Investigating the Role of Chromium-Coated Clad During Accident Progression for an SMR Using MELCOR

The need to develop more economical and safer nuclear reactors has led to the emergence of small modular reactors (SMRs), most notably the NuScale design, and the use of accident tolerant fuel, e.g. chromium-coated Zircaloy (Cr-coated Zr) cladding. The former (SMR design) is of interest because of its scale, cost, and passive cooling capabilities. The latter is of interest because of its characteristics of improved accident tolerance compared to the Zircaloy (Zr-based) cladding. In fact, researchers suggested that the implementation of Cr-coated cladding has the potential to provide more economic benefits for SMRs. However, our focus here is on studies exploring its role during accident progression.

This paper investigates the significance of Cr-coated cladding for a loss-of-coolant accident under severe accident conditions. This investigation is performed by developing two NuScale plant models using the MELCOR-2024v code: (1) NuScale design with Zr-based cladding and (2) NuScale design with Cr-coated Zr cladding, where the oxidation model of the Cr-coated Zr cladding is captured using a homogeneous-material model approach.

The presence of Cr-coated Zr reduces H₂ generation by a factor of 4. The Zr-based model displays four distinct heating phases: (1) initial slow heating; (2) accelerated heating influenced by oxidation, ending with loss of the Zr-metallic form; (3) settling behavior due to radionuclide release outside the core and constant evaporation of the coolant; (4) final slow heating after complete dryout of the cooling channel. The Cr-coated Zr model differs significantly featuring a shorter Phase II and parabolic behavior in Phase III. To summarize, Cr coating is advantageous in reducing the overall amount of H₂ generation and providing a longer coping time since Zr-metallic cladding is preserved for longer duration.