇、丁酸、丁醛等组分在无硝酸的条件下无显著放热现象，但蒸气压快速增加，使得红油爆炸事故更加剧烈；硝酸饱和TBP溶液的热失控危险性更高；对热失控危险性高的硝酸饱和TBP溶液的安全泄放设计计算发现，单一的泄放尺寸过大并不适合于工程实际应用；预测热失控反应共经历3个阶段：①TBP在硝酸的作用下发生分解反应；②硝酸本身具有强氧化性，受热分解又可以产生羟基自由基和亚硝基自由基，继而发生自由基链增长反应、氧化反应生成亚硝酸丁酯、丁醛、丁酸等物质；③中间产物受热分解生成C1~C4的烯烃、烷烃等小分子烃类物质。.(3)基于理论计算和实验研究结果，提出了防止后处理厂“红油”爆炸的安全预防与控制措施。. 以2,3-二硝基-2,3-二甲基丁烷等4种硝基化合物模拟红油关键组分的热爆炸初步实验研究。发现当体系中含有硝酸铈和2,3-二硝基-2,3-二甲基丁烷时，体系热感性可能会大大提高，金属硝酸盐与多硝基烷烃都有可能是危害性较大红油组分。. 基于上述基础及应用基础研究，根据乏燃料后处理流程特点，提出了防止“红油”爆炸事故相应安全预防与控制措施。