A Good Practice Guide has been written by a working group of the Airborne Radioactivity Monitoring Users Group, under the auspices of NPL – Andy Pye takes a look at it.
The guide provides recommended procedures for examining, testing and calibrating tritium-in-air monitoring instruments for radiation protection.
Most portable and many installed tritium-in-air monitors use ionisation chambers as the detectors. The test procedures described in the guide are appropriate to such instruments, but do not apply to instruments that use scintillation detection or proportional counting.
The guide is aimed at nuclear industry operators, but may also be of interest to suppliers and users of ionising radiation instrumentation in the healthcare sector. It is designed to provide technicians who use tritium-in-air monitoring systems with simple testing and calibration procedures to enable them to comply with the Ionising Radiations Regulations 1999 and ensure that the instruments remain fit for use. It is the responsibility of the employer to define the frequency of Periodic Tests based upon considerations such as the age of the equipment, the environment in which it is used, and the frequency of use.
Some types of ‘tritium-in-air’ monitoring equipment are either installed or not readily transportable to a suitable calibration laboratory. Consequently, periodic examination and testing of such equipment may have to be carried out in the workplace. This approach also minimises the risk of damage caused by removal, transport and re-installation of equipment. It also permits tests of auxiliary indicators, such as remote warning lights, audio-visual alarms or the removal of key components (eg electrometers for independent calibration).
Tests covered include Test Before First Use (TFBU), Routine Function Test, Periodic Test and Retest after Repair/Adjustment
Test Before First Use
Instruments may be damaged through use or in transit. Even in store, batteries can deteriorate and battery terminals may corrode. Thoroughly check the condition of any instrument before any tests or calibrations are carried out – check for mechanical damage to the instrument case, meter, etc., and perform any repairs required.
Instruments that appear well maintained are generally much better looked after than poorly maintained examples.. The user may consider additional checks that should be made prior to the test or calibration.
In a pre-calibration check, the instrument should be checked carefully for contamination. Any contamination should be noted and if above any limit should be reported in line with the organisation’s procedures. If permissible, attempt to remove any loose contamination.
Routine Function Test
A routine function check is carried out to ensure, with the minimum of interference, that the instrument is working correctly.
If the function check results fall outside prescribed limits, the instrument should be submitted for a Periodic Test. If the instrument requires repair or maintenance, the Qualified Person should consider whether another TBFU is required.
Because the instrument may have suffered from wear and tear or misuse, attention should be paid to the performance and the condition of its electrical and mechanical systems. For example, the pump within the air circuit is particularly vulnerable and should be examined at least annually. The condition of any warning lights/audible alarms should also be checked. Other system components should be examined, such as filters for potential blockages, switch failure and earth current loops.
Retest after Repair/Adjustment
After any repair, adjustment or modification that could affect the performance of the instrument, it may be necessary to repeat some of the Test Before First Use (TFBU) tests. The degree of testing depends on the nature of the change. Certain changes may be so fundamental that some parts of the Type Test should be repeated – for example, replacement with a detector from an alternative manufacturer.