05000277/LER-2017-001

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LER-2017-001, Emergency Diesel Generator Exhaust Stacks Nonconforming Design for Tornado Missile Protection
Peach Bottom Atomic Power Station Unit 2
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
2772017001R00 - NRC Website
LER 17-001-00 for Peach Bottom Atomic Power Station (PBAPS), Unit 2 Regarding Emergency Diesel Generator Exhaust Stacks Nonconforming Design for Tornado Missile Protection
ML17068A019
Person / Time
Site: Peach Bottom Exelon icon.png
Issue date: 03/08/2017
From: Navin P D
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
CCN: 17-33
Download: ML17068A019 (5)


comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (1--5 F53), U.S, Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects Resource@nrc gov and to the Desk Officer Office of nformabon and Regulatory Affairs, 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.

05000-277

3. LER NUMBER

001 0 2017 Unit Conditions Prior to the Event Unit 2 and Unit 3 were both operating in Mode 1 at approximately 100% rated thermal power. There were no structures, systems or components out of service that contributed to this event.

Description of Event

On January 9, 2017, based on an engineering analysis, it was determined that the site's four Emergency Diesel Generators [EIIS: DG] do not conform with the licensing basis for protection against tornado generated missiles. As a result, one or more of the diesel generators may not have been able to perform their safety design function if a tornado were to impact the site and if debris of sufficient mass and velocity were to strike the diesel engine exhaust stacks.

As a result of the non-conforming condition, on January 9, 2017, at 1530, all four emergency diesel generators were declared inoperable in accordance with Technical Specification (TS) 3.8.1. With two or more diesel generators inoperable, TS 3.8.1 Condition F requires all but one to be restored to an operable status within two hours, or per Condition G, be in Mode 3 in

12 hours
1.388889e-4 days
0.00333 hours
1.984127e-5 weeks
4.566e-6 months

. Compensatory measures were verified as being in place in accordance with NRC guidance contained in EGM 15-002, "Enforcement Discretion for Tornado-Generated Missile Protection Noncompliance" (ML15111A269). As allowed by the EGM, the Emergency Diesel Generators were immediately returned to an operable but non-conforming status.

EGM 15-002, along with Interim Staff Guidance DSS-ISG-2016-01, "Clarification of Licensee Actions in Receipt of Enforcement Discretion Per Enforcement Guidance Memorandum EGM 15-002" (ML15348A202), provide guidance from the NRC for acceptable compensatory measures that licensees can put in place if equipment is determined to be inoperable as a result of tornado generated missiles.

Peach Bottom's compensatory measures are in accordance with the NRC guidance. They include verification of the following:

a) Procedures are in place which instruct personnel to identify and secure potential missile hazards if severe weather is forecasted for the site.

b) Procedures require that planned maintenance of AC power sources be deferred in the event that severe weather is forecasted. Work on other equipment is controlled to minimize risk during severe weather.

c) If severe weather has occurred, procedures call for an inspection of the emergency diesel generator exhaust stacks for damage. If damage is observed and operation of the diesel engines is adversely impacted, the procedure has the damaged portion of the exhaust stack removed using a high temperature cutting torch. Two of these torches are located on site and all equipment operators have been trained on their use. They are part of the FLEX equipment that is stored in a reinforced concrete building that is designed to withstand earthquakes, tornadoes and other severe natural phenomena.

comments regarding burden estimate to the FOIA, Privacy and Information Collectors Branch (T-5 F53), 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 05000-277

3. LER NUMBER

001 0 2017 d) Increasing the level of awareness and preparedness by communicating the issue to Operations personnel though shift manager turnover notes.

Analysis of the Event

Four emergency diesel generators are located on-site which provide backup power in the event of a loss of all offsite power. Each diesel generator is rated at 2,600 kW for continuous operation at 4,160 volts.

The diesel generators provide sufficient power to safely shut down both units in the event of a design basis accident in one unit and a failure of one of the diesel generators. Each diesel generator is housed in a seismic Class I structure and designed to be protected against earthquakes, floods, tornadoes and other natural phenomena. However, the diesel engine exhaust stacks, which extend approximately seven feet above the roof of the building, are not protected against tornado generated missiles. The exhaust stacks are 24-inch diameter steel pipes with a 3/8-inch wall thickness. It was determined that, if the stacks were struck by a design basis tornado missile, they could be crimped in such a manner as to prevent the diesel generator from performing its design function. The design basis tornado missile for the site includes a wood plank that is 4 inches thick, 12 inches wide and 12 feet long traveling at 300 mph.

There were no actual consequences resulting from this non-conforming condition. A tornado that could produce the design basis missile is highly unlikely in the mid-Atlantic region. The probability of missiles causing more than one diesel generator to become inoperable is also very low.

This condition is being reported in accordance with the following criteria:

  • 10CFR 50.73(a)(2)(v)(D) for any condition that could have prevented the fulfillment of the safety function of systems that are needed to mitigate the consequences of an accident.
  • 10CFR 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.

Cause of the Event

This condition was part of the original plant design and has existed since the initial construction and licensing of the plant. Peach Bottom Unit 2 and Unit 3 received their operating licenses in 1973 and 1974, respectively, based in part on the design of the emergency diesel generators as described in the Final Safety Analysis Report (FSAR). This report did not address the fact that the exhaust stacks are not protected against tornado missiles and no other documented engineering analysis has been identified. It is not known if this was not evaluated at the time of the original design and licensing process, or if it was considered acceptable due to the low likelihood of damage to the exhaust stacks from tornado missiles.

comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, used to impose an information collection does not display a currently va'id OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection Past perception of this condition was that only one tornado generated missile needed to be considered and only one diesel exhaust stack would be impacted. Since only three emergency diesel generators are needed to safely shut down the plant, such a condition was considered to be included in the licensing basis of the plant. Additional industry and NRC information on this issue pointed to the need to have all diesel generators survive a tornado with the capability to perform their safety function intact.

Corrective Actions

Compensatory measures were implemented in accordance with NRC guidance contained in EGM 15-002 and DSS-ISG-2016-01 as described earlier. Enforcement discretion provided by EGM 15-002 remains in effect for five years for Peach Bottom, at which time modifications, license amendments or other actions must be implemented to fully resolve the issue. Peach Bottom intends to request a license amendment to allow for probabilistic methods to be applied to demonstrate that the risk associated with tornado missiles on specific components is sufficiently small such that plant modifications are not warranted.

Previous Similar Occurrences No previous similar events have occurred at the site.