ML19134A248

27 - Control Rod Drive Mechanism (CRDM) Thermal Sleeve Flange Wear Analyses

ML19134A248
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Issue date:	05/22/2019
From:	David Rudland Division of Materials and License Renewal
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Control Rod Drive Mechanism (CRDM)

Thermal Sleeve Flange Wear Analyses May 22, 2019 David L. Rudland Senior Technical Advisor for Materials Division of Materials and License Renewal U.S. NRC Office of Nuclear Reactor Regulations

Purpose of Sleeve:

• Guides rod cluster control assembly (RCCA) drive rods into the head Wear remnant can penetration tubes during reactor vessel cause control rod to head installation stick

• Provide thermal shielding of the head penetration tubes 2

• Information on wear rates uncertain

- 95% at 0.03/year, 99% at 0.04/year from limited US data -

French saw 0.12/year

• Unknown if wear was uniform or location specific

• Wear will continue until failure occurs

• Deterministically thermal sleeve failures is predicted after 12.5 years with multiple sleeve failures - possible large safety impact

• Using probabilistic analyses with risk insights provide a more appropriate prediction of behavior and treatment of uncertainty

• Impacts on defense-in-depth and safety margins?

3

• LIC-504 - Integrated Risk-Informed Decision-Making Process for Emergent Issues

- Compliance with Existing Regulations

- Consistency with the Defense-in-Depth Philosophy This

- Maintenance of Adequate Safety Margins presentations

- Demonstration of Acceptable Levels of Risk focus

- Implementation of Defined Performance Measurement Strategies

• Cross-discipline integrated review team -

materials, systems, risk 4

Materials staff focused on development Risk and systems staff focused on of initiation event frequency, i.e., CRDM core damage frequency analyses thermal sleeve failure using material staffs input Probability Wear rate, in/yr Probability of failure Many unknowns, e.g., wear between outages rate, etc lead to conservative assumptions Number of failures 5

Conclusion:

Five principles were met, but additional data needed to confirm assumptions - smart sample (OpESS) inspection recommended

• 62% of most susceptible plants were sampled by Regions Licensees followed guidance appropriately, NRC assumptions were conservative 6

• Effort completed in 7 months using integrated decisionmaking and integrated review team

• Five principles of integrated risk-informed decisionmaking satisfied

• Smart sample inspection verified NRC analysis assumptions

• Will continue to monitor industry operating experience on this issue 7