Tertiary Radon Chamber Accreditation

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A.C. George
A tertiary radon chamber is a facility that must among other things, successfully inter-compare with an accredited secondary chamber in order to qualify for accreditation. Accuracy of ±10% is expected. A tertiary chamber may be used for initial evaluations of devices by manufacturers, for spiking and calibration as required by US EPA Radon Protocols, AARST standards and the Radon Proficiency Program. Calibration in a tertiary radon chamber is not a sophisticated and detailed device evaluation as required for device approval.

The use of tertiary chambers for testing or calibrating devices for the measurement of radon decay product concentration is not addressed in this document as there is little demand for this kind of service. Tertiary radon chambers should not be used for proficiency testing and detailed device evaluation as required for device approval purposes because more stringent standards and capabilities are required, found in primary and secondary accredited radon chambers. If proficiency testing was allowed, in a tertiary radon chamber, there could be potential for conflict of interest in manufacturers or vendors testing devices that they may sell or rent to their clients.

The criteria for tertiary radon chambers are less stringent than those of secondary and primary radon chambers. In terms of size, they can be much smaller. They must be specific for the device being evaluated, spiked or calibrated. If several devices are tested simultaneously, a large volume chamber ranging from 500 – 1500 liters is required to accommodate all of them. A large volume chamber is required for spiking or calibrating charcoal devices and multiple units of continuous radon monitors. The concentration of radon inside the chamber must be maintained homogeneous, well defined and either kept stable or varied in some deliberate manner for some applications. For the evaluation or calibration of a single device a smaller chamber could be adequate as long as the radon concentration is homogeneously mixed and well defined during the test period.

Active continuous radon monitors that use a pump to sample test air through a probe, or a single entry point such as a valve on a scintillation cell, may require a very small volume chamber or perhaps not even a chamber. If a monitor is being evaluated or calibrated by pumping air through it, it can be placed outside the radon chamber and pump intake air from inside the chamber through a port and recycle the air back into the chamber. This may not be feasible for many types of monitors because accessing the exhaust port may be very difficult or impossible.

Active continuous radon monitors that use a pump to sample test air through a probe (point source), require essentially no volume. In this case, the radon generator airflow rate must be sufficiently higher than the combined air-flow rate of the two devices one being tested and one being the reference device. As an example the RAD 7 sniffer, has a probe that can be connected to the output of the radon generator. The tertiary radon facility used for evaluating or calibrating this type of monitor would be evaluated on case-by-case basis.

If a tertiary radon chamber is used to test or calibrate devices that are affected by conditions of temperature and humidity, such as charcoal devices and some continuous monitors, then the chamber must be capable of controlling these conditions over some reasonable range typically found indoors. For example, the tertiary chamber should be capable of maintaining the temperature and relative humidity at values ranging from 65 oF to 80 oF and 30% and 70%, respectively. The tertiary chamber should be capable of maintaining well defined conditions for a period of time sufficient for the type of test being conducted.

Typically, radon devices are exposed in the field for a minimum of 48 hours. Chamber tests of short-term devices, are expected to be conducted over a period in the range of 2-7 days. Tests of long-term devices could be conducted over a period ranging from weeks to months depending on the concentration of radon in the test chamber.

A tertiary chamber must be monitored continuously to measure the average concentration of radon as a function of time over the period of a test or calibration. A continuous radon monitor is the most appropriate monitor for measuring the radon concentration in the chamber. The continuous monitor must successfully pass a blind inter-comparison test at least annually in an accredited secondary chamber. Documentation must be maintained to show proof of traceability to an accredited secondary radon chamber. The sensitivity, or precision error of the device used to monitor the chamber must be at least as good as, and preferably better than the devices being tested or calibrated. It is advisable to use duplicate continuous radon monitors if possible to determine precision and whether the performance of either one of the collocated devices is acceptable. If the two traceable radon monitoring devices do not agree within the prescribed precision limits, then there is an option to investigate and find the source of the problem.

Although, a continuous radon monitor is preferred, properly calibrated charcoal canisters or electret ion chambers, reserved specifically for use in tertiary radon chambers, may be used to monitor the radon concentration. These devices must first pass successfully blind inter-comparison tests at least annually in an accredited secondary radon chamber.

If grab samples such as scintillation cell devices are used to monitor a radon chamber during calibrations, they must be very frequent, as a minimum on hourly basis. Their calibration must be traceable to a documented reference laboratory inter-comparison in a secondary accredited radon chamber.

The operator of a tertiary radon chamber must ensure that personnel and equipment are protected from radiation exposure and contamination from the radon generator source. The radon sources used in tertiary radon chambers must meet the radiation regulations which differentiate between licensed and exempt radium sources. Appropriately sealed and shielded 226Ra sources in dry form are commercially available and are preferred. However, in-house prepared radon generator sources that use small activities of radium or natural ores such as uranium ore or tailings are sometimes used to avoid State licensing or certification.

If such a source is used, it must be adequately sealed against leakage of the source into the nearby environs. Usually, double filtration will keep the dry medium in its container but allow the radon to emanate freely into the test chamber or into the device being tested. The operator must demonstrate and document that the contribution of thoron is negligible if a source such as a natural ore is used that potentially could contain 224Ra. (In other words, if someone is using a commercially available 226Ra source, there is no need to demonstrate and document that the contribution of thoron is negligible). The exhaust air from the radon generator and/or chamber must be vented into an unoccupied environment usually outdoors, where it is diluted very quickly by the essentially infinite volume of outdoor air. Also consideration of the point of exhaust must be taken to avoid re-entry indoors.

A worker safety program must be in place to protect personnel from radon and radon decay products that may be leaking into occupied areas. A personal or area radon monitor such as a long-term alpha track detector can be used near the tertiary chamber for quarterly or annual determination of exposure to radon. Documentation of exposure must be available and maintained for a minimum of five years. Other applicable safety considerations, such as electrical safety, fire safety, safety from tools and confined space environments must be addressed and maintained at all times.

An operator of a tertiary radon chamber must have a Quality Assurance Plan, Standard Operating Procedures and Worker Safety Plan in place when applying for certification. The personnel responsible for the operation of the tertiary chamber and for the acquisition of data and results must be qualified by means of participating in radon courses and by being successfully certified as a National Radon Safety Board radon measurement specialist. As a minimum, a radon measurement specialist must be supervising the entire operation.

There must be operating procedures documenting the steps taken to provide the appropriate exposure environment for the calibration of a specific device and what criteria are used to determine the successful calibration of the device and what criteria were used to determine its successful calibration. There must also be procedures documenting how the device that is used to monitor the tertiary chamber is calibrated and compared with an accredited secondary radon chamber. The accreditation organization reviews the documented information and acts accordingly to see that the required criteria were addressed.

NRSB cannot anticipate every tertiary facility or application thereof and should review each application on a case-by-case basis to ensure that the tertiary facility meets the appropriate criteria required for the stated application.