Incident Time History Instrumentation

4.8Incident Time History Instrumentation

Fixed instrumentation (or instrumentation that is already in place at specific sites) that may be moved to a specific site can also be used to continuously record the time history of the exposure rate. An example of deployable instrumentation would be a continuous air sampler and/or a gamma radiation rate meter with a recorder and its own portable power supply. This system could be mounted on a stand, carried to a location by truck and set up in the field to make continuous measurements. The disadvantage of deployable instrumentation of this type is that it may miss the onset of the release. However, this disadvantage is offset by lower maintenance costs and more control over the sitting of the instrumentation in the plume compared to a stationary instrumentation system. The preceding remarks on the cost effectiveness of stationary instrumentation also apply to deployable instrumentation.

An application for a time history of the exposure rates from the plume is to follow the course of the accident and to assess the offsite consequences over the length of the release. Most facilities maintain some stationary and/or moveable instrumentation for assessing the offsite consequences for both routine and accidental releases. If time history data is needed for protective action decision making, for example, during a very prolonged release, the State should plan to obtain this data from the facility and the Federal support teams (FRMAP) who should have the necessary instrumentation.

4.9Aerial Radiological Monitoring

The AMS is potentially very useful in defining the boundaries of the contaminated area, especially if a large contaminated area is expected. AMS may utilize either helicopters or fixed-wing aircraft which are equipped with a detector array having twenty 5" x 2" NaI(Tl) scintillation crystals which are equally distributed within two cargo pods.^liv This system is effective for detecting and identifying gamma emitting radionuclides which have energies greater than 50 keV. The helicopter mounted detector system has a sensitivity range of less than 0.1 to 1.0 µCi/m² for gamma energies greater than 50 keV. The fixed-wing aircraft detector system has a sensitivity ranging from 1.0 to 10 µCi/m² for similar gamma energies.^lv However, at gamma energies between 50 and 100 keV, the detection limit is highly dependent on the geometry of the source and its distribution in the soil. The difference in sensitivity between these two aerial systems is due to the helicopter's ability to be flown at lower altitudes and at lower air speed than the fixed wing aircraft.

The normal data output from AMS is in units of microroentgens per hour extrapolated to one meter above ground level.^lvi The data from the aerial radiological survey is recorded on magnetic tapes for further data reduction using a ground based computer system.

Generally, aerial radiological surveys are capable of: 1) detecting areas of enhanced radiation, 2) determining the average surface area exposure rate, and 3) identifying the specific radionuclide(s) responsible for any observable anomaly.^lvii However, this system has the following limitations: 1) it may be grounded by inclement weather conditions either at the home air base or at the accident site, 2) it can only detect gamma emitting radionuclides, 3) it is unable to distinguish between contamination on crops and contamination on the ground, 4) it is of little or no value for detecting waterborne releases, and 5) it may underestimate the magnitude of localized sources, since aerial detection systems tend to average gamma exposure rates over a large area.

An aerial radiological monitoring capability has been developed by FEMA for use by the States using Civil Air Patrol and other volunteer civil aviation groups. These aircraft and personnel, if located within an hour's flying time from the nuclear site and equipped with properly sensitive instrumentation, can fill this interim plume tracking need. Special training of the volunteer pilots and observers is required. The pilots should be instructed to fly above the plume, e.g., about 3,000 to 5,000 feet above the ground, to avoid contaminating the aircraft, and along predetermined flight paths using visual navigation points. The CDV-781 Aerial Radiological Survey Meters must be replaced with a more sensitive gamma survey meter which can be carried within the aircraft and visually observed. A 0-50 mR per hour GM or scintillation survey meter (Section 4.3, "Low-Range Gamma Survey Instruments") should be adequate for this purpose. Observations should be reported by radio directly to the EOC or to a radio-telephone relay point.

The greatest value of an aerial radiological monitoring capability is for aerial surveys of deposited materials after the airborne release has dissipated. The AMS Program is designed for these purposes, and the State agency should plan to utilize this Federal capability (Appendix A).