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Calculated Neutron Dose Equivalent Rates - doe-std-1128-98_ch10165
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Table 6.7.1 Spontaneous Fission Nuetron Yields - doe-std-1128-98_ch10167


DOE-STD-1128-98
H = 0.0097 S/r2
(6.4)
where H = dose equivalent rate, mrem/h
r = distance from the center of the source, cm
S = neutron emission rate from the plutonium source.
The total neutron emission rate, S, is the product of the mass of
plutonium (in grams) times Y, the total neutron yield per gram of
plutonium (neutrons/second/gram) from spontaneous fission, ( α ,n)
reactions with low atomic number elements in contact with the
plutonium, and fission-induced neutrons. Generally, fission-induced
neutrons are required because stringent criticality safety rules prevent
accumulation of enough moderator and plutonium to significantly
increase neutron emission rates. But kilogram quantities of metals or
compressed oxides can have significant multiplication and increased
emission rates. For example, neutron emissions from 1 kg plutonium
metal "buttons" are generally measured to be 30% higher than the
calculated neutron emission rate.
6.2.3.2 Neutron Emission Yields
The neutrons produced by spontaneous fission and α ,n reactions can be
estimated from the following information. Most neutrons from
spontaneous fission originate from the even plutonium isotopes: 238Pu,
240Pu, and 242Pu. Because it is the most abundant, the isotope 240Pu is the
most important source of spontaneous fission neutrons. Decay progeny
of plutonium have very low spontaneous neutron emissions. Table 6.7
contains spontaneous fission yields for plutonium and other isotopes that
may be found in plutonium facilities within the DOE complex. These
data are taken from NUREG/CR-5550 (Reilly et al., 1991) and are
believed to be more current then the previously published PNL values
(Faust et al. 1977, Brackenbush et al., 1988). As a rule of thumb,
nuclides with even numbers of protons and neutrons have the highest
spontaneous fission neutron emission rates. The spontaneous fission rate
for odd-even nuclides is about 1000 times less, and the rate for odd-odd
nuclides is about 100,000 less. Spontaneous fission neutrons are emitted
with a Maxwellian energy distribution given by the equation:
N ( E ) = ( E ) Exp ( E / 1.43 MeV )
(6.5)
where N(E) is the number of neutrons as a function of the energy E in
MeV.
6-14


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