ML19351A377
| ML19351A377 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 10/12/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19351A376 | List: |
| References | |
| NUDOCS 8910200145 | |
| Download: ML19351A377 (3) | |
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~ SAFETY EVALUATION BY THE OFFICE _OF NUCLEAR REACTOR REGULATI%
RELATED TO AMENDMENT NO.100TO FACILITY OPERATING LICENSE NO. NPF-6 ARKANSAS P3WER AND LIGHT COMPANY r
ARKANSAS NUCLEAR ONE UNIT NO. 2 DOCKET NO. 50-368
1.0 INTRODUCTION
h By letter dated June 15, 1989, Arkansas Power and Light Company (AP&L), the licensee, submitted proposed Technical Specification changes revising the control element assembly (CEA) drop time limits for Arkansas Nuclear One, Unit 2 (ANO-2).
Specifically, the proposed amendment would expand Technical Specifica-tion 3.1.3a4.to include the average drop time of all full length CEAs, which must be no greater than the 3.2 second limit currently applied to individual CEAs. The maximum CEA drop time for any individual full length CEA would be changed from 3.2 seconds to 3.5 seconds.
The: reason for these changes is due.to the results of the ANO-2 Cycle 7 startup testing where the maximum drop time for individual CEAs exceeded the Technical Specification maximum value. This adverse change in the measured CEA drop times was revealed by a new measurement methodology. The testing method used times involved interrupting the power to the l
previously.for measuring CEA drop (CEDM) from each individual CEDM breaker.
f control element drive mechanism The y
'new. test method, which is consistent with the actual CEA scram sequence.
l involved interrupting.the power to all the CEDMs simultaneously via the main trip breakers. The additional delay time is associated with the difference
.between.the electromagnetic decay time of multiple CEDM coils and the decay time of an individual coil.
l A revised. analysis of all events was made oreviously by the licensee to support i
a CEA drop time Technical Specification increase from 3.0 seconds to 3.2 seconds.
l The revised. analyses credited space-time kinetics in conjunction with the new CEA. drop' time curve to calculate the time dependent scram reactivity insertion.
'The. core protection calculator (CPC) power uncertainty penalty was also increased L
in support of the revised analyses.
As a result of the Cycle 7 drop time testing, the margin between the slowest CEA and Technical Specification CEA drop time was 20 milliseconds (3.20 - 3.18 seconds) which is comparable to expected cycle-to-cycle variations. Since l
, failure to pass the CEA drop time test precludes entering the startup operational mode, Ap&L would'like to increase this margin before the Cycle 8 startup without any further penalties. The proposed method for increasing the time between the measured CEA drop time and the Technical Specification drop time of 3.2 seconds is to credit the measured spatial distribution of CEAs about an 8910200145 891012
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average position as opposed to the present safety analysis assum) tion that all x
CEAs drop at the same speed and therefore are at the same axial 1eight as the
- slowest CEA. This proposed analysis method is evaluated below.
i 2.0- EVALUATION The current ANO-2 safety analyses assume that all CEAs drop into the core at
. the same time and at the same rate following a reactor trip. Therefore, every l'
CEA is at the same axial height at any time during a trip. The drop time is assumed to be governed by the slowest CEA, which is limited to no longer than 3.2 seconds. Therefore, current Technical Specifications require that all CEAs, fall within the 3.2 second drop time.
v The reactivity worth of a CEA is a function of the power or neutron flux l.
. environment surrounding the CEA. During a reactor trip, the faster CEAs will be-in higher flux regions sooner and wi11 therefore make a greater relative l
contribution to the net negative reactivity insertion than the slower CEAs.
Therefore, the licensee contends that the negative reactivity insertion for L
- any' reasonable distribution of CEAs is more directly correlated to, and can be j'
represented by, the average CEA insertion rather than by the slowest.
L Based on ANO-2 measured CEA drop patterns presented by the licensee, the CEAs 1e do not fall at the same time and at the same rate during a reactor trip.
The
- scatter in the drop time about the average increases with CEA insertion and varies with individual CEA. This is primarily due to the distribution of CEA
..; extension shaft weights in ANO-2 The longer heavier extension shafts located at the core interior cause faster CEA drop times which become progressively slower towards the core periphery where the CEA extension shafts are shorter and less heavy. The staff concurs that the ANO-2 measured CEA drop time test data:shows.the CEAs have a predictable spatial distribution about the average during a reactor trip.
CombustionEngineering(CE)hasperformedasetofthree-dimensionalspace-time
- calculations using the NRC-approved HERMITE computer program. The staff has reviewed the initial conditions assumed in the HERMITE calculations and finds that they' adequately cover the range of operating conditions and the limits of the as-measured CEA distributions. These calculations show that essentially
- tb same reactivity will be inserted by CEAs falling in a reasonable distribution about an average CEA position as the reactivity inserted by all CEAs falling at the same average position, the so-called " window shade" case. This is true for any: reasonable family of CEA distributions similar to those measured at ANO-2.
However, if the distance between the fastest and slowest CEAs becomes too large or the distribution of CEAs deviates significantly from that modeled by CE in this study, then the average CEA position (window shade) may not be representa-tive of the time dependent reactivity insertion. Therefore, a limit will be placed on the CEA drop time distribution. This will be expressed as a maximum drop time limit on the slowest CEA in the revised Technical Specification. The staff concurs that this will ensure that the safety analyses remain valid for the average CEA drop time Technical Specification and finds the proposed Technical Specification changes acceptable.
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t The staff has reviewed the proposed ANO-2 Technical Specification changes which would include an average drop time of all CEAs of no greater than 3.2 seconds e :! a maximum drop time for any individual CEA of 3.5 seconds.
Based on the 0 -! CEA drop test data and the results of the CE calculations which were su h.t.ted to the staff, the time dependent reactivity insertion of a window shade scram at the average CEA drop time will provide the same reactivity insertion as the more realistic distributed case about the same
-average. The staff therefore finds the proposed Technical Specification changes acceptable for ANO-2 with the following conditions:
(1) Any fuel management change that significantly affects the core wide axial or radial power profiles, such as axial blankets or ultra-low leakage fuel management, may necessitate reverification of the average CEA drop time analysis.
(2) Changes that would significantly affect the CEA drop time distribution, such as changes to the CEDM circuits, large increases in the core flow pressure drop, changes in the total drop weight of the CEAs or changes in the location of the CEAs, may also require reverification of the average CEA drop time concept.
Barring these type of changes or failure to meet the new Technical Specification limits, reverification of the average drop time analysis will not be required on a cycle-by-cycle basis.
3.0 ENVIRONMENTAL CONSIDERATION
The amendment involves a change in the installation or use of a facility i~
component located within the restricted area as defined in 10 CFR Part 20.
The staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant l
increase in individual or cumulative occupational radiation exposures.
The Comission has previously issued a proposed finding that the amendment involves no significant hazards consideration and there has been no public comment on such finding. Accordingly, the amendment meets the eligibility l
criteriaforcategoricalexclusionsetforthin10CFRSection51.22(c)(9).
l Pursuantto10CFR51.22(b),noenvironmentalimpactstatementorenviron-l mental assessment need be prepared in connection with the issuance of the amendment.
L CONCLUSION The staff has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the will not be endangered by operation in the proposed manner, and (2) public such L
activities will be conducted in compliance with the Commission's regulations, and the issuance of the amendment will not be inimical to the common defense i
and security or to the health and safety of the public.
Date: October 12, 1989 j
Principal Contributor:
L. Kopp t