Palladium

DOE-HDBK-1129-99

heating/cooling cycle. Active impurity gases, such as oxygen and nitrogen, are irreversibly

removed by reaction with the uranium.

Disadvantages to using uranium tritide beds are: 1) uranium powder is pyrophoric; 2) the

generation of significant tritium pressure requires a high temperature that results in permeation of

tritium through the vessel wall; and 3) the capacity is also permanently reduced by exposure to

active impurity gases.

2.5.2.b(2) Palladium

Palladium is a metallic element of group 8 in the Periodic Table. The symbol for palladium is Pd,

the atomic number is 46, the atomic weight is 106.42, and the melting point is 1554.9C.

At room temperature, palladium absorbs up to 900 times its own volume in hydrogen. It diffuses

easily through heated palladium; this is one means of purifying the gas. Finely divided Pd is a

good catalyst, and is used for hydrogenation and dehydrogenation reactions. Palladium metal is

used in dentistry, as an alloy in making jewelry, and in making surgical instruments, watches, and

electrical contacts.

Palladium powder is currently the second most used material for general-purpose tritium storage

beds. Palladium can be obtained in powdered form and loaded directly into the container used for

the metal tritide bed. Palladium was used extensively at both LLNL and SNLL in the tritium

storage beds.

When the tritium is exposed to the powder, it dissolves in the palladium powder with a maximum

Pd:T ratio of approximately 0.7. Palladium powder is not pyrophoric, but it has a higher tritium

partial pressure than uranium at room temperature.

At room temperature, tritium, deuterium, and protium dissolve in the palladium powder and the

tritium partial pressure in the gas over the powder is approximately 50 torr. The over pressure

increases as a function of temperature. As the temperature of the palladium is increased by

heating the bed, the tritium partial pressure increases as a function of the temperature and reaches

a pressure of around 750 psia at 350oC. The general form of the equation for the dissociation

pressure, P, in millimeters of mercury (mm) for palladium hydride, deuteride, and tritide is:

log Pmm =

(-A/T + B)

10 (-A/T + B)

Pmm =

Where T=

temperature (K)

The values for A and B for hydrogen and deuterium determined by different investigators are given

in Table 2-4, and the equations developed by the different experimenters over the temperature

range they investigated are plotted in Figure 2-6.