Criticality Accident Dosimetry cont'd - doe-std-1128-98

DOE-STD-1128-98

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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