FNSF Maintenance and Research Strategy

1. FNSF Maintenance and Research Strategy Siegfried Malang and Mohamed Abdou FNST Meeting held at UCLA, August 3, 2010

2. FNSF Maintenance and Research Strategy • Principal function of the vacuum vessel: • Vacuum boundary Maintain reliable ultra-high vacuum, provisions for bake-out. • Other potential functions of the vacuum vessel: • Mechanical support of the in-vessel components (Disruption forces!) • Containment of activated inventory (tritium!) • Contribution to radiation shielding of TF- and PF-coils • Pressure boundary in case of in-vessel LOCA (up to 0.5 MPa ?) • Heat sink in case of complete loss of ancillary cooling loops

3. Principal layout and operating conditions of the VV: • Located between HT shield and TF-coils • Operating temperature ~ 200 C (water cooled!) • What is the maximum dpa value to keep the fracture toughness sufficiently high during the anticipated life-time? (Strongly dependent on material and irradiation temperature!) • If repair or partly replacement of the VV during the life-time of the plant is required, what are the re-welding limits? - Max. He concentration < 1appm? (material and welding method dependent !) Candidate materials for VV: • Austenitic steel (SS 316) (activation allowable?) • Ferritic/Martensitic steel (F82H, Eurofer) (embrittlement at low irradiation temperature, complicated Post Weld Heat Treatment required) • Simple ferritic steel (Fe with small amounts of C, Mn, Si…widely used in industry, without complicated PWHT, tolerable activation?) • Others (Inconel, Cu-alloys, Al-alloys…)

4. Configuration and Maintainability • Should we try to get the lay-out of the in-vessel components similar to a power plant in order to get relevant information’s about Mean Time To Replace/Repair (MTTR) ? • Or should we design a FNSF for minimized time to replace in-vessel components by using particular features of the FNSF, for example the possibility to open normal conducting TF coils? • Is there a strong impact of the type of the fusion plant on the optimum maintenance concept? • Which are the envisaged maintenance concepts for the following candidate FNSF facilities: • Spherical Tokamak with normal conducting TF coils, • Standard aspect ratio Tokamak with normal conductive TF coils, • Compact Stellarator, • Tokamak with S/C TF coils?

5. Reliability, Availability, Maintainability, Inspectability • Plant availability is mainly the result of failure rate and the down time to replace or repair components. • An exceptionally low failure rate for all in-vessel components is a feasibility issue for any fusion plant because: • There is nearly zero tolerance for failure (for example the smallest coolant leak inside the VV shuts the plant down!) • Any repair or replacement of in-vessel components requires cool down of the power core, many operations of disconnent (e.g. manifold connections) opening of the VV, and insertion of handling tools, then in-situ repair or removal, then the processes are repeated in reverse (insertion of module, connections of maniflolds, etc) all are time consuming tasks. • In order to achieve a sufficiently high availability of a fusion plant, it is mandatory already for a FNSF but much more for a power plant to strive for extremely reliable components (MTBF extremely long!) and short down times for replacements (MTTR much lower than in presently envisaged plants!)

6. What are necessary goals for MTBF and MTTR ? • Example: • There are 200 blanket modules, and it is assumed that each of them fails once during its anticipated life-time of 5 years (MTBF = 9 days). If this would be the only reason for shutting the plant down, and if it would take 1 week to be back in operation (MTTR=7 days), the resulting availability would be 56 %. • Would this be realistic? • Un-availability of 44 % caused by the failures in blankets alone would be fare to low even for an experimental machine. There are many additional components where a failure causes a shut down, for example divertor targets. • On the other hand, a MTTR of one week to replace an in-vessel component is already very optimistic (ITER assumes that it will take ~ 2 weeks to replace a TBM, which is located in a test port and is designed for fast replacement).

7. What can be done to improve this situation? • Get the MTBF much longer by developing and testing the in-vessel components to the maximum extent possible. • This means for example for blankets an extensive fabrication and testing program prior to the start of the construction of a fusion plant, and a gradual increase of the performance in such a plant. • The suggestion is to start with robust low performance versions of a concept, use test blanket modules in dedicated test ports as fore-runners for more advanced versions, and increase the performance of the base blanket gradually up to the status desired for a following power plant. • Design the next step fusion facility for fast maintenance. Short MTTR in such a facility is considered by many to be more important than similarity to a following power plant. (However, other experts are also very concerned about not getting prototypical data on MTTR in FNSF). • The missing possibility to extrapolate the experience for maintenance to the power plant has to be compensated for by an extensive development and qualification program for maintenance methods and tools needed for such a plant.

8. Suggested Subjects for Discussions • If we would like to operate the VV at temperatures > 350 C, how would we cool it : • Water at P > 20 MPa, Helium, organic fluid, liquid metal? • Can we attach mechanically “hot” in-vessel components to a “cold” VV? (Differential thermal expansion, thermal insulations, disruption forces) • Alternative: Design in-vessel “hot skeleton rings” composed of HT shield and coolant manifolds, attaching blanket modules to this rings which can freely expand relative to the “cold” VV. • Do we have to design the VV ports for the attachment of transfer cask to avoid the spread of in-vessel radio-activities (Tritium, dust) into the building atmosphere? • Do we have to design the VV for repair, partial replacement, and what would be the requirement on re-weldability? • What is the impact of mechanical strength, operating temperature, activation, required PWHT, on the selection of a suitable material for the VV? • For the selection of the maintenance method for a FNSF, what should have the higher priority: • Similarity to a following power plant or short MTTR in the FNSF? • Are there inherent differences in the maintainability of the different types of candidate concepts for a next step fusion facility? If yes, should this be a major criterion for selecting a concept?

9. Suggested Methods for Maximizing the Reliability of the In-Vessel Components • Design for high reliability by proper selection of concepts, materials, margins to given limits, fabrication methods, quality assurance programs, analyses methods… • Considering the available data base on the failure rates of nuclear quality pipes, welds, and other elements of in-vessel components (in addition to initial failures at the start of operation), should the complexity (number and kind of internal pipes, coolant channels, welds…, be a criteria for the selection of concept for FW/blankets, divertor target plates...? • Use all thinkable methods, tools and facilities for testing the components and the fabrication methods to the maximum extent possible prior to the insertion of any component into the fusion plant. • Develop suitable method for monitoring the performance of the components in order to detect and localize potential components for failures before they fail (“replacement before failure”). • Use the test ports for testing advanced versions of the concept used for example as base blankets.