Mass of Contained Materials

DOE-STD-3013-2000

has a 0.05 margin in keff is calculated to be 4.53 kg (9.98 lb.). The parameter keff

indicates the criticality status of an assembly of fissile and possibly other materials

a value of 1.0 indicates the critical condition (a self-sustaining reaction) and values

less than 1.0 indicate subcriticality, with smaller values signifying greater departures

from criticality. A margin of 0.05 (a keff of 0.95) is commonly used to assure

subcriticality. In other words, the smallest amount of plutonium that could go critical

is somewhat more than 4.53 kg. The 4.4 kg limit specified corresponds to the limit

for some shipping packages and allows a modest additional margin of safety. Note

239

Pu. This constraint

that the mass limit applies to all fissile species and not just

233

prevents potential criticality incidents involving stored fissile materials (i.e.,

235

237

Np, or higher plutonium isotopes) because the critical masses of these fissile

239

Pu.

radioisotopes are greater than that of

In terms of plutonium mass, 5.00 kg (11.02 lb.) of plutonium oxide is equivalent to

4.40 kg (9.70 lb.) of plutonium metal. The oxide weight limit refers to the total mass

of the plutonium-bearing materials present, not just to the plutonium oxide content.

This constraint provides additional assurance of subcriticality by making the

conservative assumption that all the contents are pure plutonium dioxide. Mass

limits may be further limited by facility-specific considerations including

administrative criticality, radiation, and wattage (heat output) constraints. Note that

the mass limit does not imply subcriticality of arrays and the normal, more facility-

specific analyses are required to demonstrate criticality safety in storage and

transportation.

For consistency, and through a similar reasoning process, the total mass of metal,

including alloying additions and other non-fissile species, is also limited to 5.0 kg

(11.02 lb.).

Finally, keeping the mass of the contents at or below 5.0 kg (11.02 lb.) ensures that

the safety envelope established through the container certification (drop testing)

program is maintained.