(A) physics study on moderated SiC block for high-temperature gas-cooled reactors

Abstract

The growing interest in High-Temperature Gas-cooled Reactors (HTGRs) has highlighted several challenges in their deployment. Notably, economic considerations related to the fuel cycle duration and fuel utilization have become prominent, particularly for prismatic HTGRs. In response to these challenges, the exploration of advanced materials such as Silicon Carbide (SiC) and Yttrium Hydride (YH$_x$) has been proffered, to tackle some of the key limitations of such reactors due to the extensive graphite employment. Two distinct configurations, centred on pure SiC and SiC fibres, have been systematically developed, optimized, and comprehensively characterized. Due to the partial or complete substitution of graphite, an efficient moderator should be implemented as well, which led to YHx use. A common issue in the reactor core is the presence of large excess reactivity. To mitigate this, the incorporation of a burnable absorber (BA), Boron Carbide (B$_4$C), has been investigated in realistic fuel assembly designs. The impact of the BA on the Moderator Temperature Coefficient (MTC), one of the important safety parameters in reactor design, has been scrutinized, encompassing a detailed examination of its contributions and the consequences arising from the BA insertion. The assessment of discharge burnup emerges as a pivotal feature in the devised layouts, with the pure SiC block exhibiting promising performance despite the considerable penalty incurred due to extensive SiC absorption, which is the primary drawback of the material utilization. However, the intrinsic fragility of the pure SiC fuel assembly raises concerns that may impede the realization of its promising results. Consequently, the SiC fibers-based layout emerges as the most viable candidate, albeit with trade-offs concerning structural integrity and performance. Furthermore, the degradation of MTC resulting from the formation of fission products has been verified, accentuating safety concerns in the final design. A comprehensive study aimed at characterizing MTC behaviour has been concluded to facilitate a nuanced understanding of the major contributors to the MTC value, thereby providing essential insights to support the whole core design phase. The presented work is expected to support the development of HTGRs while it aims to characterize concurrent effects which are reflected within the MTC value.