IA-97-455, Submits Bases for Plant TSs on Control of Explosive Gas Mixtures in Waste Gas Sys in Order to Aid W/Solution of Related Problems Elsewhere

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Submits Bases for Plant TSs on Control of Explosive Gas Mixtures in Waste Gas Sys in Order to Aid W/Solution of Related Problems Elsewhere
ML20198M060
Person / Time
Site: Three Mile Island Constellation icon.png
Issue date: 07/22/1981
From: Boegli J, Willis C
Office of Nuclear Reactor Regulation
To: Gammill W
Office of Nuclear Reactor Regulation
Shared Package
ML20198L994 List:
References
FOIA-97-455 NUDOCS 9801160180
Download: ML20198M060 (9)
v • d • e


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.f ',.h S KHORANDUM FOR: William P. Camill, Chief -

6' Ju . W 1981 * 'S .

Effluent Treatment Systems Branch, 051 .." ,'qi 0 2

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FROMt. Charles A. Willis, Leader, Sectier. B Effluent Treatment Systens Branch, 0$1 f /

Jacques 5. Boeg11, Section 8

[ Effluent Treatment Systems Branch, OSI SUBJECT TECH $PEC ON CONTROL OF EXPtfS!VE CAS MIXTURES IN THE WA$TE GAS $YSTEM AT THI-1 In attempting to update (to current requirenents) the_ Tech Spec of THI-1, a problem was encountered in the control of explosive gas mixtures in the wast?

gas system (WGS). Af ter some effort, a position was found which was accept-able to the utility and which seems to provide as much protection against a l H2 -- 02 explosion as would the provisions of the model RETS (NUREG-0472).

The bases for the T'11-1 Tech Spec are reported here because they nav help with the solution of related problems,elsewhere.

Insufficient H2 -- 02 Monitoring The basic problem at TN!-1 was insufficient monitoring equipment. Their nonitoring system consists of a single analyzer for H2 and another for Op.

Samples are taken sequentially from 10 locations as indicated in Figure I.

While each point is monitored every-20 ninutes, the monitoring is not actually continuous. Neither is it dual and redundant, so it does not meet m

the requirements of the SRP 11-3. Thus, the model RETS requirements do not fit the situation and become quite burdensome.

Other Safety Features Incorporated in the Desinn

We conclude that the system could be deemed to meet the intent of the NRC

< requirements only if the design includes safety features which are not re-

. quired. Thus, considerable effort was expended to gain a detailed under-standing of.the system and its operation.

A principal unrequired safety feature is that both 112 and 02 are controlled.

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Dilution by N2 is provided to keep the concentrations below 31112 and 0.1% 02 .

Thus, neither a failure which allows excess 112 nor a failure which allows g)!/

  • excess oxygen will result in a combustable mixture (Fiqure 2). }/

Furthemore, special provisions are made to limit the probability of oxygen $8 ' 1. g' entering the system. First, the compressors are equipped with water seals and operated under a N2 blanket so a leak would not bring air into the g . T,/

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-o W1111am P. Caus111- 22 g system. Second . the system is maintained at a potitive pressure,15.5 to l 16.4 psia so, ag,ain, a leak would not mean air inleakage. Finally, the oxygen l content of waste gases is limiteds.even the gas from the miscellaneous waste  ;

L evaporator and the miscellaneous waste tank cover gas are held to 0.1% 02 ' l

consequently, a high concentration of oxygen is highly unlikely. l The relatively large volume of the waste gas system also contributes-to j i= i

-of safety.

11,050 - Thi5 ft each.Even volume with is all provided 3 bleedbytanks the three 'f::11*-ofreactor coolant liquid, 3000bleed f t of tanks,3 ,

- gas space rereins. The header, pipingI Misc. Waste Tank gas space, and waste {;

gas delay tank provide another- jo00 ft . The three waste gas decay tanks

! each have a capacity of 1150 ft et 85 psig (7800 scf). The large volume  !

' limits the rate at which concentrations can change.  ;

Evidently _ the system did contain unrequired features so the task became one- l of finding-approoriste Tech Specs.  ;

Limit Both H -- 2 and 02 Concentrations t

p The first step was to limit both constituents with the following liniting  ;

i conditten for operation * -  !

'The. concentrations in the waste gas system shall be limited l to less than or equal to 21 oxygen by volume and 41 hydrogen  ;

by vol me." ,

A principal value of this requirement is that it ensures retention of the important safety feature now provided by procedures (which are subject to l

- change without notice). Also it provides an acceptable point of departure t for safety analysis. No single failure produces unacceptable conditions.

This requirement also has the virtue of being essentially ' free". It only  !

requires something less than is already being done. .

Action Required for Inoperable Monitoring Equipment  ;

The model RETS requirements for action in response to loss of monitoring  !

capability are relatively severe, including shutdown if the equipment is

- 1 ' corresponds to NUREG-0472 LC0 3.11.2.5.

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V1111am P. Canmill not returned to operation in a timely r.anner. The NRC position (SRP 11-3) if that the probability of a forted shutdown should be ninimited by providing

  • dua) monitors but many OR$, including THI-1, do not have dual monitors.

t.icensees find these requirments objectionable and contend that more appro-priate actions can provide equivalent Safety.

For this specific system it was concluded that less onerous requirenents could

. provide equivalent safety. First, it wcs recognized that only 2 precautionary measures are available if monitoring capablitty is lost, vir.:

1. grab samples, and
2. dilution Second, since the waste gas system encounters very dif ferent conditions in the dif ferent operational phases (startup, steady t,peration and shutdown), it is reasonable to establish dif ferent action requirements for the different con-ditions.

During nomal operations additions to the waste gas systens are relatively small.

t.etdown to the bleed tarts will be the principal source and even r.;ar the end of core 11fe bleed will not exceed 4000 gpd. At most3 this could add only 21.4 SCF/ day of 112 With minimum gas *.'olume (9000 f t ) and this noxinum rate of hydrogen introduction, it would take 4.4 days to increase the average hydrogen concentration one percent. Since a 1% increase would bring the con-centration to 4% or less, no potential hazard is created. Thus the action iten* requires taking and analyzing greib samples every 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and, if necessary, diluting to 3% 112 and 0.1% 02 '

This assures that flamable concentrations will not be reacted durin;l nomal operations even if the monitoring system is inoperable.

During startup the bleed rate is nuch greater than during non al operation, pririartly because of deboration. With the nsxinun biced rate (45 qpn) and a mintrum gas volume,112 concentration can be increased it in only 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />.

Accordingly, the action item requires sariple analysis prior to startup and every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> during startup wtth dilution requirec any time the concentra-tions exceed 3% 112 or 2% 02 '

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- This provides safety during startup, even with the H2 and 02 monitors increrable.

The most dif ficult operation for H2 control is degassing (af ter shutdown).

The purpose of this operation is to remove gases,' including hydrogen, from the coolant. Therefore, a large quantity of hydrogen is deliberately added to the waste gas system. The coolant can contain up to 500 $CF of dissolved hydrogen and there can be another 400 $CF in ttM makeup tank. Furthennore, much of the hydrogen from the enkeu,s tank is cdded to the waste gas system quite soon af ter the isolation valve is opened. To assure safety of this operation, before de-gassing, a grab sempie is analyzed and (* necessary, dilution is provided to ensure thdt the concentrations are less than 1% 112 and 2% 20 . Then during degassing at least 4 tires as much nitrogen as waste gas is added to the system.

Of course, this degree of' dilution may nos keep the H concentration below the Tech Spec limit, but it does ensure a mixture of H2 e 2 and air cannot be flamable if the 112 and the air comprise only 20% of he volume. To ensure

t. hat the hydrogen concentration does not remain high and assure that dilution is ef fective, grab samples are to be analyzed every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

These requirements er.wre safety during degassing, even if the monitoring syster.1 is inoperable.

Finally, it is recognized that the monitoring equipment should not be out of service for an extended period. In principle, reliance on e.dministrative controls is not preferred practice. In the model P. CTS 'this principal is reflected in an action" requirement that the plant be shut down if the monitors are not made operable in 30 days. Since a shutdown is likely to cost a million dollars or more, the shutdown requirement provides a strong incehtive to repair the instruments (guickly. On the other hand, licensees contend that (1) a forced shutdown is an excessive penalty for inability tn repair an instrteent that is not safety related, and (2) shutdown is not an appropriate response to loss of waste gas system monitoring instruments l because shutdown tends to increase the addition of waste pas to the system.

Therefore, an al'. rnate requirement was imposed. The THI-1 Vice President (or his designate) must call the l*C and explain the delay any time the weste gas H2 -- 02 monitor is out of service for more than 7 days.

l Thus a provision is included to pronote timely repair of the instrumentation.

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William P. Gamill Conclusion For the specific waste gas systees of TM!-1 the new Tech Spec provision  !

(Enclosure 1) provides the same degree of protection against waste gas  ;

explosions as do the provisions of the mJdel RETS. Therefore, these Tech Specs are adjudged to neet the intent of standard NRC requirements in this area.

Original signed by:

Charles A. Willis, Leader, Section B Ef fluent Treatment Systems Branch O! vision of Systems Integration CHg!bal signed by Jacques S. Boegli, Section R Ef fluent Treatment Systems Dranch  !

Division of Systems Integration Enclosure As noted cct R. Hattson '

W. Kreger .

I F. Congel '

O. Ottanni J. Buchanan R. Bangart CTSB Staff DISidlBUTION:

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, Central File ETSB Reading File ETSB Subject File 5.4 CAWillis JSBoegli i

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- Assumed petonable 10 90 tiaits .

20 -go 30 70 40 / c 60 .

50 50 0xygen,v/o 60, - 40 70 t 30 btr Mtut 20

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80 90 100 40 50 60 70 0 10 20 30 11ydrogen, v/o 9

FICURE 2. FIAMMABILITT LIMITS OF ;IYDROGEN-0XYCEN-HITROGEN HIXTURES

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TM!-1 TECH $PEC REQUIREMENTS FOR CONTROL Of EXPLOSIVE GAS MIXTL'RES 3.22.2.5 The concentration in the waste gas holdup system shall be limited to less than or equal to 25 0xygen by volume and 45 Hydrogen by vol ume.

APPLICABILITY AT ALL TIMES.

ACTION At With the concentration in the waste gas holdup system greater than ,

25 0xygen by volume or 4% Hydrogen by volume, immediately minimize additions of waste g:ses to the system and rdduce the concentra- I tions to less than the aforementioned values within one hour.

ACTION 30: With the number of channels OPERABLE less than that required by the Minimum Channels Operable requirement (a)* (1) Within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, take and analyre an initial grab sample. ,

Grab samples shall be taken every 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> until the monitor is declared operable. ,

(ii) If the sample indicates greater than 25 0xygen or 35 Hydrogen, within one hour begin to add Nitrogen to reduce ..

the concentration to less than .1% 0xygen or 31 flydrogen.

-(iii) Prior to Startup if the Hydrogen concentration is greater than 31, Nitt ogen shall be added to reduce the Hydrogen concentration to 3%. During Startup operations, a grab sample shall be taken and analyzed every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. .

'If the hydrogen monitor is operational, hydrogen montor results may be used .

rather than grab sample analysis for hydrogen. Alternately, if the oxygen  !

monitor is operational, oxygen monitor results may be used rather than grab i J

sample analysis for oxygen.

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(iv) Prior to degas operations, a grab sample shall be taken and analyzed. If the Hydrogen concentration is greater than ll, Nitrogen shall be added t'o reduce the Hydrogen concentrations to less than it. During the degas operation, ,

a grab sample shall be taken and analyzed every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

6 During degas Operat.fon add four times as much N2 as waste gas to the system (waste gas addition inay be estimated).

, (b) If the channel is not OPERABLE within seven days a telephone call i

will be made by the Vice President of TH!-1 (or his designate) to ,

i the Director of Region 1 Of fice of Inspection and Enforcement (or . .

his designate) describing the reas9ns for the delay and the cor.

rective actions being taken. A written report documenting reasons l

for the delay and corrective actions taken will be forwarded within 1 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> of the telephone notification.

NOTE: These requirements corre; pond to LCO 3.11.2.5 and to Action 39 in the model RETS (NUREG-0472).

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