ML20127N302

Responds to Requesting Info Re Increased Background Radiation Readings Detected by Monitoring Instruments Operated by Citizens Radiological Monitoring Network,Pilgrim Inc

ML20127N302
Person / Time
Site:	Pilgrim
Issue date:	11/20/1992
From:	Murley T Office of Nuclear Reactor Regulation
To:	Salter H CITIZEN'S RADIOLOGICAL MONITORING NETWORK, PILGRIM
References
NUDOCS 9212010169
Download: ML20127N302 (4)

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e November 20, 1992 Harry W. Salter, President Citizen's Radiological Monitoring Network, Pilgrim inc.

45 Oldfield Drive Kingston, Massachusetts 02364

Dear Mr. Salter:

I am responding to your letter of October 28, 1992, in which you requested information regarding increased background radiation readings detected by monitoring instruments operated by you and members of your organization.

Specifically, you asked (1) if two events reported at Pilgrim by Boston Edison Company (BEco) to the Nuclear Regulatory Commission (NRC) were related to the it. creased background readings and (2) if other monitors in the area showed similar increases. On the basis of an independent NRC inspection and discussion with BEco representatives, we have concluded that the increased monitor readings were not the result of either of the two events mentioned above and were most likely associated with rainfall.

The two Pilgrim events, recorded by NRC Event Reports 24491 and 24494, did not cause the increased background radiation readings. Shutdown of the Pilgrim reactor was started on October 23, 1992, to begin a planned 30-day mid-cycle outage. The first event was an unplanned automatic actuation of the reactor protection system (RPS) that occurred at 11:10 a.m on October 24, 1992. The actuation was the result of a spurious neutron monitoring system signal, rather than any change in the actual neutron flux level. The reactor had been subcritical for more than 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> before the event, the reactor mode sslect switch had been placed in the shutdown position, and all control rods had already been fully inserted during the nonnal course of the scheduled shutdown. No control rod motion occurred,- and the RPS actuation had no effect on plant emissions as measured at either the reactor building vent or main stack.

The second event was an unplanned automatic actuation of the Group I primary containment isolation system (PCIS). The group I PCIS actuation provides isolation or closure signals to the main steam iso 1' tion valves (MSIVs),

drain line main steam isolation valves, and reactor sample line isolation valves. The event occurred at 10:28 p.m. on October 24, 1992, again with -

reactor subcritical and almost fully depressurized at about 2 pounds per square inch gauge and the reactor coolant system temperature about 165'F. The actuation occurred because of a " spike" in a reactor vessel water level instrumentation signal. The actual reactor ~,sel water level did not undergo the indicited change (i.e., the " spike") and remained within the normal operating range. However, before the event, all MSIVs had been manu.!1y closed during the normal course of the plant depressurization and cooldown to isolate the four 20-inch main steam lines. In response to the event, only the .

isolation valves for the smaller 3-inch main steam drain line and 1-inch i reactor sample line repositioaed to the closed position. As with the previous . p 9212010169 921120 PDR P

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A Harry W. Salter Novenber 20. 1992 event, this actuation had no effect on plant emissions; therefore, no effect on background radiation readings.

Regarding your second question, we found that other radiation instruments showed increases in the background radiation similar to the increases recorded by your instruments. These instruments included monitors operated by EEco and by local high schools.

The increases in background radiation, detected by your monitu s, coincided closely in time to increases detected by these other radiation monitor,.

The NRC has reviewed data from the background radiation monitors, and also data from plant effluent release monitors. We have reasonable assurance that the effects of rainfall caused the increased background radiation readings and that the Pilgrim Nuclear Power Station was not the source of the increase.

This conclusion is based on the NRC's review of the following:

Data from the monitoring network in the vicinity of the Pilgrim site show elevated background readings consistent in magnitude and timing with your readings. This characteristic, however, is indicative of environmental effects rather than a plant release. A plant release would be associated with a point release (i.e., a plume), which would be expected to result in the potential for increased readings in the downwind direction. The simultaneous registering of the increamd background levels by all monitors would be inconsistent with dose assessment diffusion models used in determining the travel of a plume.

Effluent data from the Pilgrim main stack and reactor building vent show no correlation with the elevated offsite background radiation readings.

The reactor shutdown (which occurred before the elevated readings) typically decreases normal effluent releases through the main stack and reactor building vent. When the effluent release rates decreased, no corresponding change or ru ponse was noted on the offsite background radiation monitors. This is expected since normal effluent releases do not significantly affect background levels. When the offsite background radiation monitors showed elavated readings, the release rate from the plant was less than normal and almost below minimum detectable levels.

Similar effects from weather conditions associated with the October 24 and 25 rainfall were r.oted at other sites with monitoring networks.

NRC review of data concerning background radiation around these sites revealed similar elevated readings, suggesting a similar phenomenon involving the effects of weather conditions. As you noted in your letter of October 28, 1992, there was rainfall that occurred at the time of the elevated background radiation readings. Although rainfall does not always cause increased background readings, a sufficient rain, coupled with certain environmental conditions can cause increased

November 20, 1992 liarry W. Salter radon emanation and can result in a natural increase in the indicated background radiation. Such radon concentrations have been frequently observed and well published (e.g., "Some Observations of the Variations in National Gamma Radiation Due to Rainfall" by Susumu Mirato, Department of Energy Conference 780-422, Volume I, 1978). (

l trust the information provided has fully addressed your concerns with the elevated readings detected on your monitoring instruments.

Sincerely, Origireal cir,ue' by, rwxas E. ErloI/

Thomas E. Murley, Director Office of Nuclear Reactor Regulation DISTRIBUTION:

Docket file 50-293 (w/ incoming) S. Varga NRC & Local PDRs (w/ incoming) J. Calvo EDO #8252 W. Butler J. Taylor OGC J. Sniezek OPA J. Blaha OCA H. Thompson NRR Mail Room (EDO#8252 w/ incoming)

T. Murley/F. Miraglia C. Norsworthy W. Russell B. Clayton J. Partlow R. Eaton w/ incoming F. Gillespie T. Clark D. Crutchfield J. Linville, R1 PDI-3 Reading (w/ incoming) PDI-3 GT File SECY (CRC-92-0923) T. Martin, R1

• See previous concurrence Off1CE LA:PDI-3 _ PM1Phl/r/ Rhjp // ,0:PDI-3 17

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• E TClard[v REh mW GKN1y WButler DME /[/0/92 d AR /92 Il /h/92 . }\ /4 /92 11/18/92 offitE *ADRI *D:DRPE ADP b "dhb3NM WE JCalvo SVarga JPartlkA TH[rley DME 11/18/92 11/18/92 II /'*J2 // / 0/92 2 / /

OFFICIAL RECORD COPY Document Name: GT8252.PI d g,M s-I

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.s radon emanation and can result in a i :u al increase in the indicated backgcound radiation. Such radon con entrations have been frequently

c obserW and well published 'e.g., "Str.e Observations of the Variations in Natianal Gansna Radiation Due to Rainfall" by Susumu Minato, Department of Energy Conference 780-422, Volume I, 1978).

-. rust the information provided has fully addressed your concerns with the y,L] 1 2vated readings du.tected on your monitoring instruments.

Sincerely, F ,..

[V Thomas E. Murley, Director ,P' :

Office of Nuclear Reactor Regulati ,

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