Health and Safety Considerations Associated with Sodium Debris Bed Experimental Nuclear Assembly Dismantlement

摘要

As the end of World War II approached, scientists who developed the processes to generate nuclear material for war-time use grew interested in applying those technologies to the generation of power for cities across the United States. Concerns about the availability of natural uranium led to the development of self-fueling fast neutron breeder reactors. In order to produce plutonium at a rate greater than the consumption of uranium during a nuclear chain reaction, high-energy “fast” neutrons are required. Fast neutron reactors utilize low melting-point metals, like sodium, to provide cooling.After the earliest reactors experienced trouble providing adequate cooling during upsets, resulting in partial melt-downs, experiments were designed to test fuel and cooling material arrangements that would improve cooling characteristics, even in the event of partial damage to reactor components. In a sodium-cooled reactor, damaged components from a high temperature excursion may involve nuclear fuel, liquid sodium coolant, thermal insulation, and other instrumentation. This combination of damaged material is referred to as a sodium debris bed.Between the mid-1970s and the mid-1980s Sandia National Laboratory constructed eleven experimental assemblies to simulate debris beds formed in a sodium-cooled fast breeder reactor. All but one of the assemblies were irradiated. The experimental assemblies were transferred to the Idaho National Laboratory (INL) in 2007 and 2008 for storage, dismantlement, recovery of the uranium for reuse in the nuclear fuel cycle, and disposal of unneeded materials. The effort to recover this fuel is termed the INL Sodium Debris Bed Material Treatment Project.The assemblies are comprised of nested layers of metal and thermal insulation innervated by a variety of monitoring instrumentation. In the center of the assembly is a primary containment vessel holding a crucible of enriched uranium oxide fuel surrounded by a blanket of metallic sodium. Due to the additional time required to design and install equipment necessary to separate the sodium from the uranium product, the project was broken into two phases. The first encompasses dismantlement and recovery of the primary containment vessel for temporary storage. The second phase involves separation of the sodium from the uranium product. This report deals with the dismantlement phase of this project.After identification of all disciplines responsible for, or impacted by, elements of the effort, a series of trade studies were conducted to identify appropriate facilities; means of exposure control; transfer, disassembly, and repackaging strategies; and preferred assembly processing order. Concurrently, an effort was undertaken to identify and compile all applicable regulations, codes, standards, procedures, or other requirements. Physical and chemical hazards including radiation, material handling, asbestos, and sodium, and the controls implemented are discussed.Prior to operations, full-scale mock-ups with realistic surrogate assemblies were completed, as well as internal and independent self-assessments to demonstrate the readiness of facilities, staff, and operating procedures. Dismantling operations commenced in late 2014 and are ongoing. Lessons learned from the effort to date are presented. The use of realistic mock-ups and attention to the order in which assemblies are processed are largely credited for project success to date.