ML20028B838
| ML20028B838 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 12/01/1982 |
| From: | Kay J YANKEE ATOMIC ELECTRIC CO. |
| To: | Crutchfield D Office of Nuclear Reactor Regulation |
| References | |
| FYR-82-113, NUDOCS 8212070047 | |
| Download: ML20028B838 (4) | |
Text
4 Telephone (617) 872-8100 TWX 7103801619 YANKEE ATOMIC ELECTRIC COMPANY 2.C.2.1 s,
FYR 82-113 hY.
1671 Worcester Road, Framingham, Massachusetts 01701
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~s December 1, 1982 United States Nuclear Regulatory Commission Washington, D. C. 20555 Attention:
Mr. Dennis M. Crutchfield, Chief Operating Reactors Branch No. 5 Division of Licensing
References:
(a) License No. DPR-3 (Docket No. 50-29)
(b) YAEC Letter to USNRC, dated January 4,1982 (FYR 82-1)
(c) YAEC Letter to USNRC, dated April 9, 1982 (FYC 82-4)
Subject:
Request for Exemption
Dear Sir:
The purpose of this letter is to request an exemption from the requirement of 10CFR50.44(c)(3)(ii), which ' requires that PWR plants with containments that rely on a purge /repressurization system as the primary means of hydrogen control, be provided with either an internal hydrogen recombiner or the capability to install an external recombiner, following a postulated LOCA. We do not feel that this requirement of the regulation is applicable to the Yankee Plant. Our reasons and justification are stated below.
Paragraph (g) of 10CFR50.44 states that for facilities that applied for a construction permit before December 22, 1968, only a purge system is necessary if (1) the combined radiation dose at the outer boundary of the low population zone from purging and the postulated LOCA is less than 25 rem to the whole body and less than 300 rem to the thyroid, and (2) the purge system and any filtration system associated with it are designed to conform with the general requirements of Criteria 41, 42, and 43 of Appendix A to 10CFR50 [i.e.,
General Design Criteria (CDC)].
It is our position that Yankee qualifies under the above paragraph
[10CFR50.44(g)] and, therefore, only a purge system is necessary for hydrogen control. The Yankee Plant applied for its construction permit on July 9, 1957. Analyses submitted for SEP (Topic XV-19) have demonstrated that the calculated doses are within the parameters outlined above (Reference (b)]. In addition, containment atmosphere contror systems (i.e., atmosphere recirculation and hydrogen control) conform with the general requirements of the GDC. The penetrations used for hydrogen control are dedicated and have been shown to conform to GDC 54 and 56 for single failure protection.
8212070047 821201 PDR ADOCK 05000029 P
Unitsd Stntso Nucisar Rigulctory Commicsion DIcstber 1, 1982 Attanticn Mr. Dannio M. Crutchfield Pags 2 i
In Reference (c), Yankee submitted comments on the proposed regulation pertaining to hydrogen control (46 FR 62281, 12/23/81). At that time, we urged the Commission to give further consideration to the extensive post-TMI modifications made to all plants, in order to significantly reduce the probabilities of event precursors leading to degraded-core conditions. At the Yankee Plant, these have included emergency feedwater system upgrading, instrumentation to recognize inadequate core cooling, increased containment isolation capability and dependability, and intensive operator training programs (i.e., STA training program and mitigating core damage training program). The latter training program specifically addresses accident mitigation, placing emphasis on potentially hazardous conditions that could lead to inadequate core cooling and, eventually, severe core damage.
In addition, this training includes the consequence of hydrogen gas generation and radiation hazards associated with a damaged core. Therefore, based on the above modifications and increased operator training and awareness, not to mention the additional post-TMI requirements, we believe that post-accident hydrogen control is an insignificant contributor to overall risks attached to severe-accident scenarios.
Additionally, and in support of the last statement, Yankee has performed a Probabilistic Risk Assessment of the Yankee Nuclear Power Station.
In this assessment, the potential for containment failure due to overpressure was considered. One of the mechanisms evaluated was hydrogen combustion and the presence of excessive amounts of non-condensibles. Hydrogen generation, concentration and burning, and containment pressure were conservatively calculated, assuming an adiabatic complete combustion of hydrogen from a 10%
to 0% concentration. The results show that in no case would the resulting pressure rise be in excess of the containment failure pressure. Consequently, the probability of containment failure due to the hydrogen combustion would be small. Furthermore, a more realistic analysis, assuming that hydrogen ignited at 8% and burned down to 4%, shows almost a factor of 2 reduction in maximum containment pressure. It is reasonable to conclude from this analysis that in realistic terms, the containment would not be endangered by a spontaneous combustion of hydrogen generated during the course of an accident.
The above discussion suggests that venting the containme may not be necessary. However, as required by present plant procedure, following a loss-of-coolant accident, the containment atmosphere will be analyzed for hydrogen and periodically vented to maintain the concentration below 4 v/o.
Two open-ended pipes in the containment, either of which can be isolated by shutting a solenoid valve, allow the containment atmosphere at the midplane or near the top of the containment to be directed to the sample and control station. Compressed air can be charged into the containment to provide dilution air and provide a positive pressure for venting. During charging the containment internal pressure will be monitored.
Venting will be initiated prior to reaching a hydrogen concentration within containment of 4% and sustained at a rate suf ficient to prevent further buildup. The vented gas will be passed through a filter system to remove radioactive particulates and iodine and is then vented out the primary vent stack. A radiation monitor is installed in the vent line and will provide continuous recording of activity during venting.
Unitcd Stetso Nuclacr R:gulctery Commiccion D:cember 1, 1982 a
Attention:
Mr. Dennio M. Crutchfield P:gs 3 In order to determine the associated radiation doses should venting be required, an analysis was performed based upon Regulation Guide 1.7, Control of Combustible Gas Concentration in Containment Following a LOCA. The following assumptions, as presented in the guide, were used in the analysis:
1.
Fraction of Radiation Energy Absorbed by Coolant a.
Betas from fission products in fuel rods 0
b.
Fraction from fission products mixed with coolant 1
c.
Gammas f rom fission products in fuel rods, coolant in core 0.1 G(H ) - molecules /100 ev.
0.5 2.
2 3.
Extent of metal water reaction, percent 5
4.
Fission product distribution model a.
Halogens 50% total inventory b.
Solids 1% mixed with coolant Based on the calculated hydrogen generation rate and an initial 5%
metal-water reaction, a 4% hydrogen concentration is not reached until more than 1 x 107 seconds (139 days) have elapsed. It was assumed that venting commenced at 120 days at a 0.32%/ day rate, which is sufficient to prevent a further hydrogen concentration increase. The vent flow would be directed through the system filter which is assumed to remove 90% of the iodine. No removal of noble gases is assumed. An atmospheric dilution factor of 1 x 10-4 sec/m3 was used for the site boundary dose calculation which is considered to represent a conservative value based on the on-site measurements. An atmospheric dilution factor of 1.35 x 10-5 sec/m3 was used for the low population zone dose calculation based on analysis of the on-site data.
The results of the dose calculations are shown in the following table:
POST-ACCIDENT VENTING DOSES (REM) 30-Day Vent Period 60-Day Vent Period Thyroid Whole Body Thyroid Whole Body Site Boundary 4.5 x 10-3 3.7 x 10-2 4.8 x 10-3 7.3 x 10-2 Low Population Zone 6.0 x 10-4 5.0 x 10-3 6.5 x 10-3 9.8 x 10-3 The calculated doses at the site boundary and low population zone even for the 60-day vent period are substantially below the acceptable values spectfied in 10CFR100.
i l
{
Unitsd Stetss Nucissr R:gulctory Commicsien Decsrber 1, 1982 a
Attsntient Mr. D2nnis M. Crutchfield Page 4 In summary, we believe an exemption from the requirements of 10CFR50.44(c)(3)(ii) is justified because:
(1) The plant applied for its construction permit prior to December 22, 1968 and, therefore, only a purge system is necessary. Doses have been shown to be within the acceptable valves. In addition, systems used for containment atmosphere control conform to the general requirements of CDC 41, 42, and 43.
(2) Implementation of modifications and new training programs resulting f rom post-TMI requirements aid in reducing the probabilities of event precursors to degraded-core conditions.
(3) Post-accident hydrogen control is an insignificant contributor to overall risks atrached to severe-accident scenarios.
(4) Analysis shows that the containment would not be endangered by a spontaneous combustion of hydrogen generated during the course of an accident.
(5) The purge system, if required for operation, would result in calculated doses at the site boundary and low population zone substantially below acceptance criteria in 10CFR100.
These doses are calculated in accordance with the present acceptable standards.
We believe the information discussed above providas an adequate basis for our exemption request.
If you have any questions or desire additional information, please contact us.
Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY d
J. A. Kay Senior Engineer - Licensing JAK/dd l
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