Comparing Maintenance Approaches for Tokamak Fusion Power Plants

Two generic approaches for maintaining commercial fusion power plants are compared to determine the most desirable maintenance scheme and reactor design approach to consider for the next generation, advanced tokamak power plant, the ARIES-AT¹. The scheduled and unscheduled maintenance times for the power core of fusion plants are extremely important as they directly determine the plant availability and, ultimately, the cost of electricity. The plant down time is determined by the time to access the failed or worn out part(s), the time to accomplish the replacement, and the time to verify the replacement.

The ARIES-AT power core² is the design basis for this comparison. One possible maintenance approach is the in-situ removal of moderate-sized modules of individual first wall, blanket, and divertor elements from inside the tokamak power core. This approach potentially allows smaller and lower cost toroidal and poloidal field coils that tightly fit around the outer surface of the power core shield or vacuum vessel. A second approach uses larger toroidal and poloidal field coils that will allow much larger ports to extract a complete, intact sector module of the first wall, blanket, shield, and divertor elements.

The time to access and egress the power core components is largely determined by operations independent of the maintenance approach, such as reactor cool down, draining/filling fluids, unfastening/fastening doors, vacuum leak checks, etc. Replacement time of the core elements was found to significantly favor the modular sector approach because there are fewer and more accessible coolant and structural joints to unfasten and fasten. For the in-situ maintenance approach for ARIES-AT, there are more, but smaller, modules to handle than with the modular sector approach. Verification of the successful refurbishment is a distinct advantage for the modular sector approach because it can be operationally tested in a remote assembly area before being installed. Only a few main coolant connections will be verified within the power core region. For these reasons, the modular sector maintenance approach was adopted for the ARIES-AT conceptual design.