8
012208
In the majority of Ronan installations, the source is
contained in a lead-filled source holder with an ON/OFF
mechanism. The holder is designed so that the radiation
field is 5 mR/hr or less at a distance of 12 inches from
the surface of the holder when it is in the OFF position.
When the source holder is mounted on the pipe or
vessel and turned to the ON position, the pipe walls,
process material and mounting bracket absorb most of
the radiation. Again, the field intensity is about 5 Mr/hr at
a distance of 12 inches from the surface of the gage.
Thus, a person would have to be within 12 inches of the
gage for 20 hours per week to receive 100 mrem. A
person would have to be within 12 inches of the gage for
five hours per week before he would be required to have
a personnel monitoring device, such as a film badge or a
dosimeter.
Long experience in the nuclear gaging industry with
hundreds of gages, where the source is contained in a
source holder, indicates that the dose received by
operators, maintenance personnel and supervisors
averages less than 25 mrem per week. Thus, for gages
where the source is contained in a source holder, it is
usually not necessary to provide any personnel with
monitors.
Whether or not monitoring devices are needed should be
determined at the gage site at the time of installation in
the form of an
occupancy evaluation.
Only those persons
specially licensed by the NRC or an agreement state to
install a radioactive device are qualified to make an
occupancy evaluation.
In some installations, it is impossible to mount the source
in a source holder. In these cases the source is usually
mounted in a source well. Installation of the source in the
well should be done as rapidly as possible. All necessary
equipment should be assembled prior to opening the
shipping box containing the source.
A trial installation using a dummy source is
recommended. A dummy source can easily be fabricated
from steel or brass using the outline drawing of the
source supplied by Ronan Engineering. When an
unshielded source is installed in a vessel or when it is
wipe tested, the radiation field is usually greater than 100
mR/hr. Thus, personnel monitoring in the form of film
badges or dosimeters is required. A record of the film
badge or dosimeter reading must be kept on Form NRC-
5. Since records must be kept, the employer must
furnish to the employee, if requested, a record of the
employee’s radiation exposure annually and on
termination of employment.
Additional precautions are required when a gage is used
on a vessel large enough to permit entry of personnel.
With the source holder in the open position or when the
source is not removed from the source well, the radiation
field intensity inside the vessel can be high. A procedure
must
be established so that personnel cannot enter the
vessel until the source holder is in the closed position or
the source is removed from the source well. The use of
padlocks on all man-way and access port covers is
acceptable. The key or combination for the locks should
be kept by the person responsible for radiation safety.
In some cases, when the vessel or pipe is empty, the
radiation field intensity of the outside of the pipe or
vessel will be such that personnel monitoring is required.
For installations using source holders, this problem can
easily be solved by turning the source holder to the OFF
position. For installations using sources in source wells,
where the radiation cannot be turned off, it may be
desirable to remove the source temporarily and return it
to its lead shielded shipping and storage container.
11.1 Field Intensity Calculation
The best method for determining the radiation field
intensity is by measurement with a survey meter.
However, the field intensity can be calculated fairly
accurately without a survey meter.
The radiation field intensity can
be calculated from:
D = dose rate, mR/hr
mCi = millicurie value of source
d = distance to source in inches
k = constant 0.023 for AM-241
0.5 for CS-137
2.0 for CO-60
Suppose that for a certain installation the estimated
exposure time to the unshielded source is 10
minutes at an average body-to-source distance of
20 inches. The source is 10 millicuries of CS-137.
The dose rate would be:
And the dosage received would be:
To calculate the radiation field intensity on the
outside of the vessel with the source installed in the
vessel wall, a set of transmission curves is needed.
The graph in Figure 2 shows the percentage of
transmission of CS-137 versus material, thickness
for lead, steel, concrete and water.
1000
X
2
(d)
mCi
k
D
=
mr/hr
12.5
1000
X
400
5
1000
X
2
(20)
10
X
0.5
rate
Dose
=
=
=
mrem
2.08
mR/hr
12.5
min/hr
60
min
10
dose
Total
=
=
=
