Minimizing Fuel Assembly Distortion in LWRs to Prolong Life and Increase LWR Sustainability

Topic 20c of DOEs SBIR solicitation seeks grant applications to develop technologies for the assessment and mitigation of materials degradation in Light Water Reactors in order to extend the service life including methods that can mitigate or predict irradiation and aging effects An important degradation phenomenon that limits the service life and energy production of Light Water Reactor (LWR) fuel is fuel assembly distortion. Specific manifestations include bowing of the overall assembly structure in Pressurized Water Reactor (PWR) fuel assemblies and channel distortion in Boiling Water Reactor (BWR) fuel assemblies. Such bowing and distortion can lead to core unloading and reloading difficulties, fuel assembly damage, or incomplete rod insertion in PWRs and premature channel replacement or fuel assembly damage in BWRs. These issues impede the core refueling process and increase outage times. They also have long-term negative consequences. A fuel assembly that is damaged during one outage may fail during subsequent reactor cycles, thereby releasing fission products to the reactor coolant. Also, since the distortion gets worse the higher the fuel burnup, solving this problem is an important prerequisite towards achieving higher fuel burnup, thus reducing the amount of spent fuel to be treated and/or disposed of by the DOE. It is known that the stiffness and creep properties of fuel rods and control rod guide tubes within PWR fuel assemblies plays a dominant role in determining the amount of distortion that occurs. Zircaloy 4, Zirlo and M-5 fuel rods and control rod guide tubes, now in use, are relatively flexible, and prone to creep under load. This SBIR project seeks to determine if replacing these existing metal tubes with an advanced ceramic material, which does not creep under load, and is stronger than the zirconium alloys, can mitigate or eliminate the distortion problem. Specifically, we will measure the key mechanical properties of our new silicon carbide (SiC) three layer tubular material (already partly developed via other DOE industry supported programs) and also modify and apply an array of unique analytical computer codes designed to predict the distortion of PWR fuels assemblies as a function of the material properties of the fuel rods and guide tubes. If the analyses proves that these new materials will indeed mitigate or eliminate the distortion prevalent with existing materials, we will propose a specific test program for Phase II involving scale model testing, and also examine similar solutions to Boiling Water Reactors (BWRs) which use channel boxes instead of control rod guide tubes, as the main structural backbone of the fuel assembly. Development and use of these innovative technologies would directly support DOEs LWR sustainability program goals.Commercial Applications and Other Benefits: This proposed research can be applied directly in the manufacture and operation of commercial Light Water Reactor fuel assemblies for the existing 104 U.S. plants and future Generation 3+ plants and Small Modular Reactors now being planned.