Year
2003
Abstract
The National Spent Nuclear Fuel Program, located at the Idaho National Engineering and Environmental Laboratory, coordinates and integrates national efforts in management and disposal of U.S. Department of Energy (DOE)-owned spent nuclear fuel. These management functions include development of standardized systems for packaging, storage, treatment, transport, and 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 a corrosion resistant, neutron absorbing structural material for nuclear criticality control. These materials offer distinct advantages over existing neutron absorbing materials available to the commercial nuclear industry for use in spent nuclear fuel pools, transportation systems and storage casks. This paper will outline the results of a metallurgical development program 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 neutron absorption cross section and low solubility in the expected repository environment. The nickel-chromiummolybdenum alloy family was chosen for their known corrosion performance, mechanical properties and weldability. The workflow of this program 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 show that the gadolinium addition is not soluble in the primary austenite metallurgical phase and is present in the alloy as gadolinium-rich second phase. This is similar to what is observed in a stainless steel alloyed with boron. The mechanical strength values are similar to those expected for commercial Ni-Cr-Mo alloys. The alloys have been corrosion tested in simulated Yucca Mountain aqueous chemistries with acceptable results. The initial results of Varestraint weldability tests have also been acceptable. Neutronic testing in a moderated critical array has generated favorable results. The goals of this work is to qualify these materials for incorporation into the American Society for Testing and Materials standards, obtain American Society of Mechanical Engineers code qualification, and be accepted for use at the Yucca Mountain Repository.