Calculated Photon Dose Rates

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

must check the photon libraries to make certain that the higher-energy photons are

included. Also, many point kernel codes may not give accurate results for thin

shields because low-energy build-up factors are not very accurate.

There are only a few codes specifically designed for plutonium dose calculations in

the ORNL code center(1); they include the following:

-- PUSHLD - Calculation of Gamma Radiation Dose Rates from Three-

Dimensional Plutonium Sources and Shield Geometries at Various Distances,

HEDL-TME 73-89, Hanford Engineering Development Laboratory (Strode,

1974_).

-- BMC-MG - A Multigroup Monte Carlo Kernel Integration Neutron and

Gamma-Ray Shielding Code System for Plutonium, BNWL-1855, Pacific

Northwest Laboratory (Zimmerman, 1975_).

-- PURSE - A Plutonium Radiation Source Code, PNCT 852-78-13, Japan Power

Reactor and Nuclear Fuel Development Corp., Tokai-Mura, Japan.

The PUSHLD computer code has the advantage that the calculated results were

experimentally verified to make certain that the low-energy build-up factors were

correct. There are undoubtedly several other codes that could give accurate dose

rates from plutonium, particularly if a radioactive decay code is used to calculate

the amount of progeny as a function of time. For example, the Los Alamos code

MCNP (Briesmeier, 1986) would provide accurate neutron and photon doses if the

decay progeny were included in the calculations. Unfortunately, all of these codes

were developed years ago to operate on mainframe computers, and simple "user-

friendly" versions are not available for personal computers.

There are some empirical equations that can be used to calculate dose rate through

simple shields, such as Neoprene, when plutonium is directly handled in a glove

box. Because of the dominance of low-energy X-rays, the surface dose rates from

plutonium sources can be quite high. Roesch and Faust have derived a formula for

predicting the surface dose rate from plutonium through a 100-mg/cm2 shield:

Ds(rad/h) = 171 f238 + 0.51 f239 + 2.4 f240 + 8.7 f241

(6.2)

+ 0.15 f242 (0.074 f241)t

= the surface dose rate of plutonium metal or oxide, rad/h

where Ds

(1)RISC, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6362, Telephone (423) 574-6176.

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