Some Aspects of Fuel-Pin-Failure Propagation in Sodium-Cooled Fast Reactors

This paper discusses some aspects of pin-to-pin failure propagation in a sodium-cooled fast-reactor subassembly resulting from (a) fission-gas release, (b) a local blockage, and (c) release of small amounts of molten fuel. The consequence of a severe flow dilution due to fission-gas release from a highly burned-up fuel pin is shown to give rise to only minor overheating because of the strong effects of fuel heat capacity, radial heat conduction, and mixing. Analysis has also shown that the occurrence of local boiling due to local blockage of detectable size appears unlikely to lead to dryout and flow instability because of the large subcooling effect in the wake downstream of the blockage. Moreover, even if a pin in a fuel assembly is assumed to fail and release small amounts of molten fuel, calculations indicate that heat losses and condensation will prevent any significant pressure generation and void propagation and therefore reduce the likelihood of rapid failure propagation.