Monitoring fission products and actinide levels during pyroprocessing via UV-vis absorption spectroscopy

Keywords: Non-proliferation, nuclear safeguards, pyroprocessing, UV-vis spectroscopy

Abstract

Nuclear power is regaining global prominence as a sustainable energy source as the
world faces the consequences of depending on limited fossil based, CO2 emitting fuels. A key component to achieving this sustainability is to implement a closed nuclear fuel cycle. Without achieving this goal, a relatively small fraction of the energy value in nuclear fuel is actually utilized. This involves recycling of spent nuclear fuel (SNF)â??separating fissile actinides from waste products and using them to fabricate fresh fuel. Pyroprocessing is a viable option being developed for this purpose with a host of benefits compared to other recycling options, such as PUREX. Notably, pyroprocessing is ill suited to separate pure plutonium from spent fuel and thus has non-proliferation benefits. Pyroprocessing involves high temperature electrochemical and chemical processing of SNF in a molten salt electrolyte. During this batch process, several intermediate and final streams are produced that contain radioactive material. While
pyroprocessing is ineffective at separating pure plutonium, there are various process misuse scenarios that could result in diversion of impure plutonium into one or more of these streams. This is a proliferation risk that should be addressed with innovative safeguards technology.
One approach to meeting this challenge is to develop real time monitoring techniques that can be implemented in the hot cells and coupled with the various unit operations involved with pyroprocessing. Current state of the art monitoring techniques involve external chemical
assaying which requires sample removal from these unit operations. These methods do not meet
International Atomic Energy Agencyâ??s (IAEA) timeliness requirements.
The use of UV-vis absorption spectroscopy for live online monitoring of concentration levels of uranium, plutonium, and fission products in molten salt could address this major issue. Fission products, lanthanides and actinides typically have electronic transitions suitable for optical studies in the visible range. For example, Nd and U have signatures for direct optical and
nonlinear optical transitions. UV-vis absorption spectroscopy has been shown to detect electronic transition associated with the elements dissolved in molten salt.
Previous researchers have shown that UV-vis absorption spectroscopy is capable of measuring the concentration of uranium dissolved in LiCl-KCl molten salts. Korean and Japanese researchers have demonstrated the feasibility of this technique in static molten salt melts with low concentration levels of lanthanides and uranium. We examined the applicability of this technique to higher concentrations and its application to molten salts pools in the pyroprocessing flowsheet other than the electrorefiner. A UV-Vis system was setup inside of an argon atmosphere glove box. The sample cell was heated to 650oC in order to run experiments with molten LiCl mixed with various impurities, including actinides. The objective was to demonstrate that concentration threshold of these impurities that could be detected in the molten
LiCl. Presence of actinides in the LiCl salt used for the electrolytic reduction process would be a compelling indicator of a diversion attempt by the operator. Preliminary UV-Vis spectra will be presented with assessment of potential spectral interferences and an initial assessment of
feasibility of the method.