Criticality Accident Dosimetry - Continued

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

are based on stochastic effects, primarily cancer induction some years later. But the

doses in criticality accidents are typically so large that acute symptoms, including death,

may occur within relatively short times, and quality factors are usually not applicable.

For this reason, it is usually more appropriate to determine absorbed dose rather than

dose equivalent if a person receives more than about 25 rem. These absorbed dose

estimates to the torso are much more important for triage and treatment considerations.

The NADs are used to determine the neutron and photon dose at various locations in the

plutonium facility, as well as providing spectral and calibration data for PNADs. A

typical NAD used at the Hanford Site is shown in Figure 6.4. This unit is fixed to the

wall or posted at locations around plutonium storage areas where it is easily recovered in

the event of a criticality. The "candle" insert contains neutron- and gamma-sensitive

TLDs as well as activation foils positioned at the center of the detector. Tests at the

Health Physics Research Reactor at Oak Ridge have shown that this arrangement gives

accurate estimates of "deep" dose for both neutrons and gamma rays. A set of foils

identical to those used in the PNAD dosimeter is positioned above the moderator. These

foils provide an estimate of the average cross-section or response per unit dose, so that

the neutron dose from the foils in the PNAD can be more accurately evaluated for the

incident neutron spectrum.

The PNAD dosimeter typically consists of several activation foils. In the case of the Los

Alamos/Hanford design (Vasilik and Martin, 1981), the activation foils consist of -in.-

diameter foils of bare and cadmium-covered gold, bare and cadmium-covered indium,

cadmium-covered copper, and a sulfur pellet. Algorithms have been developed to unfold

an approximate neutron energy spectrum from the measured neutron activation products,

so that neutron doses can be calculated. Criticality dosimeters containing various

activation foils are available from vendors, but some of the commercial products do not

contain sufficient material to measure neutron doses as low as 10 rad, which is the

recommended lower detection limit for personal criticality accident dosimeters.

In the past, the Health Physics Research Reactor (HPRR) at Oak Ridge National

Laboratory was available to calibrate criticality accident dosimeters, and several

intercomparisons were held to demonstrate the accuracy and lower detection limit of

criticality alarms and nuclear accident detector systems (Sims and Raga, 1987).

However, the HPRR has been dismantled. Some dosimeter testing capability is being

established at pulsed reactor facilities at Sandia (Albuquerque, New Mexico) and at a

Department of Defense pulsed reactor at the Aberdeen Proving Ground in Maryland.

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