Self-Heating - STD-1128-98master0047

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

2.5.1

Self-Heating

Heat generated by radioactive decay in plutonium, its alloys, or its compounds can be

calculated from data provided in Table 2.4, together with the isotopic composition and

plutonium fraction. The power output of reactor-produced 239Pu metal is usually in the

range of 2 to 10 W/kg. According to Van Tuyl,3 the equilibrium surface temperature of a

metal can that contains 1.2 kg of plutonium at the higher specific power would be 150C.

This calculation is complex because it depends on the thermal conductivities and

configuration of all the materials in the shipping container. Thermal diffusivity

measurements reported by Kruger and Robbins (1975) were combined with existing heat-

capacity values to derive a curve for the thermal conductivity of the Pu-1wt% Ga alloy

from room temperature to 600C. Gram quantities of 238Pu can melt from self-heating

under poor heat-transfer conditions. The major effects to be expected from self-heating

are phase transformation, dimensional changes, chemical reactions (depending on the

gaseous environment or other materials in contact with the plutonium), and desorption of

previously sorbed gases or vapors.

2.5.2

Radiolysis

In gases, liquids, and covalently bonded solids, the chemical effects of alpha particles and

the associated recoil nucleus can cause ionization, excitation, and dissociation of

molecules. From the energy requirement for ion pair formation, only about half the

energy causes ionization; the other half goes into molecular excitation. Radiation effects

are commonly measured by a quantity called the G-value, i.e, the number of molecules

destroyed for each 100 eV of energy absorbed. For free radical production, this quantity

is expressed as the GR-value. For organic liquids, GR-values typically range from 0.85 for

carbon disulfide to 70 for carbon tetrachloride (Prevost-Brnas et al., 1952).

Although there is a considerable body of data on the radiolysis of aqueous solutions,

organic liquids, and solids irradiated by gamma rays, X-rays, and fast electrons, little has

been published on the radiolysis of plutonium compounds, solvents containing

plutonium, or radiation-induced damage in materials that come in contact with

plutonium. Nevertheless, radiation-induced damage can affect all aspects of plutonium-

handling.

Van Tuyl, H. H. 1981. "Packaging of Plutonium for Storage or Shipment." Unpublished report by the Pacific Northwest Laboratory task

force chairperson to the U.S. Department of Energy.

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