Federal emergency management agency fema rep-2, rev. 2 / June 1990

6Other Considerations for Offsite Radiation Measurement Systems

6.1Documentation

1. 
   Assessing the impact of the incident, and
   
2. 
   Measuring the effectiveness of the State radiological emergency preparedness plan for use in improving future plans.
   

Both of these efforts will involve reconstructing the incident based on the available data, and both efforts will also involve calculations of the dose received and the dose saved by implementation of protective measures. Some of the post-accident investigations may require that data be introduced as evidence at legal proceedings. Consequently, it is important that data be documented with these aspects in mind. All measurements should be fully documented; i.e., date, time, location, weather conditions, measurement readings, background readings, instrumentation identification number, transfer of data and samples, signatures of personnel and witnesses to the signatures, and other pertinent information in order that they may be used for assessment purposes.

All State radiological emergency preparedness plans, including plans for emergency radiation monitoring, should be reviewed for proper documentation. Accordingly, preprinted forms with specific directions should be developed for documenting appropriate actions.

6.2Quality Assurance

The frequency of calibration required for various types and models of instrumentation varies with their known degree of calibration stability. This frequency should be determined for each type and model of instrumentation by comparison with past calibration data and should also be based on the recommendation of the instrument supplier in accordance with the guidance in NUREG-0654 Planning Standard and Evaluation Criteria H.10.^lxviii The amount of change in radiation sensitivity detected between previous calibrations and the period of time over which these changes occur could determine the future calibration cycle.

In general, ionization chamber dosimeters and high-range survey meters show little or no change in sensitivity unless they became defective. Calibration checks of these types of devices are performed more to detect defective units than to determine sensitivity changes. The calibration frequency for such civil defense devices used for civil defense purposes is once every four years. However, civil defense instruments which are used for radiological emergency response (REP) purposes must be calibrated at least annually and this frequency should be designated in the procedures section of the State or local emergency response plan. Instrument calibrations should conform to ANSI N323-1978, Section 4.^lxix GM type exposure rate measuring instruments, TLD’s or other instruments utilizing more sophisticated electronic circuits should be calibrated more often, since slight changes in component characteristics can often significantly change their ionizing radiation response. Generally, instruments using vacuum tubes require the most frequent calibration. However, there are many exceptions to this rule.

Any time electronic components or detection devices are repaired or replaced, the instrument should be recalibrated. Instruments which exhibit battery voltage sensitivity should be calibrated every time the batteries are changed (See ANSI N323-1978, Section 3).

Thermoluminescent dosimeters have to be annealed, i.e., heated at a manufacturer's specified temperature, after each readout to remove any residual absorbed energy from exposure to radiation. Uniform annealing heating rates and post annealing cooling rates produce TLDs of uniform sensitivity. The TLDs should be calibrated after each annealing by exposing randomly selected TLDs to a radiation source for varying periods of time. The radiation source should be similar in energy to the radiation expected to be measured by the TLDs under field conditions. This radiation calibration source should be periodically tested with a secondary standard, i.e., calibrated ion chamber, to determine its exposure rate. The output reading of each TLD exposed to the calibration source should be plotted against its total accumulated exposure to produce a calibration curve of TLD output response versus accumulated exposure.

For radiation detection instruments other than TLDs, calibration history is by far the best means of determining an instrument's calibration frequency, and rule-of-thumb should only be used until such a history is established. It is therefore necessary to document and preserve calibration data.

Calibration of all instrumentation should be performed to establish linearity of radiation response as well as accuracy. However, once this linearity is determined in ion chamber type dosimeters and instruments, a one point calibration will suffice since these characteristics rarely change in these types of devices. Other types of exposure rate measuring instrumentation, particularly instruments using GM detectors, generally exhibit non-linear response. Since non-linearity can shift with changes in electronic components, more than a one point calibration should be made. In the case of GM instruments, or instruments with logarithmic and semi-logarithmic response, a calibration response curve based on several points across the meter scale, or over the range of sensitivity, should be provided with each instrument. A calibrator traceable to NBS is available in most States for the calibration of CDV-700 survey meters.

If the instrumentation is not used on a regular basis, periodic operability checks must be made. For exposure rate measuring devices, this check must determine whether the instrument is capable of accurately measuring the radiation level of an appropriate check source, or at least, that the power supply is in operating condition. In the case of ion chamber type dosimetric devices, a check for leakage of the ion chamber must be made. Thermoluminescent dosimeters should be periodically annealed to assure an up-to-date background level.

The minimum calibration and battery replacement frequency for all instrumentation must be annual. Calibration records should be maintained in accordance with ANSI N323-1978, Section 4.5.

The instrumentation shall be labeled on their exterior with the following information:^lxx

1. 
   Date of the most recent calibration and calibrator used
   
2. 
   Initials or name of person who performed last calibration
   
3. 
   Energy correction factors to be used, where required
   
4. 
   Graph or table of calibration factors, where necessary, for each type of radiation for which the instrument may be used; this should relate the scale reading to the units required if units are not provided on the scale
   
5. 
   Instrument responsiveness to an identified check source. Source must be identified and expected readings(s) indicated
   
6. 
   Special conditions (unusual) or limitations on the use of the instrument
   
7. 
   Date that primary calibration is again required
   
8. 
   Special condition identification label (if applicable); ANSI N323-1978, Subsection 4.3.1.
   

Operability checks must be performed every 90 days with the instruments allowed to operate at least one hour during each check. The 4 year calibration frequency provided by FEMA-supported State Maintenance and Calibration Shops for civil defense type instrumentation is not adequate when the civil defense instrumentation is used as REP emergency response equipment. Air sampling pumps must be calibrated annually or after each maintenance or repair. Air sampling pumps must be calibrated in accordance with ANSI N320-1979, Section 9.2,^lxxi NRC Regulatory Guide 8.25,^lxxii and American Conference of Governmental Industrial Hygienists manual “Air Sampling Instruments for Evaluation of Atmospheric Contaminants.”^lxxiii

In the case of instrumentation used to analyze samples which vary in radioactivity, a range of standards in the appropriate media should be developed which span the range of radioactivity anticipated to be measured. Sampling procedures and analytical techniques should be developed to ensure adequate sensitivity and statistical validity of the data obtained. The participation in the use of round-robin unknown samples for determining accuracy of the total analytical system should be encouraged wherever possible.