ML030910632
| ML030910632 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 09/30/2002 |
| From: | Sheron B NRC/NRR/ADPT |
| To: | |
| References | |
| FOIA/PA-2003-0018 | |
| Download: ML030910632 (28) | |
Text
Reactor Pressure Vessel Head Material Degradation Dr. Brian W. Sheron Associate Director for Project Ucensing and Technical Analysis Office of Nuclear Reactor Regulation h
U.S. Nuclear Regulatory Commission Penn State University September, 2002 k,
I
Overview of Presentation
- Description of Degradation
- Safety Implications
- NRC's Actions
- Licensee's Actions September 2002
Typical Pressurized Water Reactor September 2002
a Typical Reactor Vessel Head September 2002
Davis-Besse Reactor Vessel September 2002
- Control Rod Drive Mechanisms lI I
Nozzle =3 Fae Contl September 2002
Typical CRDM Nozzle A
SA-182 F304 SB-167 UNS N06600 (Alloy 600) v OobFR awr F I
P ERNiCr4 (Alloy 82)
Outer Surtaoe of RPV Head
. A RPV Head (SA-533 Gr. B Cl. 1)
- Iner Surtace of RPV Head
\\
(Stainless Steel Cladding)
J-Groove Weld EnICrF4 (Alloy 182)
September 2002 J-Groove Weld EnlCrFe-3 (Alloy 182)
Davis-Besse RPV Head Inspection
- Davis-Besse Visual Inspection of RPV Head per NRC Bulletin 2001 -01 o February - March 2002
- UT Inspection of All 69 CRDM Nozzles o5 Nozzles with Indications, 3 with Throughwall Cracks 0 Cavity Found Adjacent to Nozzle #3
- Degraded Area Near Nozzle #2
September 2002
CRDM Nozzle Map Axial Cracks m
September 2002 J-Groove Weld EriCrFe-3 (Alloy 182)
RPV Head Degradation September 2002
Contributors to Degradation
- Primary Water Stress Corrosion Cracking o Susceptible Materials o Alloy 600 Nozzles o Alloy 82/182 Welds o Highly Susceptible Heat of Material o 4 Out of the 5 Nozzles with M3935 Were Cracked o High Tensile Stress Because of Weld Residual Stresses (J-Groove Weld) oAggressive Environment, High Head Operating Temperature o Leakage Through Cracks in the Nozzles September 2002
Davis-Besse.RPV Head September 2002
Opportunities to Identify Degradation
- Boric Acid Buildup on RPV Head
- Corrosion on Vessel Flanges
- Containment Radiation Monitor Filter Clogging
- Containment Air Cooler Clogging a
Septenber 2002
Safety Implications
- Structural Margins Were Significantly Degraded
- Potential for Loss-of-Coolant Accident
- Plants Are Designed to Handle Loss-of-Coolant Accidents
- Prompt Action Warranted to Verify Condition of Other Plants September 2002
NRC Actions - Davis-Besse
- Special Inspections to Assess Compliance with Regulations
- Special Inspections for Modifications, Repair, or Replacement of RPV Head
- Confirmatory Action Letter o Determine Root Cause o Evaluate Rest of Reactor Coolant System for Corrosion o Obtain NRC Approval for Any Repair or Modifications o Obtain NRC Approval for Restart Septefter 2=
A NRC Actions - Generic
- Ongoing Evaluation of Other Plants o Most Licensees Repair Leaks and Remove Any Deposits o Some Licensees Leave Minor Debris and Isolated Boron Deposits on RPV Head
- Reevaluating the Basis for Licensee's RPV Head Inspection Programs o Bulletin 2002-02 Issued in August 2002 o Programs that Rely on Visual Examinations May Need to Be Supplemented with Non-Visual NDE September 2002
NRC Actions - Internal
- Identifying Improvements to Regulatory Processes
- Identifying Improvements to Inspection Programs
- Identifying Improvements to Regulations
- Initiated Confirmatory Research Studies September 2002
2 Davis-Besse Actions
- Plant in Safe, Shutdown Condition
- Investigating Extent and Cause of Degradation
- Conducting Root Cause Analysis oTechnical o Management and Human Performance
- Replacing RPV Head with One from a Canceled Nuclear Power Plant slot REC@7¢ 1.-
September 2002
Licensee's Investigation September 2002
Licensee's Investigation V4 _
I I
September 2002
Licensee's Investigation hi l WI/-PLs September 2002
Licensee's Investigation Crack In Cladding
_ "Bulged" Area September 2002
Licensee's Investigation Crack In Cladding
[
I 7
September 2002
Licensee's Investigation,
Carbon Steel RPV Head V S Stainless Steel Cladding September 2002
Davis-Besse Actions - Root Cause
- Pressurized Water Stress Corrosion Cracking of Control Rod Drive Mechanism Nozzle o Leakage Onto Head
- Licensee's Inspection Program Inadequate and Not implemented Properly
- Changes in Plant Conditions Inadequately Investigated oArmount and Color of Deposits on RPV Head 0 Clogging of Air Coolers o Fouling of Radiation Monitor Filters I
September 2002
Davis-Besse Actions - Repairs &
Corrective Actions
- RPV Head Being Replaced
- Management Team Has Been Replaced
- Procedures for Detecting and Correcting Problems Is Being Enhanced
- Safety Focus of Plant Personnel and Managers Is Being Addressed
- Plant Safety Systems Being Inspected and Repaired, If Needed September 2002
Davis-Besse Actions Licensee has to demonstrate its readiness to operate the plant safely.
September 2002
Addational Information A
I http://www.nrc.gov/reactors/operating/ops-experience/vessel-head-degradation.html
.O l
a September 2002