Metals - h3010v10101

DOE-HDBK-3010-94

4.0 Solids; Metals

4.2

M ETALS

4.2.1

T h erm al S tres s

Most metals react with oxygen in some fashion, with some reactions occurring at room

temperatures. At low temperatures, the reaction rates may be so slow that the oxidation is

not readily recognized, or a protective oxide film may form that limits/reduces additional

oxidation. Many metals generate heat from the oxidation reaction. Metals that attain a self-

sustaining reaction at ambient temperature are called pyrophoric. Some nuclear metals

(uranium, plutonium) can achieve a self-sustaining reaction at elevated temperature dependent

upon surface-to-volume ratio and heat transfer conditions.

Initial oxidation rate is a function of the temperature; when heat is externally supplied and

generated by the oxidation reaction, the kinetic controlled regime exists. At some

temperature (a function of the balance between the heat available and the heat loss), the

reaction becomes self-sustaining (plutonium ignition temperature ~ 500 oC), and the reaction

rate and the temperature become limited by the diffusion of the oxygen to the reaction

interface and the overall heat transfer characteristics of the system. Under these sustained

conditions, known as the diffusion-controlled regime, temperatures range from 900 to

1100 oC and plutonium (mp 641 oC) is molten but uranium (mp 1132 oC) is not. Both metals

may form protective suboxide films at the interface that are adherent, but more stable oxides

are formed as the depth of the oxide layer increases. The matrix spacing for some oxides is

sufficiently different from the metal phase spacing that the oxide is non-adherent and can be

made airborne by sloughing of the oxide from the oxidizing mass and entrainment in the

convective currents generated by the heated metal. Other heat sources such as fires may also

generate convective currents that may carry the airborne materials once ejected from the

oxidizing mass. High temperatures (>1000 oC) may coarsen the size distribution of the

residual powder or reduce suspension by sintering the powder oxide.

4.2.1.1

P lu ton iu m

Haschke (July 1992) reviewed and evaluated the data on the oxidation of plutonium. "The

oxidation is a 'paralinear' process involving three stages. ... has the functionality

characteristic of a diffusion-controlled process. As the thickness of the adherent oxide layer

increases on the metal surface, the rate of oxygen diffusion through the layer decreases

according to a parabolic curve like observed for stage I ... formation of oxide particles

begins during stage II ... characterized by linear rate ... at onset of stage II, the thickness of

the inherent product layer attains a critical value determined by buildup of stress induced by

forming of low-density oxide (molar volume = 23.67 cm3/mol) on the high-density metal

(molar volume 12.10 cm3/mol). This stress is relieved by cracking and spalling of oxide

Page 4-11