Plutonium metal phase changes

DOE-STD-3013-2004

The 19-watt limit also caps temperatures that may be reached under normal and

off-normal conditions. Calculations performed at the Savannah River Site indicate

that the plutonium metal-stainless steel container interface temperature will not

exceed approximately 189C (372F) even when the container is placed in a

9975 transportation package, exposed to diurnal solar heating and an ambient

temperature of 37.8C (100F), provided the heat generation rate of the contents

does not exceed 19 watts [Hensel 1998b]. These calculations have also shown

that the centerline temperature of the plutonium metal will not exceed

approximately 202C (397F) under the same conditions. These and other 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 1999].

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