Calculated Neutron Dose Equivalent Rates

DOE-STD-1128-98

Experience has shown that only spontaneous fission and alpha-neutron reactions

are important. Because of strict criticality controls, most forms of plutonium have

very little neutron-induced multiplication. Induced fission seems to be a problem

only in metal (1 kg or more) or in very large, high-density arrays of plutonium

oxide with an additional moderator.

Plutonium-238 used for heat sources deserves special attention. Even sub-gram

quantities of 238Pu produce appreciable neutron doses because of the extremely

high spontaneous fission rate in 238Pu. Also, the high specific heat of 238Pu creates

handling problems; small microspheres of 238Pu can melt through gloves in glove

boxes and produce contamination problems.

Plutonium compounds created during the plutonium manufacturing process can

produce very high neutron dose rates, especially PuF4 created during the

separation and purification of plutonium. Fluorinator glove boxes typically have

the highest neutron dose rates in a plutonium processing line. Although PuO2 is

the preferred form because of its chemical stability, the oxide emits almost twice

as many neutrons as pure metal. Neutrons are produced in alpha-neutron

reactions with 17O and 18O. Some PuO2 sources used in medical applications are

prepared with enriched 16O to reduce neutron dose rates, but isotopic enrichment

is generally not used to reduce neutron doses from plutonium compounds.

6.2.3.1 Calculated Neutron Dose Equivalent Rates

Neutron dose equivalent rates can be calculated accurately with

computer codes, such as MCNP (Briesmeier, 1986). The MCNP code

has the advantage that it can calculate both neutron and photon doses

through shielding and in complex arrays. The Monte Carlo codes can

also calculate the effects of neutron multiplication in systems containing

large amounts of plutonium.

However, neutron dose equivalent rates can also be calculated from

simple empirical formulas. Unlike gamma doses, there is very little self-

shielding for neutrons in subkilogram masses of plutonium. In most

instances, a canister containing plutonium can be treated as a point

source at the geometric center of the plutonium. The neutron dose

equivalent rate from a plutonium source can be calculated by:

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