Table 6 -4. Spontaneous Fission Neutron Yields

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

nuclides is about 100,000 less. Spontaneous fission neutrons are emitted with a Maxwellian energy

distribution given by the equation:

(6.2)

where N(E) is the number of neutrons as a function of the energy E in MeV.

Table 6 -4. Spontaneous Fission Neutron Yields

Spontaneous Fission

Spontaneous

Isotope

Total Half-Life

Half-Life,

Fission Yield,

years

n/sec- gram

232

8 x 1013

71.7 y

1.3

233

1.59 x 105 y

1.2 x 1017

8.6 x 10-4

234

2.45 x 105 y

2.1 x 1016

5.02 x 10-3

235

7.04 x 108 y

3.5 x 1017

2.99 x 10-4

236

2.34 x 107 y

1.95 x 1016

5.49 x 10-3

238

4.47 x 109 y

8.20 x 1015

1.36 x 10-2

Energetic alpha particles can overcome coulomb barriers in low-atomic -number elements and

create an unstable nucleus that emits neutrons. Because of the high alpha activity of uranium, this can be a

significant source of neutrons. There are two nuclear reactions that are of importance:

Table 6-5 lists the alpha-neutron yields for oxides and fluorides for the uranium isotopes. Note

that the neutron yields are normalized per gram of nuclide, not per gram of compound. These data are

taken from NUREG/CR-5550 (NRC 1991).

The total neutron yield per gram of uranium can be found by summing the contributions from:

spontaneous fission (from Table 6-4)

alpha-neutron reactions in oxides or fluorides (from Table 6-5)

neutrons from low-atomic -number impurities (from Table 6-6).

Multiplying the specific neutron yield (neutrons/second-gram of uranium) by the mass of

uranium (grams) gives S, the neutron emission rate (neutrons/second).

6-9