Electrorefining Process for the Treatment of Zr-Alloy Cladding Hull Wastes from Used Nuclear Fuels

Abstract

The pyroelectrochemical process has been widely used for the treatment of used nuclear fuel owing to its high proliferation resistance as well as a substantial reduction capability of high level nuclear wastes by recovering valuable materials such as uranium (U) and transuranic (TRU) elements. The pyroprocess developing at KAERI consists of a head-end process that produces feed materials through a disassembly/chopping/decladding of used fuel assemblies and a back-end process that recovers U and TRU from the feed materials by a series of molten salt processes such as electroreduction, electrorefining, and electrowining. At the head-end process, metal wastes including cladding hulls and hardware are generated in amounts of about 2.5 tons and 1.0 tons per 10 tons of U, respectively. In particular, the cladding hull wastes are expected to be categorized as an intermediate- or a GTCC level waste owing to the radioactivated elements produced during the reactor operation and residue of used nuclear fuel adhered to the surface. Although the commercialized methods used for the treatment of cladding hull wastes such as a simple compaction and melting process are able to reduce the volume of wastes, the activity level of all waste cannot be changed. Therefore, alternative approaches that can recover a major element of hull waste, zirconium (Zr), have been studied to lower the activity as well as reduce the volume/weight of the waste. This study demonstrates a Zr electrorefining process for the treatment of cladding hull wastes in molten salt systems. The electrolytes for the experiments include chloride-, fluoride-based, and mixed salts with initiators. The operating conditions are varied to enhance the morphology of recovered Zr, thereby reducing the incorporated salt into the deposit. By electrorefining Zr from fresh cladding hull tubes such as Zirlo and Zircaloy-4, the characteristics of Zr deposits will be examined using SEM, XRD, and ICP-AES.