05000289/LER-2017-004

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LER-2017-004, Items Nonconforming to Design for Tornado Missile Protection
Three Mile Island Unit 1
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications

10 CFR 50.73(a)(2)(vii)(D), Common Cause Inoperability

10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident

10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
2892017004R00 - NRC Website
LER 17-004-00 for Three Mile Island, Unit 1, Regarding Items Nonconforming to Design for Tornado Missile Protection
ML18037A705
Person / Time
Site: Three Mile Island, Three Mile Island Unit 1 Exelon icon.png
Issue date: 02/05/2018
From: Callan E W
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
TMI-18-010
Download: ML18037A705 (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.

3. LER NUMBER 1. FACILITY NAME 2017 004 - 00

A. EVENT DESCRIPTION

Plant Conditions before the event:

Babcock & Wilcox — Pressurized Water Reactor — 2568 MWth Core Power Date/Time: December 6, 2017 / 09:58 EST Power Level: 100% Mode: Power Operation There were no structures, systems or components (SSCs) out of service that contributed to this event.

On December 6, 2017, the plant was operating at steady state full power operation. TMI-1 was concluding a systematic review of the plant protection for tornado generated missiles against the plant design and licensing basis. It was determined that a non-conforming condition exists with the Emergency Diesel Generator (EDG) Fuel Supply Tank Vent Stack with respect to tornado missile protection. Since the fuel oil supply tank and vent stack is a common integral component of the EDG's and is not protected against tornado missiles, the design does not meet the licensing basis and a non-conforming condition exists.

TMI-1 entered this condition into the corrective action program and notified the NRC via Event Notification 53106 on December 6, 2017.

The extent of condition review identified three additional non-conforming conditions: Vent Stacks for each Emergency Diesel Generator Day Fuel Tanks, the Borated Water Storage Tank and steam piping near the main feed pumps that could affect Secondary Pressure Control. This concluded the TMI-1 evaluation of plant protection for tornado generated missiles against the plant design and licensing basis which results in the reporting of this event as a total of four (4) non-conforming conditions.

The event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications; 10 CFR 50.73(a)(2)(ii)(B) as an event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety; 10 CFR 50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident; 10 CFR 50.73(a)(2)(vii)(D) for any event where a single cause or condition caused at least two independent trains to become inoperable in a single system designed to mitigate the consequences of an accident.

B. CAUSE OF EVENT

These conditions were part of the original plant design and have existed since the initial construction and licensing of the plant. TMI-1 received its operating license on September 19, 1974, based in part on the design as described in the Final Safety Analysis Report (FSAR). The NRC (at that time the Atomic Energy Commission, AEC) issued its safety evaluation for TMI-1 on July 11, 1973. It is not known whether these 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.

3. LER NUMBER 1. FACILITY NAME 2017 004 00 recently identified non-conforming SSCs were considered acceptable due to the low likelihood of damage to the SSCs from tornado-generated missiles during the time of the original design and licensing process.

TMI-1 identified four SSCs as non-conforming conditions to the plant design and licensing basis because of and in concert with a collective industry effort to address the NRC issuance of Regulatory Issue Summary 2015-06 "Tornado Missile Protection".

C. ANALYSIS / SAFETY SIGNIFICANCE

There were no actual consequences resulting from these identified non-conforming conditions. A tornado that could produce the design basis missile is highly unlikely in the mid-Atlantic region. The probability of missiles striking any one of the identified non-conforming SSCs in a fashion to render the SSC inoperable is also very low.

The TMI-1 FSAR described characteristics of the design basis tornado (DBT) wind hazard and tornado generated missile hazard, design requirements are stated as:

  • Tangential wind velocity of 300 mph. (A gust factor of 1.3 has been used for those structures designed for tornado loadings.)
  • An external vacuum of 3 psig.
  • Missile equivalent to a utility pole 35 foot long, 14 inches in diameter, weighing 50 pounds per cubic foot, and traveling at 150 mph.
  • Missile equivalent to a 1 ton automobile traveling at grade at 150 mph.

TMI-1 reviewed these requirements against the plant SSCs needed to achieve safe shutdown in the event of a tornado. The result was the identification of four non-conforming conditions described in the event description which were declared inoperable.

NRC Enforcement Guidance Memorandum EGM 15-002, Revision 1, provides for application of NRC enforcement discretion which allows the four identified non-conforming conditions to be returned to an operable status based on the application of compensatory measures which are intended to reduce the likelihood that the tornado event would result in a failure of any one of the identified non-conforming conditions. TMI-1 applied the guidance in EGM 15-002, Revision 1, and, NRC DSS-ISG-2016-01 Revision 1 Appendix A, to determine the acceptable initial and comprehensive compensatory measures necessary to ensure that the identified nonconforming condition could be treated by the NRC with enforcement discretion.

D. CORRECTIVE ACTIONS

Compensatory measures were implemented in accordance with NRC guidance contained in the EGM 15- 002 (revision 1) and DSS-ISG-2016-01 (revision 1) as described earlier. Enforcement discretion provided by EGM 15-002 remains in effect until June 10, 2020, for TMI-1, at which time modifications, license amendments or other actions must be implemented to fully resolve the issue. TMI-1 intends to request a 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.

3. LER NUMBER 1. FACILITY NAME 2017 004 - 00 license amendment to allow for probabilistic methods to be applied to demonstrate that the risk associated with tornado generated missiles on specific components is sufficiently small such that plant modifications are not warranted.

E. PREVIOUS OCCURENCES

No previous similar events have occurred at the site.