Composition Measurements

DOE-HDBK-6004-99

Composition Measurements

This method is used for measurement of gaseous samples only. A representative sample of the gas

is taken. The gas that is to measured must be mixed well. The volume, pressure, and temperature

must be measured accurately. The temperature is difficult to measure accurately because of

temperature gradients caused by the heat of decay of tritium. The composition of the gas in the

sample is then measured using a mass spectrometer or a laser Raman spectrometer.

The mass spectrometer will measure all gas species. A high resolution mass spectrometer is required

to distinguish between different molecules with the same mass number. For example HT and D2 have

the same mass number, but must be separated to determine the tritium concentration. All species that

can contain tritium must be measured. This includes, water as HTO, methane as C(H,D,T)4, ammonia

as N(H,D,T)3, etc. If the approximate gas composition is known, the sum of all the species

containing tritium can be determined. If the approximate gas composition is unknown, the use of the

mass spectrometer may be difficult.

The laser Raman spectrometer is a relatively new system that can be used to measure molecular

concentrations in a gas mixture. The sample is placed in a cell with optical windows. The laser

excites the rotational or vibrational atomic levels in the gas molecules. The light emitted as the

excited levels decay back to the ground state are detected using a photodetector system. The

measurement is absolute in that the frequency spectrum of each molecule is unique. The intensity is

proportional to the amount of gas present. The disadvantages of the Raman method is that the

amount of inert gases can not be determined. Common inert gases at fusion facility are the isotopes

of helium.

Both of these techniques can be used for real time measurements. For the mass spectrometer system

a sample is bled to a high vacuum system for measurement. The Raman system is easily adopted to

real time measurements. The gas stream at atmospheric pressure is passed through an optical cell.

The spectrum for a mixture hydrogen isotopes can be determined in approximately one minute. The

total accuracy of these measurements is approximately 3 to 5 %.

The mass spectrometer technique has been the standard method that DOE facilities have used for the

determination of the tritium inventory. It is a proven system although it requires a expensive

spectrometer ($200k) and accurate determination of the temperature, pressure and volume.

The Raman system has not been accepted. Experiments are currently being performed to demonstrate

that this will be an acceptable technique.

Thermal Methods of Inventory Measurement

The primary method to inventory large quantities of tritium in the liquid or solid form is to use a

calorimeter. The sample is placed in a thermally isolated contained. The power required to maintain

the temperature of the container is then a measure of the amount of tritium in the sample. Containers

that can accept samples that vary from several inches in diameter up to a 55 gallon drum.

The lower limit of accuracy can be as low as 100 Curies. Calorimeters are expensive (>$200k).

There require high tech electronics. They are the primary methods used to measure tritium in waste

such as HTO on molecular sieve. They have not been used to measure process tritium except in very