Plutonium metal phase changes

DOE-STD-3013-2000

results are given in Table A-1. In evaluating temperature dependent phenomena

in plutonium metal, it was conservatively assumed that the plutonium and the

plutonium-steel interface were at 250C (482F), thereby providing considerable

margin to the calculated maximums [Williamson 1990]. Note that thermal

analyses such as those performed by Hensel should be performed for actual

storage configurations to verify the conservatism of the 250C assumption made

for this evaluation.

Potential metal storage issues related to metal temperature include 1) volume

changes associated with plutonium metal phase transitions and 2) metallurgical

interactions between plutonium metal and the container walls. These two issues

are discussed below.

2) Plutonium metal phase changes

The alpha to beta phase transition of plutonium metal, which occurs near 119C,

is accompanied by a significant volume increase [ANS 1980, Spearing et al.

1999; Spearing/Veirs 1999; Flanders/Krishnan 1999]. This volume change

typically is not fully recovered when the metal is returned to the alpha phase by

cooling below the transition temperature. Concern that cycling of alpha

plutonium metal through the alpha-beta phase transition could cause enough

radial growth in the contained metal to damage or breach the container led to

recent experiments to address this issue [Flamm 1997; Spearing/Veirs 1999;

Spearing et al. 1999]. A recent peer review of these experiments concluded that

"the only potential failure mode that we could anticipate is one of fatigue

resulting from repeated cycles" [Hecker/Stevens 1999]. Experimentally, it is

observed that plutonium volume expansion occurs anisotropically in a cylinder

with more expansion in the axial direction than in the radial direction. Also, the

fraction of expansion occurring in the axial direction increases as the strength of

the can increases. The peer review concluded that cycling through the beta-

gamma transition alone near 185C would be less demanding on the container

than the alpha-beta transition cycling because 1) the volume change is

significantly less for this transition and 2) the strength of stainless steel

decreases more slowly with temperature than the strength of plutonium. Finite

element analysis using the alpha-beta transition experiment data evaluated the