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The effects of core barrier coatings, void spaces, and surface-cleaning techniques on the quality of Zircaloy-clad flat-plate U0>^ fuel elements prepared by gas-pressure bonding were investigated in a continuation of the development program reported in RMI-1374. Techniques were developed for the application of barrier layers of chromium by a vapor-deposition proces'i and of crystalline carbon by a pyrolytic process. These coatings, together with a graphite coating developed previously, were then evaluated in pressure-bonded fuel elements for their effectiveness in preventing core-to-cladding reaction and for their general production feasibility. Crystalline carbon coatings 15 to 10 pin. thick and chromium coatings 25 to 40 p. in. thick were determined to be satisfactory. In the study of the flow of cladding-plate material into void spaces in the picture-frame assembly, it was established that excessive flow, and consequent thinning of the cladding, can be minimized by individually compartmentalizing the cores with Zircalov ribs. This design also results in maximum restriction of the effects of a cladding failure in service. Quantitative data on the maximum amount of void space resulting from manufacturing tolerances or from chipped fuel cores that is tolerable in elements of this design were obtained. Studies of surface-cleaning techniques were limited to refinement oj procedures established previously. It was found that a final multistep rinsing cycle ,esuited in bonds consistently free of evidence of contamination.

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