On-Line Monitoring for Control and Safeguarding of Radiochemical Streams at Spent Fuel Reprocessing Plant

摘要

There is a renewed interest worldwide to promote the use of nuclear power and close the nuclear fuel cycle. The long term successful use of nuclear power is critically dependent upon adequate and safe processing and disposition of the spent nuclear fuel. For large throughput nuclear facilities, such as commercial reprocessing plants, it is difficult to satisfy the IAEA safeguards accountancy goal for detection of diversion. Process monitoring is used in safeguards as an additional measure to nuclear material accountancy. Process monitoring consists of utilizing process control measurements to detect abnormal plant operations. Liquid-liquid extraction is a separation technique commonly employed for the processing of the dissolved spent nuclear fuel. Our approach is based on prerequisite that real time monitoring of the solvent extraction flowsheets provides unique capability to quickly detect unwanted manipulations with fissile isotopes present in the radiochemical streams during reprocessing activities. The instrumentation used to monitor these processes must be robust, require little or no maintenance, and be able to withstand harsh environments such as high radiation fields and aggressive chemical matrices. Our team experimentally assessed the potential of Raman and vis-NIR spectroscopic techniques for on-line real-time monitoring of the U(VI)itrate ionitric acid and Pu(IV)/Np(V), respectively, in solutions relevant to spent fuel reprocessing. Our ability to identify material intentionally diverted from a liquid-liquid solvent extraction contactor system was successfully tested using on-line process monitoring as a means to detect the amount of material diverted. A chemical diversion and detection from a solvent extraction scheme was demonstrated using a centrifugal contactor system operating with the simulant PUREX extraction system of Nd(NO_3)_3itric acid aqueous phase and TBP/dodecane organic phase. During a continuous extraction experiment, a portion of the feed from a counter-current extraction system was diverted while the spectroscopic on-line process monitoring system was simultaneously measuring the feed, raffinate and organic products streams. The amount observed to be diverted by on-line spectroscopic process monitoring was in excellent agreement with values based from the known mass of sample directly taken (diverted) from system feed solution. We conclude that near real-time spectroscopic process monitoring is a useful tool for the immediate detection of diverted material. This paper summarizes our methodology of on-line process monitoring and shows results of specific examples.