Paul E. McConnell

Nickel-based Gadolinium Alloy for Neutron Adsorption Application in Ram Packages

Abstract The National Spent Nuclear Fuel Program, located at the Idaho National Laboratory (INL), coordinates and integrates national efforts in management and disposal of US Department of Energy (DOE)-owned spent nuclear fuel. These management functions include development of standardised systems for long-term disposal in the proposed Yucca Mountain repository. Nuclear criticality control measures are needed in these systems to avoid restrictive fissile loading limits because of the enrichment and total quantity of fissile material in some types of the DOE spent nuclear fuel. This need is being addressed by development of corrosion-resistant, neutron-absorbing structural alloys for nuclear criticality control. This paper outlines results of a metallurgical development programme that is investigating the alloying of gadolinium into a nickel–chromium–molybdenum alloy matrix. Gadolinium has been chosen as the neutron absorption alloying element due to its high thermal neutronabsorption cross section and low solubility in the expected repository environment. The nickel–chromium–molybdenum alloy family was chosen for its known corrosion performance, mechanical properties, and weldability. The workflow of this programme includes chemical composition definition, primary and secondary melting studies, ingot conversion processes, properties testing, and national consensus codes and standards work. The microstructural investigation of these alloys shows that the gadolinium addition is present in the alloy as a gadolinium-rich second phase. The mechanical strength values are similar to those expected for commercial Ni–Cr–Mo alloys. The alloys have been corrosion tested with acceptable results. The initial results of weldability tests have also been acceptable. Neutronic testing in a moderated critical array has generated favourable results. An American Society for Testing and Materials material specification has been issued for the alloy and a Code Case has been submitted to the American Society of Mechanical Engineers for code qualification.

EXTENDED DRY STORAGE OF USED NUCLEAR FUEL: TECHNICAL ISSUES: A USA PERSPECTIVE

Used nuclear fuel will likely be stored dry for extended periods of time in the USA. Until a final disposition pathway is chosen, the storage periods will almost definitely be longer than were originally intended. The ability of the important-tosafety structures, systems, and components (SSCs) to continue to meet storage and transport safety functions over extended times must be determined. It must be assured that there is no significant degradation of the fuel or dry cask storage systems. Also, it is projected that the maximum discharge burnups of the used nuclear fuel will increase. Thus, it is necessary to obtain data on high burnup fuel to demonstrate that the used nuclear fuel remains intact after extended storage. An evaluation was performed to determine the conditions that may lead to failure of dry storage SSCs. This paper documents the initial technical gap analysis performed to identify data and modeling needs to develop the desired technical bases to ensure the safety functions of dry stored fuel.

An evaluation of the use of depleted uranium as a structural component for transport casks

Depleted uranium (DU) alloys are currently used for gamma-ray shielding in casks and as shielding blocks. For the transport cask application, a significant weight and dimensional penalty exists when using the DU solely for shielding. If credit could be taken for the structural use of the DU for containment in a transport cask, greater payloads may be realized. Mechanical property measurements of several uranium alloys and finite element analyses of prototype transport casks assumed to be constructed, in part, from selected uranium materials were performed to evaluate the potential for the use of DU alloys for cask containment. These data and analyses support the concept of the use of DU alloys for the containment function even under hypothetical accident conditions. A conclusion is that the properties of certain DU alloys are therefore sufficient to warrant further consideration of the material for this purpose.

Transportation operations model analysis for removal of stranded fuel from shutdown reactors

Abstract The transportation operations model was used to identify options for removing stranded fuel currently in dry storage at nine shutdown reactor sites to a hypothetical consolidated storage facility. The logistical variables included the campaign duration, fuel selection priority, consist size and location of the consolidated storage and maintenance facilities. The major factors affecting the logistics of fuel removal were identified. Recommendations for optimal strategies for the transport of stranded fuel from shutdown sites are made.

A New Ni-Cr-Mo-Based Gadolinium Structural Alloy for Neutron Adsorption Application in Radioactive Material Packages

In order to avoid restrictive fissile loading limits in some types of US Department of Energy (DOE) spent nuclear fuel, nuclear criticality control measures are needed in standardized waste packages for long-term disposal of high-level radioactive materials in the proposed Yucca Mountain repository. This need has been addressed by development of a corrosion-resistant, neutron-absorbing structural alloy for nuclear criticality control. Applications for the alloy include internals (e.g., baskets) for storage, transportation, and disposal of radioactive material (RAM). This paper outlines results of a metallurgical development program that alloyed gadolinium into a nickel-chromium-molybdenum matrix. Gadolinium was chosen as a neutron-absorption alloying element due to its high thermal neutron absorption cross section and low solubility in the anticipated repository environment. The nickel-chromium-molybdenum alloy family was chosen for its known corrosion performance, mechanical properties, and weldability. An ASTM International Material Specification has been issued for the alloy and a Code Case has been approved by the American Society of Mechanical Engineers for RAM package applications. Mechanical and physical property data for the alloy are provided in this paper.Copyright

MITIGATION OF CAESIUM AND COBALT CONTAMINATION ON THE SURFACES OF RAM PACKAGES

Abstract Techniques for mitigating the adsorption of 137Cs and 60Co on metal surfaces (e.g. RAM packages) exposed to contaminated water (e.g. spent-fuel pools) have been developed and experimentally verified. The techniques are also effective in removing some of the 60Co and 137Cs that may have been adsorbed on the surfaces after removal from the contaminated water. The principle for the 137Cs mitigation technique is based upon ion-exchange processes. In contrast, 60Co contamination primarily resides in minute particles of crud that become lodged on cask surfaces. Crud is an insoluble Fe–Ni–Cr oxide that forms colloidal-sized particles as reactor cooling systems corrode. Because of the similarity between Ni2+ and Co2+, crud is able to scavenge and retain traces of cobalt as it forms. A number of organic compounds have a great specificity for combining with nickel and cobalt. Ongoing research is investigating the effectiveness of chemical complexing agent EDTA with regard to its ability to dissolve the host phase (crud) thereby liberating the entrained 60Co into a solution where it can be rinsed away.