ML19309E780
| ML19309E780 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 04/16/1980 |
| From: | Counsil W NORTHEAST NUCLEAR ENERGY CO. |
| To: | Ziemann D Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8004240373 | |
| Download: ML19309E780 (4) | |
Text
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C':C,L"'":::::l April 16,1980 Docket No. 50-2h5 Director of Nuclear Reactor Regulation Attn:
Mr. D. L. Ziemann, Chief Operating Reactors Branch #2 U. S. Nuclear Regulatory Commission Washington, D. C.
20555
References:
(1)
D. C. Switzer letter to D. L. Ziemann dated March 20, 1978.
(2)
W. G. Counsil letter to D. L. Ziemann dated October 31, 1979 Gentlemen:
Millstone Nuclear Power Station, Unit No.1 Primary Containment Continuous Leak Rate Monitoring In Reference (1), Northeast Nuclear Energy Company (NNECO) proposed changes to the Millstone Unit No.1 Technical Specifications, deleting the Su"veillance Requirement for primary containment continuous leak rate monitoring via the inerting system makeup. The initial Staff review of the submittal prompted verbal requests by the NRC Project Manager for additional bases, alternatives,
and further direction. Accordingly, in Reference (2), NNECO requested a post-ponement of Staff review until such time as the requested information could be provided. The purpose af this letter is to provide the additional information requested so that Staff re~iew of the proposed Technical Specification changes may proceed.
(1) The reasons for deleting Technical Specification Surveillance Requirement 4.7.A.3.g for primary containment continuous leak rate monitoring via inerting system makeup, were summarized in References (1) and (2) as follows:
This specification vu or.;inally developed because the nitrogen inerting system was envisioned to be capable of serving an additional function.
In practice, this has proved impractical. Nitrogen has been required for the following additional purposes:
(1) re-inerting in order to control oxygen build-up during operation, (2) maintenance of the differential pressure between the dryvell and torus, and l
(3) replacement for nitrogen tank "beil-off".
8004240 p 3
e Containment leak rate testing as specified in 10CFR50, Appendix J, does not require a continuous leak rate monitor. Also, other BWR's do not have means to monitor containment leakage on a continuous basis.
- However, if any gross out-leakage from the drywell should occur, this would be detected by abnormal operation of the drywell/ torus differential pressure system.
The current method of continuous containment monitoring is nitrogen
^
makeup; obtaining accurate results has been compromised by drywell/
torus differential pressure requirements and by containment purging.
(2) For more than a year, NNECO has been evaluating the plant changes that would be necessary to make the monitoring, via inerting system makeup, sufficiently accurate. The evaluations included the following:
a) Currently, the torus is being vented daily to the standby gas treatment system in order to maintain the Technical Specification requirement of one (1) psid between the drywell and torus. The large volumes of makeup nitrogen necessary to repressurize the drywell to maintain the required one paid overrides the continuous leak rate monitoring capability and acceptance criteria. That is, any volume of nitrogen required to compensate for drywell leakage is masked by the large volumes of nitrogen required for establishing the one psid. The nitrogen makeup volumes for drywell inerting and differential pressure requirements are frequent and substantial, and completely screen the detection of drywell leakage trends. Thus, the existing nitrogen makeup system cannot provide accurate data for compliance with the subject Technical Specification.
b) To verify containment integrity, a continuous monitoring system must utilize thermodynamic parameters such as temperature, pressure, free volume, and atmospheric mass. These parameters vary with time. The drywell temperature depends upon reactor operations i
and drywell cooling. Torus temperature varies with changes in pool temperature. Dryvell pressure varies as a result of main-taining the one psid drywell/ torus pressure requirement and inerting requirements. Drywell free volume changes as the drywell sump water level changes, and as thermal expansion occurs during reactor operations. The torus free volume changes with variations in suppression pool water level. The mass in the drywell atmosphere changes continually due to steam leaks and service air leaks.
The Technical Specification Surveillance Requirement that NNECO proposed to delete stipulates, ".
. the containment shall be continuously monitored for gross leakage by review of the inerting system makeup requirements". Gross leakage may be defined as the maximum allowable Technical Specification 4.7.A.b.1 leakage, of 1.2 weight percent per day at 43 psig.
If so, then a containment leakage of less than 0.25 cfm must be measurable when drywell
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/ pressure is 1.8 psig (alarm setpoint). Such measurements require extremely accurate monitoring of all thennodynamic parmaeters in the drywell and torus, which continuously vary with time. Also, the torus is maintained between 0.0 and 0.25 inches W.G. vacuum.
When the torus is at the same pressure as the reactor building, torus leakage cannot be measured.
c) A number of engineering modifications were considered to improve the capability of containment leakage monitoring via nitrogen makeup. The installation of a gas flowmeter and recorder in the atmospheric control system piping would provide data on the volume of gas vented. Various other existing systems and Technical Specifications discussed below were assessed, with the conclusion they provide indirect monitoring of containment integrity. The upgrading of an existing roots blower system would make it easier to establish and maintain the required dryvell/ torus differential pressure with less nitrogen makeup required. The blower would take suction from the torus and discharge into the drywell.
If these modifications were all implemented, extensive instrumentation backfits to very accurately monitor the dryvell and torus for all thermodynamic parameters would still be required. Even if this were done, the continual variation in these parameters makes it questionable that the leakage could be measured sufficiently accurate, compared to 0.25 cfn discussed in Part b, above.
d) Primary containment integrity is monitored continously via another existing Technical Specification requirement, 3 7.A.2.a.
The drywell and torus pressures are continually monitored during compliance with the Technical Specification requirement that a one psi differential pressure be maintained between the torus and drywell. A gross leak from the drywell vill cause a decrease in the required differential pressure.
A gross leak into the torus (the torus is kept under a slight vacuum) will also cause a decrease in the required differential pressure.
Primary containment integrity is also ensured by the testing required by 10CFR50, Appendix J.
Each operating cycle, containment penetrations are tested. Also, integrated leak rate tests (ILRT) are performed three times every ten years. An ILRT is currently scheduled to be performed during the upcoming 1980 refueling outage. In the unlikely event gross leakage from the containment does occur, area radiation monitors located in the reactor building, in the reactor building ventilation system, and in the standby gas treatment system vill alamn.
In summary, based upon the preceding, the existing Technical Specification for continuous leak rate monitoring via nitrogen makeup cannot be performed with either existing equipment or utilizing other instrumentation. Alternate methods of improving the existing capability have been evaluated, with the conclusion that the nitrogen makeup method is non-viable, although it was thought to be L
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_4-possible ten years ago when the Technical Specifications for this method were first envisioned.
NNECO, therefore, requests that the proposed changes in Reference (1) be approved in order to eliminate the non-viable requirement in the Technical Specifications.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY b
W. G. Counsil Vice President