Figure C-1. Change in mole percent with time in a mixture of 40% T2, 40% 3He and 20% Ar

DOE-HDBK-1129-99

the container and sample change with time. Figure C-1 shows an example of the changing

moles in a gas mixture.

m% He3

m% T2

m% Ar

M%He3At-t

M%T2At-t

M%ArAt-t

100

120

140

160

180

200

220

240

260

280

300

Elapsed Time In Days

FIGURE C-1. Change in mole percent with time in a mixture of 40% T2, 40% 3He and 20% Ar

The total number of moles in the container is known at the time of the sampling but not at the

time of the analysis. If the gas analysis is performed on the same day as the sampling, then,

since the half-life of tritium is 4500.88 days, the error caused by the decay is small (< 0.015

percent for 1 day, approximately 1 percent in 67 days). Some facilities are equipped to perform

the sample analysis within a few minutes or on the same day as the sampling. Others facilities

must depend upon collecting a gas sample, on the sampling date, for analysis at a later date.

The following equations include three significant dates. The Sample Date is the day the gas

sample for analysis is collected, and the pressure, volume, and temperature of the container are

measured and recorded to determine the total moles of gas. The Analysis Date is the date the

gas sample is analyzed to determine the mole percent of tritium per mole in the sample. The

Book Value Date is a date on which the quantity of tritium in the container is known from a

previous assay. Additionally, it assumes a gaseous mixture at the start of 100% tritium and

other stable gases; i.e., it does not contains HT, DT, HTO, or other radioactive or non-inert

gases.

The process steps are as follows:

1. Calculating the number of moles of gas in the container on the Sampling Date.

2. Determining the percent of tritium in the collected sample on the Analysis Date by gas

analysis.

3. Using the two values in steps 1 and 2, the time period, in days, between the two dates and

the half-life of tritium, calculating the number of grams of tritium in the container on the

Sampling Date.

C-7