ML20209G689
| ML20209G689 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/25/1985 |
| From: | Cintula T NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20209G683 | List: |
| References | |
| TASK-AE, TASK-T506 AEOD-T506, NUDOCS 8509190653 | |
| Download: ML20209G689 (7) | |
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AEOD TECHNICAL REVIEW REPORT
- UNIT: San Onofre-3 TR REPORT NO. AE00/T506 DOCKET N0. : 50-362 DATE:
LICENSEE: Southern California Edison EVALUATOR / CONTACT: T. Cintula NSSS/AE Combustion Engineering /Bechtel
SUBJECT:
PRIf1ARY SYSTEM RELEASE DUE TO PRESSURIZER DEGAS RELIEF VALVE LIFTING EVENT DATE: October 2, 1984 4
SUMMARY
A disturbance in the instrument air system at San Onofre-3 caused two pressure regulating valves in the pressurizer degas system to fail to control downstream pressure for approximately one minute. During this-malfunction, approximately 40 gallons of primary system fluid was released to the environment. This event was investigated because: (1) San Onofre-3 is the only modern PWR with the subject degas system; (2) its potential safety implications, including LOCA, seismic considerations, potential for radioactive releases, etc., had not been analyzed in the FSAR; and (3) system operation was not addressed by technical specification requirements.
We found that instrumentation (i.e., radiation monitors outside of contain-ment) and operating procedure guidelines have not been provided to assure system isolation if the pressure regulating valves should again fail to control downstream pressure. We believe this is less than fully consistent with ALARA guidelines. However, the event was of limited actual significance and no unanalyzed safety concerns were raised. We also concluded that it would be unlikely for subsequent releases from the. pressurizer degas valve to be of-radiological significance as this event caused a maximum reposition-ing of the flow opening in each pressure regulating valve and because of subsequent licensee modifications to the system to minimize recurrence.
No further action appears necessary at this time.
- This document supports ongoing AE00 and NRC activities and does not represent the position or requirements of the responsible NRC program office.
8509190653 850725 PDR ADOCK 05000362 S PM
DISCUSSION On October 2, 1984, with Unit 3 of the San Onofre Nuclear Generating Station operating at 100% power, a low level radiation alarm was received from the 4
main vent stack radiation monitor (3RE-7865). Tha sharp spike on the monitor recording trace indicated that the release was of a short duration.
An investigation by the plant operating staff found that a relief valve (3PSV-0245) in the Pressurizer Degas System (PDS) lifted for approximately one minute. During the time the valve was open, approximately 40 gallons of condensed pressurizer steam was released from the primary system. The radioactive effluent was collected by the Miscellaneous Waste Tank (T063).
Tne waste tank is vented to the gas collection header which is piped to the plant vent stack. The release was calculated to be approximately 97.5 curies of noble gas to the environment and the release concentration was determined to be within the plant technical specification limit.
A review by tne Southern California Edison Company concluded that the ,
event was caused by a momentary pressure instability in tne instrument air system that subsequently affected the two pressure regulating valves (3PV-0248 and 3PV-0249) upstream of relief valve 3PSV-0245 (see Figure 1). The instrument air system pressure perturbation allowed the degas system pressure, downstream of the pressure regulating valves, to briefly increase above the 200 psi lift setting of the relief valve.
The purpose of the PDS is to reduce primary system gaseous activity that is generated by defective fuel assemblies in the initial core at San Onofre-3..
Control of radicactive gases in the reactor coolant system (RCS) is desirable to provide a proper working environment within the facility in order to minimize occupational radiation exposures, and to minimize releases of radioactive materials in effluents to unrestricted areas to levels'as low as reasonably achievable (ALARA).
The PDS is designed to transfer noncondensible gases from the pressurizer steam space to the vapor space of the VCT. During power operation, the pressurizer steam space is continuously supplied witn a small flow of sub-cooled reactor coolant from a cold leg of the RCS via the pressurizer spray system. Tne spray action has the effect of stripping noncondensible radioactive gases from the coolant spray allowing the gases to accumulate in the steam space of the pressurizer.
During power operation's, chemical and radiological sampling of the pres-i surizer steam space is performed by the Nuclear Sampling System (NSS). The NSS is designed to vent steam and noncondensible entrained gases on an intermittent basis from the pressurizer steam space at temperatures and I pressures up to 700*F and 2485 psi respectively, and perform up to 13 discrete analyses on the captured sample. The NSS is described and analyzed in Chapter 9.3.2 of the San Onofre-3 FSAR.
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As shown in Figure 1, the NSS, as installed at San Onofre-3, starts with a 3/4 inch line that is tapped into the pressurizer steam space. A flow reducing orifice (3F0-0119) is installed in the line to limit the potential for excessive mass loss from the RCS. The area of the orifice is designed so that, in the event of a complete break in the line downstream of the ori-fice, the resultant flow would be less than the rapacity of one charging pump.
A break of this magnitude is analyzed in Chapter 15 of the FSAR.
Downstream of the orifice are two containment isolation valves (3HV-0510 and 3HV-0511) and a sample cooler (3AX-0493). The containment isolation valves were designed to meet' the requirements of 10 CFR 50, Appendix A, General Design Criterion 54. The containment isolation valves close autom-atically from a safety injection actuation signal or a high containment pressure signal. They do not close automatically on a high radiation signal from a radiation monitor located either inside or outside of the containment bounda ry. '
Reduction of sample tenperature and change of phase is accomplished by a downstream sample cooler. Upon leaving the cooler, the primary system steam is condensed and subcooled to less than 140*F and contains radioactive gases in concentrations up to fifty times that of RCS fluids.
At San On6fre-3, the PDS begins as a separate system with a line that tees off the NSS downstream of their shared sample cooler (3AX-0493). Pressure reduction in the 1/2 inch diameter PDS supply line begins with two in-series instrument air operated pressure regulating valves (3PV-0248 and 3PV-0249).
The two pressure regulating valves reduce PDS pressure from a maximum of 2485 psi to 350 psi and 110 psi, respectively. A final system pressure reduction to 60 psi is achieved by a downstream pressure control valve (3PCV-0246).
The degas system is normally aligned to flow into the CVCS through the VCT inlet spray line, where the stripping action of the VCT spray has the effect of removing the dissolved gases from the fluid. The VCT is vented periodically to the GRS for routine collection and treatment of the radioactive geses on a batch basis. The GRS has adequate capacity to process the additional volume of radioactive gas from the PDS without reducing the effective holding time for the collected gases. The greater radioactive gas concentration being vented from the VCT may improve GRS effectiveness.
During operation, the flow reducing orifice limits tne flow through the PDS to an equivalent of 1/2 GPM from the pressurizer steam space. .The PDS is designed to provide a continuous flow to the VCT from the pressurizer steam space. However, when discrete sampling of the pressure steam space is required, the NSS is aligned for sampling and the PDS is isolated.
Procedures are in effect so that pressurizer vapor space sampling and degasification are not operational concurrently.
3
The PDS was installed in early 1984 and placed into operation in September 1984. Apparently, San Onotre-3 is the only large, modern PWR with an operational PDS. The safety aspects of this system were evaluated by the licensee under the provisions of 10 CFR 50.59, " Changes, Tests and Experiments." The PDS is not described in the updated FSAR. The licen- :
see has not submitted technical specification amendments to identify operability requirements of the PDS components. -
In reviewing the 10 CFR 50.59 proposed f:cility change for the pressurizer degasification modification, the licensee concluded that the PDS modifica-tion to the design of the NSS would not reduce the margin of safety of any plant system. In addition, the PDS was designed in accordance with existing
- seismic criteria and would not change the seismic qualification for other connected piping systems. The licensee concluded that the probability of an accident or malfunction of any equipment previously evaluated in the FSAR would not be increased and that all accident scenarios, probabili. ties and consequences remain bounded by existing analyses. The licensee also concluded the PDS does not require any additions to the technical specifica-tions for Unit 3.
1 In analyzing the event of October 2, 1984, the licensee attributed the 4
lifting of the pressurizer degas relief valve (3PSV-0245) for approximately one minute to a local disturbance in the instrument air system which directly affected the two pressure regulating valves (3PV-0248 and 3PV-0249) upstream of relief valve 3PSV-0245. The opening of each of these pressure regulating valves is controlled by a diaphragm operator with instrument air piped to both sides of the diaphragm. Air pressure on the bottom of the diaphragm, ,
tending to open the valve further, is balanced by a controlled pressure derived from a combination of instrument air and sensed downstream PDS pressure, tending to close the valve. Therefore, increases in downstream PDS pressure should close the valve further. These valves will fail clu>ed on a complete loss of instrument air supply. The licensee postulated that the release from the PDS may have been initially caused by the instrument air supply to the two pressure regulating valves dropping to zero pressure.
This would have caused the two pressure regulating valves to fail to their closed position. Concurrently, pressure control valve 3PVC-0246, which regulates only on sensed downstream pressure, would have traveled to the full open position in response to the loss of downstream pressure. Then, as instrument air pressure began -to recover, the two pressure regulating valves would have started to open. With all three valves partially open, downstream PDS pressure briefly increased above the 200 psi lift setting of relief valve 3PSV-0245 causing the release. The licensee has installed throttle valves in the instrument air supply line to the two pressure regulating valves to slow their response to instrument air system pressure changes and thereby minimize the potential for possible pressure overshoot.
Also, a design change will be implemented to relocate the inlet line (from the 1/2 inch diameter piping of the PDS to the 3 inch piping of the CVCS) of relief valve 3PSV-0245, so it will be less sensitive to abrupt pressure increases in the PDS.
FINDINGS Ine PDS design is such that it can provide a direct relief path from the primary system to the plant vent stack (i.e., to the environment) on system overpressure. However, the same relief valve also provides overpressure protection for tne purification ion exchangers, an existing component that is part of the CVCS. The CVCS also processes radioactive gases, so the same path for venting radioactivity to the environment through this relief valve has existed prior to the PDS modification. However, the PDS modifica-tion has the potential to increase the relief valve actuation frequency and, therefore, the total releass of radioactivity to the environment.
The loss and subsequent recovery of the instrument air supply to the two pressure regulating valves caused the PDS to operate in a nonconservative manner. During the October 2, 1984 event, the valve repositioning sequence allowed system pressure to increase sufficiently to cause the overpressure protection device to operate.
The control room operators took no action during the event to minimize the release of radioactivity to the environment. Rather, the duration of the release was limited by the recycle time of the two pressure regulating valves and the downstream pressure control valve (to reduce PDS pressure below 200 psi). The existing plant emergency procedures do not specifically address the conditions or situations for which the PDS isolation valves are to be manually closed in the event of an alarm from the main vent stack radiation monitor.
The licensee did not discuss the likelihood or analyze possible radiological consequences to the plant or the environment for the lifting of a PDS relief valve in tneir 10 CFR 50.59 facility change. The licensee has not proposed technical specifications on PDS operability limitations to assure that releases or radioactive material in gaseous effluents will be consistent with the ALARA goal.
CONCLUSIONS The PDS modification at San Onofre-3 has the potential to increase the release of radioactive material to unrestricted areas by the actuation of relief valve 3PSV-0245. The radioactive release of October 2, 1984 was of limited actual significance, however, because the PDS pressure regulating valves recycled quickly to reduce systen pressure below the relief valve lift setting. Consequently, the release was of short dura-tion. The PDS design makes it unlikely that subsequent malfunctions and releases from the PDS relief valve would be of significantly longer duration because this event likely resulted in the maximum amount of repositioning of the flow opening of each of the PDS volves, l
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The plant vent stack radiation monitor performs only an alarm function.
The monitor does not isolate the plant vent stack or any of the systems j which tie into the stack. The control room operators do not have proce-dural guidelines to close the vent stack or PDS isolation valves en a high radiation alarm. The lack of such precautions (to shorten the duration of a possible release of radioactive materials in effluents to unrestricted areas to less then the arbitrary recycle time needed for the valves to regain their pressure control function) would appear to be less than fully consistent with ALARA recommendations.
, The modification to the instrument air supply to the pressure regulating valves should lessen the impact of instrument air system pressure fluctua-
, tions on the pressure regulating valves. Similarily, relocating the inlet line of the PDS relief valve to a much larger diameter line should lessen the frequency of abrupt pressure increases in the PDS. ,
In conclusion, although the arrangements provided to ensure timely isolation of the PDS would appear to be less than that which would be needed to meet the ALARA goals, we would conclude that no significant safety concerns were raised by the event at San Onofre-3. Finally, no further action appears necessary at this time.
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g 110P Ion Exchangers 350P 140F . (Letdown F1ow)
700F 2485P 2285P Nuclear <e 653F 2235P Sainpling System Gaseous Radwastec Containment System f g 3 Pressurizer Volume Control Tank Figure 1. Pressurizer Degas System at San Onofre 3
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