Microstructural characterization of an irradiated RERTR-6 U-7Mo/AA4043 alloy dispersion fuel plate specimen blister-tested to a final temperature of 500 °C

The Material Management and Minimization (M3) Reactor Conversion Program, in the past called the Reduced Enrichment for Research and Test Reactor (RERTR) Program, is developing low-enriched uranium (LEU) fuels for application in research and test reactors. U–Mo alloy dispersion fuel is one type being developed. Blister testing has been performed on different fuel plate samples to determine the margin to failure for fuel plates irradiated to different fission densities. Microstructural characterization was performed using scanning electron microscopy and transmission electron microscopy on a sample taken from a U-7Mo/AA4043 matrix dispersion fuel plate irradiated in the RERTR-6 experiment that was blister-tested up to a final temperature of 500 °C. The results indicated that two types of grain/cell boundaries were observed in the U-7Mo fuel particles, one with a relatively low Mo content and fission gas bubbles and a second type enriched in Si, due to interdiffusion from the Si-containing matrix, with little evidence of fission gas bubbles. With respect to the behavior of the major fission gas Xe, a significant amount of the Xe was still observed within the U-7Mo fuel particle, along with microns into the AA4043 matrix. For the fuel/matrix interaction layers that form during fabrication and then grow during irradiation, they change from the as-irradiated amorphous structure to one that is crystalline after blister testing. In the AA4043 matrix, the original Si-rich precipitates, which are typically observed in as-irradiated U-Mo dispersion fuel, get consumed due to interdiffusion with the U-7Mo fuel particles during the blister test. Finally, the fission gas bubbles that were originally around 3 nm in diameter and resided on a fission gas superlattice (FGS) in the intragranular regions of as-irradiated U-7Mo fuel grew in size (up to ∼20 nm diameter) during blister testing and, in many areas, are no longer organized as a superlattice.

材料管理与最小化（M3）反应堆转换计划，过去被称为研究与试验反应堆低浓化（RERTR）计划，正在研发用于研究和试验反应堆的低浓铀（LEU）燃料。铀 - 钼合金弥散燃料是正在研发的一种类型。已对不同的燃料板样品进行了起泡试验，以确定辐照到不同裂变密度的燃料板的失效裕度。使用扫描电子显微镜和透射电子显微镜对从RERTR - 6实验中辐照的U - 7Mo/AA4043基体弥散燃料板上取下的一个样品进行了微观结构表征，该样品经过起泡试验，最终温度达到500°C。结果表明，在U - 7Mo燃料颗粒中观察到两种类型的晶界/胞界，一种含钼量相对较低且有裂变气泡，另一种由于与含硅基体相互扩散而富含硅，几乎没有裂变气泡的迹象。关于主要裂变气体氙的行为，在U - 7Mo燃料颗粒内仍观察到大量的氙，并且在AA4043基体中也有几微米的渗透。对于在制造过程中形成并在辐照过程中生长的燃料/基体相互作用层，它们在起泡试验后从辐照后的无定形结构转变为结晶结构。在AA4043基体中，原始的富硅沉淀物（通常在辐照后的铀 - 钼弥散燃料中观察到）在起泡试验过程中由于与U - 7Mo燃料颗粒相互扩散而被消耗。最后，在辐照后的U - 7Mo燃料的晶内区域，原本直径约为3nm且位于裂变气体超晶格（FGS）上的裂变气泡在起泡试验过程中尺寸增大（直径可达约20nm），并且在许多区域不再呈超晶格排列。