Fixed and Personnel Nuclear Accident Dosimeters

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

method is less sensitive than the use of indium foils and even a small reading can indicate a significant

exposure. An approximate equation to calculate worker dose (D) based on body weight and instrument

reading is shown in Equation 7.1:

(7.1)

Differences in incident neutron energy spectrum, orientation, and measurement techniques relative

to conditions used to develop activity-dose correlations can cause significant errors in estimated radiation

dose based on quick-sort surveys. Swaja and Oyan showed radiation doses estimated from induced body

activity can vary by a factor of approximately 2 because of neutron energy spectrum or orientation effects

and by as much as 30% due to probe position. Doses based on indium foil activity can vary by a factor of

approximately 9 due to neutron energy spectrum effects, a factor of 3 depending on foil orientation relative

to the incident field, and a factor of approximately 2 due to probe window setting. Swaja and Oyan

recommended those count rates above background during quick-sort techniques should be initially

interpreted only as an indication that the person has been exposed.

7.4.2.2 Fixed and Personnel Nuclear Accident Dosimeters

A comprehensive nuclear criticality dosimetry system should consist of stationary (fixed- location,

area) dosimeters, neutron and gamma dosimeters worn by personnel (i.e., PNADs), and specialized

laboratory equipment to evaluate the dosimeters.

Fixed nuclear accident dosimeter units should be capable of determining neutron doses in the range of

10 rad to 10,000 rad with an accuracy of 25%. They should also be capable of providing the approximate

neutron spectrum to permit the conversion of rad to rem. The gamma-measuring component of the

dosimeter should be capable of measuring doses in the range of 10 rem to 10,000 rem in the presence of

neutrons with an accuracy of about 20%. The number of fixed dosimeter units needed and their

placement will depend on the nature of the operation, structural design of the facility, and accessibility of

areas to personnel. Generally, dosimeters should be placed so there is as little intervening shielding and as

few obstructions as possible. The number and placement of dosimeters should be periodic ally reverified to

reflect changes in building design and operations. Ease of dosimeter recovery after a criticality event

should be considered in their placement, including the possible need for remote retrieval.

PNADs should be worn by all individuals who enter a controlled area, with locations requiring an

installed criticality alarm system. The PNADs should be capable of determining gamma dose from

10 rad to 1000 rad with an accuracy of 20% and neutron dose from 1 rad to 1000 rad with an accuracy of

30% without dependence upon fixed-unit data.

The general criteria of ANSI N13.3 for nuclear accident dosimeters are reviewed below.

Dosimeters, both fixed and personnel, should be protected against radioactive contamination to avoid

false measurements. Periodic inventory methods should be established and audits made to ensure the

dosimeters are not removed or relocated without appropriate approvals. Techniques for estimating the

effect of body orientation at the time of the exposure should also be developed.

Neutron-Measuring Component of Dosimeter.Criticality accidents create a wide range of neutron

energies. Since the neutron dose per unit fluence is strongly dependent on neutron

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