ML19296B886
| ML19296B886 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 02/14/1980 |
| From: | Counsil W, Fee W NORTHEAST UTILITIES |
| To: | Ziemann D Office of Nuclear Reactor Regulation |
| References | |
| TASK-15-19, TASK-RR NUDOCS 8002220328 | |
| Download: ML19296B886 (10) | |
Text
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NORTHEAST UTILITIES fey [9;516%-
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L J :::= r?,".'.".:..'=;'l February 14, 1980 Docket No. 50-245 Director of Nuclear Reactor Regulation Attn: Mr. D. L. Zie= ann, Chief Operating Reactora Branch #2 U. S. Nuclear Regulatory Commission Washington, D. C.
20555
References:
(1)
J. F. Opeka letter to B. H. Grier dated January 31, 1980, transmitting LER #RO-80-02/lP.
(2)
J. F. Opeka letter to B. H. Grier dated February 14, 1980, transmitting LER #RO-80-02/lT.
(3)
R. F. Thibault (GE) letter to W. C. Mihal (NUSCO) dated February 4,1980 (attached).
(4)
A. D. Vaughn (GE) to M. P. Hills (NUSCO) dated February 8,1980 (attached).
Gentlemen:
Millstone Nuclear Power Station, Unit No.1 Small Break Loss of Coolant Accident Analysis This letter provides the results of a small break Loss-of-Coolant Accident reanalysis that has been performed specifically for Millstone Unit No.1.
The reanalysis was necessary due to a non-conservative assumption that was discovered in the generic small break accident analysis.
On January 30, 1980, Northeast Nuclear Energy Company (NNECO) representatives me:: with the General Electric Company to review the performance of the Low Pressure Coolant Injection loop selection logic.
It was concluded that one of the assumptions in accident analysis for low pressure ECCS injection was non-conservative. This was subsequently reported to I&E in References (1) and (2), and the NRC Staff Project Manager for Millstone Unit No. I was informed by telephone on January 31, 1980.
It was agreed with the the Proj ect Manager that 'NNECO would submit the results of the reanalysis, in accordance with.
10CFR50.46 and Appendix K.
The analysis has been performed and the results are provided as attachments to this lettdr (References (3) and (4)). The non-conservatism was also presented to the Staff in a GE presentation in Bethesda on January 31, 1980, wherein the effects on the generic product line were discussed.
Plant-specific analysis for Millstonc Unit No.1 showed that corrective action was necessary, as discussed further below.
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. The non-conservatism is described briefly as follows: Diff erential pressure instrumentation is used in the LPCI loop selection logic to detect the broken recirculation loop and inject LPCI flow to the intact loop.
It was discovered that the sensors would be insensitive to small pressure diff erentials, such that the logic would be incapable of reliably selecting the intact loop for break sizes less than 0.1 ft2 The sensors are capable of detecting the broken loop for all other break sizes greater than or equal to 0.1 f t2 Selection of the wrong loop for a small discharge side break, combined with a loss of all off site power and the worst single failure (loss of gas turbine emergency power) would result in less LPCI flow getting into the reactor vessel than assumed in the accident analysis.
Upon learning of the non-conservatism, Reference (1) was submitted to I&E and the plant took corrective administrative action. The thermal limits (MAPLHGR's) were reduced and the automatic timer delay for the Automatic Depressurization System was reduced.
Both administrative actions were within Technical Specification limits and were confirmed to be the necessary corrective action based on the reanalysis. The attached results show (1) the effect of the LPCI loop selection logic failure for the affected break sizes; (2) the necessary MAPLHGR multiplier; and (3) effect of ADS timer setback.
The administrative MAPLHGR reduction and ADS timer setback are interim corree-tive actions that will remain in effect until the appropriate Technical Specifica-tion changes are made for the long term. NNECO plans to submit proposed Technical Specification changes for MAPLHGR Ifmits that will conservatively bound the more limiting small break accident results. These Technical Specification changes will analytically incorporate the Isolation Condenser System into the ECCS systems, such that credit can be taken for the performance of the Isolation Condenser. We intend to justify ECCS credit for the Isolation Condenser for the remainder of the current Cycle 7 and for the balance of commercial plant lif e.
NNECO plans to propose the necessary Technical Specification changes by March 15, 1980.
Should you have any questions, please contact us.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY M
W. G.Gdounsil Vice President By:
W. F. Fee Vice President Attachment 0
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GEN ER AL h ELECTRIC
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GENERAL ELECTRIC COMPANY.
1 WASHINGTON STREET DEPARTMENT W E L L E sL E Y. M Ass ACHusETTS 02181 Phone (617) 237 2o50 cc:
R. Mc Guiness T.J. Dente G-EH-0-17 R.J. Herbert February 4,1980 Mr. W.C. Mihal Northeast Utilities Services Company P.O. Box 270 Hartford, CT 06101
SUBJECT:
SMALL RECIRCULATION DISCHARGE BREAK ANALYSIS WITH A GAS TURSINE FAILURE AND LPCI INJECTION INTO THE.
RECIRCULATION LOOP WITH THE BREAK As a follow-up to the January 30, 1980 meeting in Chicago on the subject matter, the following information is forwarded for your use. A Millstone specific ECCS analysis has been conducted for the subject small break scenario which was not previously analyzed.
ASSUMPTIONS--The assumptions involved in this postulated scenario are as follows:
- 1) A small break between 0.04 ft.2 and 0.1 ft.2 occurs in recirculation loop A in the discharge side of the loop-
- 4) The loop selection system selects the broken loop because the break is very small;
- 5) A significant portion of the LPCI flow is lost out the break in the discharge side of recirculation loop A.
The assumptions involved in the amount of LPCI flow lost out the break are listed below; a--The pressure at the break is higher than vessel pressure due to forward LPCI flow through the jet pump nozzles; b--Calculated LPCI flow out the break is maximized with the standard subcooled water relationship based on the pressure at the break as discussed in (a).
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GENER AL $ ELECTR,1C c--The break flow is 120 F.
COMPARISON WITH PREVIOUS ANALYSIS:
Analysis was performed in October W8 for the Millstone plant
- small breaks and transmitted in a letter to P.S. Check (of the U.S.
NRC) from R.E. Engel (General Electric Co.) dated November 1,1978 in the context of D.C. power failure" analysis. This analysis assumed the correct systems remaining operable for a Millstone postulated gas turbine failure but was presented in the context of D.C. power failure.
In other words, the effect of a Millstone gas turbine failure on ECCS performance is identical to the bounding D.C. power failure assump-tions of the November 1978 letter.
The only change in the analysis presented here relative to the, November 1978 analysis is the postulated injection of LPCI.into a broken recirculation loop for break sizesfo.1 ft.2 ANALYSIS RESULTS:
Six break sizes over the potentially affected range of the small break spectrum were analyzed specifically for Millstone with the approved SAFE, REFLOOD, and CHASTE models.
The results of these analyses show that for the a:sumption of LPCI injection into the recirculation line with a postulated break size below the detection capability of the loop selection system, the small break accident becomes more limiting than the large break accident which was previously limiting.
New bounding MAPLHGR limits were calculated for Millstone in the form of a multiplier to be applied to current MAPLGHRs to reduce calculated temperatures below 2200* F.
The most limiting fuel type and exposure were used in this analysis and justification that these results are bounding is given in Attachment 1.
Results are tabulated in Table 1.
It was determined that a reduction in the ADS timer delay from 120 seconds to 45 seconds, (which affects only the time of automatic ADS actuation) would produce a mitigating effect that results in earlier reflooding of the core without any other significant effect.
The calculated water level, pressure, and peak cladding testperature as a function of time for the 'no;t limiting break size and.for the ADS timer set at(45 seconds are given in Figures 1 and 2.
- Millstone was the bounding pIant used in this analysis.
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GENER AL h ELECTR,1C t
MODEL CONSERVATISMS:
Currently approved models contain assumptions that are appro-priate for large limiting breaks that are insppropriately and overly conservative for small limiting breaks.
The two most significant of the'se c"onferTatisms are the absence of steam cooling and the presence of a 20%' addition to the ANS decay heat curve for uncertainty in the core heatup analysis with CHASTE.
For the small breaks of. concern here the core is not calculated to uncover until approximately 200 seconds or more after scram. At this point on the decay heat curve the uncertainty has been known to be much less than 20% for many years. Also the presence of more coolant in the reactor core for the long period before and.during uncovery of the core will produce steaming rates large. enough to give significant core cooling.
Taking into account either one of these two effects.would prevent the calculated peak cladding temperatures from exceeding limits with current MAPLHGR limits (i.e., with no reduction in MAPLHGR).
If we can be of any furtherhelp, please let us.know.
Very truly yours, R.F. Thibault Service Man ~ager - Nuclear RFT:MAC:js Attachments e
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TABLE 1
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t ADS TIMER $120 SEC MAPLHGR MULTIPLIER =.80*
Break Size Uncovery Time Reflooding Time Total Uncovered Time
( FT.2 )
(SEC)
(SEC)
(SEC)
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.04 293
'5 84 291
.05 263
- 569, 306
.07 221 541 320
.085 197 539 342'
.10**
186 539 353
.15 189 458 269 ADS TIMER;6 45 SEC MA/LHGR MULTIPLIER = 0.86*
.07 206 507 301
.085 185 497 312
.10**
168 489 321 Multiplier based on 8 x 8 R fuel with current MAPLHGR'of 10.66KW/FT.
Limiting break 4
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SLIBJECT: JUSTIFICATION FOR BOUNDING MAPLHGR MULTIPLIER AND PU CALCULATION FOR SMALL BREAKS The following discussion explains why the MAPLHGR Multiplier and PCT calculated usin'g the highest 8x8 retrofit MAPLHGR is bounding for all fuel types, at all exposures, in the core.
The primary determinant of PCT for a small break LOCA is the MAPLHGR. The stored energy in the fuel, which is a function of exposure, is not importans because it is removed from the fuel during the long time that the fuel is covered by the coolant.
This is evidenced in the MAPLHGR limits of plants limited by small breaks (i.e. none jet ptrnp ;,lant), these limits are practically independent of exposure.
The MAPLHGR limits derived for a large break LOCA are typically as shown on Figure 1, be!ow.
The maximum value occurrs at mid-life, where the stored energy is minimum (i.e. gap conduchnce is the highest).
Therefore, for a particular fuel geometry, the highest MAPLHGR will yield the 0
highest PCT for a small break LOCA. Also, if PCT exceeds the limit of 2200 F, for a small break, the per-cent MPLHGR reduction required to maintain the small break 0
PCT below 2200 F will be greatest for the highest MAPLHGR, this is shown on Figure 2, below. Therefore, for a particular fuel geometry, the reduction factor for the highest MAPLHGR may be conservatively applied to all exposures.
Sensitivity ~ studies have been conducted which show that the multipli>.. for 8x8 retrofit fuel is 1 to 21 smaller than 8x8 standard fuel multiplier and 6 to 71 smaller than 7x7 fuel multiplier. This is attributed to the higher dependance of the 7x7 fuel PCT to stored energy under large break LOCA conditions, with respect to 8x8 fuels, and.
to the smaller heat capacity of the 8x8 ' retro, fit fuel with respect to the 8x8 standard fuel.
j Therefore. 8x8 retrofit fuel will have the smallest multiplier of all fuels 1
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F9 ruary 8, 1980 cc: RR Calabro b
FP Felini ADV:80-017 RL McGuinness WC Mihal JF Opeka CH Stoll Mr. M. P. Hills HORTHEAST UTILITIES SERVICE COMPANY 9 rW fl D " '{ J" '(
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SUBJECT:
Millstone 1 Low Core Flow MAPLHGR Multiplier
REFERENCE:
Letter to W. C. Mihal from R. F. Thibalt (G-EH-0-17), "Small Recirculation Discharge Break Analysis With a Gas Turbine Failure and LPCI Injection Into the Recirculation Loo ~p With the Break". Dated February 4,1980.
Dear flike:
This letter is in response to your question concerning the applica-bility of the Low Flow Multiplier for MPLHGR in the Technical Specifica-tions for Millstone 1 (Bases Section, page 11.1). The statement in question reads that "for flows less than 90 percent of rated, the. accept-able MAPRAT must be less than 0.95."
The current MPLNGR multiplier that is being applied because of the consideration of Gas Turbine Failure and the LPCI Loop Section (reference) provides sufficient margin under low core flow conditions such that the
" Low Core Flow Multiplier" does not need to be applied.
Please be informed that the analysis defined in the reference letter has been internally reviewed with the same Quality Assurance as is main-tained in the normal reload licensing schedule.
If you have questions on this matter, please contact me.
Very truly yours, WYY A. D. Vaughn Fuel Project Manager Millstone M/C 174; (408) 925-1618 Veri fication:
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(, L, Casillas Engineer R. E. EngeV, tianager
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