ML20057C120
| ML20057C120 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 09/14/1993 |
| From: | Richard Anderson NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9309270094 | |
| Download: ML20057C120 (4) | |
Text
n-I Northem States Power Company 414 Nicollet Mall Minneapolis Minnesota 55401-1927 Telephone (612) 330-5500 September 14, 1993 F
U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR CENERATING PIANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Recuest For 7he Exercise of Enforcement Discretion The purpose of this letter is to confirm the results of discussions between Northern States Power Company and the NRC Staff on September 13, 1993, in which Northern States Power requested the exercise of enforcement discretion for the noncompliance with an action statement contained in Section 3.7.B.6 of the Prairie Island Technical Specifications. A Regional Notice of Enforcement Discretion was verbally issued by the NRC Staff at 2037 on September 13, 1993.
Reautrements for Which Relief is Requested Technical Specification 3.7.A.2 requires that 4 kV safeguards buses 15 and 16 shall be energized whenever the reactor coolant system average temperature is greater than 200*F.
On September 13, 1993 a penetration into the room containing 4160V Bus 16 was determined to not be protected from steam impingement which could result from a main steam line break in the turbine building. As a result of this unprotected penetration, Unit 1 4160V Bus 16 was declared inoperable at 1715 on September 13, 1993 and the action statement contained in Technical Specification 3.7.B.6 was entered.
Technical Specification 3.7.B.6 allows 4160V Bus 16 to be inoperable for a maximum of eight hours provided the redundant 4160V safeguards bus and its associated 480V safeguards bus are verified oparable and the diesel generator and safeguards equipment associated with the redundant train are operable.
All of the required redundant equipment was verified to be operable upon entry into the action statement. However, because the installation of an impingement barrier over the penetration could not be completed within the eight hours allowed by Technical Specification 3.7.B.6, it was necessary to request the exercise of enforcement discretion to allow 4160V Bus 16 to be inoperable for an additional 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> beyond the specified eight hour allowed out of service time.
GiIngp. stances Leading to Recuest and Need for Prompt Action The temporarily sealed penetration between the Unit 1 turbine building and the Bus 16 room was identified, on September 13, 1993, as a potential steam impingement issue as part of the planned walkdown of steam exclusion
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USNRC Septenber 14, 1993 Nodhem States Power Company Page 2 i
boundaries resulting from the -event notification of September 3, -1993.
The penetration is located on the 715 elevation of the turbine building near the south end of the 8 line wall, in an area that is not protected by_ originally installed impingement barriers. The subject penetration was installed as part of the SB0/ESU project (DS/D6 diesel generator addition and electrical safeguards upgrade project). The SB0/ESU project design included an impingement barrier to enclose this penetration; however, installation was not complete at the time of this identification.
The walkdown of steam exclusion area boundaries began as a result of the event identified on September 3, 1993.
This walkdown was planned to first inspect the auxiliary building steam exclusion boundary to identify any additional openings. The next phase was initiated to include steam exclusion boundaries where openings would likely impact the operability of a single train of equipment. This additional temporarily sealed penetration was identified early in this second phase.
Evaluation of Safety Sinnificance and Potential Consecuences Even though Unit 1 4160V Bus 16 was declared inoperable at 1715 on September 13, 1993, the bus was fully energized and its related controls remained available and capable of responding to any plant transient. Additionally, as stated above, all of the required redundant equipment required by Technical Specification 3.7.B.6 was verified to be operable.
The inoperability of 4160V Bus 16 was solely based on the potential for a postulated main steam line break at a specific location in the turbine building impinging on an unprotected penetration into the room containing Bus 16.
This is a very low probability event, and allowing Prairie Island Unit 1 to remain at power for an additional 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> under this condition does not significantly increase the risk to the public health and safety.
- Further, because all of the equipment redundant to 4160V Bus 16 has been verified to be operable, the redundant train of safeguards equipment would be available to mitigate the consequences of a steam line break and thus the potential consequences of such an event would not be different than those previously evaluated.
As discussed above, corrective actions were initiated to install an impingement barrier over the subject penetration following discovery of this condition.
Protection of the penetration will increase and the risk to 4160V Bus 16 will decrease as the installation of this barrier progresses.
The risk of a main steam line break is still present until the unit reaches cold shutdown, and 4160V Bus 16 would still be required to mitigate such an event even though the unit is being shutdown. If a shutdown were initiated at the end of the eight hour allowed out of service time, the installation of the penetration impingement barrier woald be complete prior to reaching cold shutdown. Therefore, initiation of a plant shutdown under these circumstances will not place the unit in a condition where Bus 16 is not required to be operable prior to the bus being restored to operable status, i
i USURC September 14, 1993 Northern States Power Company Page 3 4160V safeguards Bus 16 does not actively support power operation of Unit 1.
Iloweve r, once the unit is in the hot. shutdown condition, 4160V Bus 16 is required to power the Unit 1 motor driven auxiliary feedwater pump and to therefore support decay heat removal from the reactor.
Therefore, placing Unit 1 in the hot shutdown condition will place the unit in a condition where the relative importance of Bus 16 is increased.
The initiation of a Unit 1 shutdown as a result of the inoperability of 4160V Bus 16 would potentially increase the risk of a steam line break occurring for the following reasons:
- 1. As the unit is reduced in power, main steam pressure increases.
This increase in steam pressure will increase the risk of a steam line break, and
- 2. The shutdown of the unit introduces incretsed potential for transients or fluctuations in main steam line pressure which could increase the risk for a main steam line break.
Therefore, since the bus is energized and capable of responding to any plant transient and is only inoperable due to the potential for a postulated main steam line break at a specific location in the turbine building impinging on an unprotected penetration, the increase in risk as the result of a shutdown is greater than the risk from staying at power until the impingement barrier can be installed.
Discussion of Compensatory Actions 4160V Bus 16 has remained fully energized and its related controls available and capable of responding to any plant transient.
Even though there is a high probability that 4160V Bus 16 will not be adversely affected by a steam line break during the time required to complete the installation of the impingement barrier, the following compensatory actions were implemented until the barrier installation is complete:
- 1. Both turbine building cranes were secured to minimize the potential for a heavy load being dropped on the portion of the main steam line which could affect the subject penetration.
- 2. All surveillance testing which has the potential for introducing a plant transient or reactor trip has been suspended,
- 3. The portion of the main steam line which could affect the subject penetration will be walked down every four hours.
US!)RC September 14, 1993 Northern States Power Company
- Page 4 Justification of Duration of the Recuest This. request for the exercise of enforcement discretion was necessary to allow 4160V safeguards Bus 16 to be inoperable _for an additional 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> beyond the eight hour allowed out of service time specified in Technical Specification 3.7 B.6.
This additional time was necessary to complete the installation of a a seismically qualified impingement barrier over the unprotected penetration l
into the 4160V Bus 16 room. This effort included the installation of brackets to support the barrier plates and the installation of the five plates which l
made up those portions of the impingement barrier necessary to protect the subject penetration. The plate installation included alignment, welding of the plates to the brackets, drilling and bolting.
Operations Committee Review and Approval The Plant Operations Committee has reviewed and approved this request for the i
exercise of enforcement discretion.
Impinr_ement Barrier Completion Installation of those portions of the impingement barrier necessary to protect the subject penetration was completed at approximately 1225 on September 14, 1993 and 4160V Bus 16 was subsequently declared operable.
l In this document, we make no new long term NRC commitments.
Please contact Gene Eckholt (612-388-1121, Ext. 4663) if you have any questions related to this request for cne excicise of enforcement discretion.
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mpa g Roger d' Anderson Director Licensing and Management 1ssues c: Regional Administrator - Region III, NRC Senior Resident Inspector, NRC NRR Project Manager, NRC J E Silberg j
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