Liquid-Metal Bond for LWR Fuel Rods

A liquid metal (LM) consisting of one-third weight fraction each of Pb, Sn, and Bi has been investigated as the bonding substance in place of He in the pellet-cladding gap of light water reactor fuel elements. The LM bond eliminates the large T over the preclosure gap that is characteristic of helium-bonded fuel elements. Because the LM does not wet either UO₂ or Zircaloy, simply loading fuel pellets into a cladding tube containing LM at atmospheric pressure leaves unfilled regions (voids) in the bond, which lead to local fuel hot spots. 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. Fabrication of a 4-m-long full-scale fuel rod using this technique was successfully demonstrated. A destructive examination revealed numerous breaks in the frozen alloy bond, but all of these appeared to result from handling the fuel rod. Application to commercial fuel manufacturing should require only minor modifications to existing fabrication lines. The most suitable nondestructive examination technique utilizes a collimated X-ray beam to probe edge-on the region between the pellet surface and the cladding inside diameter.