Table 2-1. Derived air concentrations for tritium and tritiated water

DOE-HDBK-1129-99

TABLE 2-1. Derived air concentrations for tritium and tritiated water

Bq/m3

Ci/ml

5E-01

2E10

HTO

2E-05

8E05

Unlike the situation for oxide and elemental, DACs for tritides (e.g., titanium tritide or hafnium

tritide) are currently not defined in relevant regulations and, therefore, the process for radiation

posting at 10 percent of the DAC level cannot be finalized for these tritiated compounds. The

Office of Worker Protection Programs and Hazard Management within the Office of Worker Health

and Safety (EH-5) and Environmental Management's Office of Safety and Health (EM-4) at DOE

Headquarters are currently pursuing this issue, and a white paper, entitled Workplace Indicators

and Bioassay Limitations When Dealing With Stable Metal Tritides (SMTs), was authored at

Mound in September 1998. The Mound paper uses the terms stable and unstable to refer to the

measure of the degree of tritium releasability from the metal. Easily releasable tritium is unstable,

and is not a workplace monitoring or bioassay problem, as the tritium is more reasily released as

elemental or oxide. The tritides in which the tritium is difficult to release, or stable, are the topic of

interest here. The biological effect resulting from these tritides is also not as well understood or

modeled as those from oxides and elemental. The biological half-life of some tritides is at least an

order of magnitude higher than oxide (see Appendix A-3). Tritium that is adsorbed or contained

within respirable particulates (e.g., TiT2, or tritium gas in glass microspheres) presents unique

dosimetry problems. The physical configuration of such material will affect the uptake, distribution,

retention of the tritium, and will also affect the amount of energy deposited from each

transformation. Other complexities (e.g., production of bremsstrahlung radiation) may have to be

considered.

The possible dose pathways from inhalation of tritiated particulates are direct irradiation of the

surrounding tissue; irradiation of the surrounding tissue by bremsstrahlung from the particle;

uptake of any tritium oxide (HTO) contamination from the surface of the particle; and absorption

(leaching) of tritium from the particle into the bloodstream.

The distribution and retention of tritiated particulates is largely dictated by the chemical and

physical characteristics of the particles. For example, to gain a perspective on this issue,

distribution and retention of inhaled TiT2 has been modeled using a modification of the ICRP-30

Respiratory Tract Model with modified deposition fractions and compartmental half-lives of several

hundred days. [2]

The factors, which affect the distribution and retention of such particulates, can act to both reduce

and enhance the dose received per unit intake. For example, a very large fraction of inhaled

activity is quickly eliminated from the body if the particle size distribution is relatively large. The

small residence time, coupled with the effective encapsulation of the tritium activity and the short

range of the beta particle emitted, all lead to a significant reduction in dose per unit intake (not

uptake). Conversely, a postulated or observed long residence time in the lungs may lead to

significant lung doses.

Due to the high degree of self-absorption of beta energy that takes place within the particle, only a

very small fraction of the beta energy from each transformation is expected to escape the particle.

It has been estimated [3] that this self-absorption may reduce the effective dose by as much as a

factor of 2,000. Additionally, macrophages are expected to envelop particles in the lungs and add

tissue shielding around the particle.