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Failure modes, effects and rates and remote maintenance/downtime

Introduction – Failure is defined as the ending of the

ability of a design element to meet its function before its design lifetime is achieved.

– Even the material has a lifetime of ~ 8 years (for a neutron wall load of 2.5 MW/m2), the FNST component of which is constructed from the material could fail earlier than the material lifetime.

– Components tend to have higher failure rates during early development phase.

A broad knowledge of failure modes, effects, and rates during the (e.g. , DEMO) design process is critical for creating reliable components.

Reliability/maintainability/availability plays a key role on determining the potential of the use of fusion for energy.FNSF provides a test bed for exploring, verifying, and quantifying FNST component failure mechanisms and rates while exercising design practices per reliability and maintainability perspective.

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The 1993 analysis on blanket MTBF shows that the MTBF for blanket system is less than 0.2 year*. Similar analysis can be performed based on the new failure rates data while differentiating failure modes.

SC failure = 1.58x10-8/h.weld

Total failure (without SC)= 1.247x10-9 /h.weld

Pipe failure rates and rupture frequencies for PWR RCS Published at 2004

Data from OPDE (an international database on pipe degradation and failures in commercial nuclear power plants in OECD member countries.)

EBS1 (50 < ≤150 mm) and ∅EBS2 ( > 150 mm).∅

• Failure rate can be reduced by proper prevention measures

FAC: flow accelerated corrosion

Published at 2008

• Failure rates are categorized according to failure modes

• Stress corrosion cracking has the highest failure rate

However, Failure Rate in Fusion Could Be Higher:More damaging higher energy neutronsUnique conditions and use of special materials

Fission Technology Failure Rate Could Be Used to Estimate FNST Component Reliability.

*The MTBF estimated for a blanket system SG was ~ 0.28 year. – FZKA 5581 (Nov. 1995)

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The reliability and maintainability requirements on the FW/Blanket are challenging and pose critical concerns. These must be seriously

addressed as an integral part of the R&D pathway to DEMO.

(1)* Number estimated based on the data presented in the paper, "Reliability and Availability Issues in NET" by R. Buende in Fusion Engineering and Design 11 (1989). The calculations assumed that reactor components other than the blanket component are all well developed.

Requirements on Blanket Module MTBF for Different MTTRn

A MTBF of 11.4 years for the blanket module with a mean MTTR of 135 hours was derived by J. Sheffield (memo to the Dev Path Panel) to achieve a reactor availability of 50%.

For a long MTTR, reduce the number of the modules (go with a larger size per module) can relax the MTBF requirements

  Required MTBFn (Years) Required MTBFn (Years)

  ABS = 0.5, Reactor Availability = 37% (1) ABS = 0.8, Reactor Availability = 50% (1)

MTTRn n= 80  n= 32  n= 80  n= 32 

2 weeks 3.1 1.23 12.3 4.92

1 month 6.6 2.66 26.6 10.66

2 months 13 5.33 53 21.3n = number of blanket modules in the Blanket System (BS) Shaded boxes: no failure is allowed during the life time.

Time estimated for DCLL AEU replacement was 388 p-hours.

Unscheduled replacement

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The challenge and the way moving forward

• For most products, testing is used during design and early production phases to precipitate failures, followed by analysis of the resulting failures and corrective action through redesign to eliminate the root causes.

• However, prior testing in simulating fusion environment is severely limited.

• A remedy to achieve high reliability/low failure starts with design for high reliability and high quality assurance in mind- do it right at the first time:– Allow amble design margin to start with– Assess alternative (e.g., coolant plate versus pipe)– Need to design for maintainability and reduced MTTR (smart

attachment scheme, larger replacement units, etc.) – Use automatic fabrication process (reduce human errors)– Comprehensive design analysis

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Failure modes, effects and rates and remote maintenance/downtime

Near-term R&D Objectives• Develop Design for Reliability and

Maintainability Practices for FNST components

• Identify critical constituents of FNST components through implication of fault tree analysis (or similar methodology) using knowledge base from existing technologies

• R&D for critical constituents with achieving high reliability in mind

Interface conductance

Coolant blockage

Local hot spot

In-vessel leakInlet manifold weld failure

Outlet manifold weld failure

Brazed joint failure

Blanket failed

Pipe break (to be distinguished fromlow performance)

A fault tree analysis for FNST reliability and R&D needs assessment (example)

FNSF Objectives• Identify and Characterize failure modes, effects, and rates for FNST critical

constituents in fusion environments • Study engineering feasibility and conduct (better minded design, reduced test

and fix) reliability growth test program for FNST components• Gain knowledge base for Inspection techniques (to identify failed components)• Accumulate data on remote maintenance procedures under irradiation

What failure and how significant of the failure lead to reactor shutdown? (e.g., a pinhole leak?)