ML20009B152
| ML20009B152 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 06/29/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20009B150 | List: |
| References | |
| NUDOCS 8107140883 | |
| Download: ML20009B152 (2) | |
Text
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NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 g.
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REGARDING THE AFWS RECOMMENDATION GL-2 BALTIMORE GAS AND ELECTRIC COMPANY CALVERT CLIFFS NUCLEAR POWER PLANT, UNITS NO. 1 AND 2 DOCKET N05. 50-317 AND 50-318 One of the long-term recommendations published in NUREG-0635 by the Sulletins and Orders Task Force is that licensees with plants in which the primary auxiliary feedwater system (AFWS) water supply passes through valves in a single flow path, but the alternate AFWS water supplies connect to the AFWS pump suction downstream of these valves, should install redundant valves parallel to these valves or provide automatic opening of the valve (s) from the alternate water supply upon low pump suction pressure (Recommendation GL-2).
The NUREG-0635 requirements were transmitted to Saltimore Gas and Electric Company (BG&E or the licensee) by our letter of November 17, 1979.
Recommendation GL-2 was developed because of the potential for inadvertent closure or the failure of single suction valves to cause the failure of all trains of the AFWS, Such valves in the suction lines of the AFWS have even greater significance cecause they not cnly can cause tem:orary flow interruptions, but also have the potential for causing the destruction of the downstream pumps by cavitation when they are needed for a demand event.
In letters dated November 18, 1980 and February 2,1981, BG&E proposed to modify their AFWS design such that the suction line valve for each unit (1-AFW-161 and 2-AFW-161) that is upstream of the alternate water supply connection have control room-located position indicators provided for them and additional valves installed in parallel around them.
By letter dated March 5, 1981, BG&E requested an exemption from this recommendation based upon the cost and reliability data that was supplied in their letter.
In this letter, BG&E proposed making all of the other AFWS modifications detailed in NUREG-0635 except for that of GL-2.
Their basis for this request was a reliability study, performed in conjunction with our Interim Reliability Evaluation Program (IREP).
The event with the greatest potential adverse consequences was found to be a steam generator rupture. The BG&E results for differential cost and AFM un-availability are shown below in the first two columns.
Steam Generator Rupture Transient Cost AFW Unavailability Probability Priority of Meltdown Score
~
Present System N/A 1.2 x 10' 3.57 x 10-4 N/A Modification A
$15-20 x 100 7.7 x 10-5 2.34 x 10-4 6.4 x 104 3
Modification B
$40 x 10 7.1 x 10-5 2.34 x 10~4 0
8107140803 810629 PDR ADOCK 05000317 P
.. P DR
. Modification B is the GL-2 recommendation and Modification A is all other AFWS modifications.
It should be noted that Modification B is a system change in addition to, not instead of, Modification A.
The probability of meltdown (frequency of the event) data is the result of an event tree analysis performed by the staff.
Using the above data, and gpproximating the magnitude of radioactive release for the event as 2 x 10o curies and assuming the cost of the necessary NRC resources to be insignifican com-pared to the industry cost, we calculated the priority scores for Modifica-tions A and B.
As can be readily seen, for Calvert Cliffs Modification S would result in no substantive reduction in consequcnces for this transient which then results in a priority score of zero.
However, Modi fication A has a substantial safety benefit and, even with its large cost, results in a priority score of 64,000.
This score is about in the middle of the other issues / requirements that we have prioritized.
The reason for the very small reliability benefit of installing the parallel valves is, as previously indicated, when the proposed modifications for the Calvert Cliffs AFWS are completed the design for each unit will include position indication in the control room for the suction line valve upstream of the alternate AFWS supply connection.
The design will also include a cross-over line from each unit's new motor driven pump to the other unit's AFWS and the use of one of the two turbine-driven pumps per unit in the standby mode during AFWS demands.
The position indication in the control room is of benefit because it tends to reduce the likelinced of inadvertent valve closures (typically a major contributor to such a valve's unreliability) remaining undetected for prolonged periods.
The inclusion of the cross-over line is particularly valuable since it e. iminates a unit's complete depen-dence on flow through its unit's suction valve.
The use of one turbine-driven pump in the standby mode is valuable because it provides protection in the event of a closed valve causing the failure of the pumps actually started.
The combined effect of these proposed modifications offsets the concerns of Recommendation GL-2 and, thus, the additional reliability improvement of installing redundant suction valves is very small.
We have concluded that the combined effect of the modifications that have been proposed by BG&E, and that form the basis of the staff's AFWS safety evaluation contained in License Amendments 54 and 37 issued on May 8,1981, satisfy the intent of Recommendation GL-2.
We, therefore, find that the installation of redundant suction line valves to valves 1-AFW-161 and 2-AFW-161 is not necessary for the Calvert Cliffs units.
Date:
JUN 2 9198)
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