Appendix C: Assay Methods

DOE-HDBK-1129-99

APPENDIX C: ASSAY METHODS

There are a number of different assay methods used at the DOE tritium facilities. Most facilities

need assay equipment capable of measuring tritium in gaseous, solid, and liquid form.

Gas Analysis: For assay of gaseous tritium, most facilities use some form of mass

spectrometer ranging from quadrapole to large sophisticated light isotope drift tube systems.

Gas analysis equipment; especially gas analysis equipment which will measure the low

molecular weight gases like H2, HD, HT, D2, DT and T2 accurately; is very expensive and

requires a high degree of expertise to operate. These systems can cost from fifty to several

hundred thousand dollars.

Solid (Metal Tritide): Tritium stored in solid form such as a metal tritide must either be

decomposed to return it to the gas form for analysis, or the heat output of the solid caused

by the decay of tritium can be measured in a constant heat flow calorimeter. In order to use

calorimetry to measure the tritium quantity, it must be known that the item being assayed

does not contain any other radioactive component and that no chemical reactions are taking

place in the container. Constant heat flow calorimeters vary in chamber size from a few

cubic centimeters up to a few liters, and the item to be assayed must be small enough to fit

inside the calorimeter chamber. The constant heat flow assay process is the most accurate

assay method available if the chamber size and item to be assayed are well matched.

Liquid (HTO, DTO, T2O): Tritium at high concentrations in liquid form is generally measured

using calorimetry. Low concentrations of tritium in liquid form are generally measured by

using a scintillation counter. A sample of the liquid is mixed in a scintillation cocktail, and the

quantity of tritium in the sample is measured.

C.1 Measurement Accuracy and Safeguards and Security

DOE requires that tritium be accounted for to the hundredth of a gram. However, most of the

equipment and the techniques used cannot accurately determine the tritium quantity to a

hundredth of a gram once the quantity assayed exceeds about one half gram. This appears to

be inconsistent with the DOE accountability requirements. Measurement accuracy better than

that implied by the previous statement is certainly achieved but requires expensive,

sophisticated equipment and world-class operators that are only justified by special process

needs. Even the more sophisticated equipment does not measure the quantity of tritium

accurately to a hundredth of a gram once the quantity exceeds about five grams. The assay

technique to be used in an operation or facility should be frankly discussed with DOE

safeguards and security to make sure that it will meet the DOE needs for the facility safeguards

and security category and the activities performed in the facility.

C.2 Tritium Assay Analysis by PVT Mass Spectrometer

The most common method of assaying tritium in gaseous form, i.e., T2, HT, and DT, mixed with

other gases such as Ar, N2,, O2, and 3He, is referred to as "PVT mass spec." The total number

of moles of gas, n, in a container is calculated using the equation,

n = PV/zRT

where P = pressure in the container in torr

C-1