A liquid metal (LM) consisting of 1/3 weight fraction each of Pb, Sn, and Bi has been proposed as the bonding substance in the pellet-cladding gap in place of He. The LM bond eliminates the large AT over the pre-closure gap which is characteristic of helium-bonded fuel elements. Because the LM does not wet either UO(sub 2) or Zircaloy, simply loading fuel pellets into a cladding tube containing LM at atmospheric pressure leaves unfilled regions (voids) in the bond. The HEATING 7.3 heat transfer code indicates that these void spaces lead to local fuel hot spots. Surface treatments yielded no improvement in wetting properties. Voids were eliminated during fabrication by first evacuating the rod loaded with solid alloy and a fuel stack, melting the alloy, pushing down the fuel stack to drive the LM into the gap, and finally applying at least 5 atm He overpressure. A 4-m long full-scale fuel rod using this technique of fabrication was successfully demonstrated. A destructive examination showed that the bonding in at least 25% of the fuel rod remained completely intact (the remaining fuel rod awaits the destructive examination). The pre-closure reduction in fuel temperature in the LM-bonded rod slowed fission-gas diffusion in the grain and so increased the time required to saturate the grain boundaries. Numerical calculations compared the delay in the onset of fission gas release in LM-bonded rods compared to He-bonded rods. The delay afforded by LM-bonding could be as high as approximately 1 year and as low as 2 days. Application to commercial fuel manufacturing requires only minor modifications to the existing fabrication line. The most suitable NDE technique utilizes a collimated X-ray beam to probe edge-on the region between the pellet surface and the cladding ID.
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