Publications
Nuclear Fuel Cycle (NFC) Technology and Policy Program
On the Use of High Performance Annular Fuel in PWRs
Bo Feng, Pavel Hejzlar and Mujid S. Kazimi
MIT-NFC-TR-100 (June 2008)
Abstract
Recently, MIT’s Center for Advanced Nuclear Energy Systems developed a new high burnup annular fuel that features both internal and external cooling. Implementation of this fuel design in current pressurized water reactors (PWRs) will allow power uprates up to 50% while maintaining or improving the existing thermal and safety margins. Each annular fuel assembly is arranged in a 13x13 array but has the same side dimensions as a 17x17 solid fuel assembly. Even at much higher power densities, the peak fuel temperatures are substantially lower and the MDNBR is comparable to that of solid fuel at 100% power. The main motivation for utilizing this fuel is the lower capital construction cost per kilowatt of electrical production compared to reactors using solid fuel.
To elaborate on the previous work, three remaining issues were addressed: the reduced reactivity shutdown margin at 50% uprated power, effect of inner channel flow restrictions due to crud buildup and obstructions, and the economic impact of a fleet of reactors using high burnup annular fuel. All of the work was done using computer codes specializing in core neutronics, thermal hydraulics, and fuel cycle analysis.
The deficit in shutdown margin was found to be caused mainly by a reduction in control material volume coupled with a higher power density. This issue was resolved by changing the control material from traditional Ag-In-Cd to 25 wt% B-10 enriched B4C. Increasing the control rod surface area was also investigated as a possible solution but it was revealed that any departure from the cylindrical shape would lead to a reduction in control volume which resulted in decreased rod worth.
Simultaneous oxide growth and crud buildup on the inner cladding of the annular fuel was simulated in a whole core thermal-hydraulics model to determine the maximum thickness that the annular fuel could tolerate while maintaining an MDNBR greater than 1.3 under transient overpower conditions. Under very conservative conditions, the maximum tolerable thickness was calculated to be a uniform 50 μm layer of combined oxide and corrosion on the inner and outer cladding surfaces of the hot rod. Under full power conditions, the fuel was found to be able to tolerate a 35-40% blockage of the hot rod’s inner channel. However, plugging can be regarded as hypothetical since debris filters in PWRs have a mesh spacing smaller than the fuel’s inner channel diameter.
The fuel cycle analysis code CAFCA SD was modified to enable modeling the effect of LWRs using high burnup annular fuel on the US fuel cycle. At a given date, annular fuel can be introduced to the once-through fuel cycle via Generation II reactor uprates or construction of Generation III reactors. Results showed that constructing new 1.5 GWe reactors using annular fuel resulted in the greatest reduction in the cost of electricity due to its low capital construction cost. Total spent fuel was also reduced due to the reduced amount of reactors required to fulfill power demand. However, compared to traditional 4.5 wt% enriched solid fuel, using the higher enriched annular fuel for an entire fleet of LWRs would require a greater uranium mining rate.
