FNSF Maintenance and Research Strategy
Siegfried Malang and Mohamed Abdou
FNST Meeting held at UCLA, August 3, 2010
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
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:
a) Austenitic steel (SS 316) (activation allowable?)
b) Ferritic/Martensitic steel (F82H, Eurofer) (embrittlement at low irradiation
temperature, complicated Post Weld Heat Treatment required)
c) Simple ferritic steel (Fe with small amounts of C, Mn, Si…widely used in
industry, without complicated PWHT, tolerable activation?)
d) Others (Inconel, Cu-alloys, Al-alloys…)
Configuration and Maintainability
a) 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) ?
b) 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?
c) Is there a strong impact of the type of the fusion plant on the optimum
maintenance concept?
d) 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?
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:
a. There is nearly zero tolerance for failure (for example the smallest coolant
leak inside the VV shuts the plant down!)
b. 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!)
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).
What can be done to improve this situation?
A.
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.
B.
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.
Suggested Subjects for Discussions
A. 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?
B. 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.
C. 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?
D. Do we have to design the VV for repair, partial replacement, and what would be the
requirement on re-weldability?
E. What is the impact of mechanical strength, operating temperature, activation,
required PWHT, on the selection of a suitable material for the VV?
F. 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?
G. 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?
Suggested Methods for Maximizing the
Reliability of the In-Vessel Components
1.
Design for high reliability by proper selection of concepts, materials,
margins to given limits, fabrication methods, quality assurance programs,
analyses methods…
2.
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...?
3.
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.
4.
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”).
5.
Use the test ports for testing advanced versions of the concept used for
example as base blankets.