The US Nuclear Regulatory Commission (NRC) has approved a technical report submitted by Framatome concerning high burnup limits for pressurized water reactors.

This decision follows six years of engineering work to support increased limits for the GAIA and HTP fuel designs. These updates allow plant operators to extend reactor cycles from 18 months to 24 months. 

The NRC also recently approved the use of U-235 enrichment levels above 5% and granted a license amendment for the domestic transport of fuel assemblies with enrichments reaching 8%.

The fuel assemblies will be manufactured at a facility in Richland, Washington, which has been in operation for 55 years. 

Designed for high-burnup cycles

The GAIA design is used for high-burnup cycles and includes mixing vane grids to stir coolant as it flows past fuel rods. This mechanism improves critical heat flux performance and maintains safety margins during operation. 

The design also features guide tubes made of Q12 alloy to resist fuel assembly bowing and lateral deformation. To manage fission gas release at the end of the fuel’s life, the assemblies can utilize chromia-doped pellets containing 500 to 1600 parts per million of chromia.

The High Thermal Performance (HTP) platform is used for various reactor lattice types in the United States and Europe. It features a spacer grid with an 8-line fuel rod support concept to prevent grid-to-rod fretting wear. 

Many of the technologies used in the GAIA design were established and tested within the HTP line. These designs are part of a fuel management program that provides technical methods and codes for the existing nuclear fleet to support higher burn-ups and higher enrichment levels.

Reducing the amount of waste produced

The approval of the topical report allows operators to manage fuel utilization and reduce the total amount of waste produced. By using fuel that remains in the reactor for longer periods, plants can maintain energy production with fewer refueling outages. 

The recent suite of NRC approvals includes methods for applying codes to operating conditions that involve higher enrichment standards. These efforts are directed at maintaining the mechanical reliability of fuel as the industry moves toward these expanded operational limits.

The use of Q12 alloy for guide tubes is a measure to address physical stability within the reactor core. Lateral deformation and bowing can impact the placement and performance of fuel, and the monobloc construction of these tubes is designed to mitigate those issues. 

The mixing vane grids serve as structural components that replace standard spacer grids, providing both mechanical support and active coolant management. These technical components work together to meet the safety and performance requirements established by the NRC for the high-burnup limit increase. 

“The approval of this report demonstrates our expertise in fuel design and commitment to our customers and the industry in bringing economical solutions to the market,” concluded Lionel Gaiffe, Framatome’s Senior Executive Vice President, Fuel Business Unit.