To address the energy needs of developing countries and remote communities, Oregon State University has proposed the Multi-Application Small Light Water Reactor (MASLWR) design. This design uses 8% enriched fuel to achieve five years of operation without refueling. The specific operational conditions (lower pressure and temperature of fuel and coolant), increased enrichment of fuel, and extensive use of gadolinium burnable absorbers lead to significantly different neutron physics compared to conventional pressurized water reactors. In particular, spectrum hardening due to increased thermal neutron absorption, changes in kinetic parameters due to the isotopic content of the fresh and irradiated fuel, and fuel and control rod shadowing by burnable absorbers are consequences of the design requirements. Enhanced neutron leakage from the small MASLWR core also adds complexity. Neutron reflectors and a unique fuel-loading pattern compensate the pronounced axial and radial gradients of the neutron flux and power generation.

This paper discusses the neutron physics and thermal-hydraulic issues of the core design for a small reactor with increased fuel enrichment and natural circulation of the coolant. The paper describes three evolutionary steps of the MASLWR core design process and discusses core parameters, advantages, disadvantages, and design limitations as they appeared during the core design feasibility study. The paper demonstrates the feasibility of the core design for five effective years of nonrefueled operation with 8.0% enriched UO2 fuel.