05000250/LER-2014-002
| Turkey Point Unit 3 | |
| Event date: | 3-19-2014 |
|---|---|
| Report date: | 5-15-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| Initial Reporting | |
| ENS 49933 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 2502014002R00 - NRC Website | |
DESCRIPTION OF THE EVENT
On March 17, 2014 with the Unit 3 reactor [AC, RCT] in Mode 5 (Cold Shutdown) boric acid deposits were was in Mode 5 in preparation for refueling. Further investigation on March 19, 2014 revealed that the boric acid deposits were due to primary coolant leakage from the annular space surrounding the heater sleeve [AB, PZR, EHTR, SLV] at penetration 11. No other heater sleeves were identified with boric acid deposits.
The condition is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(A). Event Notification 49933 was made on March 19, 2014 in accordance with 10 CFR 50.72((b)(3)(ii)(A).
Relief was authorized to leave the flaw in place for one operating cycle.
CAUSE OF THE EVENT
The root cause of leakage cannot be determined because of limitations in examination techniques. The most likely root cause is an original fabrication welding defect in the heater sleeve partial penetration weld further impacted by stress corrosion cracking and/or thermal fatigue.
ANALYSIS OF THE EVENT
Background
The Pressurizer provides a high point in the Reactor Coolant System (RCS) [AB] where a steam bubble can be drawn and used to establish system pressure, maintains RCS pressure during steady-state operation, limits pressure variation caused by coolant thermal expansion and contraction during normal load transients, accommodates expansion and contraction of the RCS coolant volume caused by load transients, and relieves excessive pressure to the Pressurizer Relief Tank [AB, PZR,TK] to prevent RCS pressure from exceeding the design limit.
The Pressurizer contains replaceable direct immersion heaters [AB, ETHR] at the bottom and a spray nozzle at the top. The surge line which is attached to the bottom of the Pressurizer connects the Pressurizer to the hot leg of the B loop in the RCS. During an outsurge caused by an increase in plant load, pressure is kept above minimum allowable limit by the immersion heaters cycling on. There are 78 individual heaters inside the Pressurizer.
The Pressurizer is a cylindrical pressure vessel installed with its longitudinal axis in a vertical position. It is 39 feet 7 inches high and 7 feet 8 inches in diameter; it has a volume of 1300 cubic feet. The vessel consists of a cylindrical barrel shell with a hemispherical head welded to each end. To minimize corrosion, the entire inside surface of the vessel is clad with two austenitic stainless steel layers, each 3/16 inch thick, and all internals exposed to primary water or steam is constructed of austenitic stainless steel.
The lower head is constructed of cast carbon steel. It contains one surge penetration nozzle and 78 penetrations for the immersion heaters. It also contains one sample penetration nozzle, one temperature instrument penetration, and three penetrations for level and pressure instruments. There is a retaining basket above the surge line nozzle that prevents any foreign matter in the Pressurizer vessel from entering the RCS piping. There are two baffle/supports that maximize in and outsurge contact with the heaters. The baffle/support plates also provide support for the heaters. The heaters enter the Pressurizer through a stainless steel (316) sleeve that is mechanically rolled into the Pressurizer shell. The pressure boundary is a weld on the inside of the Pressurizer at the cladding and around the sleeve.
Analysis Boric acid deposits were discovered on the bottom head of the Unit 3 pressurizer while conducting a VT-2 visual inspection. This inspection is performed each refueling outage.
After removal of the heater element, eddy current testing confirmed that there was no flaw in the heater sleeve indicating that the through-wall leak path was in the partial penetration weld joining the heater sleeve to the stainless steel cladding on the inside surface of the Pressurizer. A borescope was inserted into the heater sleeve to visually examine the surface of the sleeve to cladding weld. An area of discoloration approximately '/2 inch along the reinforcing fillet weld face was noted. However, no indication of cracking was identified in the discolored region or any other part of the weld that was viewed. In addition, there was no sludge build-up on the surface in the vicinity of the sleeve. The exact failure mechanism is not known because there was no available non-destructive examination method capable of interrogating the partial penetration weld, and removal of the weld for destructive examination was impractical.
No similar leakage has been reported at either Unit 3 or 4 Pressurizer heater sleeves since unit operation began.
Reportability An unacceptable flaw in the reactor coolant pressure boundary is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(A).
ANALYSIS OF SAFETY SIGNIFICANCE
This event resulted in a minor loss of primary coolant at a RCS pressure boundary. Although there was pressure boundary leakage, it was undetectable in the daily RCS leakrate investigations while the unit was in operation during the past cycle. Pressurizer base metal thickness was unaffected and thus did not challenge the pressure boundary. The flaw remaining in place has been judged to be acceptable for one operating cycle until more extensive analysis is completed.
CORRECTIVE ACTIONS
Corrective actions are in accordance with condition report 1949021 and include the following:
1. The leak was corrected with a half nozzle ASME Code repair of Pressurizer heater sleeve 11, which moved the RCS pressure boundary to the outside of the Pressurizer at the penetration.
2. A visual inspection of the remaining 77 heaters in the Unit 3 Pressurizer confirmed there were no other heater sleeve penetration leaks.
3. A VT-2 inspection at Normal Operating Pressure (NOP)/Normal Operating Temperature (NOT) in preparation for unit startup was performed with satisfactory results.
ADDITIONAL INFORMATION
component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED: In-vessel Pressurizer heater sleeve attachment weld PREVIOUS SIMILAR EVENTS: None