Speaker
Description
As water circulates through the core vessel of spallation neutron systems, it becomes radioactive due to spallation and transmutation reactions induced by high-energy protons, neutrons, and other particles. The activated coolant then flows through secondary components, such as pipes, tanks, and pumps, located outside of the core vessel. These secondary components require adequate shielding to ensure personnel safety and protect sensitive facility electronics from radiation damage. Unique challenges arise in modeling coolant activation in spallation systems because several key radionuclides originate in significant quantities from high-energy spallation reactions with oxygen. We devised the short- and long-lived method to accurately calculate the activity of the water coolant as it flows between the primary components within the core vessel and the secondary components outside the core vessel. The method uses two activation calculations. The first calculation is tailored to long-lived radioisotopes whose half-lives are longer than loop circulation times. The second calculation focuses on radioisotopes that decay significantly between circulations. We compared this method to the already existing method that assumes stagnant irradiation but dilutes the activities of the radioisotopes produced within the core vessel by the total loop volume to account for water circulation. Comparison of the two methods revealed significant differences in calculated radioisotope inventories, decay photon spectrum, and dose rates through concrete shielding. We validated both methods by calculating dose rates near the Oak Ridge National Laboratory Spallation Neutron Source (SNS) proton beam window water coolant pipe and comparing them to measurements. The existing dilution method underestimated the measured dose rate by a factor of ~14, while the short- and long-lived method yielded results within 20% of the measured value. Based on these findings, we recommend adopting the short- and long-lived method for future shielding design efforts in spallation systems, including those at SNS Second Target Station and other facilities.