Liquid-Metal-Bonded Gap for Light Water Reactor Fuel Rods

摘要

In light water reactors, a helium-filled gap between the fuel and the cladding accommodates fuel swelling and cladding creep down. However, the low thermal conductivity of helium results in a large AT 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℃), its lack of chemical reactivity with UO_2 and water, and its high thermal conductivity (～100 times that of He). The thermal resistance of the LM-bonded gap is virtually nil. Prior to closure of a helium-bonded gap, the centerline fuel temperature can be hundreds of degrees hotter than that with a LM-bonded gap at the same linear heat rating. In addition to reducing the stored energy in the fuel, the LM bond permits initial gap thicknesses sufficiently large to totally avoid fuel-cladding mechanical interaction over the fuel element lifetime. With modestly creepdown-resistant cladding, a 150 μm initial cold radial LM-bonded gap will not close until ～ 60 MWd/kgl). Liquid metal in the gap can also prevent massive secondary hydriding by impeding steam ingress into the gap following cladding failure. The objectives of the work are: i) To produce an intact LM bond between glass and UO_2 and Zircaloy and UO_2; ii) To determine the effect of the LM bond on reducing (or delaying) fission gas release.