The relationship between the irradiation time and the calculated specific 99Mo generation (generated 99Mo activity per 1 g of molybdenum) is shown in Fig. 3. When the irradiation time is 6 days (144 h), the specific 99Mo generation is 0.286 TBq/g-Mo as shown in Fig. 3.

Irradiation time (h)

Fig. 3. Relationship between irradiation time and specific 99Mo generation

Using the specific 99Mo generation of 0.286 TBq/g-Mo, 99Mo production rates are estimated. In the case using the aqueous (NH4)6Mo7O24-4H2O solution as the irradiation target,

(99Mo production in the case using the aqueous (NH4)6Mo7O24-4H2O solution)

= 372.8 g x 0.286 TBq/g-Mo = 106.6 TBq = 2,881.9 Ci

In the case using the aqueous K2MoO4 solution as the irradiation target,

(99Mo production in the case using the aqueous K2MoO4 solution)

= 702.7 g x 0.286 TBq/g-Mo = 201.0 TBq = 5,431.7 Ci

Here, the dilution effect by the unirradiated aqueous molybdate solution and the decay time of 99Mo from the generation to the shipment are considered. It is assumed that the volume of the aqueous molybdate solution in the capsule and the pipes in the irradiation system and the supply and circulation system of the 99Mo production system is about 2,500 cm3 and that the time from the post-irradiation to the shipment is one day. After one day, 99Mo decays to 0.78 times. Time from the irradiation to the shipment is one week. The 99Mo production rates at the shipment are estimated as follows:

(99Mo shipping activity in the case using the aqueous (NH4)6Mo7O24-4H2O solution)

= 2,881.9 Ci x 1,663/2,500 x 0.78 = 1,495.3 Ci/w

(99Mo shipping activity in the case using the aqueous K2MoO4 solution)

= 5,431.7 Ci x 1,663/2,500 x 0.78 = 2818.3 Ci/w

The 99Mo production rate in the case using the aqueous K2MoO4 solution is about twice compared with that in the case using the aqueous (NH4)6Mo7O24-4H2O solution. It is a distinct advantage of the aqueous K2MoO4 solution. However, in order to aim to provide 100% of the 99Mo (5,000 Ci/w) imported into Japan and to increase the production rate, some ideas such as the concentration of 98Mo are needed.