ADS design concept for disposing of the U.S. spent nuclear fuel inventory

Abstract The United States inventory of the spent nuclear fuel (SNF) is ~80,000 metric tons of heavy metal (MTHM) at the end of 2018, which contains ~131 tons of minor actinides (MAs) and ~669 tons of plutonium. This paper describes a conceptual design of an accelerator-driven subcritical (ADS) system for disposing of this SNF inventory by utilizing the 131 tons of MAs inventory and a fraction of the plutonium inventory for energy production and transmuting some long-lived fission products. An ADS system with a homogeneous subcritical fission blanket was first examined to define the main system parameters. A spallation neutron source is used to drive the fission blanket and it is generated from the interaction of a 1-GeV proton beam with a lead–bismuth eutectic (LBE) target. The fission blanket has a liquid mobile fuel using LBE as the fuel carrier. The fuel materials are dissolved, mixed, or suspended in the liquid fuel carrier. Monte Carlo simulations were performed for three fission blankets to determine the main concept parameters. The loaded amount of actinide materials in the LBE carrier is either 5, 7, or 10% of the total volume of the fission blanket, respectively. The plutonium fraction in the MAs of each fission blanket was selected so that the neutron multiplication factors of the three fission blankets are ~0.980. In addition, Monte Carlo burnup simulations using the MCB5 computer program were performed to analyze the performance of the three conceptual ADS systems. During operation, fresh fuel is fed into the fission blankets to adjust its subcriticality and to control the system power. The burnup results show that the total MA materials consumed are about 30.6, 35.3, and 37.2 tons, after operating the three fission blankets for 35 full-power years with 25-MW 1-GeV proton beam. Thus, the corresponding numbers of ADS systems to utilize the 131 tons of MA materials of the SNF inventory are 4.3, 3.7, or 3.5, respectively. ADS systems with tube bundles inserted in the fission blanket were analyzed to overcome the disadvantages of the homogeneous fission blanket concept. The liquid lead is used as the target material, the mobile fuel carrier, and the primary coolant to avoid the polonium production from bismuth. Physics and thermal–hydraulic analyses were coupled to determine the parameters of the heterogeneous fission blanket. The engineering requirements for a satisfactory performance of the HT-9 ferritic steel structure material have been realized. Two heterogeneous fission blanket concepts with the liquid lead mobile fuel inside or outside the tube bundles were considered. The heterogeneous fission blanket with the mobile fuel inside the tubes shows better performance than the fission blanket with mobile fuel outside the bundle tubes and it is presented in this paper. MCB5 and Serpent, the Monte Carlo burnup computer programs were both used to simulate the fuel burnup in the ADS concept with the mobile fuels inside the tubes for 35 full power years. The results show that five ADS systems can dispose of the total United States inventory of the spent nuclear fuel.