In-pin fuel motion dynamics for beginning-of-life core in fast breeder reactors

Highlights: • Modelling of nuclear fuel melting and motion in annular fuel pin of fast reactor. • Impact of fuel melting on fast reactor dynamics in BOL and equilibrium cores. • Negative reactivity feedback due to in-pin fuel motion is weaker for BOL core. • Possibility of molten fuel vaporization during UTOPA is greater for BOL core. - Abstract: Melting of nuclear fuel can occur during any unexpected accident where there is rise in power. Specifically, in the unprotected transient overpower accident (hereafter UTOPA), fuel melts and flows axially inside the fuel pin. A previous study of this phenomenon, known as in-pin fuel motion, in a medium-sized fast reactor with an equilibrium core concluded that it basically enhances the inherent safety features. The objective of the present study is to confirm whether such an enhancement also occurs for the beginning of life core (hereafter BOL) and to quantify the differences. Therefore, this article reports the best estimate and conservative UTOPA analyses for both BOL and equilibrium cores. A fuel mass relocation analysis of both cores shows that due to the absence of fission gas perturbations upon melting, the axial relocation of molten fuel is least in the BOL core. This leads to a smaller relocation feedback in the BOL core for the same amount of melting. Results show that while the molten fuel relocation feedback does act as an inherent feedback for the BOL core, it is unable to prevent extensive fuel melting, especially under the conservative UTOPA analyses. On the contrary, such an extensive amount of melting is altogether avoided in the equilibrium core due to greater relocation feedback for lesser amount of melting. A thermodynamic analysis of molten fuel during UTOPA is carried out and it is found that there is a greater possibility of vapourization of molten fuel in the BOL core as compared to the equilibrium core.