05000306/LER-2017-002

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LER-2017-002, Reactor Coolant System Shutdown Communication Live Vent Through Wall Defect
Prairie Island Nuclear Generating Plant
Event date: 10-16-2017
Report date: 12-11-2017
Reporting criterion: 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
3062017002R00 - NRC Website
LER 17-002-00 for Prairie Island, Unit 2, Regarding Reactor Coolant System Shutdown Communication Live Vent Through Wall Defect
ML17346A079
Person / Time
Site: Prairie Island Xcel Energy icon.png
Issue date: 12/11/2017
From: Northard S
Northern States Power Company, Minnesota, Xcel Energy
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
L-PI-17-049
Download: ML17346A079 (5)


comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

On October 16, 2017, with Prairie Island Nuclear Generating Plant (PINGP) Unit 2 shutdown for refueling outage 2R30, an indication of leakage was found on the pipe socket weld upstream of valve 2RC-8-37 [VTV] during boric acid corrosion examinations. Valve 2RC-8-37 functions as a vent path on the normally isolated 3/4 inch Loop A shutdown communication line [AB]. Subsequent non-destructive examination confirmed a pressure boundary leak existed, which was not found acceptable under ASME Section Xl.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(A), “Any event or condition that resulted in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded” because the weld defect in the primary coolant system was not found acceptable under ASME Section XI. In addition, the pressure boundary leakage exceeded the zero reactor coolant system operational leakage limit specified in Tech Spec 3.4.14 and therefore was reportable per 50.73(a)(2)(i)(B) as a condition prohibited by plant technical specifications.

There was no actual safety consequence associated with this event. The quantity of dry boric acid at the location was small (estimated at 1/2 teaspoon in volume). Even if the weld had experienced a complete circumferential failure, the leak was isolated by closed valves that would have limited primary coolant leakage to within the capacity of the charging system when the reactor coolant system was pressurized (see figure next page).

CORRECTIVE ACTIONS

Valve 2RC-8-37 and the associated weld were replaced and the reactor coolant pressure boundary was restored.

PINGP intends to perform phased array ultrasonic inspections of socket welds on similar Class 1 piping containing stagnant water during future PINGP Unit 1 and 2 refueling outages.

PREVIOUS SIMILAR OCCURRENCES

A similar boric acid deposit was previously identified during refueling outage 2R29 in 2015 on valve 2RC-8-37 at the same location. However, visual and dye penetrant testing performed in accordance with plant procedures showed no indication of flaws or an active leak. The origin of the boric acid residue on the valve at the time was incorrectly attributed to a prior leak (e.g. reactor coolant pump seal leak) or maintenance activity.

SAFETY SIGNIFICANCE

CAUSE

Laboratory analysis determined the cause of the leak to be stress corrosion cracking. Contributing causes included weld material sensitization and the presence of sulfur and oxygen on internal surfaces. Oxygen is introduced from containment air when the system is in use during outages and is not removed by the normal primary system oxygen scavenging mechanisms because the line is normally isolated / stagnant.

DESCRIPTION OF EVENT

EVENT ANALYSIS

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

Figure