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On-Site Air Radiation Monitoring Systems


On-site radiation monitoring systems, operated by Pilgrim are described below (radiation detectors at exit points and the chemical and gamma spectroscopic analysis techniques used to estimate release rates of individual nuclides). Source: Preliminary Draft Report, “Survey of Documents Concerning the Operation of Pilgrim Nuclear Power Station” by Ellen B. Cargill, R.T., PhD.    
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Radiation Monitoring Systems

Radiation detectors are located at every exit point from the plant (see following comment) to measure gaseous radioactive effluents. These detectors monitor the gross gamma radiation of gaseous effluents as they pass by. These readings are monitored and recorded in the control room, and when the radiation level approaches release limits, either the effluents can be diverted to another system for further processing, or the power level of the reactor can be reduced in order to reduce the amounts of radioactivity produced. The radiation detectors are sensitive only to the total amount of radiation impinging on them, they don’t differentiate between one isotope and another, since there are substantial assumptions regarding short half-lives of isotopes entering the systems. One fundamental limitation to measuring gamma radiation levels exiting the plant ventilation systems is that a small perturbation in the total amount of radiation detected, since the decay rate is so much lower compared to short half-life isotopes. In this way, a leak of long half-life isotope could go undetected by a radiation detector. The use of chemical and gamma spectrographic analysis is designed to augment the stack radiation monitoring program.

Chemical and gamma spectroscopic analysis techniques used to estimate release rates of individual nuclides

Periodic sampling and analysis techniques are employed to determine the relative abundance of various isotopes that are being released. This is very important since the biological action and possible impact is quite different for different isotopes. The way this is carried out is that radioactive effluent is sampled by systems that employ filters and charcoal to draw air through them. After a given period of time, the contents of the filters and charcoal are analyzed by measuring the radioactive decay rate as a function of disintegration energy. Since isotopes decay by emitting radiation of characteristic energies, the amount of a given isotope present in the sample can be estimated by the magnitude of the number of disintegrations at characteristic energies.  The uncertainties associated with this method are that in general isotopes emit a spectrum of radiation frequencies, and in a case where there are a large number of unknown isotopes present in the sample, the energy peaks can overlap for different species and it may not be possible to assay many isotopes with any accuracy. Another problem that can occur is that the efficiency of the charcoal absorber is strongly a function of relative humidity, so in cases of high humidity, the amount of a given isotope present in the charcoal may not at all reflect the concentrations in the sampled effluent. Detectors used to perform these measurements have non-uniform responses to different energy peaks, and calibration of these sensitive instruments should be conducted frequently. Finally, the raw measurements from these instruments are entered into equations to estimate actual release rates, so the associated uncertainties may be quite high.

Are there monitors at all exit points?

Unknown: For example, the public was assured when the reactor went on line in 1972 that there were radiation detectors located at every exit point from the plant to measure gaseous radioactive effluents. However, it was not true. Citizens brought to the NRC’s attention in 1991 (NRC Report, 50-293/92-02,sec. 2.3) that the nine vents in the Turbine Building did not have either monitors or filters and vented to the atmosphere during the 20 years since Pilgrim went on line.


NRC and Massachusetts Department of Health’s Radiation Control Program Oversight
Staff members of the NRC and Radiation Control Program of the Massachusetts Department of Public Health conduct inspections at the Pilgrim Nuclear Power Station. The purpose of these inspections is to review the strip chart recordings of the emission monitors – Pilgrim’s reported data.



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