Calculated Neutron Dose Equivalent Rates

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

-- alpha-neutron reactions with low-atomic-number elements, including oxygen and

fluorine in plutonium compounds and impurities in metals

-- neutron-induced fissions.

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

238

Pu 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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