Resolution of Generic Safety Issues: Issue 69: Make-Up Nozzle Cracking in B&W Plants (Rev. 11) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
Cracks were found in the normal make-up high pressure injection (MU/HPI) nozzles of several B&W plants following an inspection of the 8 B&W plants licensed to operate. These cracks appeared to be directly related to loose or missing thermal sleeves. As a result, a B&W Owners' Group Task Force was established to identify the cause of the failures and recommend modifications to eliminate future failures.513
The B&W Task Force has completed a generic investigation of the MU/HPI nozzle component cracking problem and has submitted a report containing the findings of that investigation.514 The report presents relevant facts and probable failure scenarios, as well as recommended modifications to thermal sleeve designs, make-up system operating conditions, and ISI plans. Failure analysis indicated that the cracks were initiated on the inside diameter and were propagated by thermal fatigue. Recent inspections at Midland have also shown that gaps may be present between the thermal sleeve and safe-end in the contact expansion joint. These findings along with stress analysis and testing have implicated insufficient contact expansion of the thermal sleeves as the most probable root cause of the failures. The NRC staff has evaluated the B&W Owners' Group report and has found the report acceptable.
Safety Significance
On the B&W plants with 145, 177, and 205 fuel assemblies, four HPI/MU nozzles (one per cold leg) are used to: (1) provide a coolant source for emergency core cooling, and (2) supply normal make-up (purification flow) to the primary system. In general, one or two of the nozzles are used for both HPI and MU, while the remaining nozzles are used for HPI alone.
The incorporation of a thermal sleeve into a nozzle assembly is a common practice in the nuclear industry to provide a thermal barrier between the cold HPI/MU fluid and the hot high pressure injection nozzle. This helps prevent thermal shock and fatigue of the nozzle. The purpose of the safe-end is to make the field weld easier (pipe to safe-end) by allowing similar metals to be welded. The dissimilar metal weld between the safe-end and the nozzle can then be made under controlled conditions in the vendor's shop. The use of the safe-end also eliminates the need to do any post-weld heat treating in the field. Failures in these HPI/MU nozzles may preclude the proper functioning of the ECCS and/or the normal fluid makeup to the primary system.
Possible Solutions
As a result of their investigation, B&W made the following recommendations514 as the solutions to the problem:
(1) Reroll the upstream end of the thermal sleeve, when inspections indicate that a gap exists, or repair and/or replace damaged components
(2) Maintain a continuous MU flow greater than 1.5 gpm
(3) Implement an augmented ISI plan
(4) Perform a detailed stress analysis of a nozzle with a modified thermal sleeve design to justify long term operation.
The staff reviewed and evaluated the recommendations of the Task Force and supported all four recommendations. However, the staff concluded that the implementation of Recommendations (1), (3), and (4) were adequate for resolving the issue.
CONCLUSION
All licensees participating in the B&W Owners' Group Task Force performed the repairs to damaged components outlined in Recommendation (1). The augmented ISI program in Recommendation (3) was voluntarily implemented. The stress analysis of Recommendation (4) will be done by the affected licensees and will require an MPA for follow-up staff verification.667 Thus, this issue was RESOLVED and no new requirements were established.
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