Methodologies and Shielding Analysis for the Design of Activated Cooling Water Components in Spallation Neutron Systems

The Second Target Station (STS) at the Oak Ridge National Laboratory Spallation Neutron Source (SNS) uses light water to cool the primary components inside the core vessel, including the target, proton beam window, moderators, beryllium reflectors, and other structures. As water circulates through the core vessel, 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. These processes are largely absent in fission and fusion reactors, where the maximum radiation energies are approximately two orders of magnitude lower. The goal of this paper is to demonstrate how to perform shielding analysis for the secondary components of water-cooling loops in spallation neutron facilities. It describes the method used during the original SNS design, which has also been adopted at the European Spallation Source (ESS). This “dilution method” 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. A new method developed during the SNS Proton Power Upgrade (PPU) project is introduced. This “short- and long-lived (S&L) 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. The two methods are compared by calculating radioisotope inventories, decay photon spectra, and dose rates in a concrete shield surrounding an infinite pipe. To validate both approaches, dose rates were also computed for one of the SNS coolant pipes and compared against facility measurements. The dilution method underestimates the measured dose by a factor of approximately 14, while the S&L method produces results within 20% of observed values. The S&L method is then applied to assess the shielding requirements of key secondary components of the water coolant loops at STS, including the delay tanks, hydrocyclone, and gas-liquid separator tanks. Using realistic STS operational design parameters, the required concrete wall thickness for the delay tank vault is estimated to range from 160 to 190 cm. These results provide design guidance for STS and align with scaled estimates based on PPU shielding analyses.