Performance of internally and externally cooled annular fuel in a loss of coolant accident

To realize significantly larger increases in core power density while maintaining safety margins radical changes in PWR fuel design are necessary. An innovative design recently proposed at MIT departs from the traditional solid rod design by employing annular elements with both internal and external cooling. The annular rods have larger diameter than the current fuel rods, thus using a significantly smaller number of pins in a regular size PWR assembly. The key benefits of the new fuel include (1) very low operating peak fuel centerline temperature (1000 deg. C than a solid fuel pin) even for fuel assembly powers 100% higher than those of current PWRs, (2) small cladding temperature rise in a LOCA due to limited stored energy in the fuel, (3) appreciable increase of DNBR margin due to the reduction of surface heat flux, allowing a substantial power density increase of up to 50%, and (4) small fission gas release due to low fuel temperature and reduction of pellet cracking due to the small temperature gradient. This paper focuses on the LOCA performance for a Korean Standard Nuclear Power Plant (KSNPP) where the reference 16-16 fuel assemblies were replaced with 15-15 bundles of annular fuel pins,. The standard RELAP5/MOD3.2 code was modified to extend the gap conductance model and the metal-water reaction model to the inner gap and the inner cladding, respectively. The peak cladding temperatures (PCT) during a LB-LOCA event were calculated. During the depressurization phase, the peak annular fuel reached a PCT of only 606 deg. C, which is about 320 deg. C less than calculated for the reference solid fuel pin at the same core power and far below the 1200 deg. C limit. The sensitivity of the PCT to fuel conductivity, gap conductance and core power were also investigated. The results confirm superior LOCA performance of the annular fuel and its potential for a significant power density increase while maintaining large safety margins. (authors)