Scan Survey for Fixed Contamination - doe-std-1128-98

DOE-STD-1128-98

If contamination is detected during a scan survey for fixed contamination, a

survey for removable contamination should be performed to determine if the

contamination is fixed and to quantify any removable contamination. The

survey should be performed using a small piece of absorbent material, such

as a standard paper smear. This type of survey for removable contamination

is often called a technical smear survey. If no contamination above the

guideline values for removable contamination in Table 4.1 is detected during

the smear survey, the contamination is fixed, and the area should be posted

appropriately.

A technical smear survey may be used routinely to detect removable

contamination, especially for contamination surveys of radiological areas.

Scan Survey for Fixed Contamination

A scan survey for fixed contamination requires passing a portable instrument

over the surface of the area being surveyed at a fixed, known scan speed and

at a specified distance from the surface. Typically, the scan speed is 2 in./s (5

cm/s) and the maximum distance is 0.25 in. (0.6 cm) for alpha contamination

instruments, but this can vary depending on the instrument, probe

configuration and background. A scan survey should be used to survey

material that resides in an area controlled for contamination purposes, an area

where unsealed radioactive sources are used, or a radiological buffer area

surrounding an area controlled for contamination purposes. A scan survey in

conjunction with a wipe survey should be used to release from radiological

control material with a total surface area less than 5 ft2 (0.46 m2). A

statistically based survey, which will be discussed later, should be used to

release from radiological control material with a surface are greater than 5 ft2

(0.46 m2).

During the performance of scan surveys, the audible response of the

instrument is faster than the needle deflection. Therefore, audible response

should be used in conjunction with meter readings. For alpha surveys, the

surveyor should pause for 3 to 5 seconds each time an individual pulse is

detected in order to allow a longer count time at the location of the detected

pulse, until it is determined whether the response indicates random

background noise or detected contamination.

Several important factors affecting scan survey detection sensitivity are:

instrument detection efficiency, background, size of the effective probe area,

and the speed at which scan surveys are performed. For a given instrument,

scan speed can be a critical factor as counting time is inversely proportional

to scan speed. For instruments with larger detector faces, the scan speed is

faster for a given rate of meter movement because a point on the surveyed

surface remains beneath the window longer. To ensure that low levels of

contamination can be detected, it is necessary that a maximum scan speed be

mandated and that this speed be implemented during field measurements. As

noted above, a typical scan speed for instruments in current use is 2 in./s (5

cm/s). However, the scan speed for a specific application should consider the

instrument, probe, guideline value, and confidence level desired. The

MARSSIM (NRC 1996) contains guidance for determination of scan rates. It

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