Mo-99 is priced and sold based on units of radioactivity (or activity) calibrated to a certain future time. Time calibration is necessary because of radioactive decay. The unit of activity used by Mo-99 producers to price and sell this isotope is the unit curie (Ci), which is equal to 37 billion disintegrations per second. Most producers, and all large-scale producers, calibrate the sale price to the number of curies present in a shipment of Mo-99 6 days after it leaves the producer’s facilities. This quantity is referred to as 6-day curies.
The 6-day curie concept is schematically illustrated in the figure below, which shows the buildup and decay of Mo-99 during target irradiation, processing, and shipping. During the 5- to 7-day period of irradiation in the reactor (left side of figure) Mo-99 builds up in the target and eventually approaches a maximum as Mo-99 production is balanced by Mo-99 loss to radioactive decay. Mo-99 continues to be lost to radioactive decay after the targets are removed from the reactor, and some additional losses are incurred during target processing because of process inefficiencies (middle of figure). The amount of Mo-99 available for sale as 6-day curies (right side of figure) is only a fraction of the isotope present in the targets at the end of bombardment (EOB) by neutrons in the reactor.
The current global demand for Mo-99 is about 12,000 6-day curies per week. To produce this quantity of isotope, producers would need to irradiate enough U-235 targets to obtain about 77,000 curies of Mo-99 in the targets at EOB (left side of figure). About 54,400 curies of Mo-99 will be recovered from processing these targets, assuming a Mo-99 recovery efficiency of 90 percent (Chapter 2) and a processing time of 1 day (Table 3.4). The 12,000 6-day curies represent about 17 percent of the Mo-99 present in the targets at EOB.
The weekly global demand for Mo-99 can be supplied by the fission of about 2 g of U-235. The 54,400 curies of Mo-99 available at the end of target processing would have a mass of about 0.11 g. This mass of Mo-99 is about the amount contained in a cook’s “pinch of salt.” The remainder of the U-235 ends up as waste.