ML20031E163
| ML20031E163 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 09/22/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20031E152 | List: |
| References | |
| NUDOCS 8110150161 | |
| Download: ML20031E163 (3) | |
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SAFETY EVALUATION REPORT REVISED BORON DILUTION ANALYSIS FORT CALHOUN STATION, UNIT NO.1 -
Introduction During a reanalysis of the Boron Dilution Event for hot and cold shutdown modes at the Millstone Unit 2 facility in early 1980, it was discovered that an incorrect assumption had been made (Reference 1).
The analysis of this event while in Mode 5 (cold shutdown) had assumed a full reactor coolant system and a 1% sbutdown margin. The results of this analysis showed that it would take 20 minutes of uncontrolled boron dilution before the reactor returned to criticality.
This satisfied the 15 minute time interval recommended in Standard Review Plan 15.4.6 for operator action.
However, the analysis had not considered that Mode 5 operation is permitted with the reactor coolant system drained to the centerline of the hot leg. Assuming a reduced volume of reactor coolant to be diluted results in a time to reach criticality of less than 15 minutes.
The Millstone facility corrected this problem by revising their Technical Specifications so that a greater shutdown margin existed while in Mode 5.
By increasing the minimum shutdowd' margin from one to two percent, a time to criticality of greater than 15 minutes was established.
The staff has requested ths Fort Calhoun licensee to review their boron dilution event to verify that they have assumed a reduced reactor coolant system volume in their analysis. The licensee's response (Reference 2 and 3) states that a Technical Specification change needs to be made to ensure that an adequate shutdown margin exists while in Mode 5 operation.
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l Evaluation The previous boron dilution event analyzed for Fort Calhoun at cold shutdcwn conditions assumed a full reactor coolant system volume (5506 ft ).
The cold shutdown margin of 2.0% aK/K allowed a dilution 3
time to criticality of 31.4 minutes.
A change in the cold shutdown margin from 2.0% 4K/K to 3.0% aK/K is necessary to remain above the recommended 15 minute dilution time to criticality when the.RCS is not completely full. This can occur under certain conditions such ~as replacing a reactor coolant pump seal which i
could allow the RCS to be drained to.a level which is less than one-half the normal volume.
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. To incorporate a conservative analysis, the baron dilution incident 3
was reanalyzed using the minimum coolant volume (2036 ft ) allowed, shutdown banks A and B withdrawn, and all regulating rods inserted except for the most reactive regulating rod which was left stuck out.
Assuming the above rod contigurati' n and coolant volume (which will o
yield the minimum dilution time to criticality), a 3.0% aK/K yields a time to criticality of 16.53 minutes. Normally, all regulating rods and shutdown groups would be inserted in the cold shutdown mode, resulting in a-time to criticality greater than 16.53 minutes.
Operator recognition of boron dilution is achieved through the audio response of the wide range logarithmic channels measuring neutron Four independent logarithmic channels continuously monitor fl ux.
the neutron flux during the cold shutdown mode, with a single channel selection providing the audible counts /sec in the control room.
Upon the start of boron dilution, the logarithmic flux monitor would announce pn increasing count rate to the operator who would then take a'ppropriate action to stop dilution.
Based on this revised analysis, the licensee has proposed revising the The licensee has pr<cposed (Reference 2 and 3)
Technical Sp'ecifications.
raising the minimum shutdown margin from 2.0%' to 3.~'% aK/K while in 0
the cold shutdown mode.
We have reviewed the licensee's submittal and conclude that a co.1servative boron dilution event has been analyzed. Assuming a conservatively low value for the reactor coolant system volume, the licensee has shown that raising the cold shutdown margin to 3.0% aK/K (which allows 16.53 minutes for corrective action) provides adequate protection to avoid a return to criticality.
Additional conservatism was included by assuming that the shutdown control rods were withdrawn from the core.
Based on our review, we conclude that the proposed Technical Specifica-tion changes are acceptable.
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g 3-Environmental Consideration We.have determined that the amendment does not authorize a change in effluent types or total amounts nor an increase in power level and Having made will not result in any significant environmental impact.
this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR 551.5(d)(A), that an environmental impact statement or negative declaration and environ-mental impact appraisal need not be prepared in connection with the issuance of this amendment.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the pu' lic o
will not be endangered by operation in the proppted manner, and (3) such activities will be conducted in compliance Mith the Commission's regulations and the issuance of this amendment will not be inimical to tne c0mmon defense and security or to the health and safety of the oublic.
Date: September 22, 1981 References 1.
Millstone Unit 2 LER 80-05/1T-0.
2.
Letter from LeBoef, Lamb, 'Leiby and MacRae (Attorneys for Omaha l
Public Power District) to NRC, dated March 30, 1981.
l 3.
Letter from Omaha Public Power District to NRC (W. C. Jo Robert A. Clark) dated August 3,1981.
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