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Surveys for Removable Contamination cont'd - doe-std-1128-98_ch10099
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Release Criteria - doe-std-1128-98_ch10101


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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