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.
22.214.171.124 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: