3.3 Light-Element (a, n) Reactions

Additional biological hazard in the handling of plutonium recovered from irradiated uranium or of uranium from irradiated thorium arises from fast neutrons produced by (a, n) reaction. Alpha particles from actinide decay react with light elements—lithium, beryllium, carbon, oxygen, etc.—to produce energetic neutrons such as

2Be + ?He-> *?C + in (8.69)

The fast neutrons are very penetrating and may require some hydrogenous shielding for protection of operating personnel. Also, techniques to ensure low concentration of light-element contaminants in the recycled actinide material may be required.

The allowable concentration of light elements in recycled fuel depends on the alpha-decay rate in the material, the energy of the alpha particle, the probability of an (a, n) reaction, the energy and relative biological effectiveness of the neutron produced, and the allowable surface dose rate of these (a, n) neutrons. The average energies of neutrons from (a, n) reactions in light elements are listed in Table 8.13 along with the tolerance flux for these neutrons. Also listed in Table 8.13 is the neutron emission rate per gram of uranium or plutonium metal that would result in a dose of 1 rem per 40-h exposure at the surface of a kilogram of this metal. This dose rate is about 30 percent less than the official tolerance for radiation exposure localized to the hands and forearms of radiation workers.

The rate of neutron generation from (a, n) reactions in a fuel containing alpha-emitting actinides and various light elements is predicted from

where n = the neutron production rate

Xi = the concentration of the light element;

+Based on data by Arnold [Al, A2].

Ф Reaction constant, a.

§ Based on 1010 alphas directly from 228 Th, but includes effect of 228 Th daughters.

a, y = the number of (a, n) neutrons per alpha disintegration per unit concentration of the light element, with / identifying the energy of the alpha particle Aj = the alpha-disintegration rate of actinide /

Values of a for various light elements, calculated from the data of Arnold [A2], are listed in Table

8.14. The values of a for 239Pu and 240Pu are assumed equal for a given light element because of the nearly equal energies of the alphas from these two isotopes. Similarly, the a for 242 Pu should be very nearly the same as that for 233 U.

For a given mass and isotopic composition of plutonium and contaminant concentration, neutron production rate and allowable concentration of a given contaminant can be estimated from data in Tables 8.13 and 8.14. Estimates for 239Pu containing 1000 ppm 238Pu and for 233U containing 100 ppm 232U are given in Table 8.15. By comparison, plutonium undergoing

Table 8.15 (a, n) surface dose from light elements in recycled plutonium and uranium

Light-element concentration (ppm)l to give
1 rem/40-h wk at surface of 1-kg sphere

final purification by the typical technique of oxalate precipitation may contain the contaminant concentrations listed in Table 8.16. Boron, sodium, and calcium are easily found above the allowable concentration. Either neutron shielding or special precautions to maintain low contaminant concentrations are necessary.

A special problem arises when chemical compounds of plutonium with light elements are handled as massive solids. For example, Pu02 fuel will produce above-tolerance fluxes of (a, n) neutrons at surface contact. Even greater neutron production occurs with PuF4, which is an intermediate in the conversion of plutonium compounds to plutonium metal. Approximately 12 neutrons are produced per 106 alphas in 239PuF4, and some thickness of neutron-shield material may be required.