ML20204J252

Safety Evaluation Supporting Licensee 840510 Submittal Re Testing of Facility Dewpoint Moisture Monitors (Dpmms) at 70% of Full Power.Restriction Re Min Flow to Low Level Monitors Under Restricted Flow Conditions Specified

ML20204J252
Person / Time
Site:	Fort Saint Vrain
Issue date:	03/23/1987
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20204J175	List: ML20204J169 ML20204J252
References
TAC-55053, NUDOCS 8703270204
Download: ML20204J252 (3)

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UNITED STATES NUCLEAR REGULATORY COMMISSION

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.j WASHINGTON, D. C. 20555

\...../ SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO REVIEW 0F DEWPOINT MOISTURE MONITORS RESPONSE TESTING AT 70 PERCENT OF POWER FORT ST. VRAIN NUCLEAR GENERATING STATION DOCKET N0. 50-267

1.0 INTRODUCTION

By letter dated May 10, 1984, the Public Service Company of Colorado (the licensee) requested the NRC staff's concurrence on the results of the tests performed on the Fort. St. Vrain Dewpoint Moisture Monitors (DPMMs). {

Testing of the monitors was performed with the plant operating at 70 percent of rated reactor power.

2.0 DISCUSSION Fort St. Vrain Technical Specification (TS) LC0 4.9.2 requires the performance of response time tests on the plant protective system's dewpoint moisture monitoring system. Tests at 5 and 25 percent of rated reactor power were performed previously and it was demonstrated that the response times for the DPMMs at these power levels were satisfactcry.

The results of these tests were reported to the NRC by letter dated June 30, 1977.

The licensee originally planned to perform the final test of the DPMMs at full rated reactor power. However, due to technical considerations, the licensee decided to change test conditions and perform the test at 70 percent of rated reactor power. The NRC concurred with this change in a letter dated July 25, 1978. The test procedure consisted of introducing water vapor-saturated helium into the primary cooling system of the plant and measuring the response times of the DPMMs one at a time. Six low level and two high level DPMMs were tested in this way. Some of the DPMMs were tested in two modes of operation, with unrestricted and restricted sample flow rates. The results of the tests indicated that, in general, the measured response times for the tested DPMMs met the acceptance criteria. Moreover, it was found that the minimum sample flow rates for the low level DPMMs with restricted flow should not go below 50 sec/sec.

This value for sample flow is currently specified for 70 percent rated reactor power in Note (t) of TS LCO 4.4.1.

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l 3.0 EVALUATION The series of time response tests performed on the DPMMs at'70 percent of rated-reactor power and reported in the licensee's submittal dated May 10, 1984, completed the requirement of TS LCO 4.9.2 for testing of the dewpoint moisture monitoring system. The licensee has shown in their submittal of June 2,1978, that the time response tests perfonned at 70

(' percent of rated reactor power would validate the responses of the DPMMs I at full reactor power because at higher levels, the response curve is basically a straight line. Furthermore, the difference in response times is small; consequently any extrapolation of 70 percent power data to full power ccnditions can be accomplished with a high degree of confidence in the results obtained. Thus, the 70 percent power tests, together with the tests already completed at 5 and 25 percent of rated reactor power, provide adequate confirmation of the dewpoint moisture monitor system response.

The response time tests at 70 percent of rated reactor power were of three different kinds: 1) base case tests providing basic response times for the six low-level and two high-level monitors, 2) repeatability tests (performed twice) providing verification of the results on each of the two-high-level monitors, and 3) restricted flow tests, performed on one low-level monitor for each loop and one high-level monitor, providing the results under reduced flow conditions.

It should be noted that the purpose of the restricted flow tests was to verify that the low-level DPMM, located in a potentially leaking primary coolant loop, with its sample flow restricted due to partial blockage of the gas flow channel, would actuate before the similar DPM with an unrestricted sample flow, located in an unaffected loop. This was achieved by assuring that the response time for the restricted DPM was less than the response time for the unrestricted flow DPMM, plus the time required for transfer of the wet helium between the loops. In the restricted flow tests, sample flow was reduced to 40 scc /sec and the bypass valve was set at the minimum stop position.

The test procedure for all three of the above test types consisted of introducing helium gas containing a known amount of moisture into the primary coolant system. This helium, after passing through the circulator, was directed through the sample lines to the DPMM undergoing testing.

The time required for the DPMM to react to the change of moisture content was recorded. In general, it was not possible during the test to produce concentrations of moisture approaching the concentrations predicted to occur during an actual water leak in the operating plant. Thus, since lower water concentrations required longer transfer times from the bulk of gas to the instrument's mirrar, the times of response for the DPMPs were longer during the tests. To account for this effect, a suitable correction was applied. The corrected results were subsequently compared to the calculated response times for a given set of operating conditions, which constituted the acceptance criteria. Comparison of the response times from *he unrestricted flow monitors indicated that the corrected

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response times were within the range of the acceptance criteria.

However, the response tiraer for the monitors with a restricted sample flow fell slightly cutside the acceptance criteria range. The licer.see determined that in order for the DPMMs to stay within the acceptance criteria range under restricted flow corditions, the minimum monitor sampic flow rate should be at least 50 scc /sec.

4.0 CONCLUSION

Based en the above considerations, the staff concludes that the response times for the dewpoint moisture monitors comply with the response times specified in the acceptance criteria, with the qualification that the minimum flow be maintained at or above 50 scc /sec to the low-level monitors under restricted flow conditions. The staff, therefore, finds that with the above restriction, the performance of the dewpoint moisture monitors is acceptable.

Review: K. Parczewski, DPWR-F Date: March 23, 1987 i

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