ML20246E712
| ML20246E712 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, 05000000 |
| Issue date: | 08/21/1989 |
| From: | Chrzanowski R COMMONWEALTH EDISON CO. |
| To: | Murley T Office of Nuclear Reactor Regulation |
| References | |
| 0258T, 258T, NUDOCS 8908290268 | |
| Download: ML20246E712 (8) | |
Text
, - _ __. -
M.
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Commorc;ccith Edison q/g,
,4
.,., 72 West Adams Street. Chicago, Ilhnois
- . / Address Repi to: Post office Box 767 l\\
/
Ad Chicago, Illinois 60690 - 0767 -
August 21, 1989 Dr. Thomas E. Murley, Director Office of Nuclear Reactor Regulation
.U.S.-Nuclear Regulatory Commission.
Hashington, DC. 20555
Subject:
. Byron Station Units 1 and 2 Braidwood Station Units 1 and 2 Containment Hydrogen Monitoring System NRC Docket Nos. 50-454/455 and 50-456/457
Reference:
(a) July.25,.1989, letter from LN 01shan i
to TJ Kovach
Dear Dr. Murley:
The referenced letter. identified a difference.between the Byron and Braidwood design of-the containment hydrogen monitoring system and the original Safety Evaluation Report-and the Updated Final Safety Evaluation.
-Report.. The system design utilizes two containnient isolation valves in' series on each line with one valve powered from an ESF division 11 power' supply and the other valve powered-from an ESF division 12 power supply.
The Safety
~ Evaluation Report and the UFSAR describe the design of the system to satisfy the single failure criterion.
The difference exists if the postulated accident assumes the failure of one electrical ESF division that would prevent the re-opening of one of the two isolation valves in each line and thus result in the loss of the hydrogen monitoring system.
Commonwealth Edison has carefully reviewed the hydrogen monitoring system and believes that the existing design is acceptable and meets the-intent of the applicable regulations. Attachment A to this letter contains the detailed justification of the existing design.
Please address any further questions on this matter to this office.
Very truly yours, h
d**RU R. A.
brzanowski Nuclear Licensing Administrator L
0258T:52 1
cc: Byron Resident Inspector l
Braidwood Resident Inspector v#jI i
L N. 01shan - NRR sh S.
P.. Sands - NRR l
Office of Nuclear Facility Safety - IDNS
-8906290268 890821
~PDR ADOCK 05000454 P
, f,fjb ; y X
ATTACHMENT A; Reiponse to Nuclear Regulatory Commission Letter dated July 25. 1989 Desian of Containment Hydroaen Monitoring Syst1m Justification of Existina DesiaD The practice of using two separate Class IE power supplies to power redundant containment isolation valves in series, is commonly used
-throughout the plant.
It meets the requirements of the applicable General Design' Criteria (56) and the NRC guidance in Standard Review Plan Section 6.2.4.
The UFSAR' criteria applicable to containment isolation valves (reference UFSAR Section 6.2.4.1.2.d) is that "in lines where two automatic valves are provided, each valve operator is actuated by an independent signal, and each operator is also supplied from a separate emergency power supply".
The intent of this design critetla is to ensure that no single failure will prevent containment isolation.
Therefore the.
diversity'of.the isolation valve power supplies in a particular line normally takes precedence over the electrical independence of the redundart system lines. The original design of the post accident monitoring system in 1981 utilized diverse isolation valve power supplies.
and this configuration has been retained through construction and plant-operation. The FSAR and UFSAR do not accurately reflect the system configuration shown on the design drawings. However, it is CECO's position that the system as installed is acceptable. This position is supported'by the reliability and diversity of' power sources available to each of the subject isolation valves. The probability of a single failure resulting in loss of an entire' electrical-ESF division is very low based on.the existing design features of the Class IE dc Power System and as evidenced by its operating history.
Figure 1 is a simplified single line diagram of the ESF Division 11 i
auxiliary power system desico and power supply configuration relied upon l
to remotely operate the sub_ect isolation valves.
(A similar power system L
configuration exists for electrical ESF Division 12, 21, and 22).
The following can be seen from this diagram:
The primary source of Class IE dc power to the Division 11 isolation valves is the 125Vdc battery charger 111.
The battery charger can be powered from either of three highly reliable safety related sources:
the Unit 1 Station Auxiliary Transformer (SAT),
a the Diesel Generator a
the Unit 2 SAT via the 4.16KV switchgear bus cross-tie breakers.
q The second source of Class IE dc power to the Division 11 isolation valves is the 125Vdc Battery 111 located in a separate room from the batter charger.
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AUAQitlERLAlcantinuedl Re_snons.e_to htleALRegulatory comis11onle11er_dAteLitulY_2L_13E9 De11gn of Containment Hydroatn Monitoring _SyJigm The tie breakers between the Unit 1 and Unit 2 125Vdc buses provide a third source of power to the subject isolation valves from the Unit 2 ESF Division 21 power system (consisting of a separate battery, battery charger, diesel generator and SAT that are interconnected in a similar manner as that shown on Figure I for electrical ESF Division 11)..
These tie breakers can be manually closed in the miscellaneous electrical equipment room utilizing the existing procedural and administrative controls.
It is evident from the above that a significant number of independent and diverse power supplies are available to support operation of the subject isolation valves.
The only single failure that could result in loss of both hydrogen monitoring :ubsystems would be a failure of the 125Vdc ESF Bus / distribution panel 111 and/or its main breaker feeding the isolation valves. Upon a containment isolation signal, the valves in question are required to close.
Hydrogen monitoring is required to begin thirty minutes after SI initiation and continues isolation.
Since these valves are " fail as is", the above thirty minutes duration is the window during which the postulated failure must occur which we believe is highly unlikely.
This belief is supported by industry data on breakers and 125Vdc buses (relative to probability of failure upon demand) and by past operating performance at the Commonwealth Edison Company Stations.
The existing design is considered to be acceptable for the following reasons:
In the event of a Loss of Coolant Accident (LOCA), containment isolation is required to be accomplished immediately.
The current Byron /Braidwood design accomplishes this function.
The hydrogen monitoring function is not required until 30 minutes after the accident.
In the event of the highly unlikely failure described above (occurring within 30 minutes after the LOCA), tire is available to utilize alternative design features.
For example, the hydrogen recombiner could be started to accomplish the dual functions of a) reducing and hydrogen inventory present in the containment atmosphere, and b) providing indication of hydrogen concentration through the hydrogen monitoring instrumentation which is integral with the recombiners.
The hydrogen concentration as a function of time after an accident is shown in Section 6.2.5 of the UfSAR. As can be seen, the concentration builds slowly with time.
This allows times for manual actions, prior to the hydrogen concentration reaching the level where an explosion would occur.
i
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- AUAQJMENT A (coottinuftdl Reipan51 _to
.hC31pLResulatorv_ commission Letter dated July 25. 1989 Dfsign of Containment HvAtogenJonitorina Syltstn In summary, we believe that the existing design is acceptable and meets the intent.of the applicable. regulations.
In this case, in view of the conflicting requirements for' system operability and containment isolation, it was judged that containment isolation was the more important of the two requirements.
The subject isolation valve power supplies have been
. selected so as to favor completion of the containment isolation function while'at the same time provide sufficient independence and reliability to ensure availability of the hydrogen monitoring system following a LOCA.
Even in the event of-the postulated single failure, alternative ESF equipment is available'to perform the' function of the hydrogen monitoring system, and because of the long time over which the hydrogen concentration builds up, sufficient time.is available to restore the system to operable status via' manual operator actions.
Commonwealth Edison Company concurs that the existing design of the hydrogen monitoring system has not been described in sufficient detail and/or accurately in all of the. pertinent UFSAR sections.
It appears that because of this lack of specificity and detail _in the UFSAR, the NRC misinterpreted the existing design as presently described and accepted in the original Byron Safety Evaluation Report (SER), issued in February 1982.
He, therefore, propose'to revise the affected UFSAR pages as indicated on Attachtrent 6 to agree with the actual plant configuration.
Commonwealth Edison Company believes that the existing design of the hydrogen monitoring system is acceptable and consistent with the applicable regulatory requirements.
1 i
i
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4 ATTACHMENT B COMMORREALTH EDISON COMPANY ElRDE/3_RAIDH000 STATIONS - UNITS 1 & 2 11ESALCHANCtES_EAS ED_DILEKISIING e-QESIGtLDL HYDE 0 GEN HQNIIORUiG_SlSIEM (Pages 6.2-68 and E.30-7)
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/ 77%Cn%'E~tV7~8 A
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B/B-UFSAR k
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.. d.
Indication of hydrogen concentration is available in the main control room when the monitors are operating.
e.
The hydrogen monitors are located in the auxiliary-building elevation 401 feet.
Samples are piped from containment penetration to the monitors.
The accuracy of the monitors is 22.5% of full scale
'(dry basis).
Operation of.the hydrogen monitors is independent of.the hydro-gen recombiner and its associated hydrogen analyzer since both systems use separate piping and containment penetrations and are not dependent upon.the other to operate in any way.
The hydrogen monitoring system consists of two independent, physically separated and redundant subsystems,2:0 th;;.;e..
the cir:10 f iltre crit:ri:.. Separate piping penetrations of the containment are utilized by each train of this system.
contuhme tula% powered from A separate IE power. source,excepf d><,fd Each train is hias.One *t' de % cmtskmerr es.tsk. 4 ; g (njet ye, The portions of the hydrogen monitoring piping. system which ducfaf-i form the containment atmosphere isolation barrier.are desig-yssm 4 nated Seismic Category I, Quality Group B.
The remainder of pauv=/ 4en the system outside the containment is Seismic Category I, (Acc QualityGroupBuptothehydrogenmonitoringinstrumentation.4Wofoh.oshy Piping internal to the instrumentation is classified as ANSI go.gr B31.1.
The piping from the containment to the first isolation s [Pj '
j valve is designed to the requirements of SRP 3.6.2.
A sample of the containment atmosphere is taken at or near one of the containment penetrations and another'approximately 180 degrees away on the other. side of the containment.
The samples taken are representative of the containment atmosphere due to the mixing system effects.
The mechanical piping penetrations used for the hydrogen moni-toring system at Byron are IPC-12 and 1PC-31 for Unit 1 (IPC-45 and IPC-36 for Braidwood) and 2PC-12 and 2PC-31 for Unit 2 (2PC-45 and 2PC-36 for Braidwood).
Penetrations IPC-12 and 2PC-12 (IPC-45 and 2PC-45 for Braidwood) are for the Train A monitors and 1PC-31 and 2PC-31 (lPC-36 and 2PC-36 for Braid-wood) are for the Train B monitors.
Additional information concerning the mechanical penetration's elevations and azimuths are listed in Table 3.8-1.
6.2.5.2.3 Hydrocen Mixino System Desian The function of the mixing subsystem is to ensure that local concentrations with greater than 4% hydrogen cannot occur within the primary containment following a LOCA.
The mixing is achieved by natural convection processes, containment fan cooler operation, and the containment spray system.
6.2-68 l
AT7AdqNrWT 6 B/B-UFSAR as ANSI B31.1.
The piping from the containment to the first isolation valve will be designed to the requirements of SRP 3.6.2.
Operation of the hydrogen monitors is independent of the hydrogen recombiner since both systems used separate piping and contain-ment penetrations and are not dependent upon the other to operate in any way.
The hydrogen monitoring system consists of two inde-pendent, physically separated and redundant subsystems.endr-thesy meets th; ;ingic f;ilurc critcric.
Separate piping penetrations of the containment are utilized by each train of this system exce/7t de tde tephelf Each train is pokw red f rom a separate IE power source lt /shg_ n ef sasy,y MxS Cne*T hva sos tkohmT na/1+M v6 b-1 si f& okf isEloM g
Byron /Braidwood stations meet the requirements for continuous d j6snqf indication in the main control room with IEEE 323-1974 qualified hvy v4, indicators.
The monitors may be controlled f rom the control room. gyu,'e SAMPLE CONDITIONING W Sa The Model 225CM monitoring system is designed to monitor contain-ment gas for percentage by volume of hydrogen (dry analysis).
The operating range is -5 to +50 psig, 40*F to 445'F and relative humidity from 10 to 100%.
A sample of the containment atmosphere will be taken at or near one of the containment penetrations and another approximately 180 degrees away on the other side of the containment.
The samples taken are representative of the contain-ment atmosphere due to_the mixing system effects, which is dis-cussed in Subsection 6.2.5.2.3.
Radioactive sample gas is drawn from the containment vessel by means of a sample pump into the analysis unit precooler where it is lowered from temperatures as high as 445'F to ambient temperature of the analyzing unit.
A solid state self-regulating thermoelectric cooler further reduces the gas temperature to below analysis unit ambient; after which 0.4 scfh of sample gas is directed to the sample measuring cell maintained at 170*F.
After the gas passes through the cell, it is returned to the containment via a pressure regulating network which maintains pressure above containment assuring return of the sample gas.
Any condensation formed in either of the coolers is gravity drained to a water trap which is automatically purged back to the
{
containment with the aid of the pressure regulating network.
j CALIBRATION Instrument calibration is performed by actuating the appropriate solenoid valve directing zero or span gas with a known concentration through a flow controller and into the cell.
GAS MEASUREMENTS - GENERAL DISCUSSION Analysis is accomplished by using the well established principle of thermal conductivity measurement of gases.
This technique utilizes two pairs of self-heating filaments fixed in the center of separate cavities inside the analyzing cell housing.
One of E.30-7 1
I',
COMMONWEALTH ~ EDISON COMPANY
+
BYRON /BRAIDWOOD STATIONS - UNITS I'& 2 PROCESS SAMPLING _
POST' ACCIDENT H2 MONITORING SYSTEM i
S B-OPEN S B-OPEN S B-OPEN S B-OPEN S A-CLOSE S A-CLOSE S A-CLOSE S A-CLOSE IPS22BA IPS229A IPS228B IPS229B ESF DIV.li ESF'DIV.12 ESF DIV.ll ESF DIV.12 l
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ESF DIVISION II SINGLE LINE DIAGRAM
's FIGURE I
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