ML19305C287
| ML19305C287 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 03/17/1980 |
| From: | Mills L TENNESSEE VALLEY AUTHORITY |
| To: | Ippolito T Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8003260480 | |
| Download: ML19305C287 (9) | |
Text
6-9 -
TENNESSEE VALLEY AUTHORITY CH ATTANOOGA. TENNESSEE 37 dot 400 Chestnut Street Tower II March 17, 1980 Director of Nuclear Reactor Regulation Attention:
Mr. Thomas A. Ippolito, Chief Branch No. 3 Division of Operating Reactors U.S. Nuclear Regulatory Commission Washington, DC 20555
Dear Mr. Ippolito:
In the Matter of the i
Docket Nos. 50-259 Tennessee Valley Authority
)
50-260 50-296 Your letter to H. G. Parris dated October 25, 1979, requested that we discuss the provisions made to ensure that the containment purge and vent system at the Browns Ferry Nuclear Plant has been designed in accordance with each paragraph and subparagraph of Branch Technical Position CSB 6-4.
Enclosed is our evaluation of the Browns Ferry containment purge system performed in response to your letter.
Very truly yours, TENNESSEE VALLEY AUTHORITY
~ 1 L. M. Mills, Manager Nuclear Regulation and Safety Enclosure Ao31 s
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ENCLOSURE s
EVALUATION OF THE BROWNS FERRY PURGE SYSTEM PER NRC BRANCH TECHNICAL POSITION CSB 6-4 The purge system at the Browns Ferry - Nuclear Plant has been evaluated based on the positions of Branch Technical Position (BTP) CSB 6-4, revision 1.
An introduction is provided which describes the systems physical features and its primary modes of o peration.
This is followed by a point by point discussion of each requirement in the BTP.
Introduction The purge system at Browns Ferry consists of two large supply ducts and two large exhaust ducts.
The drywell is provided with one supply and one exhaust duct.
The other two ducts are used to purge the torus.
Redundant containment isolation valves are provided on all paths from containment.
Figure 1 is a schematic of the purge system.
In addition to the lines di.scussed above a 10-inch nitrogen supply line tees into the main purge supply header.
The purge system performs three basic functions:
A.
To inert the drywell and torus with nitrogen at the end of an
- outage, B.
To purge nitrogen from the containment just prior to reactor shutdown, and C.
To provide a safe atmosphere in containment for plant personnel when the plant is shutdown or in hot standby and containment access is required.
The purge system is not used during routine power operations.
Ecwever, various inspections and tests must be performed inside containment with the reactor in the hot ecndition.
The risk of less of life to plant personnel frem working in a nitrogen atmosphere is unacceptable.
This is the primary reason to purge with the reactor in modes cther than cold shutdown.
For additional considerations refer to enclosure 1 of our letter from J. E. Gilleland to T. A.
Ippolito dated June 12, 1979.
The following is a detailed discussion of the CSB 6-4 technical positions.
Discussion of the Technical Positions l
1.
The on-line purge system shculd be designed in accordance l
with the following criteria:
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a.
The performance and reliability of the purge system isolation valves should be consistent with the operability assurance program outlined in MEB Branch Technical Position MEB-2, Pump _ and Valve Operability D"D
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_2 Accurance Program.
(Also see SRP Section 3.9.3.) The design basis for the valves and actuators should include the buildup of containment pressure for the LOCA break spectrum, and the purge line and vent line flows as a function of time up to and during valve closure.
Response
An evaluation of the operability of the Browns Ferry purge valves is underway.
This evaluation is being conducted in response to and in accordance with an NRC letter from Darrell G.
Eisenhut to All Light Water Reactors, dated September 27, 1979.
TVA is working with various valve vendors to assess the effects of fluid flow through a closing purge valve during the initial stages of a LOCA.
Upon completion of the operability study TVA will inform the NRC of the results.
In the interim TVA has committed to limit the maximum opening of the valves to a position which reduces the potential accident loads on the valve.
b.
The number of purge and vent lines that may be used should be limited to one purge line and one vent line.
Response
Figure 1 provides a layout of the purge system.
As shown a single supply line splits to provide one inlet to the torus and one to the drywell.
There is one exhaust line from the torus and one from the drywell.
During normal conditions there is no direct communication between the drywell and torus necessitating the present configuration.
Most importantly the analysis presented in section Sa shows the offsite dose is not significantly increased due to a LCCA during purge.
Therefore the present system design meets this criterien of CSB 6-4.
c.
The size of the purge and vent lines should not exceed about 8 inches in diameter unless detailed justification for larger line sizes is provided.
Response
The size of the purge lines are shown on Figure 1.
The line sizes are in excess of 8 inches.
However, the dose analysis (see section Sa) provides the justification for a largre line size, d.
The containment isojation provisions for the purge system lines shoulf meet the standards appropriate to engineered safety features; i.e., quality, redundancy, testability, and other appropriate criteria.
Response
Etub purge line is provided with two automatic isolation valves in series.
The valves are leak tested as required by 10 CFR 50, Appendix J.
.The isolation valves and the purge lines between the outer isolation valve and the primary containment meet the standards for engineered safety features at Browns Ferry.
The piping and valves meet ASME Section III Class B, Seismic Class I requirements.
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e.
Instrumentation and control systems provided to isolate the purge system lines should be independent and actuated by diverse parameters; e.g., containment pressure, safety injection actuation,-and containment radiation level.
If energy is required to close the valves, at least two diverse sources of energy shall be provided, either of which can affect the isolation function.
Response
Automatic isolation of the purge valves occurs when a low reactor water level, high drywell pressure, or reactor zone high radiation signal is present.
The purge valves are pneumatically actuated and fail closed on loss of power.
f.
Purge system isolation valve closure times, including instrunentation delays, should not exceed 5 seconds.
Response
The maximum closure time for a purge valve at Browns Ferry is 15 seconds.
The dose analysis performed as required showed low offsite doses even with a 15 second closure time.
However, TVA will modify the valves to be in compliance with this position.
g.
Provisions should be made to ensure that isolation valve closure will not be prevented by debris which could potentially become entrained in the escaping air and steam.
Responce:
TVA is adding debris screens where each line penetrates the drywell or torus.
Figure 2 shows the anticipated screen configuration.
The screens are designed to be able to withstand the maximum LOCA pressure while completely clogged with debris without failure.
2.
The purge system should not ce relied on for temperature and
~
humidity control within the containment.
Response
The purge system is not used for containment temperature or humidity control during power operation.
Normal uses of the purge system are discussed in the introduction.
3.
Provisions should be made to minimize the need for purging of the containment by providing containment atmosphere cleanup systems within the containment.
. Response:
The purge system is not used for atmospheric cleanup.
The addition of atmospheric cleanup units would not reduce che need for purging.
'4.
Provisions should be made for testing the availability of the isolation function and the leakage rate of the isolation i
valves, individually, during reactor operation, j
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Response
The system can be leak rate tested during power operation and functional tests of the isolation channels are presently performed once a month.
During power operations, the purge system is used only at the start or end of an outage period, so there is no need to routinely leak est the purge isolation valves during power operation.
S.
The following analyses should be performed to justify the containment purge system design:
a.
An analysis of the radiological consequences of a loss-of-coolant accident.
The analysis should be done for a spectrum of break sizes, and the instrumentation and setpoints that will actuate the vent and purge valves closed should be identified.
The source term used in the radiological calculations should be based on a calculation under the terms of Appendix K to determine the extent of fuel failure and the concomitant release of fission products, and the fission product activity in the primary coolant.
A pre-existing iodine spike should be considered in determining primary coolant activity.
The volume of containment in which fission products are mixed should be justified, and the fission products from the above sources should be assumed to be released through the open purge valves during the maximum interval required for valve closure.
The radiological consequences should be within 10 CFR 100 guideline values.
Response
We have performed an analysis of the radiological consequences of a LOCA during purge operations for a 5 second and 15 second valve closure time.
The analysis performed was based on the event (DBA) that produces the highest drywell and torus pressure.
To maximize the offsite dose the following assumptions were made:
A.
The purge valves remain fully open until closure time is reached when the flow area is instantaneously reduced to zero.
B.
No flow losses were as.;umed for pipe friction, bends, valves, entrance or exit losses, or debris screens.
C.
The flow area was based on the nominal line size.
D.
Three 18-inch lines and one 20-inch lines were open.
E.
Only steam was vented from the drywell.
F.
No mixing with containment atmosphere was considered.
In the Mark I Containment all steam entering the torus from the drywell in the first 15 seconds of a LOCA will condense in the pool.
Therefore only air will exit from the torus.
The Moody critical flow correlation with an fL/D equal to zero was used to calculate the steam release from the drywell.
Critical flow equations for gases -were used to calculate the air loss.
The use of these correlations show the following results.
1
O.
Pive Second Closure.
Steam - 2903 lbs Air - 955 lbs Fifteen Second Closure Steam - 8425 lbs Air - 3572 lbs This is a bounding calculation for a spectrum of break sizes.
For larger breaks, the containment isolation will be initiated shortly into the incident as the drywell pressure exceeds the isolation setpoint.
For smaller breaks where containment pressurization rates are slow, the isolation logic will be tripped promptly by the reactor zone high radiation monitors.
The source term used for the reactor coolant was obtained from NUREG-0016, revision 1.
There are no fuel failures as a result of the LOCA prior to purge line isolation.
An iodine spike was assumed to be present.
The spike was considered to increase the reactor coolant concentrations by a factor of 500.
The two hour site boundary incremental doses are:
5 second Closure 15 second closure Beta Dose 9.42~x 10-3 Rem 2.74 x 10-r Fem Gamma Dose 3.95 x 10-2 Rem 1.15 x 10-1 Rem Thyroid Dose 4.01 Rem 11.6 Rem As can be seen the results of the analysis show the contribution to the of fsite dose for a LOC.A during purge to be extremely small.
b.
An analysis which demonstrates the acceptability of the provisions made to protect structures and safety-related equipment; e.g.,
fans, filters, and ductwork, located beyond the. purge system isolation valves against loss of function from the environment created by the escaping air and steam.
Response
An evaluation of the effects of a LCCA during purging on the ductuork outboard of the isolation valves has been performed.
First, the effects of the LOCA pressure on the ductwork was considered and second, the ef fects on the building environment due to steam being released inside various plant structures was determined.
The purge system ductwork is located inside the reactor building.
The ductwork outboard of the containment isolation valves is constructed of sheetmetal. The sheetmetal ductwork has a design pressure of 10 inches of water (0.36 psig).
. l 1
Failure of the ductwork releasing an air-steam mixture into the reactor building was postulated and an analysis of the resulting pressure-temperature transient in the reactor building was performed.
The analysis was performed using the COMPARE MODE 2 computer code.
The following assumptions and conditions were used in the analysis:
1.
The reactor building was modeled using 9 nodes.
2.
The purge line isolation valves close in 5 seconds.
3.
No heat sinks were modeled.
4.
The total mass release was 4000 pounds (Section Sa.
showed the LCCA mass release to be 2900 pounds of steam and 955 pounds of air).
The analysis showed that the blowout panels on the refueling floor would open.
The Standby Gas Treatment System will not be able to maintain sufficient negative pressure in the Reactor Building as a result of the blowout panels relieving.
TVA is reviewing options available to assure the maintenance of secondary containment. IVA is limiting purge operations at hot conditions until this situation is resolved.
c.
An analysis of the reduction in the containment pressure resulting from the partial loss of containment atmosphere during the accident for ECCS backpressure determination.
Response
The ECCS performance at a BWR is not strongly dependent on containment pressure as is the case with the PWR's.
d.
The allowable leak rates of the purge and vent isolation valves should be specified for the spectrum of design basis pressures and flows against which the valves must close.
Response
The allowable leak rate for the purge valves is determined as required by 10 CFR 50 Appendix J.
Leak rate testing is performed at the containment design basis pressure.
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FIGURE I
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