05000423/LER-2017-001

LER-2017-001, Loss of Safety Function - Secondary Containment

Millstone Power Station Unit 3
Event date:	01-20-2017
Report date:	03-20-2017
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
4232017001R00 - NRC Website

LER 17-001-00 for Millstone, Unit 3 Regarding Loss of Safety Function - Secondary Containment

ML17087A265
Person / Time
Site:	Millstone
Issue date:	03/20/2017
From:	Daugherty J R Dominion Nuclear Connecticut
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 17-001-00
Download: ML17087A265 (5)

20555-0001, or by Internet e-mail to Infocollects Resource @nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503 If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may collection

1. EVENT DESCRIPTION:

At 0835 on January 20, 2017 while operating in MODE 1 at 100% power, a door in the auxiliary building at Millstone Power Station Unit 3 failed to fully close following personnel passage due to the failure of the mechanical door closer mechanism. This door is part of the secondary containment boundary which must be intact for the supplemental leak collection release system (SLCRS) to perform its safety function. Operators determined that the condition of the door rendered secondary containment inoperable and resulted in a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material and mitigate the consequences of an accident. The door was repaired and the door completely closed at 1256 on January 20, 2017. This condition is being reported under 10 CFR 50.73(a)(2)(v)(C) and 10 CFR 50.73(a)(2)(v)(D).

2. CAUSE:

The cause of the door to not fully close was the mechanical failure of the door closing mechanism such that it prevented the door to close automatically or be closed manually.

3. ASSESSMENT OF SAFETY CONSEQUENCES:

SLCRS is designed to mitigate the radiological consequences of postulated accidents by filtering the exhaust air from the secondary containment. SLCRS is comprised of two redundant trains of exhaust fans and filter units. The safety function of SLCRS is to maintain a negative pressure in the secondary containment to ensure there is no unfiltered leakage to the outside environment from these areas in the event of a loss of coolant accident (other than those releases assumed in the' design basis dose calculations).

With the door stuck open, the ability for SLCRS to achieve negative pressure was impacted.

The actual reactor containment building allowable leakage (La) is much lower than that assumed in the design basis analysis. This difference provides additional margin to the design basis analysis.

4. CORRECTIVE ACTION:

The door closing mechanism was replaced and secondary containment was declared operable at

• 1256 on January 20, 2017. Additional corrective actions will be taken in accordance with the corrective action program.

5. PREVIOUS OCCURRENCES:

There are no previous occurrences with the same underlying reason or consequences.

6. ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES:

• Auxiliary Building — NF

• SLCRS — VF, VG, VH

• Secondary Containment — NH

• Door — DR