Neutron Emission Yields cont'd - doe-std-1128-98

DOE-STD-1128-98

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:

Y imp = 10 - 12 A ∑ P j I j

(6.8)

where A α =

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).

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