Interaction Layer Characteristics in U-xMo Dispersion/Monolithic Fuels

Abstract

Published data concerning the interaction layer (IL) formed between U-xMo fuel alloy and aluminum (Al)-based matrix or cladding materials was reviewed, including the effects of silicon (Si) content in the matrix/cladding, molybdenum (Mo) content in the fuel, pre irradiation thermal treatments, irradiation, and test temperature. The review revealed that tests conducted in the laboratory produce results different from those conducted in an irradiation environment. However, the laboratory testing relates well to thermal treatments performed prior to irradiation and helps in understanding the effects that these pre irradiation treatments have on in reactor performance. A pre-formed, Si-enriched IL seems to be important in delaying the onset of rapid growth of fission gas bubbles at low irradaiiation temperatures. Several other conclusions can be drawn: 1. An IL with phases akin to UAl3 is desired for optimum fuel performance, but at low temperatures, and especially in an irradiation atmosphere, the desired (Al+Si)/(U+Mo) ratio of three is difficult to produce. When the fuel operating temperature is low, it is important to create a pre-irradiation IL, enriched in Si. This pre-formed IL is relatively stable, performs well in terms of swelling resistance, and prevents rapid IL growth during irradiation. 2. At higher operating temperatures (>150–170°C), IL formation in reactor may not be so dependent on pre-irradiation IL formation, especially at high burnup; a pre-fabricated IL seems to be less stable at high burnup and high operating temperature. Moreover, the (Al+SI)/(U+Mo) ratio of three occurs more often at higher temperature. For these two reasons, it is important at high operating temperature to also have a matrix with significant Si content to create an IL in reactor with the right characteristics. 3. Out-of-reactor testing seems to indicate that Si in the matrix material is required in some concentration (2%, 5%, ?) to provide for a thin, Si-enriched IL formed before irradiation of a fuel plate. It ensures that the IL contains beneficial phases, or prevents formation of some known to promote poor fuel performance. Significant progress has been made in determining the desired characteristics of the IL. 4. The use of a fuel with stable gamma phase appears to allow more predictable performance regarding both a beneficial pre-irradiation layer, and the fuel performance (low swelling) to high burnup. Destabilization of the gamma phase may create problems with IL breakaway growth. 5. A theory whereby prevention of the U6Mo4Al43 complex phase in interaction layers formed during fabrication may be a key to good irradiation performance. Si additions to the matrix allow for solubility of Mo in the desirable (U,Mo)(Al,Si)3 or perhaps (U,Mo)(Al,Si)4 phase, helping to prevent formation of the complex phase. Keeping alloy Mo content as low as possible may also help so long as decomposition does not occur in fabrication, forcing Mo into the interaction layer. This theory may explain a number of apparent anomalies observed in testing results. 6. More work is needed in order to prescribe the conditions to best produce a beneficial IL. Another necessity is a better understanding of any correlation between beneficial characteristics of the pre-fabricated IL and the irradiation conditions to which it will be subjected.