ML20128L146
| ML20128L146 | |
| Person / Time | |
|---|---|
| Site: | Perry  |
| Issue date: | 02/08/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20128L115 | List: |
| References | |
| NUDOCS 9302190099 | |
| Download: ML20128L146 (6) | |
Text
/
UNITED STATES
[
g NUCLEAR REGULATORY COMMISSION nf "e
WASHINGTON, o C. 20556
. e s.,...../
SAFETY EVAWATION BY THE OFFICE OF NUCLEAR REACTOR RESULATION R[ LATED TO AMENDMENT NO. 46 TO FACillTY OPERATING LICENSE NO. NPF-58 THE CLEVELAND ELECTRIC ILLUMINATING COMPANY. ET AL.
PERRY NUCLEAR POWER PLANT. UNIT NO. 1 00CKET NO. 50-440
1.0 INTRODUCTION
By letter dated October 30, 1991, the Cleveland Electric Illuminating Company (the licensee) requested an amendment to facility Operating License No. NPF-58, for the Perry Nuclear Power Plant (PNPP), Unit 1.
The proposed amendment would revise Technical Specification (TS) Table 3.3,2-2 by adding new isolation signal setpoints and allowable values for temperature and delta-temperature instruments in the Reactor Water Cleanup (RWCU demineralizer rooms, demineralizer valve room,)and receiving tank room). contai The amenoment would also revise the existing delta-flow timer setpoint and allow-able value.
The new signals will initiate an RWCU system isolation based on h'
, temperature or high delta temperature in containment rooms where the
culd" portion of the RWCU piping is located.
The RWCU delta-flow timer setpoint will be extended from 45 seconds to 10 minutes.
2.0 BACKGROUND
2.1. RWCU Leakape Detection The RWCU leakage detection at PNPP Unit I features multiple, diverse sensors to detect leakage from the system. These leak detection features include reactor water level instruments, various area temperature and delta-temperature sensors i
to indicate a leak of hot water or steam, and delta-flow instrumentation to provide detection of any leakage cold enough to escape detection by the i
temperature-based methods.
The Reactor Pressure Vessel (RPV) level 2 isolation signal is the primary protection feature for a loss of reactor coolant.
This signal causes closure of selected containment isolation valves, including the RWCU valves, when reactor water level drops due to any significant loss of wate..
The temperature-based leak detection methods serve to isolate tha RWCU system in the event of hot water / steam ledge which could pose a risk of room pressurization or early offs # -
- Jose consequences (due to release of radioactive steam).
The delt,. sow instrumentation serves to provide detection of leakage from the portion of RWCU that contains relatively cold rcactor coolant (approximately 120'F) which might escape detection by the temperature sensors. The purpose of detecting cold leakage is to detect a water loss that might eventually lead to offsite dose consequences.
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With the exception of the containment pressurization case discussed in section 6.2.1.1.4.2 of the PNPP Unit 1 Updated Safety Analysis Report (USAR), the RPV level 2 isolation is the only RWCU system isolation for which credit is taken in the accident analyses of Sections 6 and 15 of the USAR. The pressurization analysis discussed in section 6.2,1,1.4.2. of the USAR takes credit only for isolating hot water or steam leaks (the function of the temperature-based detection methods). Therefore, the purpose of the RWCU delta-flow signal is not to protect the reactor fuel against a loss of coolant accident (either large or small) or to protect against containment pressurization.
Rather, the purpose of the delta-flow instrumentation is to detect cold water leakage to provide for RWCU isolation before cold water leakage exceeds a value which could result in an offsite dose concern.
2.2 RWCU Delta-Flow Isolation The cold water portion of the RWCU system consists of the section downstrean of the non-regenerative heat exchangers, through the filter /demineralizer portion of the RWCU system and back to the return side of the regenerative heat exchangers, where the water is reheated for return to the reactor system. This cold portion also includes the blowdown line to the main condenser and radwaste system, which is used-to adjust reactor system water inventory.
The RWCU delta-flow logic system compares inlet (suction) RWCU flow versus the sum of RWCU return flow to the reactor plus RWCU system blowdown flow (going to either the main condenser or to radwaste).
If a flow mismatch exists which exceeds the trip setpoint, indicating there is more flow into the RWCU system than accounted for in the return and blowdown lines, a timer and an alarm actuate.
If the trip setpoint is exceeded for longer than 45 seconds, the timer causes an : solation signal to be sent to the RWCU isolation valves.
Since PNPP Unit I received its operating license in March 1986, it has experienced numerous (39) spurious isolations of the RWCU system resulting from high delta-flow signals. Numerous corrective actions have been taken (including the addition of a dynamic compensator to the electrical summing network, relocation of the blowdown path flow orifice, and replacement of square root extractors for the delta-flow instruments) to prevent the spurious isolat'ons. Although a significant reduction in the number of isolations has been achieved, complete elimination of these spurious events has not been realized.
The licensee conducted extensive invcstigations into these events and concluded that voiding in the regenerative heat exchangers was the root cause of the four most recent spurious system isolations.
Several of the isolations occurred during operation in the Reduced Feedwater Temperature mode. The Reduced Feedvater Temperature mode is used during reactor shutdowns and cooldowns, when the temperature of feedwater returning to the reactor vessel is colder than during normal operation (because the feedwater heaters are not in service when the nain turbine is off-line).
Since the return flow from RWCU to the reactor flows into the feedwater line, the Reduced Feedwater Temperature mode of RWCU is used to bypass flow around the shell side of the regenerative heat exchangers. This lineup returns flow to the feedwater system with minimal reheating in order to minimize thermal stratification of the feedwater piping.
O
. -i Operation in the Reduced feedwater Temperature mode introduced the potential for an unforeseen RWCU system event. While operating RWCU in the Reduced Feedwater Temperature mode with the system flow bypassing the regenerative heat exchangers, water on the shell side of the regenerative heat exchangers is stagnant at reactor temperature and very close to saturation conditions. As plant cooldown progresses, slight pressure transients can occur which allow the water in the heat exchangers to flash to steam as pressure is reduced below saturation conditions.
If such flashing occurs, water is displaced out of the RWCU system, but reactor operators have no indication that the RWCU system has partially voided until sometime later when the void collapses.
The collapse of this void causes some bypass flow to be diverted into the heat exchangers, because the void location is at a lower pressure than the feedwater system.
This results in a significant decrease in system outlet (return) flow.
During this time, system inlet flow remains nearly constant or increases slightly.
The differential flow signal that results will actuate the delta-flow timer, and if tha voids are not filled prior to exceeding the 45-second setpoint, the RWCU system will isolate.
3.0 [VALVATION The proposed changes consist of the following revisions to TS Table 3.3.2-2:
1.
The addition of new RWCU system isolations from redundant and diverse temperature and delta-temperature sensors located in the containment valve room, demineralizer rooms, and receiving tank room where the " cold" portion of the RWCU piping is routed. The ambient temperature isolation setpoint and allowable value in each of these rooms will be 137.9'F and 143.7'F, respectively.
The delta-temperature isolation setpoint and allowable value in each of the rooms will be 82.C*F and 85.l'F, respectively. Alarm setpoints are also provided at lower values.
2.
The extension of the RWCU delta-flow timer length.
The timer setpoint and allowable value will be 10.0 minutes and 10.85 minutes, respectively.
l The allowable value is based on an analytical limit of 11 minutes, with an adjustment for the manufacturer's stated timer device accuracy.
The setpoint of 10.0 minutas was chosen for human factors considerations, and I
balances a desire to eiiminate spurious isolations of RWCU against a l
desire to minimize releases from an actual break. A significant margin i
is provided between the setpoint and the analytical limit, and, therefore, the allowable value.
The PNPP Unit 1 RWCU systcm was designed in accordance with the General Electric " Leak Detection System" Design Specification, which states that "The cleanup system shall have a means of flow comparison between the system inlet and outlets.
The alarm and isolation setpoints shall be established at a differential between inlet flow and outlet flow which equals 20% of system rated flow. A bypass timer shall be provided to override the isolation during-system pump and valve surge conditions." The Leak Detection System Design Spec Data Sheet (DSDS) states that "The RWCU system does have a portion of its piping that contains cold reactor coolant and therefore temperature monitoring for leakage would not be responsive.
For this reason, a flow comparison
between inlet and outlet flow is monitored for this piping." The DSDS also identified that the timer setpoint should be 45 seconds. This 45-second timer length was conservatively chosen by GE based on engineering judgement, with the intent of ensuring, without the need for any plant-specific dose calculations, that any offsite dose effects associated with leakage would be acceptable, The proposed char.ge will extend the length of the timer (while also adding ambient temperature and delta-temperature isolations into the cold portions of the piping).
The monitored portion of the system is outboard of the second containment isolation valve and therefore can be isolated from the reactor coolant pressure boundary in the event of a guillotine break.
Credit is taken for detection and isolation of such breaks by the RPV Water Level 2 signal in order to protect the reactor core, and isolations of the system for breaks large enough to leak hot water or steam and pressurize the containment are provided by the ambient temperature and delta-temperature instruments.
For small breaks in the cold portion of the system, the delta-flow instruments will still alarm and start the timer at the same setpoint as before. The instruments will still isolate the cold water leakage prior to significant offsite dose consequences, even assuming no operator action is taken to isolate the system prior to expiration of the timer.
In order to estimate the significante of a non-guillotine, cold water break that could potentially exist for the length of the proposed timer, the licensee performed an offsite dose evaluation.
The analysis assumed that the break occurrea while the reactor was operating at full power, and the break occurred upstream of the filter-demineralizers, as downstream breaks would reduce the dose rate by a factor of 100 due to iodine removal.
The analysis assumed the maximum flow rate from a break which would escape detection by the temperature detectors was 150 gpm.
For breaks resulting in a greater leak rate, water would pass through the heat exchangers so quickly that cooling would be less effective.
The temperature of the water leaving the break would exceed 150*F, and would be detected by temperature mor.itors.
The licensee performed radiological calculations which showed that for cold leakage from the RWCU piping, which is presumed low enough to not trip the temperature-based isolations, the resultant Exclusion Area Boundary (EAB) inhalation dose would be orders of magnitude less than 10 CFR Part 100 limits. Whole body doses would be negligible and were not calculated because iodine is *.he source of concern.
Calculated inhalation doses for the EAB, Low Popult.. son Zone (LPZ),
and control room were all a very small fraction of the 10 CFR Part 100 acceptance criteria.
The resultant radiological effects are bounded by both the design-basis and realistic analyses for the Hain Steam Line Break Outside i
Containment event (USAR Tables 15.6-8 and 15.6-11).
Inhalation doses for the l
RWCU case that assumes no filter-demineralization occurring prior to the break location produces doses more than a factor of 100 helow the licensee's realistic analysis value for the Steam Line Break event. The staff agrees that l
the potential offsite dose consequences of a cold leg RWCU break are likely to l
be bounded by the MSLB event, since the coolant from the RWCU break will remain in a liquid state.
i l~
i 2L.
The ambient temperature allowable values and setpoints were based on detecting breaks greater than 150 gpm (a limit conservatively chosen by the licensee to ensure that leakage temperature would not exceed 150*F--a value well below the 212*F boiling point temperature at which the leakage would flash to steam).
The delta-temperature analytical limit of 89'F was based on the ambient temperature analytical limit of 149'T and on a normal supply air temperature of 60*F. The setpoints associated with these new analytical limits will isolate hot water breaks (above 212*F) in a similar manner to the comparable existing temperature isolations in the RWCU rooms.
Since the doses from hot water / steam breakt of the RWCU system have been considered to be bounded by radiological calculations for the Main Steam Line Break Outside Containment, it was not necessary to perform additional dose calculations for the addition of the new temperature-based isolations.
The containment pressurization analysis in USAR 5ection 6.2.1.1.4.2 also remains valid and bounding for large hot breaks, due to the isolations from the ambient temperature and delta-temperature instruments.
The staff agrees with the licensee's rationale and justification for these changes and finds that the rroposed changes will reduce the number of unnecessary challenges to tne RWCU system isolation components. The staff also believes that these changes will result in increased RWCU system availability, which will help to minimize the radiation dose build-up of contamination in plant systems thus lowering area dose rates.
Tnerefore, the staff finds the proposed changes to be acceptable.
4.0 ETATE CONSULTATION In accordance with the Commission's regulations, the Ohio State official was notified of the proposed issuance of the amendment. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
This amendment involves a change to a requirement with respect to the instal-lation or use of a facility compnnent located within the restricted area as defined in 10 CFR Part 20 or a change to a surveillance requirement. 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 thr.re is no significant increase in individual or cumulative occupational radiation exposure. The tcmmission has previously issued a proposed finding that this amendment involves no significant hazards consideration and there has been no public comment on such finding (56 FR 64649). Accordingly, this amendment meets the eligibility criteria for categorical exclusion set f'rth in 10 CFR Sl.22(c)(9).
Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of this amendment.
6.0 CONCLUSION
The staff has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors: J.R. Hall H.K. Webb Date:
February 8, 1993 h
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