ML20024G964

Gaseous Radwaste Sys Mod Rept

ML20024G964
Person / Time
Site:	Monticello
Issue date:	03/31/1971
From:	NORTHERN STATES POWER CO.
To:
Shared Package
ML20024G273	List: ML20024G272 ML20024G964 ML20090L474 ML20090L479
References
NUDOCS 9105030427
Download: ML20024G964 (41)

Text

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MSP MONTICEL'LO NL~CLEA A GENERATING PLANT Monticello, Minnesota UNIT I

( USAL C DOCKET t.0 - 263 )

a G ASEOUS RADWASTE SYSTEM MODIFICATION REPORT MARCH 1971

.e NORTHERN STATES POWER COMPANY MINNE APOLIS, MINNESOT A l

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TABLE OF CONTENTS l

l Fage l

1.0

SUMMARY

1

2.0 INTRODUCTION

3 I

3.0 DESIGN BASIS 4

1 4.0 SYSTEM MODITICATION DESCRIPTION 10 4.1 Summary 10 l

l 4.2 Hydrogen Dilution and Recombiner 10 Sub-system t

4.3 Gas Compressor and Stor60e Tank 14 Sub-system 4.4 Instrumentation and Control 14

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4.5 Arrengement, Structures, and 16 Ventilation 4.6 Codes and Standartis 17 5.0 SYSTEM OPERATION 18 6.0 SYSTEM PERTORMANCE 22 7.0 SATETY ANALYSIS 24 7.1 Accident Analysis 24 7.2 flydrogen Handling 25 7.3 Shielding 27

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Proposed Changes to Technical Specifications Appendix 4

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. _ _.. _ ~ _. _ _. _ - - - - -

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s LIST OF TABLES i

n Page 1.

Monticello Offgas Modification Design 13 l

and Operating Parameters j

11.

Effect of Off-gas System Modification 23 on Noble Gas Nuclide Release Rate and Boundary Dose Rate 111.

Worst Case Radiation Release from 26 Accident Resulting in Simultaneous l

Rupture of All Off-Gas Storage Tanks t

LIST OF TIGURES I

1.

Effect of Monticello Off-gas System 6

Modification On Radiation Dose Con-tribution at Plant Boundary 2a.

P&ID off Gas System Modification -

11 Recombiner Sub-system l

2b.

P61D Off Gas System Modification --

12 L

Gas Storage Sub-system l.

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j GASEOUS RADWASTE SYSTEM MODIFICATION REPORT 1.0

SUMMARY

This report describes a plant modification being undertaken by the Northern States Power Company at the Monticello Nuclear Generating Plant for the purpose of reducing the quantities of radioactive gaseous effluents released to the atmosphere at the plant site. The modification consists bas : ally of the addition of equipment to increase the holdup time of the non-condensible gases removed by the niain condenser air ejectors from 30 minutes to a minimum of 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> during normal plant operation ;.

The design objective of this modifcation is to reduce the plant boundary radioactive dose rates to about one percent of the dose rates that would be experienced without the modification. Based on AEC-DRL boundary dose calculations which yield 0.5 Rem / year for an annual average re-lease rate of 0.270 Ci/sec, the boundary dose contribution from the plant operating under design conditions following the modification is about 0.004 Rem / year, corresponding to an annual average release rate of 0.012 Ci/sec. This dose value is less than variations in the yearly dose from natural cosmic and terrestrial sources existing prior to con-t,(,

struction of the plant.

The Technical Specifications for the Monticello Plant permit a stack

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release rate of 0.270 Ci/sec to the atmosphere. This is equivalent i.

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i to an annual release of about 8.53 megacuries. Following accomplish-ment of this modification, and taking into account expected operating 4

conditions it is expected that the average annual release rate will be reduced to 0.005 C1/sec or 0.16 megacuries per year. A continuous release of 0.005 Ci/sec results in a boundary dose contribution of about 0.001 Rem /yr.

Northern States Power Company will maintain and use the equipment de-n:cribed in this report in such a manner as to reduce the release of radio-active materials to the atmosphere to the lowest practicable levels. This,

it is expected, will limit atmospheric releases from the p' ant to an annual aver' age of 0.16 megacuries. At the same time, certain flexibility of operation is required compatible with consideration of health and safety to assare that the public is provided a dependable source of power, even under the unusual operating conditions which may temporarily result in releases higher than those described above, but still well within the limits specified in 10 CTR Part 20.

The proposed changes to the Technical Specifications which would be associated with this modification are contained in the attachment to this report.

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2.0 INTRODUCTION

The gaseous radweste system initially installed in the Monticello plant provides for disposal of potentially radioactive gases through the plant stack, with appropriate monitoring, dilution, and automatic shutof t facilities. There are normally three sources discharged to this stac<:

offgas from the main steam condenser air ejectors, offgas from tb main steam turbine gland seal system, and dilution air from the turbine build-ing ventilation system. During plant startup the condenser mechanical vacuum pump is also discharged to the stack, and during some operating conditions offgas from the llPCI turbine and the SGTS systems may be discharged to the stack, as described in the l'inal Safety Analysis Report, USAEC Docket 50-263. Very small quantities of radioactive gases may also be released with ventilation air from the non-contained portions of the plant, but these releases are monitored and controlled, and are rarely significant when compared to the stack releases.

4 Operating experience with plants similar to the Monticello plant design has shown that about 99% of the radioactive gases discharged come from the main steam condenser air ejectors. These gases are currently delayed for a minimum of 30 minutes prior to their release at Monticello and at other similar plants. The radioactivity of these gases 4 -1d the resultant boundary dose rates can be further reduced by retaining the gases for an additional period of time. This report describes modifications being undertaken voluntarily by Northern States Pcwcr Company at the Monticello

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Plant to retain the radioactive gases from the main steam condenser air ejectors for an additional period of time, so as to reduce the plant bound-ary doses to the lowest practicable levels.

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3.0 DESIGN BASIS The gaseous radwaste system initially installed in the Monticello plant was designed to collect, process, store, monitor and dispose of radio-active gaseous wastes generated in the operation of the plant. The system was designed such that radioactive gases could be discharged without exceeding the annual environs radiation dose rate as set forth in 10 CFR 20.

The modified system as described herein is dealgned to further reduce the gaseous radioactivity release rates to the lowest practicable levels commensurate with the state of technology, the economics as they are related to the degree of public benefit, and the availability and reliability of the process and the equipment for timely incorporation into an operating power plant.

Tour processes for krypton and xenon radioactivity reduction were con-sidered for application to this modification: removal by cryogenic distillation, removal by fluorocarbon absorption, selective retention by enarcoal adsorption, and total gas retention by compressed storage. The first two of these processes were eliminated fro:n further consideration for incorporation into an operating plant based on the state of those techno -

logies, or more specifically, on the absence of operating experience with the proposed equipment in radioactive service of this nature. The two remaining processes for retention of radioactive gases were judged essentially identical with regard to environmental effects, based on equal retention times, with both processes capable of decaying all of the noble 4'

gas isotopes except Krypton 85.

1 4

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The length of retention time required was deternmd by computation of e

the potential dose at the nearest plant boundary under the worst combina-tion of operating and meteorological conditions that will exist for any significant period of time. The computation was based on the Technical Specification limit of 0.27 C1/sec at the air ejectors, measured af ter 30-minute sample decay. The result of this computation, which is for a diff usion mixture, is shown graphically in figure 1.

1he releases from other sources were also examined, but not included in the computation

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because their contributions to the boundary dose were not considered significant, for example, the release rate from the turbine gland seal system was estimated to be 0.1% of the 0.27 C1/sec, or 0.00027 C1/

sec (see FSMO, which would produce a boundary dose rate contribution of about.0005 Rem /yr. The ventilation contribution is expected to be less than this amount. I ntermit te nt, short-term releases from the stand-by gas treatment system and the main steam condenser mechanical vacuum pump (used during plant startup only) could result in short-term releases near the Technical Specification limits, but experience with operating BWR's has indicated that the infrequency of these releases results in insignificant contributions to average boundary doses.

This evaluation revealed that by modification of the air e)cctor offgas system ta obtain a minimum of 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> holdup,the typical dose would be only about 1% of the typical dose from the plant prior to such a modi-fication. This dose would be about 0.004 Rem / year and should occur only if and when the following events occurred simultaneously:

e Maximum fuel clad failure condition at which the plant could be operated sustained for one year.

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plant boundary for one year, it was noted that under th9se extreme conditions the dose received from the plant release would constitute only about 4% of the total dose from natural sources, rurthermore, the dose from the plant release would be almost wholly from Xenon 133, which has a half life of 5.3 days. Neither of the processes for retention can hold Krypton 85 for a period sufticient to reduce its activity appreciably, but the Krypton 85 contributes less than 1/500 of the reduced boundary dor,e from the plant release; thus it does not present a significant local problem.

Based on the foregoing considerations, a minimum retention time of 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> was selected as a design basis for modification of the air ejector offgas system. Modification of the systems handling offgases from sources other than the air ejectors was concluded to be unnecessary because cf the relatively low dose rate contributions from these sources when compared to the air ejector offgas releases or to natural background levels.

Both the charcoal and compressed storage processes were found to require hydrogen-oxygen recombination to eliminate combustion hazards and to reduce component sizes to the most economical ranges. In the final analysis, the compressed gas storage process was selected, based on the following additional considerations:

e Gas storage is a passive process involving methods and equipment which r wpresent a highly developed s t at e-of-t h e-a rt.

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Performance of a compressed gas retention process for radioactive decay can be accurately predicted for all con-ceivable operating conditions and is not subject to significant variation due to component configuration or leakage character-istics, nor is it subject to deterioration with use, Design information, performance characteristics, and e

component reliability data were all freely available from U.S. manufacturers for gas storage system components, o

Positive segregation of the offges in separate tanks in a passive state permits any short-term releases of higher-than-normal radioactivity levels to be selectively decayed for longer periods of time, without unavailability of generating a

capacity, Positive segregation of the offgas in separate tanks provides o

an opportunity, under abnormal operating conditions, for more complete analyses of the gases prior to their release, The compressed gas storage process is the least subject to e

possible degradation from gaseous contaminants that could l

be present in the air leaked into the condenser, i

The compressed gas storage system could be designed and e

installed in less time than was required for the other pro-f' Cesses.

The desion objective for the modification is a reduction of the maximum air ejector offgas release rate from 0.27 Ci/sec to 0.012 Ci/sec, S

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i with a corresponding maximum boundary dose rate reduction from 0.500 Rem / year to 0.004 Rent / year. These rates are based en e !r ejector dis-charge rate of 0.27 Ct/sec (30 minute sample decay) with a condenser in-leakage rate of 28 scf m.

liowever, as stated in Section 1.0, it is expected the annual average release rate will be reduced to 0.005 C1/

sec. corresponding to an off-site dose contribution of 0.001 Rem /yr. - The calculation of these rates and the average quantity of each of the principal radionuclides of the gases expected to be released annually are presented in Section 6.0 of this report.

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4.0 SYSTEM MODIFICAT!ON DESCRIPTION 4.1 Summary The modification to the originally installed offgas system consists of a hydrogen dilution and recombiner sub-system inserted in the 6" offgas line from the condenser air ejectors immediately upstream of the 30 minute holdup pipe, and an underground gas compressor and storage tank sub-system inserted at the outlet to the 30 minute holdup pipe. Figure 2 is a P61D of this modification. The originally installed system, as depicted in the PSAR Figure 9-3-1, P&lD Off-Gas Systom, will be otherwise unchanged except for the electrical signal to the emergency shut-off valves in the off-gas line to the stack, which is discussed below.

4.2 Hydrogen Dilution and Recombiner Sub-Cyste n This sub-system consists of two parallel flow paths for hydrogen dilution and recombination, each capable of operating independently of the other and each capable of handling the condenser combined offgas and vapor design flow rate of 146 scfm (166 cfm at 130 F). The major components of each flow path are a steam Jet eductor, a preheater, a hydrogen-oxygen recombiner, a desuperheater, and a condenser. Pertinent design and operating parameters for the sub-system are given in Table 1.

The steam jet eductor is designed to dilute the hydrogen content of the mixture to a value below the flamma.ae limit. The preheater downstream of the eductor is used to assure that the vapor entering the recombiner is slightly superheated for effective operation of the recombiner, which is i'

a catalyst bed type. The spray desuperheater is designed to bring the vapor temperature, substantially increased by the heat of recombination, 1

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down to saturation to enhance heat transfer in the condenser. The con-denser is designed to remove the heat of recombination and to condense the diluent vapor from the stream. The condensors discharge the offgas into the originally installed underground holdup pipe. This entire sub-system will be operated at sub-atmospheric pressures to prevent out-leakage of radioactive gases.

4.3 Gas Compressor and Storage Tank Sub-System This sub-system consists of two parallel gas compressors and five parallel gas storage tanks, with sampling facilities and a controlled rato discharge station. The gas compressors are multi-stage piston-type with zero out-leakage provisions. Each compressor is capable of handling the system design flow rate, and one compresror is normally maintained in standby. The five tanks are located in a below-grade building near the plant stack. All valves are either hermetically sealed or capped to reduce leakage. The compressors are rated at 30 scfm each at 300 psig discharge pressure. The tanks are designed for 330 psig at -50 to 250 F.

4.4 Instrumentation and Control Each recombiner train is equipped with sufficient remote instrumentation and control equipment to permit remote operation from the reactor control room. Alarms are provided to alert the control room operator to abnormal conditions of hydrogen concentration in the recombiner train inlet and outlet, low recombiner inlet temperature, high or low recombiner outlet temperature, high desuperheater outlet temperature, high condenser off-gas temperature, and high condenser cooling water outlet temperature.

The recombiner will be automatically bypassed back to the main steam l

l 14 l

condenser if the inlet superheat is insufficient. The train will be auto-matically isolated if the hydrogen content at the inlet to the recombiner exceeds 4%, if the hydrogen content at the train outlet exceeds 2%, if the train outlet gas exceeds 200 r, or if the train static pressure exceeds 20 psia.

The gas compressors may be operated in parallel or with either unit in standby. They automatically maintain the pressure in the 30 minute pipe between 10 and 12 psia during normal operation. A signal as sounded in the reactor control room when the pressure of the storage tank being filled approaches 300 psig, and the operator remotely diverts the flow to another tank.

Alarms are sounded in the control room for high and low compressor suction pressure and high discharge pressure or temperature.

Each gas storage tank is remotely monitored for pressure and radioactivity level. Interlocks are provided to prevent simultaneous filling and dis-charge of any tank. Tank discha.rge rate is remotely set from the reactor control room and automatically controlled and recorded. Alarms indicate when drain tanks serving the plant stack and the compressor suction moisture sepalator are full.

The previously installed radiation monitors in the air ejector off-gas header will be modified to close the recombiner train inlet valves. The stack 'lonitors will be modifed to close the stack isolation valves, the gas storage tank discharge header valve (to prevent blowing the loop I'

sealsh and to sound an alarm in the reactor control room. If the Technical t

Specificauan release rate limits are exceeded at either the air ejector or the stack the associated isolation will occur automatically. Alarms will sound in the control room when the high alarra settings are reached by these monitors.

i b

15

l i

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Area radiation monitors will be provided in the new buildings and monitors will also be installed in the ventilation discharge ducts from these build-ings. All radiation monitors will have both local and remote (reactor control room) alarms.

4.5 Arrangement, Structures, and Ventilation The recombiner trains will be located in a new building located near the air ejector room. The building will be divided by a shield wall so that one train can be maintained while the other is operating. The offgas will be brought from the air ejector room and returned there after recombination for entry into the t,xisting 30 minute pipe. Ventilation from the building will be designed to maintain the recombiner building at a slightly negative pressure and will discharge into the condenser space of the turbine building.

Doors to the recombiner building will be kept locked except during main-tenance operations cr plant outages.

The outlet from the underground holdup pipe is diverted to a new gas storage building located near the base of the stack. The storage tank room is located below grade and is separated from the valve room by a radiation shield wall: the valve room is separated from the compressor room by a partition to permit ventilation flow control. The tank storage room will not be accessible during plant operation because of the high radiation levels. Ventilation from the building will be designed to main-tain the storage building at slightly negative pressure, and will discharge to the stack to serve as part of the 4000 cfm dilution flow. Air will be directed from the compressor room, to the valve room, and then to the 1

b 16

l tank room. A flapper valve will protect the discharge duct from over-i pressurization in the event of storage tank ruptures.

4.6 Codes and Standards The gas storage tanks and all piping and components which ers not normally isolated or remotely isolable from the tanks will be designed, fabricated, and installed per ASME Boiler and Pressure Vessel Code,Section III, Class C and USAS B 31.7, Class II. The remaining piping and components will be designed, fabricated, and installed per ASME Boiler and Pressure Vessel Code,Section VIII and USAS B 31.7, Class III.

The recombiner building will be a Class li structure with structural integrity consistent with that of the turbine butiding. The gas storage building will be designed for Class I seismic conditions and tornado loads. Supporting system design will conform to the standards of the previously installed systems. Quality control will be consistent with that employed for design and construction of the previously installed offgas and supporting systems.

The prime contractor for system design is Suntac Nuclear Corporation, 1528 Walnut Street, Philadelphia, Pennsylvania.

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5.0 SYSTEM OPERATION The hydrogen dilution and recombiner sub-system will receive non-condensible offgasos from the main condenser air ejectors at a maximum design flow rate of 146 scfm per train, but at a reduced pressure of about 12 psia. A steam Jet eductor will immediately dilute the offgas with about 6500 lb/hr of steam. The eductor will exhaust the gas-vapor mixture at near saturation temperature and about 14 psie, with a maximum hydrogen concentration of 4% by volume. The vapor will be heated to a few degrees superheat in the preheater and delivered to a recombiner. The recombiner will reduce the dry hydrogen content to a nominal 0.1% (maximum 2%),

while heating the mixture to a maximum temperature of 900 P.

The temperature will subsequently be reduced to near saturation by a condon-sate flow addition of about 2000 lb/hr in a desuperheater. This mixture will then enter a condenser where the majority of the vapor will be removed, leaving a maximum design gas flow of about 28 scfm at 12.5 psia and 200 P.

The other dilution and recombiner train will be in hot standby condition with reduced steam flow.

Should condenser in-leakage decrease to less than 5 scfm, instrument airwill be bled manually into the offgas stream upstream of the recombiners to assure a sufficient supply of oxygen for complete recombination and to prevent excessive concentration of the radioactive gases following recombination.

The 28 scfm offgas !!ow will then enter the previously installed 30 minute holdup pipe, but due to the decreased design flow resulting from H -0 2

2 recombination, the pipe holdup time will be increased to about 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> (about 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> with condenser air in-leakage at 10 scfm). Following this holdup, the offgas is compressed to 300 psig by the operating compressor i

and delivered to'one of the five 1250 ft holdup tanks.

i 18 e

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Under design operating conditions, each tank will undergo a minimum charging period of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> (5 to 300 psig) followed by 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> of dead

.atorage, and will then be released with approximately 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of dis-cht ge time. This yicids a minimum mecn holdup time of 40 hours4.62963e-4 days <br />0.0111 hours <br />6.613757e-5 weeks <br />1.522e-5 months <br /> in the h

tariks, plus over two hours in the holdup pipe. This holdup time will be

.ncreases' to abc ut 140 and 280 hours0.00324 days <br />0.0778 hours <br />4.62963e-4 weeks <br />1.0654e-4 months <br /> for condenser in-leakage rates of 30 cv'

, r -tm, respectively.

yk.

U Jer actual operating conditions, the tank with the lowest activity will be discm.rged, and the discharge flow rate will be selected to be only

. lig: *1y greater than the rate of of fgas flow af ter recombination. The dis-charge will be initiated from the reactor control room by opening the ap-aropriat9 tank discharge solenoid valve, followed by remote adjustment of the motor-operated thrcttle valve to achieve the desired discharge flow rate. A pressure reducing valve in the discharge line maintains a constant p

pressure through the flow ra'c sensor as. 4 an orifice limits the maximum discharge flow rate to 150 scfm in the event of reducing valve failure, f

l The charging solenoid valves are also opened from the reactor control room, following a signal indicating that the previous tank is full; charged.

An electrical interlock prevents simultaneous opening of both the charging and discharge valve on the same tank. Discharge flow can be terminated from the reactor control room by closing the tank solenoid valve or by tripping the motor operated throttle valve.

[

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I 19 8

Startup of the mositw udgas system from cold conditions requirer, local operator action. The main steam condenser is evacuated to about 5" Hg absolute with the mechanical vacuum pump. As soon as reactor steam pressure reaches 300 psig, the piping and components upstream of the recombiners are heated by bypassing the diluent steam back to the con-densers. Condensate flow is then established to the desuperheaters and the recombiner condensers, and the recombiner bypasses ere closed and flow is established through both recombiner trains. The compressor i M storage tank bypass valve at the outlet to the 30-minute holdup pl ' 's opened directly to the stack filters and the stack. The.

_...conden steam jet air ejectors are then brought on the line and the suction lines to the eductors are opened. The main st:am condenser vacuum is then pulled down to the operating level and as soon as the offgas flow drops to below 50 scfm at the recombiner outlet, the bypass around the compressors and storage tanks is closed and both compressors are started. The re-actor power can then be increased to significant power levels. The recombiner system should reach normal operating vacuum in less than one hour after the bypass valve is closed. There is no opportunity for out-leakage past the bypass valve to the stack during reactor power operation because of the sub-atmospheric pressure in the 30 minute hold-up pipe, which will be achieved in less time than would be required for flow of radioactive gases through the holdup pipe following closure of the bypass ualve.

Bypass of the compressors and storage tanks will not be permitted until the 30 minute delay pipe has been purged with at least one volume of air and a sample taken at the compressor outlet header and shown to contain less than 0.010 Ci/ft This is to prevent the gases remainirg in the 30 mi.iute pipe from being pushed out by the air ejector startup (initial 20

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discharge rate of about 400 scim) at a release rate in excess of 0.27 C1/

Failure to observe this precedural limitation would result in closure sec.

of the stack isolation valves by the stack radiation monitoring system if the limits of the Technical Specifications were exceeded.

Ofigas sampling will be performed at the air ejectors in accordance with the Technical Specifications. The quantities of the isotopes released will be recorded based on the average storage time of each tank and the most recent isotopic analysis.

21 v.

y

r 6.0 SYSTEM PERFORMANCE The modified offgas system to designed to provide holdup of the offgas from the condenser air ejectors for between 50 and 280 hours0.00324 days <br />0.0778 hours <br />4.62963e-4 weeks <br />1.0654e-4 months <br />, depending upon the quantity of condenser air in-leakage experienced. Eigure 1 show s the site boundary dose reduction as a function of holdup time.

The quantities of activation gases released from the modified system are a

insufficient to affect total release rates or the boundary dose rates.

Based on an offgas release rate at the air ejector discharge which would produce the Technical Specification limit of 0,27 C1/sec after 30 minute delay and a diffusion mixture, the activity discharged to the stack will be 0.012 C1/src at the maxir.um condenses air in-leakage of 28 scfm. It is expected the annual average stack release rate will not exceed 0.005 Ci/sec as both the air ejector release rate and the air in-leakage should

(

be less than the aferementioned values. The estimated noble gas nuclide release rates are presented in Table II for both the originally inctalled and the modified oftgas systems. It le also expected that the majority of the gaseous lodides and tritium will be removed by the recombiner, but there are insufficient data available for an accurate prediction of this removal r-efficiency.

Northern States Power Company will maintain and use the equipment de-scribed in this report in such a manner as to reduce the release of radio-active materials to the atmosphere to the lowest practicable levels. This, 7,

It is expected, willlimit atmospheric releases from the plant to an aver-t.

age annual release of 0.16 megacuries. At the same time, certain flexibility of operation is required compatible with consideration of health and safety to assure that the public is provided a dependable source of power, even under the unus Jal operating conditions which t

may temporarily result in releases higher than those described above, but still well within the limits specified in 10 CFR part 20.

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7.0 SAPLTY ANALYSES 4

7.1 Accident Analysis n

The maximum release to the environs from the modified offgas system would result if all five storage tanks were assumed to undergo instanton-4 cous and simultaneous discharge at ground level immediately after being filled to capacity with the plant operating at the Technical Specification annual average activity limit (0.270 C1/sec af ter 30 minute sample decay) at the condenser air ejectors and with maximum condenser air in-leakage (28 scfm). The calculated dose at the nearest boundary for thic instantan-cous release is 0.802 rem, which is well within the guideline of 10 CPR 100.

.f The calculated doce is ba:,ed on a tank fill time of 15.7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> (0 to 300 psig) with 28 scfm air in-leakage at the condenser and complete recombina-

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tion of the radiolytic hydrogen. This dose was found to be higher than that resulting from longer holdup time (lower condenser in-leakage), be-cause ci the greater dose contribution from the shorter lived isotopes.

Release was assumed to occur immediately after filling the !!f th tank,

-fj, with credit taken for 1.78 hours9.027778e-4 days <br />0.0217 hours <br />1.289683e-4 weeks <br />2.9679e-5 months <br /> decay 'in the' previously installed holdup pipe, for decay during the filling operation, and for deaci storage time in i

the first four tanks.

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Dose was computed by the equation:

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Curies E,f (mev y 4 B)

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7.2 Hydrogen Handling The waste gas system handlec potentially flammable mixtures of hydrogen, oxygen, and air up to the eductor of the modified system, where the mixture is diluted with steam to less than 4% hydrogen concentration by volume.

Although ignition of the mixture where it is above 4% hydrogen concentra-tion is highly improbable and has not been experienced in operating BWR's, the shock wave from such a detonation could conceivably travel through the recombiner and underground holdup pipe up to the compressor suction.

Although such a shock wave would be considerably attenuated before reaching most parts of the modified system, all new components up-stream of the compressors will be designed and tested to withstand the full force (20 atmospheres) of such a detonation.

Under normal operating conditions, the hydrogen concentration at the condenser outlet following recombination will be less than 0.1% hydrogen by volume. During transient conditions when the inlet concentration of hydrogen is very low and the air flow is simultaneously low, the recom-biner will not function as efficiently and there will be less dilution air following condensation of the steam. Under these conditions, the hydrogen concentration at the condenser outlet may rise to 0.5%. This is significantly below the flammable limit, however, and the flammable limit rises as the mixture is compressed.

'1 25

a TABLE III WORST CASE RADIOACTIVITY RELEASE FROM ACCIDENT RESULTING IN SIMULTANEOUS RUPTURE OF ALL OFF-GAS STORAGE TANKS (28 scfm condenser air in-leakage and 0.27 C1/sec after 30 minute) i Boundary Dose Tank Inventory

~ Tank No.

1 2

3 4

5 Total 1

Total C1 C1 C1 C1 Ci Ci rem /yr remhr Kr-89 1.15 E-08 1.15E-08 Kr-87

1. 41 E+ 02 3.07E-02 6.66E-06 1.41 E+ 02 0.086 0.086 Kr-8 3m 4.12 E+01 1.26E-01 3.83E-04 4.13E+ 01 Kr-88

4. 84 E+0 2

1. 01 E+ 01 2.11 E-01 4.40E-03 9.19 E-05

4. 9 4 E+ 02 0.259 0.264 w

Kr-8 5 m

2. 69 E+02

2. 29 E+ 01

1. 95 E+00 1.66E-01 1.41E-02 2.94 E+ 02 0.026 0.028 r

en K r-8 5 1.15 E+ 00 1.15E+00 1.15 E+00 1.15E+00 1.15 E+00

5. 75 E+00 Xe-137 6.11 E-07 6.11E-07 Xe-138

4. 59 E+00 1.13E-16 4.59E+00 0.001 0.001 Xe-135m 7.98E-01 1.24E-19 7.98E-01 Xe-135

1. 49 E+ 0 3

4. 60 E+ 02

1. 42 E+02

4. 39 E+ 01

1. 35 E+ 01 2.15E+03 0.201 0.291 Xe-133m

2. 54 E+ 01 2.08F+01

1. 71 E+01

1. 41 E+ 01 1.16E+01 8.90E+01 0.001 0.004 Xe-133

7. 28 E+02

6. 70 E+02 6.17 E+02

5. 67 E+ 02

5. 22 E+ 02 3.10 E+ 0 3 0.030 0.128 Xe-131m

2. 25 E+00 2.17 E+ 00 2.09 E+ 00 2.02 E+ 00

1. 95 E+00

1. 0 5 E+ 01 l

6. 3 3 E+ 0 3 0.604 0.802 Sum r

i Formula: rem /yr = 0.246 1

(Ci) (E Y+

0) eff

M%

4 Redundant hydrogen analyzers are provided in the outlet from the condensers to assure that the recombiners are operating satisfactorily. These monitors are set to alarm in the reactor control room if the hydrogen con-centration reaches 1% by volume. Shutdown of the operating recombiner train is required if the concentration reaches 2%. Other alarmed instru-ments, such as the recombiner outlet temperature, will also indicate recombiner failure as a backup to the analyzers.

7.3 Shielding The storage room of the new waste gas storage building will not be accessible when any of the tanks are pressurized. The shield wall between the storage room and the compressor room will be designed to maintain less than 5.0 mrem /hr within two feet of the compressor room side of the wall. The building will be underground with sufficient shield-t ing or area access restriction to assure personnel protection.

The new recombiner building will be designed to limit radiation levels on contact with outside walls to a maximum of 2 mrem /hr. An inside shield wall between the trains will permit limited occupancy for mainten-ance of one train while the other train is operating.

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PROPOSED CHANGES TO TECH.

JAL SPECII'ICATIONS Attachment to Gaseous Radwaste System Modification Report Monticello Nuclear Generating Plant Unit No.1 i

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Northern States Power Company Minneapolis, Minnesota

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3.0 LIMITING CONDITIONS FOR OPERATION 4.0 SURVEILLANCE REQUIREMENTS B.

Emergency Core Cooling Subsystems Actuation When irradiated fuel is in the reactor vessel and the reactor water tempera-ture is above 2120F, the limiting con-ditions for operation for the instrumenta-tion which initiates the emergency core c.>oling subsystems are given in Table 3.2.2.

C.

Control Rod Block Actuation The limiting conditions of operation for the instrumentation that initiates control rod block are given in Table 3.2. 3.

D.

Off-Gas System At least one stack radiation monitor shall be operating at any time off-gas is being discharged from the plant from sources other than the ventilation system. The trip settings for the stack monitors shall be set at a value not to exceed the instantaneous value associated with the 15-minute re-lease limit specified in Specifica-tion 3. 8.1.

3.2./4.2-2 48

3.0 LIMITING CONr. IONS FOR OPERATION 4.0 SURVEILLANCE REQUIREMENTS D.

Off-Gas System (corf t)

Both off-gas radiation monitors in the common discharge line from the con-denser air ejectors shall be operable or operating during power operations.

The trip settings for the monitors shall be set at a value not to exceed the 15 minute release limit specified in Speci-fication 3.8. A.1.

The time delay set-

+1ng for closure of the off-gas inlet valves to the recombiner subsystem shall not exceed 15 minutes.

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3.2/4.2-2.1 48a

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Table 4.2.1 - Continued Minimum Test and Calibration Frequency For Core Cooling Rod Block and Isolation Instrumentation Instrument Channel Test (3)

Calibration (3)

Sensot Check (3) 3.

Steam Line Low Pressure Note 1 Once/3 months None 4.

Steam Line High Radiation Once/ week (5)

Note 6 Once/ shift HPCI ISOLATION 1.

Steam Line Hich Flow Note 1 Once/3 months None 2.

Steam Line High Temperature Note 1 Once/3 months None RCIC ISOLATION l.

Steam Line High Flow Note 1 Once/3 months None 2.

Steam Line High Temperature Note 1 Once/3 months None REACTOR BUILDING VENTILIATION 1.

Radiation Monitorr (Plenum)

Note 1 Once/3 months Once/ shift 2.

Radiation Monitors (Refueling Floor)

Note 1 Once/3 months (4)

OFF GAS ISOLATION 1.

Radiation Moni? ors Notes (1,5,6)

Once/3 months Once/ shift Notes:

Initially once per month until exposure hours (M as defined on Figures 4.1.1) is 2.0 x 10, thereafter (1) according to Figure 4.1.1, with an interval not greater than three months.

62 3.2/4.2-16 1

.a 7-Bases Continueti:

For effective emergency core cooling for the small pipe break the HPCI or Automatic Pressure Relief system must function since for these breaks, reactor pressure does not decrease rapidly enough to allow 3.2 The arrangement of the tripping contacts is such either core spray or LPCI to operate in ti:ne.

The trip settings given as to provide this function when necessary and minimize spurious operation. Reference Section 6.2.4 and in the specification are adequate to assure the above criteria is met.

The specification preserves the effectiveness of the system during periods of main-

6. 2. 6 FSAR.

tenance, testing, or calibration, and also minimizes the risk of inadvertent operation; i.e.,

only one instrument channel out of service.

i Two air ejector offgas monitors are provided and when their trip point is reached the flow to the recombiners is terminated, resulting in subsequent reactor scram due to loss of condenser vacuum.

Two stack monitors of the plant at offgas rates in excess of plant design.

This precludes operation This are provided and when their trip point is reached, discharge of offgas to the stack is terminated.

ff-precludes discharges in excess of the limits of Specification 3.8 due to high flow rates from the o In both gas storage system or from the 30-minute pipe when the storage system is bypassed.

locations, isolation is initiated on two high

• rips, two low trips, or one high and one low trip.

Four radiation monitors are provided which initiate isolation of the reactor building and operation w

1 The monitors c.re located in the reactor building and ventila-of the standby gas treatment system.

Any one upscae trip will cause the desired action. Trip tion plenum and on the refueling floor.

settings of 26 mr/hr for the monitors in the ventilation duct are based upon initiating normal ventilation isolation and standby gas treatment system operation so as not to exceed a dose rate of Trip five percent of the dose rate allowed by 10 CFR 20 at the most restrictive site boundary.

settings of 100 mr/hr for the monitors on the refueling floor are based upon initiating normal v2ntilation isolation and standby gas treatment system operation so that none of the activity released during the refueling accident leaves the reactor building via the normal ventilation stack but that all the activity is processed by the standby gas treatment system.

Although the operator will set the set points within the trip settings specified on Tables 3.2.1, I

l 3.2.2, 3.2.3, and 3.2.4, the actual values of the various set points can differ appraciably from The deviations could be caused by inherent instru-the value the operator is attempting to set.

Therefore, these deviations have ment error, operator setting error, drift of the set point. etc.

been accounted for in the various transient analyses and the actual trip settings may vary by the following amounts.

3.2/4.2-22 68

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Bases Continued:

The most likely case would be to stipulate that one channel be bypassed, tested, and restored, 4.2 This is and then immediately following the second channel be bypassed, tested, and restored.

Note that there is no true minimum.The curve does have a definite knee and shown by Curve No. 4.

very little reduction in system anavailability is achieved by testing at a shorter interval than com-puted by the equation for a single channel.

That is, if the The best test procedure of all those examined is to perfectly stagger the tests.This is shown in test interval is four months, test one or the other channel every two months.

The difference between Cases 4 and 5 is negligible. There may be other arguments, Curve No. 5.

however, that more strongly support the perfectly staggered tests, including reductions in human error.

The conclusions to be drawn are these:

A 1 out of n system may be treated the same as a single channel in terms of choosing a 1.

test interval; and More than one channel should not bu bypassed for testing at any one time.

2.

The radiation monitors in the ventilation plenum and on the refueling floor which initiate i

building isolation and standby gas treatment cperation are arranged in two 1 out of 2 logic systems.

n The bases given above for the rod blocks applies here also and were used to arrive at the functional testing frequency.

The air ejector and stack offgas radiation monitor channel logic is so arranged that a closure of the off-gas line or the stack isolation valves, respectively, is initiated by two upscale, two downscale, Based on experience at other nuclear power plants or one upscale and one downscale trip signals.

with instruments of similar design, a testing interval of once every three months has been found to However, for additional margin a test interval of once per month will be used initially be adequate.

until a tiend is established and thereafter according to rigure 4.1.1 (see Section 3.1/4.1) with an interval not greater than three months.

The automatic pressure relief instrumentation can be considered to be a 1 out of 2 logic system and the discussion above also applies.

73 3.2/4.2-27 i

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3.0 LIMITING CONDITIONS FOR OPERATION 4.0 SURVEILLANCE REQUIREMENTS 1.

The annual average release rates of gross 1.

Station records of gross stack release beta-gamma activity, except halogens and rate of gaseous activity shall be particulates with half lives longer than maintained on an hourly basis to assure eight days, shall not exceed:

t that the specified rates are not being Average Annual Rate (Q in curies /sec):

exceeded, and to yield information concerning general integrity of the Q1 QRS fuel cladding. Records of isotopic 0.27 0.021 analysis shall be maintained. The off-gas stack and reactor building Any one fiften minute period per hour monitoring systems shall be func-(Q in curies /sec):

tionally tested and calibrated in accordance with Specification 4.2, Q1 QRS Table 4.2.1.

+

2.7 0.21 Y

Within one month of initial commercial In addition to the above limits, the service of the unit, an isotopic analy-effluent rate at the air ejector moni-sis will be made of the gaseous acti-tors, except halogens and particulates vity release rate. From this sample with half lives longer than eight days, a ratio of long-lived and short-lived shall not exceed an annual average activity will be established. Weekly rate of 0.27 Ci/sec or an instantan-samples of off-gas will be taken and cous rate of 2.7 Ci/sec for more than gross ratio of long-lived and short-15 minute per hour, based on 30 lived activ.ty determined. When the minute sample decay.

weekly samples indicate a change of greater than 20% from the previous isotopic analysis, a new isotopic j

analysis will be performed. An iso-topic analysis of off-gas will be per-formed at least quarterly. Gaseous release of tritium shall be calculated

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3.8/4.8-2 169

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.ua Release of noble gas radioisotopes will be calculated from the isotopic samples taken at the air ejectors, based on the recorded decay times p;ior to.elease of the gas to the atmos phere.

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169a 3.8/4.8-2.1

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4.0 SURVEILLANCE REQUIREMENTS 3.0 LIMITING CONDITIONS FOR OPERATION 3.

The performance and results of independent 3.

Two independent samples of each tank samples and valve checks shall be logged.

shall be taken and tinalyzed for gross beta-gamma activity and the valve t

line-up checked prior to discharge D.

Radioactive Waste Storage of liquid effluents.

(1)

A sampla from each of the Waste Sample, floor f

4.

If the limits of 3.8.C cannot be met, Drain, Condensate Storage and Waste Surge radioactive liquid effluents shall not Tanks shall be taken, analyzed and recorded be released.

every 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. If no additions to one of the above tanks has occurred since the last sample, D.

Radioactive Waste Storage that tank need not be sampled until the next addition.

The maximum amount of radioactivity in liquid storage ir. the Waste Sample Tanks, Floor Drain Sample Tanks, Waste Surge Tanks and the Condensate storage tanks shall not exceed 2 curies. If this con-m dition cannot be met, the liquids in these tanks shall be recycled to tanks within the radwaste facility until the specification is met.

E.

General The releases of radioactive materialin all effluents will be kept at small g

fractions of the limits specified in 20.106 of 10 CFR Part 20. Radioactive effluents in gaseous releases will be maintained below the design objective of an annual average rate of 173 3.8/4.8-6

i 4.0 SURVEILLANCE REQUIREMENTS 3.0 LIMITING CONDITIONS FOR OPERATION E.

General E.

General (cont.)

12,100 p C1/sec, except during periods of Operating procedures shall be developed and abnormal or emergency operation of the plant' used, and equipment which has been ins.alled to maintain control over radioactive materials when such releases may approach the limits of 3. 8.

At the same time the licensee is in gaseous and liquid effluents produced during normal reactor operations, including expected permitted the flexibility of operation, com-patible with considerations of health and operational occurrences, shall be maintained and used, to keep levels of radioactive safety, to assure that the public is pro-material in effluents released to unrestricted vided a dependable source of power even under unusual operating conditions which areas as low as practicable, may temporarily result.

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173a 3.8/4.8-6a

7 (3)

Percentage of the maximum annual !?mit released and MPC value.

(4)

Results of all isotopic analyses and estimates total curies of each identified nuclide released.

(5)

Such other information as may be required by the Commission to estimate maximum potential annual radiation doses to the public resulting from effluent releases.

(6)

Specific information will be reported if quantities of radioactive materials released during the reporting period are significantly above design objectives.

g.

Solid Radioactive Waste (1)

Total volurr, (in cubic feet) of solid waste generated.

(2)

Gross curie activity involved.

(3)

Dates and disposition of the materials if shipped off-site.

h.

Environmental Monitoring l'

(1)

A narrative summary, including correlation with effluent releases of the 5

results of off-site environmental surveys perfonred during the report period.

(2)

Tabulation of the results of the environmental monitoring program, including a figure showing location of the monitoring stations.

(3)

For any Samples which indicate statistically significant levels of radioactivity

.tbove established background levels, a comparison with applicable 10 CFR 20 limits shall be provided.

E.

Special Reports (in writing to the Director, Division of Reactor Licensing, USAEC, Washington D. C. 20545):

1.

In the event a redundant componer t (or system) covered by these Technical Specifications is determined to be out of service for periods longer than those specified in otMr sections, it shall be the subject of a special maintenance report. This report shall be submitted within seven days of the above determination and shall describe:

a.

The nature of the problem and the specific steps to be taken to remedy the situation b.

An estimate of the time required to return the component (or system) to an operable condition.

208