In light water reactors, a helium-filled gap between the fuel and the cladding accommodates fuel swelling and cladding creepdown. However, the low thermal conductivity of helium results in a large T over the gap before closure occurs. To remedy this situation, Wright et al. proposed the use of a liquid metal (LM) bond in the fuel-cladding gap. The LM (33 wt% each of lead, tin, and bismuth) was chosen for its low melting point (~120°C), its lack of chemical reactivity with UO2 and water, and its high thermal conductivity (~100 times that of He). The thermal resistance of the LM-bonded gap is nil.
Prior to closure of a helium-bonded gap, the centerline fuel temperature can be hundreds of degrees hotter than that with an LM-bonded gap at the same linear heat rating. Since the diffusion of fission gas atoms depends strongly upon temperature, it is expected that with the high thermal conductivity pellet-cladding gap, the incubation time to fission gas release should be considerably delayed. A modified Booth Sphere model, which takes into account re-solution, is adopted. The amount of fission gas atoms collected at the grain boundary is calculated using realistic time-temperature histories taken from a recent U.S. Nuclear Regulatory Commission review. The saturation value of gas at the grain boundary proposed by Dowling to fission gas release is adopted. The results show that although the temperature in the LM-bonded case is substantially lower than the He-filled case when the gap is open, the temperatures in the two cases equalize when the gap vanishes. Correspondingly, the two cases exhibit a comparable amount of fission gas at the grain boundary. Calculated differences between the times to saturation with LM and He in the gap are as high as ~1 yr and as low as 1 to 2 days.