05000336/LER-2009-002
| Docket Number | |
| Event date: | 1-3-2009 |
|---|---|
| Report date: | 09-08-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3362009002R00 - NRC Website | |
1. EVENT DESCRIPTION:
On July 13, 2009 prior to start-up from a forced shutdown, which began on July 3, 2009, while Millstone Power Station Unit 2 (MPS2) was in mode 3 (hot standby) at 0% reactor power, a reactor coolant system (RCS) [AB] pressure boundary leak the MPS2 'A' reactor coolant pump (RCP) seal cooler [P SEAL CLR] region was identified by a technician conducting a system walk-down at normal operating pressure/normal operating temperature (NOP/NOT). Chemical analysis of the fluid emitting from the RCP seal cooler leak conclusively identified the fluid to be reactor coolant. If RCS pressure boundary leakage exists, Technical Specification (TS) 3.4.6.2 action b requires MPS2 be in mode 4 (hot shutdown) within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and be in mode 5 (cold shutdown) within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. MPS2 was in mode 3 hot standby and in accordance with TS 3.4.6.2 action b, the unit immediately commenced a cool down to mode 5 (cold shutdown). This condition is being reported in accordance with 10CFR50.73(a)(2)(i)(B) as an operation or condition prohibited by Technical Specifications, and 10CFR50.73(a)(2)(ii)(A) as a degraded or unanalyzed condition.
2. BACKGROUND:
The 'A' RCP on MPS2 has an integral tube in tube heat exchanger which is permanently attached to the pump cover. This heat exchanger surrounds the labyrinth seal and provides cooling of the RCS water prior to entering the seal. This heat exchanger is referred to as the pump seal cooler and is comprised of two rows of six coils circling the RCP seal. The inner tube of the tube-in-tube configuration carries the high pressure RCS water. The outer tube carries the low pressure component cooling water (RBCCW).
Both streams of RCS fluid exit the coil at the top of the coil assembly (one from the inside coil and one from the outside coil). The outlet of the coils is directed thru either a welded 90 degree elbow or a block configuration welded to a short length of 1.5 inch diameter tubing and 1.25 diameter inch pipe, which carries the cooled RCS flow to the seal housing and seal cartridge.
The rotating assembly, the pump cover, and integral heat exchanger for the 'A' RCP were replaced in 1992 with parts provided by the original equipment manufacturer Byron Jackson:
Characterization of the Weld Flaw The leak in the RCP seal cooler was found in the weld that joins the inner tube of the inside coil to the block which turns the flow 90 degrees towards the pump seal. The leak was coming from the toe of the tube to block fitting weld. The leak rate was visually estimated to be 25 to 30 cc/minute ( Chemistry sampled the leak and determined that it was comprised of RCS coolant only; no RBCCW water was present.
Examination of the indications identified by the liquid penetrant (PT) examinations performed during excavation of the defect identified a cluster of welding porosity one sixteenth of an inch deep into the tube in the center of the leaking spot. This weld porosity acted as the nucleation site for fatigue cracks.
3. CAUSE:
The cause of the RCS leak in the seal cooler was a historic weld defect, specifically a cluster of porosity in the inner tube to block weld on the inside coil of the seal cooler. Vibration induced fatigue at the weld defect caused the defect to propagate and caused the pressure boundary leak.
This weld was made during original construction of the RCP seal cooler by the manufacturer (1992).
The weld data sheets for this weld indicated that it passed the ASME Code required PT examinations during weld preparation and upon completion of welding. No volumetric examination, such as a radiographic examination (RT) was required. The weld defect section was not available for lab analysis due to the fact that grinding was performed as part of the weld repair process.
1. The leak rate was well with-in the capacity of the charging pump(s) for make up to the RCS.
2. During excavation of the leak location the defect was noted to have propagated in an angular orientation. This specific orientation was essentially self-limiting since, as the defect propagated into the weld, the material became thicker and would function to slow the rate of growth of the flaw, such that it would not have progressed to the point of breaking.
3. In the worst case if the tube had broken completely the RCS leakage from the RCP seal cooler tube is bounded by existing Combustion Engineering Owners Group (CEOG) Task 656 Final Report, and would be classified as a small break LOCA. A small break LOCA is bounded by the design of MPS2 as described in Chapter 14 of the Final Safety Analysis Report (FSAR).
4. The extent of condition was evaluated as follows:
a. Approximately one sixth of the weld in the area of the leak was repaired and rigorously inspected utilizing the RT, PT and visual (VT) inspection methods. No indication of defects were found.
b. To the extent possible, additional weld areas on the RCS pressure boundary tubing and piping welds of the 'A' RCP seal cooler were inspected utilizing the PT and VT inspection methods. No indication of defects were found.
c. To the extent possible, equivalent weld areas on the RCS pressure boundary tubing and piping welds of the 'B'. 'C', 'D', and spare RCP seal coolers were inspected utilizing the PT and or VT inspection methods. No indication of defects were found.
d. This condition does not apply to the Millstone Power Station Unit 3 (MPS3) RCPs. The MPS3 RCPs are Westinghouse pumps with coolers internal to the pump.
5. CORRECTIVE ACTION:
The weld defect was repaired on July 17, 2009. Containment walk-downs of the weld repair were completed at NOP/NOT conditions on July 24, 2009 and no leakage was noted.
Additional corrective actions will be evaluated in accordance with the station's corrective action program.
6. PREVIOUS OCCURRENCES:
No previous similar events/conditions were identified.
Energy Industry Identification System (EDS) codes are identified in the text as [XX].