ML20127M751

Safety Evaluation Accepting Proposed Change 2 to DPR-22 TS Re Gaseous Radwaste Sys Design

ML20127M751
Person / Time
Site:	Monticello
Issue date:	01/14/1972
From:	Knuth D US ATOMIC ENERGY COMMISSION (AEC)
To:
Shared Package
ML20127M748	List: ML20127M745 ML20127M751
References
NUDOCS 9211300459
Download: ML20127M751 (13)

Text

l Docket No. 50-263 i

SAFETY EVALUATION BY Tile DIVISION OF REACTOR LICENSING U.S. ATOMIC ENERGY COMMISSION IN Tile MATTER OF MONTICELLO NUCLEAR GENERATING PLANT E-5979 PROVISIONAL OPERATING LICENSE NO. DPR-22 TECHNICAL SPECIFICATIONS CilANGE NO. 2 l

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l 9211300459 720114 PDR ADOCK 05000263 P PDR ,

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1.0 INTRODUCTION

i By letter dated April 1, 1971, as amended October 15, 1971 and i

December 10, 1971 Northern States Power Company (NSP) submitted Proposed

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Change No. 2 to the Technical Specifications of Provisional Operating l

} License No. DPR-22 for the Monticello Nuclear Generating Plant E-5979.

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! The purpose of the change was to modify the design of the gaseous radwaste

system and to incorporate in the Technical Specifications provisions l

relative to its operation.

} The design criterion for this modification is to reduce the plant 1

l boundary radioactive dose rate to less than the numerical values discussed 1

in proposed Appendix I to 10 CFR Part 50. The proposed modification will

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increase the holdup time of the condenser air ejector non-condensable gases l from 30 minutes to at least 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br />.

l The proposed system consists of redundant hydrogen recombiners and l compressors, and five gas storage tanks which will hold gases at 300 psig.

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. .hese holdup tanks containing compressed radioactive gases were not included

! in the plant design at the time of the review of the FSAR, hence the conse-quences of a posculated accident involving the rupture of these tanks were i

not evaluated dt"ing the Operating License review. On the basis of our l evaluation of Pr, posed Change No. 2 to the Technical Specifications, as modified, we hav; concluded that the proposed system can be built and

-operated without undue risk to the-health and safety of the public.

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i During our review of the proposed change, we informed NSP that.certain changes to the Technical Specifications, relating to operation of the modified i

i system, were necessary. Accordingly, pursuant to Section 50.59 of 10 CFR l,

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j Part 50, the Technical Specifications of Provisional Operating License i

{ No. DPR-22 will be changed as set forth in Attachment A.

2.0 DISCUSSION There are several expected sources of radioactive gases from a boiling I

water reactor. These sources include those.from operation of the condenser j

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-vacuum air ejector, the plant startup condenser mechanical vacuum pump, l

,; steam turbine gland seal, and plant ventilation system. Other sources include effluents from containment-purging, HPCI turbine testing, and leakage

from radioactive waste systems. The major sources of gaseous radioactive _

1 effluents are the non-condensable gases removed from the main condenser by i

the air ejector. These gases consist of air that has leaked into the i i

1 condenser, hydrogen and oxygen produced by the radiolytic decomposition of 1

water, and very small volumes of radioactive gases (primarily xenon and l

krypton). In the treatment system proposed, the gases initially pass _through

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a recombiner system which will recombine the hydrogen and oxygen-to form 4

water which is then returned to the plant. The remaining gases then pass through a gas delay line which provides a delay time of approximately two hours (the equipment originally provided to_ produce a 30-minute delay now I causes a' delay of approximately two hours because most of the hydrogen and l a stochiometric amount of oxygen has been removed from the gas _ stream) to _

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_ permit those isotopes with short half-lives to' decay. The gases are then

-compressed and stored in holdup tanks.

In designing the proposed gaseous radwaste system, the licensee has assumed an air flow rate of 28 sefm and a noble gas source term. equivalent

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to an off-gas release rate of 270,000,uci/see af ter a 30-minute decay time.

The Monticello surface condenser has two shells. Based on Standards for Steam Surf ace Condensers issued by the Heat Exchange Institute (Sixth Edition, 1970) the assumed 28 scfm air inleakage flow rate is a reasonable value for this type condenser. Based on operating BWR reactor dats, we are of the opinion that the assumption of an annual average off-gas release rate of 270,000)J:i/see as a basis for the design of the gaseous radwaste system is a conservative value.

The licensee is proposing to use a recombiner which has three times the calculated amount of catalyst required. Although the particular. catalyst proposed for use has been extensively tested for hydrogen-oxygen recombination ef ficiency and its susceptability to poisoning, the effect of a radiation field on catalysis has not been tested. Since there are other catalysts that have performed satisfactorily in actual BWR atmospheres and are readily avail-able if there are performance deficiencies with the proposed catalyst, a sub-stitution can be made. We have concluded that the proposed recombiner system is acceptable as proposed.

The proposed compressors are designed so as to have ":ero leakage".

Also, all valves used in the system are of the diaphragm type, so that the expected leakage during normal operation will be negligible.

All the equipment of the proposed waste gas system and structures in which it will be housed will be designed to withstand the design basis earth-quake (0.12 g horizontal and 0.08 vertical ground acceleration), the probable maximum flood (elevation 939.2 feet above mean sea level), and the design basis tornado (300 mph rotational and 60 mph translational winds with a 3 psi pressure drop in 3 sec).

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! - 4-i All pressure vessels, heat exchangers, pumps, valves and piping, 4 except for the two compressors and the five storage tanks, will be designed, fabricated and installed according to ASME Code Section III, Class 3, as i amended to July 1971. The compressors and storage tanks were purchased prior to July 1971. The tanks were purchased in conformance with the 1968 ASM? Code Section III, Class C with addenda. . The compressors were purchased

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I in conformance with the USAS B-31.7 Class III piping and ASME Code Section VIII pressure containing vessels with 100 percent radiography per paragraph z

UW-2a. We find the codes and standards used for the equipment and piping 4 appropriate for the intended service.

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l Even though steam is mixed with the gas stream to keep the hydrogen concentration below 4%, hydrogen analyzers will continuously monitor the i

hydrogen concentration upstream as well as downstream of the recombiner

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and instrumentation will cause valves to close automatically to stop the gas l flow whenever the hydrogen concentration exceeds safe limits. Even though i

i the instrumentation is designed to minimize the possibility of detonation.

l the Monticello offgas system will be designed to take a hydrogen explosion

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t up to the suction of the compressors. The detonation pressure from a hydro-gen mixture-is approximately 20 times the initial pressure. To ensure that no potential shock wave could reach the air compressors, the charcoal filter l element which will be installed upstream of the compressors will be designed 4

to withstand the full 350 psig overpressure without failure.

The licensee performed an analysis to determine the shock wave'attenua-J tion in the recombiner-system.- This analysis-shows that as a result of a 4

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i hydrogen-oxygen detonation, the pressure upstream of the compressors would 4

f be attenuated to 30 psig. Wa discussed this evaluation with personnel of a

! the Bureau of Mines, Pittsburgh, Pennsylvania. It is their opinion that

if the detonation starts in the 42-inch delay line, there would be very-little, if any, decrease in shock wave pressure at the suction of the com-l pressors. The rupture of the compressors would result in a dose less than 1

the rupture of the five holdup tanks evaluated in Section 3.2, Accident

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i Analysis of this Safety Evaluation. However, to mitigate the consequences s

l of such an accident, redundant hydrogen analyzers will be required to monitor the concentration of hydrogen in the stream and alarm if the 4

hydrogen concentration reaches 3%. The minimum number of hydrogen analyzers i

required for operation and the frequency of calibration of these instruments l will be covered in the Technical Specifications.

1 To preclude an operator simultaneously opening the fill and discharge f valves of a single tank, which could align the compressor discharge directly with the discharge header to the offgas stack, an electrical interlock will i

be provided, which will consist of a three-position switch. The switch has i three positions (fill, isolate, and discharge) and can be set to only one

mode at a time, so that the system will not energize both the fill and

discharge valves at the same time. To prevent opening of a tank fill valve when the solenoid operated fail-closed discharge valve has stuck open, a second electrical interlock will be provided which will prevent opening of i

i the fill valve unless the closed position limit switch on the discharge valve has been actuated. This will provide additional assurance that both valves

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will not be opened simultaneously. In addition, to reduce the conse-quences of selecting the wrong tank for discharge, a bank vault type of timing device will be provided which will serve as an interlock to prevent opening of a tank discharge valve until the control switch has been in the isolate mode for at least twelve hours. Calculations indicate that twelve hours of holdup af ter a tank has been filled (which would give an additional twelve hours of decay time) reduces the concentration of radioactivity by a factor of 20. We conclude that adequate precautions have been-taken to cope with the potential problem of venting the wrong tank.

3.0 DOSE CALCULATIONS We have calculated the potential doses at the site boundary during normal and accident conditions of the offgas system. We have used realistic assumptions for doses from normal operation, and used our standard conser-vative assumptions to calculate doses from potential accidents.

3.1 Normal Operation According to the FSAR, the worst case location is 950 meters to the south east (winds blowing from the north north west). For routine releases, we calculated the dose contributions from emission from the stack and ground level releases. Except for the turbine gland seal release where we assumed 1.75 minutes of holdup, whereas the licensee assumed three minutes of delay.

we have made the same assumptions as the licensee. We have listed our calculated values with those by the licensee in Table 3.1.1.

Based on our calculations the average dose at the site boundary from gaseous radioactive effluents is 6.5 mrem /yr. We conclude that in normal

TABLE 3.1.1 Comparison of Estimated Annual Fence Post Doses Due to Airborne Releases (in mrem /yr)

Applicant Staff

1. Steam Jet Ejector Offgas 2.4 3.4

2. Containment Purging 0.03 0.03

3. Plant Startup and Shutdown 1.3 0.85 4 Turbine Gland Seal 1.8 1.9

5. HPCI Turbine Testing Negligible Negligible

6. Offgas System Leakage Negligible Negligible

7. Plant Ventilation 0.28 0.35

8. Radwaste Vents Negligible Negligible Total 5.8 6.5 i

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l operation the gaseous releases from Monticello, after the proposed modifi-cations are in operation, will be small when compared to the natural back-ground dose rate of approximately 100 mrem /yr.

3.2 Accident Analysis i

We agree with the licensee that the maximum release to the environs from the proposed offgas system would result if all five storage tanks were assumed to undergo simultaneous discharge at ground level immediately af ter being filled to capacity with the plant operating at the Technica1 ' specifications activity limit (270,000 pCi/see after 30 minutes decay) at the condenser air ejectors and with a condenser air in-leakage of 28 scfm. The calculated dose is based on a tank fill time of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> (the licensee assumed 15.7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />). Release was assumed to occur immediately after filling the fifth tank, with credit taken for decay during the filling operation and for dead storage time in the first four tanks.

Doses were computed using infinite cloud (the licensee assumed finite cloud dimensions), Class F stability, one meter per second wind speed, 500 meters as the distance to the exclusion area boundary, and no credit for storage building wake factor. We calculate a two-hour thyroid dose of.2.4 Rem and a whole body dose'of 2.4 Rem. The calculated doses presented for this accident are well within the guideline values in 10 CFR Part 100.

4.0 CONCLUSION

S On the basis of our evaluation of_the proposed offgas system modifi-cation, we have' concluded that the offgas system can be built and operated at Monticello without undue risk to the health and safety of the public,

and that after the proposed system is in operation, the plant boundary radioactive dose rate will be small when compared to the natural background dose rate. Accordingly, pursuant to Section 50.59 of 10 CFR Part 50, the Technical Specifications of Provisional Operating License No. DPR-22 will be changed as set forth in Attachment A.

b chs M b Donald F. Knuth, Chief Boiling Water Reactor Br. No. 1 Division of Reactor Licensing Date: Jgy 14 $72

4 ATTACHMENT A

' Technical Specifications Changes 1

) to Monticello Nuclear Generating Plant No. DPR-22 i

) to be in effect after the Augmented Off-Can System is installed 2

1. Change Paragraph 3.2.D to read as follows:

"D. Stack and off-cas Systems

1. Radiation Monitors At least one plant stack monitoring system shall be oper-I able at all times. The off-gas monitoring system, including both off-gas radiation monitors shall be operable or operating whenever steam pressure is available to the air ejectors. If these requirements are not satisfied, a normal orderly shutdown i shall be initiated within one hour, and the reactor shall be 1

in the hot shutdown condition within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />.

2. Hydrogen Monitors

a. Except as specified in 3.2.D.2.b below, at least one hydro-gen monitor upstream and two hydrogen monitors downstream of the recombiners shall be operable during power operation,

b. If the above specified required hydrogen monitors are not available, an orderly reduction of power shall be initiated to bring the activity releases within ten percent of the limits of 3.8.A.1.

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2. Change the Off-Gas Isolation portion of Table 4.2.1 to read as follows:

Test (3) Calibration (3) Sensor Check (3)

Instrument Channel Off Gas Isolation Radiation Monitors Notes (1,5) Note 6 Once/ shift 1.

Hydrogen Monitors NA Weekly NA

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l 3. Change Paragraph 3.8.A to read as follows:

"A. Airborne Effluents

1. The release rates of gross beta-gamma activity, except halogens and particulates with half lives longer than eight days, shall not exceed a rate Q, in curies /sec:

Q1 + QRS 4 y

0.27 0.027

2. The release rates of gross beta-gam =a activity, except halogens and particulates with half lives longer than eight days shall not exceed 16 percent of the above 3.8.A.1 averaged over any calendar quarter, i

J 3. The activity of halogens and particulates with half lives greater than 8 days released to the environs as part of the airborne effluents shall not exceed a rate Q in microcuries/sec:

i Q1 + QRS e.

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4. If the limits of 3.8. A.1 or 3.8. A.2 are exceeded, appropriate 1

corrective action such as an orderly reduction of power shall

. be initiated to bring the releases within the-limit.

j 5. If the hydrogen concentration in the off-gas downstream of the recombiners reaches four percent, the off-gas flow shall be stopped automatically by closing the valves up-stream and downstream of the recombiners.

1 i 4. Add to Paragraph 6.6.B the following:

"4. Identify the causes if the activity release rate exceeds i

four percent of the limits of 3.8.A.1 during any 48-hour period, and describe the proposed program of action to

! reduce such release rates. Also report the flow rate of the off-gas system, and the' activity measured upstream i

of the compressor suction and downstream of the holdup tank, at a point upstream of the stack.

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