Neutron Emission Yields

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

gram of compound, multiply by 0.88 for PuO2 and 0.76 for PuF4. These data are taken

from NUREG/CR-5550 (Reilly et al., 1991).

Table 6.9 contains the neutron yields for trace amounts of elemental impurities in

plutonium metal or oxide. These data are also from NUREG/CR-5550 (Reilly et al.,

1991) and are derived from thick target yields from accelerator data. The data in

Table 6.9 differ from previous values in BNWL-2086 (Faust et al., 1977), and the

authors have not experimentally checked the accuracy of these values. Two sets of data

are included: one for alphas emitted from enriched uranium and the other for alphas

emitted from 239Pu. To determine the neutron yield from trace impurities, it is first

necessary to determine the specific alpha activity from Table 6.8, and the neutron yield

per parts per million per 106 alphas from Table 6.9 for either enriched uranium or

plutonium. The specific neutron yield from impurities can be estimated from the

following formula:

10 - 12 A "

Y imp

(6.8)

where A" = alpha activity of the plutonium nuclides

Pj = specific neutron yield from the jth element (neutrons/alpha-part per million)

from Table 6.8

Ij = elemental impurity concentration in plutonium (parts per million).

Note that this formula is valid only if the impurities are uniformly distributed with the

plutonium so that the alpha particles directly interact with the impurities. Dust layers of

plutonium oxide can also produce high neutron yields. For example, plutonium oxide

dust layers on HEPA filters with borosilicate glass can produce neutron emission rates

10 times higher than those for pure oxide because of alpha-neutron reactions with boron

in the glass fibers and aluminum spacer plates.

The total neutron yield per gram of plutonium can be found by summing the

contributions from:

-- Spontaneous fission (from Table 6.7)

-- alpha-neutron reactions in oxides or fluorides (from Table 6.8)

-- neutrons from low-atomic-number impurities (from Table 6.9).

6-16