RADIOLOGICAL SURVEY / MONITORING
By Steven H. Brown
Elevated levels of natural background radiation are seen in many occupational settings. NORM (Naturally Occurring Radioactive Material) is found in mining and mineral processing, from the mining and extraction of rare earth elements etc. This article focuses on occupational radiation safety and radiological monitoring of installations.
Radioactive decay products (like Ra 226, Pb 214, Bi 214, Pb 210, Po 210 etc.) can precipitate on internal surfaces of equipment (valves, piping, filter media, etc.) and/or can be contained in scale or other accumulations. Details of system design, fluid flow and thermodynamics affect where and to what degree the decay products accumulate and comprise elevated levels of radioactivity. Care must be exercised, particularly when opening/penetrating pressurized systems, since if significant quantities of radioactive decay products have been deposited at that location, it could be dispersible (dusting) and be released or inhaled by workers.
Determination of alpha-, beta- and gamma-radiation
However, the radiological emissions coming out of and through the materials of construction of equipment can be relatively low and may not be detectable despite their presence at high concentrations due to the self absorption of the radiation by the equipment itself. This can make “finding” contamination difficult with standard gamma survey type equipment such as sodium iodide-based scintillation detectors or similar instruments that detect only gamma rays. The radioactivity often is not observable and detectable above background levels until above mentioned systems are opened for maintenance and/or repair.
Radiological survey instruments capable of detecting beta and alpha particles are often required to detect and measure the amount of Pb 210 and Po 210 present. Commonly used and commercially available instruments that can detect alpha and beta particles include “pancake” style Geiger Mueller detectors and alpha/beta scintillation-based instruments.
Air (dust) monitoring
As previously indicated, the potential for inhalation of dusts containing Pb 210 and/or Po 210 released during maintenance activities may need to be assessed by measuring activity concentrations in air samples. This involves drawing air through a filter paper at a known rate for a known period and subsequently counting the gross alpha and beta activity collected on the filter. Air sampling for radionuclides is often classified simply as area or personnel (individual) monitoring. Area samplers are located within the workplace at fixed locations and the employee’s occupancy of the area is recorded. Personal samplers are small devices actually worn by the workers and measure the exposure in the actual breathing zone of the worker which may be more representative of potential intake in these situations where the workers are at locations where pressurized systems are opened and dust can be released into the workers face or on upper body. However, the much lower flow rates of these personal air samplers result in less sensitivity (higher detection limits) than the fixed location high volume samplers.
Surface/volumetric contamination
There are long standing contamination limits that have been applied throughout the nuclear industry for decades to control the amount of radiological contamination on equipment and materials that are released from nuclear facilities or radiological project sites that are considered acceptable for use in the public domain. These limits are typically applied in three ways:
(1) Amount of total contamination on a surface, typically expressed as decays (disintegrations per minute {dpm}) per 100 cm2 or Bq (60 dpm = 1 Bq) per cm2
(2) Amount of removable contamination on a surface that could be dispersible and therefore inhaled or ingested. Also expressed as dpm per 100 cm2 or Bq per cm2,
(3) As a volumetric concentration, typically expressed in Bq per cm3 or per gram.
The total contamination is typically determined by holding the detector probe close to (about a cm) the surface being measured since the alpha and beta particles will not travel very far in air. However, the surveyor needs to avoid actually touching the surface with the probe so as not to contaminate it. Removable contamination is determined by “swiping” (lightly rubbing) the surface with a small filter paper or cloth and then measuring the amount of radioactivity with the hand-held survey instrument probe or more sophisticated instrumentation in a laboratory. Volumetric concentrations typically need to be determined by a radiochemical laboratory.
Clearance limits
Numerical limits are assigned by regulatory agencies and other international advisory organizations to establish areal and/or volumetric concentrations specific for many radionuclides below which do not require regulatory control. Accordingly, equipment or materials containing levels below these clearance limits can be released from their facilities to the public domain for conventional disposal or recycle without any additional controls for radiation protection purposes. For example, Pb 210 and Po 210 are typically grouped with other naturally occurring radionuclides of the uranium decay series. Examples of recommended clearance limits e.g. for Pb 210 and Po 210 (note the remarkable variation with time and between different institutions) are:
· EC (2002): 5 Bq/g
· IAEA (1996): 0.1 Bq/cm2 or 0.1 Bq/g
· IAEA (2004, 2012): 1 Bq/g
· USNRC (1993): 15000 dpm/100 cm2 maximum (5000 average)
· USDOE (2009): 5000 dpm/100 cm2