Irregular, Backward-Compatible PWR Assembles with More Pins to Facilitate Uprates

Abstract

Increasing the number of pins within a pressurized water reactor (PWR) assembly reduces pin temperature for a given assembly power. In conjunction with a core retrofit, this presents a potential route to PWR uprate, which is of growing interest given recent increases in electricity prices. However, most PWRs utilize regular lattice designs with fixed guide tube positions, such as the very common 17 × 17 lattice design with 25 guide/instrumentation tubes. These tubes are aligned with penetrations in the reactor pressure vessel, which presents a prohibitive obstacle to retrofit, and more widely, may “lock” many PWRs to this particular fuel configuration. In this paper, an irregular PWR fuel assembly is proposed. It is shown that a backward-compatible lattice with 324 fuel pins per assembly (BL324), uniform enrichment, and the same hydrogen-to–heavy metal ratio as a reference 17 × 17 assembly with 264 fuel pins can achieve within-assembly power peaking within 2% of the reference assembly under equivalent conditions while fixing the guide tube positions. Power peaking can be further reduced to reach that of the existing fuel assembly by reducing the enrichment of 36 of the pins by 0.2 wt%. The fuel assembly could potentially either support a significant uprate of up to ~20% in conjunction with low-enriched uranium plus (LEU+) fuel or a more aggressive cycle design, and hence, improved discharge burnup at the same power and batch strategy. A subchannel analysis shows that the coolant heat-up distribution is comparable to the reference assembly. However, the pressure drop is estimated to be 4% higher, which would challenge the performance of transition cores containing both 17 × 17s and BL324s. Further incremental changes to BL324 may be attractive, either to improve manufacturability or to slightly improve performance through formal optimization.