Figure 2. Idealized surface showing idealized mono-molecular layers of water vapor

DOE-HDBK-1132-99

FIGURE 2. Idealized surface showing idealized mono-molecular layers of water vapor.

Tritium is incorporated first into the loosely bound, outer layers, then into the

intermediate layers, and finally into the very tightly bound, near surface layers.

When the overpressure is removed, the system experiences a new

perturbation. In this case, however, the perturbation is in the negative

direction, and the system becomes the entity that contains the excess tritium.

Le Chatelier' Principle, in this case, indicates that the tritium levels in the

mono-molecular layers of water will be shifted back to the left; that is,

2H2O + 2D2O + 2T2O

H2O + HDO + D2O + HTO + DTO + T2O.

(9)

The tritium that had previously been incorporated into the mono-molecular

layers now begins to move out of the layers, in an attempt to return to

background levels.

The movement of tritium into the mono-molecular layers of water vapor is

generically referred to as " late-out." The movement of tritium out of the mono-

molecular layers of water vapor is generically referred to as " utgassing."

Plate-Out Expectations . When the concentration gradients have been small

and/or the exposure times have been short, only the outermost, loosely bound,

mono-molecular layers of water vapor will be affected. Under such

circumstances, the surface contamination levels will range from no detectable

activity to very low levels; that is, up to a few tens of disintegrations per minute

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