NuScale LLC has proposed uprating its certified small modular reactor design from 160 to 250 MW(thermal) to gain the economic benefits of lower capital cost in terms of $/kWe. This current work explores the impact of the power uprating to 250 MW(thermal) on the transient behavior following a postulated loss-of-coolant accident with a failure of the emergency core cooling system (ECCS) using the MELCOR code.

During the transient, the uprated NuScale power module behaves in five distinct phases: (I) slow heating of the fuel due to decay heat, (II) accelerated heating due to oxidation ending with Zr cladding failure, (III) additional cooling caused by water evaporation and the establishment of vapor flow circulation between the core channels as decay heat is reduced due to radionuclides released outside the core, (IV) slow continued heating, where water in the lower plenum continues to evaporate, establishing a natural circulation through the core, and (V) complete dryout of the lower plenum water and fuel slumping. This enhanced heat transfer delays significantly (>14 h) the time of first-to-fail fuel compared to the 160-MW(thermal) design counter to our first intuition.

The 250-MW(thermal) design potentially can provide a longer time window to cope with and recover from the accident in the short term by delayed fuel failure. However, it causes higher risk in the long term since H2 generation and the radionuclides released can be greater compared to the 160-MW(thermal) design.

In contrast, the Cr coating reduces the H2 generation remarkedly (~5× lower), and the Zr-metallic form remains for a longer duration during the accident progression (~13 h). These findings help in understanding the potential behavior of the uprated NuScale design with accident-tolerant fuel cladding materials. Highlights include the following:

1. Investigating transient impact of uprating NuScale in the short term and long term.

2. Discussing the different thermal phases, which the first-to-fail fuel-clad cell displays for each model.

3. Investigating the benefits of Cr coating with uprating on accident progression.