ML20056E468

Supplemental Safety Evaluation for Topical Rept HGN-112-NP, Generic Hydrogen Control Info for BWR/6 Mark III Containments. Change Requests Consistent & Compatible W/ 10CF50.44 & Acceptable

ML20056E468
Person / Time
Site:	River Bend
Issue date:	08/05/1993
From:	Office of Nuclear Reactor Regulation
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Shared Package
ML20056E463	List: ML20056E462 ML20056E468
References
TAC-M860341, NUDOCS 9308240093
Download: ML20056E468 (3)

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ENCLOSURE SUPPLEMENTAL SAFETY EVALUATION FOR TOPICAL REPORT HGN-il2-NP l

GENERIC HYDROGEN CONTROL INFORMATION FOR BWR/6  ;

i KARK III CONTAINMENT HYDROGEN CONTROL (TAC NO. M8603401

1.0 INTRODUCTION

On August 6,1990, (letter from A. Thadani to J. Langley), the staff issued its l Generic Safety Evaluation Report (SER) regarding Mark III containment hydrogen control. The purpose of the SER was to provide regulatory guidance to licensees with Mark III containments with regard to preparation of plant specific analyses demonstrating compliance with 10 CFR 50.44, " Standards for Combustible Gas Control in Light-Water Cooled Power Reactors." In the SER, the staff provided its evaluation of the generic methodology proposed by the Hydrogen Control Owners Group (HCOG) in Topical Report HGN-ll2-NP, " Generic Hydrogen Control Information for BWR/6 Mark III Containments." On October 23, 1990, the HCOG met with the staff and presented positions contrary to staff positions described in the SER. The HCOG positions were subsequently documented in Mr. J.R. Langley's letters dated April 29, 1991, (HGN-137) and April 30, 1991, (HGN-138).

The staff positions to which the HCOG took exception relate to pressure survivability criteria for essential eqeipment and to the accident sequences to be analyzed.

This supplement discusses the HC0G positions and provides the staff responses.

2.0 PRESSURE SURVIVABILITY CRITERIA FOR ESSENTIAL E0VIPMENT Section 7.5 of NUREG-1417 states:

"In its letter of August 7, 1987, (HGN-Il8-P), the HCOG indicated equipment located inside containment is qualified to a pressure loading of at least 30 psig applied externally.

The CLASIX-3 predictions produced the most severe pressure rise of about 23 psig in the Mark III containment. The staff concludes that pressure is not a concern, pending confirmation by each licensee of the 30 psig capability. When the hydrogen ignition system is functioning, various containment subvolumes will be randomly affected by hydrogen burning; however, a large pressure spike is not expected to occur."

The staff SER thus endorsed 30 psig as a pressure criterion on the basis of the HCOG statements that 23 psig is the maximum pressure predicted by CLASIX-3 and the equipment is already environmentally qualified to 30 psig.

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In its April 29, 1990, letter, the HCOG requested that the staff revise its SER to specify 13.3 psig and 23 psig respectively as the pressure criteria for drywell and containment survivability analysis. The basis for the request was consistency with the HCOG Hydrogen Control Program Plan acceptance criteria which specifies that the results of CLASIX-3 analysis would be the basis for the survivability pressure criteria. The licensee intends, for equipment not already environmentally qualified to the specified pressure (13.3 or 23 psig),

to demonstrate survivability to that pressure.

The staff endorses the survivability pressure criteria values of 13.3 and 23 psig for the drywell and containment, respectively. As noted in the April 29, 1991, letter these values are consistent with the CLASIX-3 peak pressure data and conservative with respect to the t-scale test data.

3.0 COMPARISON OF HGE AND TBU SCENARIOS In its April 30, 1991, letter, the HCOG discusses inconsistencies between the HCOG analyzed hydrogen generation events and the TBU " acceptable" accident sequences discussed in the SER. The SER discusses an 50RV " Base-Case Scenario" in Section 5.2.1.1, and a TBU " Acceptable Sequence" in Section 5.2.1.3. The SER further states that the TBU sequence encompasses the HCOG " Base-Case" (SORV) sequence. The HCOG's April 30, 1991, letter requested the SER provide a "better definition of the station blackout sequence." A clarification is desired in order to better define hydrogen release profiles, operator actions and equipment / power availability assumptions in the models.

The SER, Section 5.2.1.3, states:

1. "The TBU sequence as described in NUREG-ll50...is an acceptable sequence leading up to core recovery,"

and

2. "The TBU is acceptable for the time sequence of events and for the hydrogen production rate and total amount."

The HCOG states that it is not their intention to encompass all NUREG-1150 TBU (blackout) sequences as most such sequences lead to a severe accident. The HCOG requested staff endorsement of the assumption of power restoration /reflood onset by 3100 seconds (51.6 minutes).

The staff position is that the reflood timing assumption should be adjusted to produce a conservative estimate of hydrogen production and establish a conservative hydrogen generation profile. Loss of AC need only be assumed to 1 the extent necessary to establish a bounding hydrogen generation release '

profile. Within this context, the staff considers the assumption, for purposes l of 10 CFR 50.44 analyses, that power restoration /reflood is initiated by 3100 l seconds is acceptably conservative. Such an assumption is consistent with 10 CFR 50.44 and does not compromise hydrogen mitigation capability.

4.0 SCOPE OF SER SECTION 8 CONCLUSION l i

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3 SER Section 8 states: "The plant-specific analysis will use test data...to confirm that the equipment will be capable of performing their functions during and after exposure to the environmental conditions created by the hydrogen in all credible severe accident scenarios.* (1talics added).

The HC0G's April 30, 1991, letter states that the scope of the conclusion should be limited to recoverable degraded core scenarios. The HCOG position is based on the 10 CFR 50.44," which refers to a " degraded core" followed by a " period of recovery." The term " severe accident" is not used.

Although 10 CFR 50.44 does not provide specific criteria as to what constitutes

" recoverability," the SER stated that the staff position interprets

" recoverability" to mean the expectation that " original core geometry is maintained," and accepted 50 percent Zircaloy melt fraction as a cutoff point for recoverability (Ref: SER Section 5.2.1). Thus, the term " recoverable '

degraded core hydrogen generation events within the scope of 10 CFR 50.44" is preferable. The issue is not significant since the analyses and thus the '

conclusion only encompass the specific sequences accepted by the staff.

5.0 SPRAY / COOLER AVAILABILITY In Section 7.4 of the August 6, 1990, SER, the staff stated its position that "BWR Mark III owners should evaluate the containment and essential equipment response to hydrogen generation events assuming containment sprays are unavailable, consistent with SB0 assumptions..." The basis for the staff position was cited as being a change of focus, resulting from recent risk analyses, to SB0 as a significant contributor to hydrogen generation events.

The HCOG took issue with this position stating that availability of spray and unit coolers should be allowed. The basis for the owner's position is that AC power availability considerations should be consistent with other assumptions (i.e., RHR, igniters availability).

The staff agrees with the HCOG position. The staff recognizes that the intent of 10 CFR 50.44 was to prescribe a higher, deterministic hydrogen generation coping capability. Probabilistic considerations for spray / cooler availability should be addressed in the context of the IPE, as with igniters. Should spray or cooler availability appear to be h significant containment performance concern, the need for such provisions ss diesel fire pumps connections to spray headers or cooler may be considered in that context.

Thus, the staff agrees that AC power recovery, RHR availability, and unit cooler / containment spray availability should be treated consistently during a hydrogen generation event.

6.0 CONCLUSION

The staff has evaluated the HCOG change requests and concludes that they are consistent and compatible with 10 CFR 50.44 and are therefore acceptable.