Process engineering for safe storage of spent nuclear fuel (SNF) is a global challenge in which Fauske & Associates, LLC (FAI) is actively involved. For example, at the U.S. Department of Energy (DOE) Hanford site, metallic SNF in degraded condition has been stored under water at a location called the K West basin. This fuel is similar to the Magnox fuel stored at the Sellafield (UK) pond designated FGMSP, and in particular one route for remediation is to move the fuel from wet pond storage to intermediate-term dry storage. Wet storage of degraded fuel has evolved a large volume of uranium-metal bearing sludge, presenting unique challenges in developing potential remediation processes.
The most difficult SNF stream to process is a stream of metallic uranium particulate created from handling the damaged SNF elements. FAI has performed basic research and development on the physical and chemical processes unique to the vacuum drying of metallic SNF particulate. The purpose of the effort was to provide the technical basis for minimal design changes to an existing cold vacuum drying (CVD) process used for damaged SNF elements. (The earlier development of CVD process specifications for intact but damaged fuel was also supported by FAI.)
FAI addressed thermal stability of the SNF particulate using a graded approach beginning with simplified models and eventually including multi-dimensional transient calculations (see illustration below). The nature of thermal instability in particulate was found to be significantly different than for large SNF scrap pieces and highly damaged fuel elements. The end products of the effort were:
A summary of the R&D effort is publicly available as “Physical and Chemical Processes During Vacuum Drying of Metallic Spent Nuclear Fuel,” Paper 59114, Proceedings of the ASME 2011 14th International Conference on Environmental Remediation and Radioactive Waste Management, ICEM2011, Reims, France, September 25-19, 2011.
Processing of sludge that contains heavy, gas-generating embedded particles presents unique difficulties. In particular, such sludge has a high yield stress, so the gas generated inside a container of sludge cannot readily escape. Rather than rise through the sludge, gas bubbles tend to coalesce within the sludge, potentially growing into a much larger mechanically stable bubble that can displace the overlying material. This can have the unwanted consequence of plugging filters or even releasing sludge from the container.
Stable bubbles are clearly undesirable, so it is important to be able to predict bubble instability (breakup) and if necessary evaluate possible container design features that can disrupt or prevent the formation of stable bubbles. FAI has extensive analytical and experimental experience in analyzing nuclear waste tank sludge.
FAI performed basic research and development on the stability of sludge gas bubbles, using Taylor stability theory to develop a criterion for sludge bubble breakup. This predictive criterion was then validated by a series of experiments using various geometries and scales (see example illustrations below). Specific design features related to the container wall were evaluated to demonstrate their effectiveness at preventing or disrupting bubble formation.
FAI has broad expertise in chemical issues that arise in decontamination and decommissioning (D&D) and nuclear waste technology. In particular, we have decades of experience in performing research and development to support remediation of the U.S. Department of Energy nuclear facilities. Our Waste Technology department has provided process engineering and safety support through analyses, software development and experiments for a wide variety of nuclear-chemical engineering issues.
Major project and facility applications related to spent fuel processing are: