ML20138F486

SER Re Allowable Control Room Inleakage to Meet Habitability Requirements.Reduction in Allowable Control Room Inleakage for Completed Control Room to 590 Cfm Recommended

ML20138F486
Person / Time
Site:	Grand Gulf
Issue date:	10/16/1985
From:	NRC
To:
Shared Package
ML20138F434	List: ML20138F430 ML20138F486
References
TAC-56445, NUDOCS 8510250274
Download: ML20138F486 (3)

Text

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ENCLOSURE SAFETY EVALUATION CONTROL ROOM HABITABILITY GRAND GULF NUCLEAR STATION, UNITS 1 AND 2 INTRODUCTION Section 6.4 of the staff's Supplement No. 6 (SSER 6) to the Safety Evaluation Report related to Operation of Grand Gulf Nuclear Station (GGNS), Units 1 and 2 (NUREG-0831) provided staff's evaluation of control room habitability for the present stage of completion of Grand Gulf Unit 2. The GGNS control room is shared between Units 1 and 2. The Unit 2 part of the control room is partially completed and construction of Unit 2 is being held in abeyance. In SSER 6 staff concluded that for operation of Unit 1, concurrent with construction of Unit 2, the allowable control room inleakage rate of 760 cfm requested by licensee in its. February 6,1984, letter was acceptable, licensee had stated in its February 6,1984, letter that after Unit 2 construction was completed the allowable control room inleakage could be reduced to 590 cfm. In SSER 6 staff indicated that it would review control room habitability systems for simultaneous operation of Units 1 and 2 with respect to allowable control room inleakage prior to operation of Unit 2. License Condition 2.C.(38) allowed a leak rate of 590 cfm for operation of Unit I with Unit 2 in its present stage of completion and no ongoing construction of the Unit 2 portion of the control room and allowed a leak rate of 760 cfm for operation of Unit I concurrent with con-struction of the Unit 2 portion of the control room.

The present safety evaluation was made to determine the allowable control room leak rate for simultaneous operation of Units 1 and 2. Based on its evaluation and conclusion described below, the staff will condition the Unit 2 operating license to limit allowable control room inleakage for the completed Unit 1 and Unit 2 control room to 590 cfm.

EVALUATION By letter dated February 6,1984, the licensee provided the results of scale model tests from which atmospheric dispersion coefficients relating accidental releases of radioactivity and control room doses may be estimated. The NRC staff independently, calculated the dispersion coefficients and control room doses.

We have reviewed the wind tunnel tr M r w lures and results. The tests were conducted using 1:240 scale modelt o @ 3: clear station structures with two complete units including cooling to, vers, and with Unit I with its natural draft cooling tower and a partially completed Unit 2. Gaseous tracers were released from the modeled auxiliary building vents and sampled at 20 locations on the west face and at 16 locations on the top of the control building. A wind velocity profile in the vertical direction was simulated in the wind tunnel.

The tests were conducted with a tunnel wind speed of 10 fps at a reference height of 9.6 inches, during which the tests are relatively independent of 8510250274 e51016 PDR ADOCK 0 % 4d6 F

friction as represented by the Reynolds number (i.e., XU/Q constant when the wind speed, u and source term, Q, are varied in proportion to one another).

The effects of wind direction were simulated by rotating the scaled structures on a turntable. Tests by wind direction were made for releases from the inboard and outboard vents of Units 1 and 2. XD/Q was calculated from measurements at each sampling location for each test. We concluded from our review of the test documentation and results that the experiment was well designed and that the resulting concentrations can be used for evaluation of control room habitability for Grand Gulf. Units 1 and 2.

The wind tunnel test results provided a basis for evaluation by the staff of atmospheric dispersion for use in the habitability evaluation of the Grand Gulf control room. The wind tunnel test results show that the highest relative con-centrations (XU/Q) usually occurred on the roof of the control room. The r maximum XG/Q value for releases from Unit 1 vents was measured as 4.2 x 10 goftgp m for the inboard vent during west winds.

Unit 2 vents was measured as 1.1 x 10y-3 Thg m - for rooftop the inboardmaximum XU/Qnorth-vent during value from northeast winds. However, the HVAC system intakes for the control room are located near ground level and on the west face of the control building. The maximum XU/Q values on thg fage of the building near the HVAC intakes were l indicated to be 1.4 x 10- m for both inboard a Unit I during south-southwest winds and 2.0 x 10-gdmcytboard vent vent

- for inboard releases from releases from Unit 2 during northeast winds. Thus, the limiting relative concentration (XGf0) {or evaluation of control room habitability fue tg inleakage is 1.1 x i

10- m- and due to ventilation intake is 2.0 x 10 m- . These Xu/Q values were each divided by wind speed (u) at the percentiles given in the Murphy-Campe methodology for pertinent time intervals following an accident. The wind speeds at the percentiles were obtained by analysis of the onsite wind data at the 49.4m level for the time periods August 1972 through July 1974 and the 1976 calendar year. The wind direction and occupancy factors were also taken from the Murphy-Campe methodology.

The results of these analyses are as follows:

Intake Time Wind Direction Occupancy Wind X/Q Period Factor 3 Factor Speed (m/s) (sec/m )

0 - 8 hr. I 1 1.2 1.7 x 10-4 8 - 24 hr. 0.88 1 1.6 1.1 x 10-4 1 - 4 days 0.75 0.6 2.1 7.1 x 10 4 - 30 days 0.50 0.4 3.0 3.3 x 10-5 It was assumed that all containment leakage in the first two minutes of a release of radioactivity would escape instantaneously into the environment, and that following this first two minute period the standby gas treatment system would filter and exhaust containment leakage at a rate of 4000 cfm. All other assumptions were as described in the SER, NUREG-0831, September 1981.

CONCLUSION The computed control room doses are 0.2 rem whole body and 20 rem thyroid. These doses are less than the dose criterion of General Design Criterion 19, 10 CFR Part 50, Appendix A, with which the control room design is in conformance. The protection of control room occupants against chlorine gas is obtained by the automatic isolation of the normal ventilation system and the provision of com-pressed air breathing apparatus, and is not affected by the inleakage rate. We conclude that the licensee's proposed control room inleakage of 590 cfm for simultaneous operation of Units 1 and 2 is acceptable.