Ronan X90 Series Manuel d'utilisateur télécharger pdf (Page 8)

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 is 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 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:

1000 X

(d)

mCi k

D =

mr/hr 12.5 1000 X

400

1000 X

(20)

10 X 0.5

rate Dose

===

mrem 2.08 mR/hr 12.5

min/hr 60

min 10

dose Total ===

1 2 3 4 5 6 7 8 9 10 11 12 13 ... 19 20

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Ronan X90 Series Manuel d'utilisateur Page 8

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