Title: Design of the Universal Neutron Irradiator

SHINE medical isotopes is developing a novel method to produce the medical isotope Mo-99, which is the parent of Tc-99m, the most widely used diagnostic compound in nuclear medicine. This method utilizes U-235 fission in a subcritical liquid target solution, driven by an accelerator-based neutron source that is based on a deuterium-tritium reaction in a gaseous target. In this approach, the neutron-generating target is surrounded by uranyl sulfate (low-enriched uranium); fission in the uranium salt solution efficiently produces the desired Mo-99, as well as other fission products. After irradiation, Mo-99 is separated and the uranyl sulfate solution can be irradiated again to produce Mo-99. This allows the solution to be recycled for a very long time and significantly reduces the amount of associated nuclear waste. SHINE has received U.S. Nuclear Regulatory Commission (NRC) approval to construct an Mo-99 production facility based on this method. In order to produce Mo-99 efficiently, the solution should be as close as possible to criticality. Because the solution is over moderated, any precipitation could push the solution to become critical and thereby hard to control. One of the possible precipitation scenarios is the formation of uranyl peroxide. Under irradiation, water decomposes into the products of radiolysis, some of which will form peroxide. To prevent precipitation, one has to develop a way to decompose the peroxide before it can react with uranium and form hard-to-dissolve precipitates. To study peroxide formation and develop methods to prevent precipitation, we are developing an experiment capable of mimicking radiation conditions in the SHINE system. This report describes the irradiation setup we designed to irradiate a small volume of uranyl sulfate solution for the duration of one SHINE irradiation (~6 days) at the relevant fission power (~0.5 W/cc)