Safety Analysis of High-Power-Density Annular Fuel for PWRs

This paper assesses the performance of internally and externally cooled annular fuel in a four-loop pressurized water reactor during a variety of transients and accidents, namely, the loss of flow accident (LOFA), main steam line break (MSLB), large break loss of coolant accident (LBLOCA), and rod ejection accident (REA). The RELAP5 code was the primary vehicle for these analyses, although the VIPRE code was also used to calculate the minimum departure from nucleate boiling ratio (MDNBR) for LOFA and MSLB transients based on the RELAP5 results. It has been found that the MDNBR for the annular fuel at 150% power was higher than the MDNBR value for the reference solid fuel at 100% power for LOFA and MSLB. For LBLOCA analysis, the RELAP5-3D code was applied twice since the code has a constraint on the reflood model, which can be applied to only one cooling surface (either the inner channel or the outer channel). The analysis, with the reflood model applied to the outer channel, showed that using the standard size (100%) accumulator but with an increased (150%) safety injection flow rate, the peak cladding temperature (PCT) for the annular fuel at 150% power would be ~1200 K (927°C). This is ~150°C higher than the PCT for the solid fuel at 100% power but 277°C lower than the regulatory limit of 1204°C. When the reflood model is applied to the inner channel, the PCT would be limited to 1100 K (827°C), which is only 50°C higher than the PCT for the solid fuel at 100% power and 377°C lower than the regulatory limit of 1204°C. The calculated fuel temperatures and enthalpies during the REA have been found to be much smaller for the annular fuel, even at 150% power, compared to that for the solid fuel at 100% power. These analyses indicate that the new internally and externally cooled annular fuel can accommodate 50% power uprate in a PWR and still maintain adequate safety margins for a variety of transients and accidents including LOFA, MSLB, LBLOCA, and REA.