ML20064A370
| ML20064A370 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 08/21/1989 |
| From: | Chrzanowski R COMMONWEALTH EDISON CO. |
| To: | Murley T Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20064A369 | List: |
| References | |
| NUDOCS 9009040116 | |
| Download: ML20064A370 (8) | |
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August 21, 1989
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l-l Dr. T3omas E. Murley, Director i
Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555
Subject:
Byron Station Units 1 and 2 Braidwood Station Units 1.and 2 Containment Hydrogen Monitoring System i
NRC Docket Not. 50 454/455 and 50-456/457
Reference:
(a) July 25, 1989, letter from LN 01shan i
to TJ Kovach
Dear Dr. Murley:
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The referenced letter identified a difference between the Byron a'nd-Braidwood design of the containment hydrogen monitoring system and the original Safety Evaluation Report and the Updated Final Safety Evaluation i
Report.
The system design utilizes two containment isolation valves in series i
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 t
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.
Cosmonwealth (dison has carefully reviewed the hydrogen monitoring system and believes that the existing design is acceptable and meets the intent of the appitcable regulations. Attachment A to this letter contains the detailed ju%1fication of the existing design.
Please address any further questions on this matter to this office.
Very truly yours, M
- fa' R. A.
hrzanowski Nuclear Licensing Administrator 0258T:52 cc:
Byron Resident Inspector Braidwood Resident Inspector L. N. Olshan - NRR S. P. Sands - NRR Office of Nu'elear Facility Safety - IDNS 9009040116 900824 PDR ADOCK 05000454 P
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l Ratnante to Noelaar Reaulaterv Committien Letter dated July 25. 1989 i
t Dation of Containment Hydrenen Monitorina Svttom i
l Justification of Existina Dettan The practice of using two separate Class IE power supplies to power redundant containment isolation valves in series, is commonly used t
throughout the plant.
It meets the requirements of the applicable General Design Criteria (56) and the NRC guidance in Standard Review Plan 56ction 6.2.4.
The UFSAR criteria applicable to containment isolation valves
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(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 criteria is to ensure that no single failure will prevent containment isolation.'
Therefore the diversity of the isolation valve power supplies in a particular Itne normally takes precedence over the electrical independence of the redundant system Iines..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 t
configuration shown on the design drawings.
However, it is CECO's position that the system as installed is acceptable.
This position is L
supported by the reliability and diver:ity of power sources available to l
each of the subject isolation valves.
The probability of a single failure I.
resulting in loss of an entire electrical ESF division is very low based l
on the existing design features of the Class IE de Power System and as evidenced by its operating history.
Figure 1 Is a simp 1tfied single line diagram of the ESF Division 11 5
aux 111ary power system design and power supply configuration rolied upon to remotely operate the subject isolation valves.
(A similar power system configuration exists for electrical ESF Divtsion 12, 21, and 22).
The following can be seen from this diagram:
l The primary source of Class IE de power to the 01 vision 11 i
isolation valves is the 125Vdc battery charger 111.
The battery i
charger can be powered from either of three highly reliable safety related sources:
the Unit I Station Auxillary Transformer (SAT),
the Olesel Generator the Unit 2 SAT via the 4.16KV switchgear bus cross-tie breakers.
The second source of Class IE de power to the Division 11 isolation valves is the 125Vdc Battery III located in a separate room from the batter charger.
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ATTAcWMENT a (eentinued) p.'
Ratnante to Nuclear Reaulaterv Committien Letter etted julv 25. 1989 Denian of Centainment Hvdronen Monitorina Svitam The tie. breakers between the dnit I and Unit 2125Vdc buses provide a o
third source of nower 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 1 for electrical E3F Division 11).
These tie breakers can be manually closed in the miscellaneous electrical egulpment room utilizing the existing procedural and administrative controls.
It is evident from the above that a significant number of independent and div6rse 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 subsystems would be a failure of the 125Vdc ESF Bus / distribution panel 111 and/or its main breaker feeding the isolatiod 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 125'!de buses (relative to probability of failure upon demand) and by past orerating performance at the Commonwealth Edison Costany Stations.
15e existing design is considered to be acceptable W the following ressons:
In the event of a Loss of-Coolant Accident (LOCA). containment isolation is required to be accomplished immediately.
The current Byron /Braldwood design accomplishes this function.
The hydrogen monttoring 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), time 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 4
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.
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i ATTACHMENT & (continued) l Reinonte to i
Nuclear Reaulatery Committien Letter dated July 23. 1989 1
Desian of Containment Hvdreann Monitorina System t
Insummary.webelievethattheexistingdesignisacceptableandmeets the intent of the applicable regulations.
In this case, in view of the conflicting requirements for system operability and containment isolation, i
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 fur.ction of the hydrogen monitoring system, and because of the long time over which the hydrogen concentration butids up, sufficient time is available to restore the system to operable i
status via manual operator actions.
L 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 l
misinterpreted the existing design as presently described and accepted in the original Byron Safety Evaluation Report (SER), issued in February 1982. We, therefore, propose to revise the affecteo UFSAR pages as indicated on Attachment 8 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.
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ATTACMMENT B p
ColGONWEALTH EDISBN COMPANY 1
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BYRON /BRAIDWOOD STATIONS UNITE 1 L2 i
i UFEAR CHANGES BASED ON EXIETING l
i DESIGN OF HYOROGEN MONITQRING EYSTEM (Pages 6.2-68 and E.30-7)
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Indication of hydrogen concentration is available i
.. in the main control room when the monitors are operating.
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The hydrogen monitors are located.in the auxiliary -
building elevation 401 feet.
Samples are piped fres containment penetrations to the monitors.
The accuracy of the monitors is a2.5% of full scale (dry basis).
Operation of the hydrogen monitors is independent of the hydro-gen recombiner and its associated hydrogen analyser 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,ead-theemusety-ti: cir:1 Pil :: ::it:rit.
Separate piping penetrations of the containatat are utilised by each train of this system.
Each train is powered from A sesarate IE power source,emespf M,2^t l
ema,a e n4t% Antas. One, e t w %. caca.
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The portions of the hydrogen monitoring piping system which ettc./t n form the containment atmosphere-isolation barrier are desig-'
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noted Seismic Category I, Quality Group 4.
yhe remainder of
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the system outside the containment is Seismic Category I, 44 eff.
Quality Group 8 up to the hydrogen monitoring instrumentation. sG w g piping internal to the instrumentation is classified.as ANSI p.,
531.1.
The piping from the containment to the first isolation valve is designed to the requirements of SRP 3.6.2. -
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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 mizing system offacts.
g The mechanical piping penetrations used for the hydrogen moni-toring system at Byron are APC-12 and IPC-31 for Unit 1 (IPC-45 4
and 1PC-36 for Braidwood) and 2PC-12 and 2PC-31 for Unit 2 (2PC-45 and 2PC-36 for traidwood).
Penetrations 1PC-12 and 2PC-12 (1PC-45 and 2PC-45 for Braidwood) are for the Train A monitors and 1PC-31 and 2PC-31 (IPC-36 and 2PC-34 for Braid-wood) are for the Train B monitors.
Additional information concerning the mechanical penetration's elevations and asinuths are listed in Table 3.8-1.
i 6.2.5.2.3 Hydrenan Mixing syntam Damien The function of the mixing subsystem is to ensure that local e.oncentrations with greater than 4% hydrog.en 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.
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3/3-UTSAR i
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as ANSI 33),1.
The piping'from the containment to the first l
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 l
in any way.
The hydrogen monitoring system consists of.two inde-pendent, physically separated and redundant subsystems.;;d, :.'. ;,
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Separate-piping penetrations of the containment art utilized by.each train of this system
,,Each train is powered f rom a, segarate IE newer source ewe M m,e WE j
hows. car 4 4 rw. m m, w,.w m M g m' Nr,w, Q, g m wsw i
Byron /Braidwood stations meet the requirementa for continuous p.(
indication in the main control room with IEEE 323-1974 qualified am, r
indicators.
The monitors may be controlled f rom the control room, gyw,
SAMPLE CONDITIONING s
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'T 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-4 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 analysing 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.
l 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 l
drained to a water trap which is automatically purged back to the containment with the aid of the pressure regulating network.
CALIBRATIQB l
i Instrument calibration is performet by actuating the appropriate solenoid valve directing zero or span gas with a known concentration through a flow controller and into the cell.
gAE MEAEUB1me-ra - CENERAL DIREUERION Analysis is accomplished by using the well established principle of thermal conductivity measurement of gases.
This technigte utilizes two pairs of self-heating filaments fixed in the center of separate cavities inside the analyzing cell housing.
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