Tritium is produced in the atmosphere by cosmic ray bombardment. Most of such tritium ends up in the oceans and in rivers. The "natural" abundance of tritium varies widely and was greatly increased by atmospheric testing of thermonuclear weapons in the '50s and in the early '60s. The order of magnitude of tritium in ordinary water is T/H - 10-18 (1 TU). Sources vary from 1 to 200 TU. The production of heavy water from ordinary water is even more efficient in the enrichment of tritium than deuterium from the feed material. Most of the heavy water currently available is produced by the H2S - H2O dual temperature exchange process (GS process). The tritium content of fresh heavy water produced by the GS process is 68 dpm/ml D2O/TU feed. Processes that are more efficient than the GS process in heavy isotope enrichment will have a minimum tritium specific activity of 50 dpm/ml D2O/TU feed. Heavy water currently being sold on the open market has a specific activity in the range 120 - 180 dpm/ml D2O. There are sources of D2O with specific activity as high as 104 dpm/ml.
Most of the work done to date on the search for tritium produced in the electrolysis of D2O in cells with palladium cathodes has been done in open cells. The measurements are frequently limited to assays of the specific activities of the starting D2O and the electrolyte after electrolysis. In general, there have been periodic additions of D2O to replace the D2O decomposed to form Pd-D and D2(gas). To determine how much tritium, if any, has been produced requires a complete inventory of the tritium at the beginning and end of the experiment. From the data on the current and on the duration of the electrolysis it is possible to estimate the amount of D2O which has been electrolyzed. Electrolysis will enrich the tritium in the D2O of an electrolytic cell. The amount of enrichment is primarily a function of the amount of water electrolyzed fur a given type of cathode. It can reach a factor of 5 when 95% of the initial charge of water is electrolyzed. Thus a careful analysis of an electrolytic experiment must be carried out if one is to interpret specific activities of tritium after electrolysis, below 1000 dpm/ml of D2O, as anything other than electrolytic enrichment [BIG].