ML20028C452
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UNITED STATES y*
'g NUCLEAR REGULATORY COMMisslON g,
j WASHINGTON, D. C. 20555 e
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JAN 15 1W9
/. L. Jordan, Assistant Director for Technical Progra MEMORANDUM FOR:
E I&E FROM:
B.-K. Grimes, Assistant Director for Engineering and Projects, D0R
SUBJECT:
PROPOSED D0R ACTION ON PREVENTION OF 0FFGAS EXPLOSIONS We have received the IE review of the licensee's responses to IE Bulletin 78-03, " Potential Explosive Gas Mixtures Accumulations Associated with BWR Offgas System Uperations." For each response, the IE review provides a sum-mary of information verified by tne IE inspector and an assessment of licen-see's response. Your memorandum transmitting the IE review includes two spe-cific recommendatioris:
(1) NRC to require reactor shutdown in the event of an offgas pressure transient analogous-to an uncontrolled hydrogen burn and (2) NRC to require upgrading the offgas systems to provide that all potential openings be equipped with positive direct-action sensors to inform the opera-tor that an offgas leak has occurred.
The D0R action on the IE Bulletin consists of establishing the NRR acceptance criteria to apply to operating BWRs for the prevention of offgas explosions. lists these NRR acceptance criteria.
Item 1-4 of enclosure 1 are the criteria, we interpret, you have already adopted in your review of the Bulletin responses.
D0R will, therefore, ;ake no further action on those ac-ceptance criteria based on your concurrence and anticipated completion of in-spection and" review of the few cases where you are taking followup action.
Item 5 of enclosure 1 is the 00R position on potential openings. 00R will review each plant and require licensees to provide a degree of offgas system integrity equivalerrt to the positions in Item 5.
It is suggested that the mechanism of informing licensees of the joint 00R/IE position will be via IE inspectors.
D0R can also send letters to specific licensees, if necessary; however this action can, hopefully, be completed ad-ministratively as an extension of the original IE Bulletin. The completion of the 00R action will result in an evaluation summary of each plant showing how those acceptance criteria are being met.
We expect tne completion of the evaluation by May,1979.
The enclosure 1, DOR position has addressed your two recommendations as follows:
l (1)
Require Reactor Shutdown Following Offaas Explosion D0R Response:
Offgas pressure transients resulting from uncon-N trolled hydrogen burn can occur, even though, in most in::tances, l
Contact:
R. Lo, EEB/ DOR 28066
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8301100192 021207 8
HIATT82-545 PDR L
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JAN 2 ? 'C:
E!. L. Jordan
' rapidly self-extinquished. The enclosure 1 position (Item 3) is that continued reactor operation is permitted provided the licen-see has adequate operating procedures, including corrective actions, following deviation from offgas system design parameters (e.g.,
pressure transients) to assure that safe offgas system operation can continue. To assure the safe operation of the system, this re-quirement is augmented by (a) technical. specifications on hydro-gen concentrations in the offgas system (see NUREG-0473) to be adopted by operating BWRs for the implementation of Appendix I to 10 CFR Part 50 and (b) Enclosure 1 position (Item 5) design fea-tures for the prevention of continuous offgas leakage from poten-tial openings.
(2)
Installation of Positive Direct-Action Sensors on potential Openings
-DOR Response: The enclosure 1 position (Item 5) contains the re-quirement that potential openings where normally explosive gas mix-tures exist should have design features to automatically prevent continuous offgas leakage. For potential openings where normally explosive gas mixtures do not exist (e.g., downstream of recombiners) licensees should either have design features to automatically prevent l
continuous offgas leakage or sensors to inform the operator that off-gas leakage is occurring so that corrective action can be taken.
Your review also identifies other considerations that could improve protection l
against ignition of offgas. These considerations are:
(1) offgas piping color coding, (2) avoid bypassing of recombiner, and (3) plant features to pre-vent explosions in mechanical vacuum pump piping.
00R has reviewed and provided initial positions on those considerations for operating reactors in enclosure 2.
A copy of this memorandum together with the enclosures has been forwarded to DSE for consideration of incorporating enclosure items-in the SRP for review of new plant application'.
s M
. 7ds Brian K. Grimes, Assistant Director for Engineering and Projects l
Division of Operating Reactors
Enclosures:
As stated cc:
V. Stello i
D. Eisenhut G. Knighton L. Barrett i
C. DeBevec 4
j J. Collins E. Adensam Section B/EEB 9
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' Enclosure i A:ceptance Critaria for Bi-:R Offgas Explosion Preventica 1.
Precautionary _ Procedures Certain maintenance operations, e.g.,' welding and system operations, e.g., switching recombiners, can involve potential ignition sources that could ina'dvertently ignite offgax mixtures.
To minimize the proba bility of these occurrences., these and similar operations should be con-trolled by written procedures."
2.
Operation of Offaas System within Desian Parameters Operating procedures related to offgas system should identify the design parameters within which safe operation of the offgas system can be as-.
sured.
Sensors, monitors and control' devices should be provided to pro-vide the operator adequate ipformation to determine and control the offgas operating parameters.
3.
Procedure on Corrective Actions Operating procedures and corrective actions should be established in the event of a deviation from offgas system design parameters (including pres-
.sure transients caused by internal detonation or burn).
These procedures should require reactortshutdown unless offgas system parameters are veri-fied to be within system design parameters (e.g., loop seals are not open).
Guidance on hydrogen instrumentation and permissible hydrogen concent.ra-tions are given in NUTEG-0473, " Radiological Effluent Technical Specifica-tions for BWRs."
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'lentilation of Conticuous Areas Adequate ventilation should be provided in the contiguous areas through which offgas system piping containing explosive mixtures of gases pass; such that possible leakage of offgas will not result in the accumulation
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of the gas mixture to detonable concentrations.
5.
Prevention of Continuous Offcas Leakaae The following design features should be provided to minimize the conse-quences following an offgas detonation or burn:
a) Rupture discs, loop seals and any other non-resealing type seals should be hard-piped back to the main condenser or portions of the system where a detonable envirdn-ment is not possible or b) self-actuating positive seals (e.g., relief valves) should be used to prevent continuous offgas leakage from poten-tial cpenings where normally explosive gas mixtures exist.
For potential coenings where normally explosive gas mixtures do not exist (e.g.,
downstream of recombiners) the above features need not be installed
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- Povided that sensors are installed at the Cotential opening 7.3 alert operators of a continuous leakage condition.
The sensors described are in addition to monitoring of hydrogen concentration downstream of recombiners required by the technical specifications to be adopted for the implementation of Appendix I.
This would alert the operators snould an explosive mixture exist at those potential openings.
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00R Positions for-Operating Reactors ^ on IE SVggested Considerations nat Could Imorove Protection Acainst Offcas I:niti;r (1).
Color Coding of Offaas'Pipino Color coding of offgas piping, and a number of similar precautionary ac-tions ('e.g., use of warning signs and labels 'on'or near equipment which contain or are in the vicinity of detonable mixtures), can aid in mini-mizing the probability of inadvertent ignition.
For operating reactors, the 00R position is to recommend these precautionary actions although they are not mandatory.
- These precautionary actions may be used to augment (but not replace) written procedures on control of maintenance thac involves po-tential ignition sources.
(2)
Avoid Bypass of Recombiners Technical specifications, to be adopted by operating BWRs for the imple-rr.entation of Appendix I, requires the operation of the offgas treatment system under specified conditions.
Plant operating procedures should control radioactive gaseous effluents'.
Any further regulctory action would not necessarily preclude the potential of an explosion.. For example, requirements on the use of an operable recombiner raay result in equipment switching, This, in turn, could create transient con-ditions which may ignite the.offgas' mixture.
(3)
Prevention of Explosions in Mechanical Vacuum Pumo Pipino Review of the reported offgas explosions did not identify any instance where an explosion had occurrei in mechanical vacuum pump piping. Vacuum pump operation is generally limited to reactor startup and shutdown oper-ations without explosive concentrations within the system.
Should further information indicate that there is a signii'icant explosion potential in the vacuum pumo system, we will investigate the need for further action on operating reactors.
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