ML20034A582
| ML20034A582 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 04/09/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20034A576 | List: |
| References | |
| NUDOCS 9004230519 | |
| Download: ML20034A582 (4) | |
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NUCLEAR REGULATORY COMMISSION i --
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WASHING TON, D. C. 20666
....*,6 SAFETY EVALUATION BY THE OFFICE OF' NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO.-
3 TO FACILITY OPERATING LICENSE NO. NPF-85 PHILADELPHIA ELECTRIC ~ COMPANY LIMERICK GENERATING STATION, UNIT 2 D_0CKET NO. 50-353
1.0 INTRODUCTION
By letter dated January 29,1990,PhiladelphiaElectricCompany-(the licensee) requested an amendment to Facility Operating. License No. NPF.-85 for the Limerick Generatino Station,(TS) to allow the required source -
Unit 2.
The proposed amendment wouldL change the Technical Specifications-range monitor (SRM) count rate to be reduced while ensuring that the design; level of counting certainty is maintained at all times for the SRMs.; To support the requested amendment, the licensee also submitted a document j
entitled "SRM Count Rate vs Signal / Noise: Ratio for Philadelphia Electric 1
Company, Limerick, Unit 2." EDE-10-0489, prepared by.the General Electric-Company (GE).
'l The same amendment requested by the. licensee for Limerick,1 Unit 2 by this-application of January 29, 1990 was approved for Limerick, Unit 1 by' 1
Amendment No. 34 to Facility Operating License No. NPF-39 on December.13, i
1989. The application for Limerick,-Unit I had been noticed in the 1
Federal Register on October 4, 1989 (54 FR 40932).-
i similar amendments were recently approved for Peach Bottom, Units 2 and 3 (Amendment Nos. 147 and 149 to Facility Operating License Nos. DPR-44 and-
.1 DPR-56 dated August 28,1989), two other BWRs operated'by'the licensee.
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Similar amendments have also been approved for other BWRs.. For each plant, the General Electric Company derived a curve of SRM count rate vs signal-4
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to-noise ratio which ensures the same level of confidence'at lower counts per second (cps) setpoints as is provided at the nominal'3 cps setpoint.
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R 2.0 OISCUSSION The Limerick Generating Station (LGS) TS presently ~ allow reduction-of the-c?;.
minimum SRM count rate required for control rod withdrawal or core M
alterations from the normal 3 counts per second (cps) to 0.7 as long as d
the signal-to-noise ratio is greater than or equal to two. ' General j
Electric Company advised Philadelphia Electric Company and other BWR 1
licensees that this provision is non-conservative with respect to.the l
E design bases of the SRM system, in that utilization of this provision could result in a reduction in counting certainty.
In order to ensure l
that the design level of counting certainty is maintained, a change to
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- 2 5-the TSLis proposed to incorporate a graph of SRM minimum count' rate versus signal-to-noise ratio such that reduction of the count rate as.
permitted by the TS-(3 cps.to 0.8 cps) is accompanied by a-corresponding-increase in the signal-to-noise ratio (2 to 30).
(The proposed amendment would also increase the minimum count rate from 0.7 to 0.8 cps).
The SRM system consists of four identical neutron detection ~ channels.c t
i Each channel contains a miniature in-core fission chamber, a. pulse-preamplifier, an electronics drawer, and remote reading indicators. -
l Each-detector is ecuipped with a motor driven mechanism to allow retraction from the-core at neutron flux levels above the SRM range.
The SRM system monitors thermal neutron flux over:a range sufficient to 1
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observe' core shutdown source level, approach to criticality, and overlap into the Intermediate Range Monitoring (IRM) system. The indicating range of the SRM may be extended by retracting the detectors from the core..The-SRM; system provides four cpnnels6 f neutron flux level information dis-pigedoverarangeof.10' 10 to 10'2 of rated thermal power)ps (approximately corresponding to:
to 10 c l
and four channels of flux level rate of. change information displayed as reactor period over a range of. -100:to
'+10 seconds.
The SRM fission chambers are operated'in the pulse counting mode and produce discrete output pulses with represent the composite effect of thermal neutron flux'and gama flux at the detector. Due to the nature 1
of the detector, the pulses produced by thermal neutrons are of much a
greater magnitude than those produced by gamma, although the number of-gamma.-pulses may far exceed the number of-neutron pulses. An' electronic circuit performs a discrimination action based on the amplitude of these pulses, thus providing an output signal proportional only to the neutron-count rate.
o The following changes to the TS are proposed by the licensee:
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i 1)
Insert Figure 3.3.6-1 on new page-3/4 3-60b, "SRM Count Rate versus Signal-to-Noise Ratio."
2)
Revise Table 3.3.6-2 on Page 3/4 3-60a, TS section 4.3.7.6 on Page~
3/4 3-88, and TS Section 4.9.2 on Page 3/4 9-4, to permit reduction i
of the minimum required SRM count rate below 3 cps "provided the Source Range Monitor has an observed count rate and signal-to-noise ratio on or above the curve shown on Figure 3.3.6-1."
3)
Revise the index on page viii to include Figure 3.3.6-1.
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i 3.0 EVALUATION a
The necessity for maintaining 'a minimum count rate on the SRMs when operating at pre-critical and low power cenditions is based on the most.
conservative evaluation which includes fresh fuel loaded:in-the initial fuel cycle' with no neutron sources.. present. 'A multiplying medium witF r.o neutrons present forms the basis for the accident scenario in which reactivity is gradually but inadvertently added.until-the medium is_in a supercritical configuration. The introduction of some. neutrons at'this point would-cause the core to undergo;a-sudden power burst, rather than a l
gradual startup, with no warning from the nuclear instrumentation. While-this scenario is of concern when a reactor is loaded with fresh fuel, it-is of less concern when loaded with-irradiated fuel.
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i Irradiated fuel continuously produces neutrons'by spontaneous fission of i
certain plutenium' isotopes, by photo fission,_.and by photo disintegration d
of deuterium naturally present in the moderator. The neutron production-in irrediated fuel is normally great enough to meet the normal minimum' SRM count rate of 3' cps for the duration of'a refueling outage, including -
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the subseouent reactor startup. However, there'is a possibility that a_
minimum count rate requirement of 3 cps could not be satisfied following an extended reactor shutdown, Providing a requirement-of at least 3 cps as measured by the SRM assures that-gny_ transient, should it occur,-begins at or-above the initial value of 10' of rated power with the reactor-critical as used in analyses of transient cold conditiors.a A review of-Chapter 15, " Accident. Analysis," (Section 15.4), " Reactivity and Power Distribution-Anomolles," of ghe LGS Final Safety Analysis Report (FSAR),
has ennfirmed the use of 10' of rated power with the reactor critical in 1
analyses of transient cold conditions.. Furthermore, the inadvertent i
criticality concerns evaluated in Chapter 15-(SectionL15.4.1) take no i
credit for the SRMs, since the SRMs are used for indication-only.
Since0.8 cps,thelowgstcountrateshownonFigure3.3.6-1, corresponds to approximately BX10~ of rated power, we have concluded that reducing i
the downscale setpoint in accordance with Figure 3.3.6-1 will not l
invalidate the assumptions used in the transient analyses. Stipulating a signal-to-noise ratio in accordance with Figure 3.3.6-1 assures that the SRMsareindeedrespondingtnneutronsandgheneutronflux.levelwith-i the reactor critical will be well above 10~ of rated. power due to subcritical multiplication.
The curve presented in Figure 3.3.6-1 was derived by General' Electric l
Company specifically for LGS Unit 2.
The technical basis for this curve is presented in General Electric Company Report EDE-10-0489, "SRM Count' Rate vs S/N Ratio fer Philadelphia Electric -Company, Limerick Unit 2',"
dated April,7, 1989. The curve ensures the same level of confidence 1
at lower cps setpoints as is provided at the nominal 3 cps;setpoint, the 3 cps setpoint is based on an assumed signal-to-noise ratio nf two, which.
yields a statistical neutron monitoring confidence of 95% that'the L!
indicated signal is correct. At lower eps setpoints, a higher signal-to-noise ratio is required to maintain the same level of counting certainty.
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The inverse relation between cps and signal-to-noise ratio depicted int J
Figure 3.3.6-1 ensures the 95% confidence-level.
The. proposed changes to the TSs ensure that design level of: counting-l certainty is maintained at all; times for the SRMs and are acceptable.
4.0 ENVIRONMENTAL CONSIDERATION
R This amendment involves a. change'to a requirement with respect to the installation or use of-a facility, component-located within the;restrictad:
area as ' defined in 10 CFR Part 20 and changes-to thei surveillance -require,
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ments. The staff has determined that the amendment. involves no significant h
increase in the amounts, and no significant change in the _ types, of any'
- j effluents that may be released offsite and that there:is no significant:
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increase in-individual or cumulative occupational: radiation exposure.,-The-a
- Comission' has previously issued a-proposed finding that: this1 amendment n
d involves no significant hazards' consideration and,there has been:no'public-coment on such finding. Accordingly, this ' amendment' meets the eli criteria for categorical exclusion set.forth in 10 CFR 51.22(c)(9) gibility Pursuantto10CFR51.22(b),noenvironmental' impact-statementnor
. environmental assessment need.be. prepared in connection with"the issuance:
of this amendment.
5.0 CONCLUSION
The Commission made a proposed determination that the amendment involv'es i
'i no significant hazards-consideration which was published in the Federal' Register.(55 FR 8231) on March 7,1990 = and: consul _ted with the<Comonwealth of Pennsylvania.. No public comments were received and'the Commonwealth of.
Pennsylvania did not have any coments.-
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The staff has concluded, based on the considerationsLdiscussed above,
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that:
(1) there is reasonable assurance:that~the health and safety of the public will not be endangered by. operation in the proposed manner, and 1
(2) such activities will be conducted in compliance with the Comission's '
regulations.and the issuance of this amendment will not be inimical to the common defense and the security nor to the health.and safety of the.public.
Dated: April 9, 1990 Principal Contributors:
U Richard Clark and Howard Richings l
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