Advanced LWR Fuel Designs with Significant Potential for Power Uprate

At Center for Advanced Nuclear Energy Systems (CANES) improving the economic competiveness of light water reactors (LWRs) has been the focal point of research. The economic enhancements in LWRs are primary investigated through increasing the nuclear core power density in form of power uprates given existing designs or future plant constructions. In order to realize a significant power uprate (> 20%) of already uprated LWRs, fuel enrichment levels of beyond 5% is required, if conventional UO2 fuel is utilized. The increase in power also must come with the ability to maintain or improve the current LWRs' safety standards and margins. The changes to the fuel geometry, materials and/or operating conditions has been previously investigated. This work focuses on changing the fuel geometry while maintaining the operating conditions similar to current LWRs, in order to achieve significant power uprate. By focusing on improving the nuclear fuel geometry, the improved fuel is still compatible with other fuel types such as high density fuels (e.g. Uranium nitride), high temperature fuels (e.g. TRISO) and/or metallic and ceramic composite claddings with improved corrosion resistance and high burnup performance. Given the current LWR fuel material, the cylindrical pin geometry is limited in providing sufficient surface area for heat transfer while maintaining desirable structural integrity. Previous parametric studies have shown that cylindrical pin geometry cannot result in beyond 20% power uprate in LWRs and other geometries needs to be explored. The Internally and eXternally cooled Annular Fuel (IXAF) is one such geometry that increases the heat transfer area of the fuel rod significantly and has shown to be able to increase the power output of existing pressurized water reactors (PWR) by 50% and boiling water reactors (BWRs) by 25%. IXAF is cooled on both the external and internal surfaces, resulting in significantly lower average fuel temperatures, even at 50% higher power rating compared to equivalent solid pin design. The larger heat transfer area and power density of IXAF, comes at the cost of reduced total fuel loading and increase in enrichment, beyond 5%. The Helical Cruciform shaped Fuel (HCF) geometry uses the strategy of fins to increase the heat transfer area and also the twisted tapes approach to increase the swirl and intra-bundle mixing of the flow to increase margin to critical heat flux compared to the traditional cylindrical fuel rod bundle geometries. The HCF design in this work is a four petal design which originates from similarly twisted three petal metallic fuel in Russian ice-breaker nuclear reactors. The HCF fuel has potential to increase the power density of PWRs and BWRs by up to 50% and 25%, respectively. The HCF concept also eliminates the need for spacer grids as the fuel rods are supported by resting on each other. Similar to IXAF concept, HCF reduces the average fuel temperature considerably compared to the cylindrical pin geometry. Though, the HCF shape is not ideal to maintain optimum neutron economy. Furthermore, the higher cladding volume along with significant power uprate requires fuel to be enriched above 5%. (author)