ML20077F903
| ML20077F903 | |
| Person / Time | |
|---|---|
| Site: | Perry  |
| Issue date: | 12/12/1994 |
| From: | Stratman R ENERGY, DEPT. OF |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| PY-CEI-NRR-1869, NUDOCS 9412150151 | |
| Download: ML20077F903 (9) | |
Text
'
-l CONTERf0R-
_A ENERGY PERRY NUCLEAR POWER PLANT Mail Address:
P.O. BOX 97 Robert A. Stratman 10 CENTER ROAD PERRY, OHIO 44081 VICE PRESIDENT NUCLEAR PERRY, OHIO 4408f (216) 259 3737 December 12, 1994 PY-CEI/NRR-1896L U.S. Nuclear Regulatory Commission Document Control Room Vashington, D.C.
20555 Perry Nuclear Power Plant Docket No. 50-440 Request for Enforcement Discretion with Respect to Control Rod Insertion Times Gentlemen:
This letter requests that the Nuclear Regulatory Commission exercise enforcement discretion vith respect to Technical Specification 3.1.3.2,
" Control Rod Maximum Scram Insertion Times", Actions c.1, c.3, and c.4.
The information necessary to support the request, as outlined in 10CFR2 Appendix C, Section VII.C is provided by Attachment 1.
This request has been reviewed by the Plant Operations Review Committee and approved by the General Manager - Perry Nuclear Power Plant Department.
If you have any questions or require additional information, please contact Mr. James D. Kloosterman, Manager - Regulatory Affairs at (216) 280-5833.
Very truly yours,
/
W Robert A. Stratman i
RASilkr Attachments cc NRC Project Manager NRC Resident Inspector NRC Region III Operatng Companies i.T ~1 Cleve:and Electnc thumenchng
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Toledo Ed: son 9412150151 941212 PDR ADOCK 05000440 l
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PY-CEI/NRR-1896L' Attachmsnt 1 Pags 1 of 8 INTRODUCTION Grand Gulf Nuclear Power Station Experience Control rod scram time tests performed at the Grand Gulf Nuclear Station (GGNS) in May of 1994 identified that 25 of 191 control rod insertion times to notch position 43 vere " slow" (e.g. 0.320 to 0.390 seconds) and that an additional 8 insertion times failed to meet the Technical Specification limits for operable rods (e.g. greater than 0.390 seconds).
GGNS root cause analysis endorsed a material problem with scram solenoid. pilot valve (SSPV) seating material located in the disk holder sub-assembly supplied in pre-assembled top head assemblies. The root cause analysis indicated that a problem may exist with seating materials (Viton) manufactured post-1991..The failure mechanism is believed to be associated with degradation of the seating material which causes it to " stick" to the seating surface. This results in a delay in venting the air actuator on the SSPV and concurrent " slow" control rod scram time to notch position 43. Testing performed by General Electric (GE) indicates a relationship between degradation in SSPV response time and the cumulative solenoid energization time.
Perry Nuclear Power Plant Experience The Perry Nuclear Power Plant (PNPP) SSPVs were replaced during RF0-4.
133 of the 177 installed SSPVs have been confirmed to contain Viton batch 314 material which is known to have been manufactured post-1991.
PNPP scram time data obtained during startup from RF0-4 includes data from sequence B scram tests conducted for 88 control rods on July 27, 1994 (SSPVs had been energized 23 days), single rod scram tests performed during the period August 2-3, 1994 for 36 rods (SSPVs had been energized <195 hours), and sequence A scram tests for 89 rods on August 4, 1994 (SSPVs had been energized 195 hours0.00226 days <br />0.0542 hours <br />3.224206e-4 weeks <br />7.41975e-5 months <br />).
The RFO-4 scram time data indicated that the PNPP control rods met the Technical Specification (TS) 3/4.1.3.2 Limiting Condition for Operation (LCO) for maximum insertion times. However, statistical analysis of the RF0-4 scram time data supported a claim that a failure mechanism may exist which results in
" slow" actuation of SSPVs.
An offset (approximately 0.010 seconds) was observed in scram time distribution from historical PNPP scram time data for control rods with SSPVs using certain lots of batch 314 Viton material.. Control rods with non-batch 314 SSPVs exhibited'a normal distribution.
In addition, a relationship was observed between the cumulative energization time of the SSPVs and the scram time in that a statistically significant upward shift in the data mean values was noted at 195 hours0.00226 days <br />0.0542 hours <br />3.224206e-4 weeks <br />7.41975e-5 months <br /> energization time.
Scram time data from RFO-4 was insufficient to project SSPV response time for solenoid energization exceeding 23 days. TS Surveillance Requirement 4.1.3.2.c requires control rod scram time testing at least once per 120 days of power operation. This is significantly longer than the maximum energization time (23 days) for which PNPP test data was available.
Since the scram time offset appears to change with SSPV energization time, additional testing prior to the 120 day surveillance interval to monitor this phenomenon was determined to be prudent.
PY-CEI/NRR-1896L Attcchm:nt 1 Paga 2 of 8 Scram t'ime test performance during plant operation requires a reduction in reactor power to maintain compliance with thermal limits and minimize cladding stress. On September 24 and 25, 1994 a significant reduction in reactor power was implemented for performance of a control rod deep-shallow exchange. The SSPVs had been energized for 49 days on September 24, 1994. Although not required'by TS, scram time testing was performed during the reduction in reactor power to determine if degradation was occurring in the response time of the PNPP SSPVs.
An initial test sample of 20 control rods was selected to provide a statistical confidence level of 95% in SSPV performance. The basis for selection of specific control rods in the test sample was biased to include: 1) SSPVs with Viton batch 314 lot 526724001, which comprise the highest percentage of SSPVs installed at PNPP, 2) SSPVs with Viton batch 314 lot 511058001, for which 5 of 12 rods tested during RF0-4 had startup scram times (to notch position 43) ranging from 0.280 - 0.290 seconds, 3) rods tested during RFO-4 vith scram times (to notch position 43) of 0.240 - 0.260 seconds, and 4) rods tested during RFO-4 with scram times (to notch position 43) of 0.270 - 0.290 seconds.
Rod selection for the test sample also accounted for the post control rod exchange pattern and attempted to minimize control rod manipulations and power fluctuations resulting from the single rod scram. This was accomplished by preferencing selection of rods on the periphery of the core and rods not adjacent to partially inserted control rods.
Data from the September 24/25, 1994 interim testing as a whole did not indicate degradation in performance of the suspect (Viton batch 314) SSPVs at 49 days of continuous energization.
In addition, based on cycle 4 scram time variances (to position 43) the data from lot 511058001 indicates no degradation of valve performance within this lots data subgrouped for lot 526724001, no clear trend or characteristic is evident; and data subgrouped for the tested population by previous times does not indicate a clear trend.
IDENTIFICATION OF THE ASSOCIATED TECHNICAL SPECIFICATION PNPP TS LC0 3.1.3.2 provides limits for control rod maximum scram insertion times from the fully withdrawn position, based on de-energization of the scram pilot valve solenoids as time zero.
TS 3.1.3.2 ACTION c. is applicable when the scram insertion time of one or more control rods exceeds the limits of the Specification as determined by demonstration of scram insertion times for 10% of the control rods.
DESCRIPTION OF CIRCUMSTANCES Control rod scram time testing commenced at 09:30 hours on December 10, 1994 in accordance with TS Surveillance Requirement 4.1.3.2.c, which requires control rod scram time testing at least once per 120 days of power operation, for the designated initial test sample of 18 control rods.
PY-CEI/NRR-1896L Attachmtnt 1 Paga 3 of 8 At,20:45 hours on December 11,11994 testing had been completed for 12 of 19 control rods, with.the following initial results.
,- 8 acceptable rods (scram times to notch position 43 met TS 3.1.3.2 LCO),
- 4 " slow" rods (scram times to notch position 43 >TS 3.1.3.2 LCO, but OPERABir ),
- 0 inor ible rods.
All rods tested met TS 3.1.3.2 LC0 times to notch positions 29 and 13.
Upon identification of each " slow" rod, testing of adjacent rods began in accordance with TS requirements concurrent with replacement of the affected SSPV prior to additional sample testing. Testing of the adjacent rods identified additional control rods which were " slow" to notch position 43.
Subsequent scram time test results for control rods with replaced SSPVs were acceptable.
Specific test results were as follows.
Sample HCU Time of Lot Scram Time Number Number Test Number To Position 43 1
46-51 09:50*
52672400118 0.332 sec *1 N/A **
46-55 11:40*
294140001 0.240 sec N/A.**
46-47 12:39*
294140001 0.320 sec *2 2
38-35 22:52*
52672400118 0.303 sec 3
42-35 23:17*
511058001#t 0.290 sec 4
58-39 01:26#
95969A 0.235 sec *3 5
58-35 01:41t 294140001 0.219 sec 6
50-27 01:569 294140001 0.230 sec 7
42-19 02:22#
294140001 0.250 sec 8
38-07 02:428 68235600198 0.351 sec *4 N/A **
42-07 04:109 95969A 0.250 see N/A **
42-11 04:261 68235600189 0.341 sec *5 9
46-07 09:596 68235600181 0.360 sec *6 10 18-51 18:34#
511017001 0.243 see 11 02-39 19:128 52672400189 0.312 see 12 10-39 20:456 52672400198 0.323 sec
- 12/10/94 1 12/11/94
- Rods tested per TS 3.1.3.2 ACTION c.3.a it Viton batch 314 " suspect" lots
- 1 SSPV replaced, retest @ 16:49*, 0.247 sec
- 2 SSPV replaced, retest @ 21:46*, 0.224 sec
- 3 Test results discounted due to interference from HCU diagnostic equipment, additional rod to be added to sample to satisfy SR 4.1.3.2.c
- 4 SSPV replaced, retest 0 07:38#, 0.240 sec
- 5 SSPV replaced, retest @ 15:241, 0.250 sec
- 6 SSPV replaced, retest @ 17:241, 0.254 sec
PY-CEI/NRR-1896L 4
Attachmsnt 1 Page 4 of 8 At 20:45 hours on December 11, 1994-the scram time for test sample rod 12 of 19 was identified as exceeding the maximum scram insertion time limits of TS 3.1.3.2.
ACTION c.1 was entered due to the number of " slow" control rods exceeding 20% of a 10% sample of the control rods, which requires the unit to be in at least HOT
- SHUTDOWN vithin 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. The unit vill.be required to be placed in HOT' SHUTDOWN by 08:45 hours on December 12, 1994.
Enforcement discretion vith respect to TS 3.1.3.2 ACTIONS c.1, c.3, and c.4 is requested to allow continued scram time testing to identify control rods that may be subject to degraded scram insertion times to notch position 43, and to provide for timely corrective action for identified rods.
' Performance of the scram time tests'for the purpose of identifying control rods virh potentially degraded scram times and implementing corrective action cannot l
be accomplished during a controlled shutdown.
Obtaining scram time data by scramming individual rods to shutdown the plant is not acceptable in that rod patterns below the low power setpoint would not be maintained as required to comply with the rod drop accident analysis. A reactor scram from approximately 20% power would provide data for only 88 control rods.
As described in the safety basis for the request, a limiting Cycle 5 analysis indicates that a 0.070 second delay for all rods in scram initiation is acceptable from a safety standpoint. This delay corresponds to the TS 3.1.3.2 ACTION a.1 "slov OPERABLE" scram time. Also, corrective actions to replace the SSPVs will be implemented for each rod with a scram time in excess of TS 3.1.3.2 ACTION a.1.
Therefore, the Cycle 5 analysis conservatively envelopes identification of individual control rods with scram times exceeding the limits of TS 3.1.3.2.
SAFETY BASIS FOR THE REQUEST As a result of the difficulties experienced at GGNS, the safety significance of delays in scram initiation times due to solenoid valve sticking was examined.
The control rod scram is designed to bring the reactor suberitical at a rate fast enough to prevent fuel damage. The accidents / transients that are " scram time sensitii ' are those where the scram time can impact on the Minimum Critical Power Ratio (MCPR) or the vessel overpressurization limit.
The limiting event for MCPR is the Load Reject with no by-pass valve actuation (LRNBP), and for pressurization it is the HSIV fast closure with a neutron flux-scram (with no credit for the scram signal from MSIV position).
Loss-of-Coolant Accidents do not set MCPR limits nor lead to overpressurization concerns, and other events such as Rod Vithdrawal Errors or Rotated Fuel Bundles are not impacted by scram times. Therefore the examination of the impact of a' delayed scram initiation concentrated on two areas: a re-analysis of the limiting scram time sensitive events, and reviews of the assumptions of the original analyses which serve as the basis for the Control Rod Maximum Scram Insertion Time i
j Specification.
Re-analyses of the Load Reject with No By-Pass and MSIV Closure events was performed, which assumed the single failure of the highest worth rod to insert, and the remainder of the rods received the Reactor Protection System scram signal with an additional delay of 70 milliseconds from that assumed in the
PY-CEI/NRR-1896L Attachmant 1 Paga 5 of 8 standard analysis. This 70 millisecond time delay corresponds to the time difference (for' notch position 43) between the standard analysis assumption and the point at which the rod would be declared " inoperable" by the TS.
As an example, the standard analysis assumption for the time to notch position 43 at 1050 psig is 320 milliseconds (0.32 seconds), and the rod would be declared inoperable if its scram time exceeded 390 milliseconds (0.39 seconds).
If any rod scram times to notch position 43 are measured during scram time testing at up to 70 milliseconds slower than the times assumed in the standard analysis, they are treated by TS as " slow" rods, but not inoperable.
The re-analysis assumption of a 70 millisecond delay simulates that every trippable rod in the core is held up by its scram solenoid to the point that they would all be as "slov" as allowed without being declared " inoperable".
The results of the analyseu of these two limiting events identified that the effects of the scram M tiation delay were minimal.
For MCPR, the change in Critical Power Ratio during the LRNBP event (the "dolta CPR") was examined.
In the original reload analysis for Cycle 5 (the current fuel cycle), the delta CPR vas 0.15.
In the re-analysis, the delta CPR vas 0.16, a change of only 0.01 from the base case. The MCPR limit for each fuel bundle type in the PNPP core is determined based on performance of various limiting transients, and for all but one of the seven fuel bundle types, the current MCPR limits are set by the Rotated Bundle Analysis, which is not scram time sensitive.
For one of the fuel types (the 10 gad rod GE10 fuel), adding the additional 0.01 delta CPR makes the LRNBP the most limiting transient for that fuel type. This has been accounted for by implementing an administrative penalty on the parameter used during power operation to ensure the MCPR limits are met, i.e., the Maximum Fraction of Limiting Critical Power Ratio (MFLCPR). The MFLCPR acceptance criteria has been administratively reduced to 0.990 from 1.00 until repairs are completed on the rods which have been identified as " slow".
Implementing the penalty in this manner applies the 0.01 delta CPR across all the fuel types, although only the one fuel type was limited by the re-analysis.
In practice this has limited impact, since the core is not currently being operated near the limits; from December 1, 1994 to date the MFLCPR has remained at or belov 0.957.
The change in peak vessel pressure during the MSIV Closure event was also examined.
In the original reload analysis for Cycle 5, the peak pressure was calculated to be 1294 psig, vell below the TS 2.1.3 Safety Limit of 1325 psig.
In the re-analysis, the peak pressure was calculated to be 1296 psig, also maintaining adequate margin to the 1325 psig limit.
Reviews were performed of the assumptions in the original analyses that support TS LCO 3.1.3.2 " Control Rod Maximum Scram Insertion Times". The purpose of these reviews was to determine the significance of any rods which might experience scram times slover than those examined in the re-analysis of the LRNBP event discussed above, i.e., with times more than 70 milliseconds slower than the original analysis values. The TS direct that these rods are to be declared inoperable. The analyses that serve as the basis for the ACTIONS of LCO 3.1.3.2 assume that there are as many as nine control rods that are inoperable (one (1) " stuck" and eight (8) inoperable but trippable) in addition to seven (7) more " slow" rods. The eight inoperable rods were assumed to scram slowly enough that they do not contribute to meeting the scram reactivity curve.
Although this analysis provides the basis for allowing up to 16 rods to be
" slow" (in the above described combination of one stuck, eight " slow" to the
PY-CEI/NRR-1896L Attachm2nt 1 Paga 6 of 8 point o'f being inoperable, and seven more " slow"), the current TS limits the total number of " sinu" (including slov to the point of inoperability) rods in the core at any one time to seven. These seven can therefore be any combination of " slow" or inoperable rods, as long as no more than one is a " stuck" rod.
This is reflected by ACTIONS a.1, a.3, b, c.2, and c.4.
Therefore, during the current scram time testing, from a safety analysis standpoint there is no significance to rods found to be merely " slow", since all of the rods in the entire core could be " slow" and the safety parameters of concern vould continue to be met.
If several rods are found to be slow to the point of inoperability, from a safety analysis standpoint, this situation is also bounded, and the existing Specification permits up to seven such rods to exist in the core at any one time.
PROPOSED COMPENSATORY MEASURES compensatory measures that vill be utilized during the duration of the enforcement discretion include: 1) corrective action to replace SSPVs will be implemented for all control rods identified as having scram insertion times to notch position 43 exceeding the scram insertion time limits of LCO 3.1.3.2, and immediate corrective action vill be implemented for control rods with scram insertion times identified in excess of TS 3.1.3.2 ACTION a.1, 2) reactor power vill not exceed 85% rated thermal power, 3) administrative reduction of the MFLCPR acceptance criteria to 0.990 from 1.00 vill be implemented until repairs are completed on the rods which have been identified as " slow".
DURATION OF THE ENFORCEMENT DISCRETION The enforcement discretion is requested to remain in effect until the completion of scram time tests for the purpose of identifying control rods with potentially degraded scram times and implementing cocrectiva action, not to exceed 5 days.
During the time the enforcement discretion is in effect reactor power vill not exceed 85% rated thermal power.
In addition, corrective action to replace SSPVs vill be implemented as described above. The enforcement discretion vill provide for 1) completion of SR 4.1.3.2.c that commenced on December 10, 1994, 2) development of an integrated test plan for the rcmainder of the control rods, and 3) implementation of corrective action.
ENVIRONMENTAL DETERMINATION The issue for which enforcement discretion is requested has been reviewed against the criteria of 10CFR51.22 for environmental considerations. The proposed discretion does not involve a significant safety hazard, nor increase the types and amounts of effluents that may be released offsite, nor significantly increase individual or cumulative occupational radiation exposure.
Accordingly, the issue for which enforcement discretion is requested meets the eligibility criteria given in 10CFR51.22(c)(9) for a categorical exclusion from the requirement for an environmental impact statement.
Pursuant to 10CFR51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with this request.
l I
4
PY-CEI/NRR-1896L AttachmInt 1 Pzge 7 of 8 SAFETY' HAZARD REVIEV 10 CFR 2, Appendix C, specifies that for issues involving enforcement discretion that the basis for the conclusion that noncompliance with the particular TS (or license condition) involved be provided, and that there vill not be a potential detriment to the public health and safety and that a significant safety hazard
~
is not involved.
i l
1.
The request for enforcement discretion does not involve a significant increase in the probability or consequences of an accident previously evaluated.
Control rods with scram times of up to 0.070 second delay are acceptable since such times are consistent with analysis done for the most limiting scram sensitive transient (Load Reject with No Bypass Valve Actuation) for PNPP. The analysis shoved that the Minimum Critical Power Ratio (MCPR) would be maintained above the safety limit for the transient conditions with administrative controls placed on the MFLCPR. The peak reactor pressure vessel overpressure on Main Steam Isolation Valve fast closure with a neutron flux scram would increase by 2 psig (from 1294 psig to 1296 psig), but minimal effect on the reactor vessel peak pressure safety limit of 1325 psig is realized. The analysis vould also allow the 0.070 second delay to be in effect during the duration of Cycle 5; however, this enforcement discretion vill only be utilized during the control rod scram time testing until replacement of the associated scram solenoid pilot valves restores compliance with the scram times specified in TS 3.1.3.2.
The request for enforcement discretion does not affect any initiator of the limiting transient, and it has been shown that the safety limits are maintained. Therefore, the request for enforcement discretion does not involve an increase in the probability or consequences of an accident previously evaluated.
2.
The request for enforcement discretion does not create the possibility of a new or different kind of accident from any accident previously evaluated.
Control rods with scram times of up to 0.070 second delay does not provide a mechanism for a new or different kind of accident. The delay has been analyzed and the results maintain the HCPR, with administrative controls placed on MFLCPR, of the most limiting scram sensitive transient above the safety limit and with minimal effect on the safety limit for peak reactor vessel pressure during MSIV closure events. No other transients are as sensitive to this delay, and the delay does not provide an initiator for a new or different kind of accident previously evaluated.
3.
The request for enforcement discretion does not involve a significant reduction in a margin of safety.
The analysis performed for the most limiting scram sensitive transients has shown that previously established safety limits, with appropriate administrative controls in place, are met and the margins of safety are not significantly reduced.
PY-CEI/NRR-1896L Attachm;nt 1 P:ga 8 of 8 The issue for which enforcement discretion has been proposed has been reviewed vith respect to the above factors and it has been determined that the enforcement discretion vill not pose a detriment to the public health and safety and that it does not involve a significant safety hazard.