ML19322C086
| ML19322C086 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 03/25/1977 |
| From: | Stoddart P Office of Nuclear Reactor Regulation |
| To: | Grimes B Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7912300074 | |
| Download: ML19322C086 (10) | |
Text
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MEf's0RAtlDU!1 FOR:
B. Grines, Ch' ', Environnental Evaluation Branch, DOR
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cit THRU:
J. Collins, Chief. Effluent Treatment Systems Ora
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P. Stoddart, Ef fluent Treatment Systems Branch, DSE SUPJECT:
TAR-6406, TP.IP REPORT, VISIT TO OC0f!EE STATION, j
!! ARCH 16,1977 i
In response to your request, I visited the Oconee Station to discuss effluent monitoring problens with station personnel. The enclosed report sunnarizes i
the results of that meeting.
Ori;I=231gn,33,
P. G. Stoddart,14uc1 car Encineer Applications-Section,
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~~ m r,.. :.mEf fluent Treatmen't!Syppms Brancti._
c o.a c c 3,u h Divisio'n of 'S'ite Safety,and 1
Environmental Anelysis i
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Enclosure:
Trip Report I
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D. Eisenhut i
K. Goller P,. Vollmer t
W. Kreger J. t!eighbors i
A. Schwencer ETSB Staff DISTRIBUTION:
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r VISIT TO OCONEE NUCLEAR PLANT MARCH 16, 1977, BY P. G. ST0DDART EFFLUENT TREATMENT SYSTEMS BRANCH, DSE (TAR-6406)
On 11 arch 16,1977, I met with members of the staff of Oconee Nuclear Station, Unit Nos.1, 2, and 3, and with representatives of Duke Power Company, as requested by EEB under TAR-6406. The purpose of this meeting was to discuss effluent radiological monitoring problems associated with the radio-active liquid effluent discharge line.
I also sat-in on a meeting of a staff task force on radiation monitoring held on flarch 16, 1977.
Members of the Duke Power and Oconee plant staff in attendance at the 6
morning meeting were:
Bob " oehler Mary Birch Mi, Tuckman Bryan Burton i-Charles Putnam "Ted McMe'ekin'33 Attending the afternoon meeting were:
Bob Koehler Ted McMeekin Charles Putnam Bill McLean Mary Birch Jim Long Bryan Burton Don Rogers Steve DeGange The following is a summary of the points discussed:
1.
Background of Liacid Waste Monitoring Problem The liauid radwaste sy3 tem effluent from all three units are discharged to the tailrace of Keowse Hydro Station. The hydro station is used
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approximately 5% of the time; during those periods, the flow is on the order of several hundred thousand gallons per minute, which allows adequate dilution to dilute effluent concentrations on the order of 10 uti/ml down to 10-10 to 10 uCi/ml.
Approximately 20%
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-II i
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of releases can be timed to coincide with Keowee Hydro Station operating periods. When the hydro station is not operating, leakage flow from valves, gates, etc., is on the order of 40 cfs, or approximately 20,000 gom. During such periods, the only dilution flow is the 20,000 gpn leakage. With radwaste system discharges at 10 to 100 gpm, this represents a dilution flow of as much as 2,000:1 or as little as 200:1.
If the radwaste system discharge is 100 gpm at
-4 10 uCi/ml, the effective dilution is 200:1, and the effective con-centration is 5 x 10- uCi/ml, which, for a typical mix of nuclides, i
approximates the limits set forth in 10 CFR Part 20.
The Oconee staff found that radioactive contamination was building-up in the discharge pi'pe'aWd'in ths ' detector"well," causing'an' increase in radiation background count at the detector and making it difficult to set the monitor to alarm at 10 CFP Part 20 limits. On November 18, 1976, Oconee requested that NRC approve a Technical Specification change which would allow the liauid effluent monitor to be set at a value which would be equivalent to 35 times 10 CFR Part 20 limits, based on a 40 cfs dilution flow.
As part of Oconee's justification for reouesting the Tech Spec change, they noted that a new 1.ic0id monitor had been procured and installed.
It was noted that the new monitor had a replaceable inner chamber or liner which could be removed for decontamination.
1 t
. 2.
Incidental Radiation Monitorinp Problems In Oconee's investigation of problems associated with the liouid effluent monitor, a number of factors were observed which may well be of a generic nature rather than being specific to Oconee, a.
Plateout. At the present time, Oconee's licuid radwaste treat-ment consists of one stage of evaporation. The resultant
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-7 condensate is typically about 10 to 10 uCi/ml. The activity that is present, however, appears to be of an ionic nature and tends to deposit or plateout on the walls of the discharge pipe and on the walls of the liouid waste effluent radioactivity monitor The contamination builds up gradually over a period of
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chamber.
time and is firmly fixed to whatever surface is present.'
Station personnel have tried polishing the inner surfaces of the monitor chamber and also have experimented with lining materials such as Teflon; in each case, the contamination buildup rate is about The the same and the contaminent is firmly fixed to the surface.
new liouid effluent monitor noted above has a polished stainless steel chamber liner which can be removed; Oconee has two re-placement liners which permit exchange and decontamination with minimum downtime.
The use of the replacement liners in the new liouid effluent monitor represents an interin solution to the problem of plateout with respect to the level of buildup of background radioactivity 4
. i and the corresponding reduction in sensitivity of the monitor.
Oconee is in the process of re-piping their liould radwaste treatment system to permit the use of a mixed-bed polishing i
f deimineralizer downstream of the radwaste evaporator.
It is considered that this augment would remove the ionic contami-nants with a high degree of efficiency and should substantially reduce the plateout problem.
b.
Scintillation Crystal Degradation.
Attempts by licensee staff d
to correlate analysis results with liquid effluent monitor e
readings showed variations and deviations which could not be, accounted-for by. expected statistical errors.
Investigation into causes of the' obs~erved discrepancies resulted in the discovery of several cases of defects in the scintillation detector crystal assemblies, including crystal fracture, separation of crystal-glass interfaces, and discoloration of crystals due to internal hydration. The outward symptoms of the conditions described were a decrease in gross count against standard radiation sources and a shif t in energy peak positions in energy spectrum analysis.
Although not confirmed by experi-mental data, it is believed that the defects are caused by thermal shock in the liquid waste monitoring application.
Under typical conditions, the monitor is at an ambient temperature of about 70 F.
Liquid effluent is discharged from the condensate tanks at 90 -100 F, following which a flush using lake water at i
. about 50 F is used to purge the discharge line.
The Harshaw Chemical Company, manufacturer of the scintillation crystals used, recommends that the rate of change of temperature of the crystal not exceeded 1 C per minute.
In reviewing the cataiog literature of two liquid effluent monitor manufacturers using similar crystals, we note that the literature does not specify a limitation on temperature change, specifying only an operating i
e range of 0 C to 50 C (32 F to 120 F).
i It is my opinion that the licensee's position that the crystal degradations,are due to thermal shock is correct.
I recommend that a bulletin be issued.to all operating plants describing the '
'l problem and asking each licensee to immediately inspect all detector crystals which are subject to such temperature changes and to report any occurrence of damaged crystals. A procedure for routine testing to identify damaged crystals without dis-assembly of the detector probe is shown in Attachment A to this mem7randum. A routine testing procedure using a pulse height i
analysis device is shown in Attachment B.
Regular inspection of detector assemblies should be required on a schedule to be determined on the basis ?f the number of defects encountered. On the basis of Oconee experience, cuarterly in-spection would seem warranted.
t 4 W"-
. On the basis of Oconee experience, it would appear that the use of multi-channel analyzers in liouid waste monitors to initiate the closure of discharge valves is not appropriate.
A small degradation in the detector crystal could result in shift of the spectral peak to a point outside the pre-determined window, making such a monitor ineffective as a safety device.
Operation in a gross count mode would make detection of abnormal releases more reliable.
Oconee has proposed a modification in detector probe design which would encase the crystal in a jacket of plastic with good thermal insulating qualities.
Such a jacket should reduce the rate of f
"U change of temperature" to the crystal / Yhis is' only a p6tential modification and may not be practicable. Another possible solution is pre-heating of the sample stream before entering the detec-tor chamber; at low flow rates, as in the case of offline monitors this could be done electrically.
At this point in time, the problem has been identified and means have been developed to identify defective crystals; however, the probable cause of the problem remains and nothing has been done to mitigate the probiem. The problem has widespread generic implications and should be resolved at the earliest possible date.
c.
Temperature Sensitiviy of Liauid Effluent Monitors. The Oconee staff also reported observation of a temperature-dependent f
- , ms l
- readout on the new liouid radwaste monitor.
In one test in-volving measurements at only two temperatures, observed count rates at 87 F and 95 F for a single test source were as follows:
87 F 57,814 cpm 95*F 43,500 cpm The results indicate a decrease in count rate of about 1,800 cpm /*F, or about 5% per *F.
This observation was reported to be quite recent and no confirnatory work had been done as of March 16, 1977.
This observation has not been confirmed and may or may not be valid.
It is a point which should be resolved since it has generic significance in liquid radwaste monitoring.
i d.
Correlation of Varying Inputs to diouid Radwaste Discharge Monitor.
One difficulty that Oconee has hab in calibrating of the liquid effluent monitor is variation of the average energy of wastes from the three Oconee plants. The applicant reports that the average energy in wastes varies from 0.2 Mev to about 2 Mev, depending on the source.
It is my opinion that while this may present difficulties in calibrating a monitor to read directly in terms of effluent concentration, there !!hould be no difficulty in preparing a calibration curve relating energy to instrument response.
While setting the monitor alarm to respond at different meter settings for each batch of liquid discharged may be a nuisance, it should not be an insurmountable problem.
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ATTACHMENT A PROCEDURE FOR TESTING SCINTILLATION COUNTER CRYSTALS TO DETERMINE CRYSTAL FRACTURE OR DEGRADATION Materials needed:
Small check source consisting of gamma emitter such as Cs-137.
Source should be small in physical size, i.e.,
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about 0.25" x 0.25".
i Procedure 1.
Remove detector assembly from monitor well. Detector cable assembly should remain connected to monitor.
Place source in first position as in diagrams, below:
2.
4 5
3 2
CRYSTAL END OF DETECTOR 1
5 3
ASSEMBLY 2
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Make count determination, i.e., count rate, for position #1.
3.
Record count rate.
4.
Repeat step 3 for positions 2, 3, 4, and 5.
Interpretation Count rates at positions 1, 2, 3, and 4 should be approximately eoual.
Count rate at position 5 should be somewhat higher.
If count rate at any point varies substantially, this is indicative of either crystal internal fracture or separation of crystal from glass face To verify, disconnect detector assembly from monitor cable and dis-pl ate.
assemble components.
Inspect crystal visually for defects. Any visible crack in the crystal, any separation of crystal from glass face plate as evidenced by bubbles under glass, or apparent discoloration (yellowing of crystal) indicates a defective crystal and that crystal should be replaced, i
ATTACHMENT B PROCEDURE FOR ROUTINE TESTING OF 4
_ SCINTILLATION COUNTER CRYSTALS TO DETERMIN Materials needed:
Small check source consisting of gamma emitters such as a.
0.25" x 0.25". Source should be small in size, i.e., about b.
Ganma pulse height analyzer (Ludlum makes small hand portable model).
Procedure 1.
Remove detector assembly from monitor.
cable from monitor.
Disconnect detector assembly 2.
Connect detector assembly cable to gamma pulse height analyzer.
3.
Place source in position 1 as in Attachment A.
4.
Record analyzer settings. Determine pulse height analyser settin Record count rate.
p 5.
Repeat for each position 2 through 5, per Attachment A.
Interoretation 1.
If PHA settings vary substantially between source locations, dis-assemble detector and visually inspect for cracks, bubbles, or discoloration.
2.
values recorded during o'rior tests, disassemble de visually inspect.
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