The selection of candidates for the irradiation target was carried out. The conditions required for the irradiation target solution are as follows:

1. The irradiation target solution has the high molybdenum content for the efficient production of 99Mo.

2. Few activation by-products are generated by target solution irradiation for the prevention of radioactive contamination.

3. The solution has good compatibility with the structural materials of the capsule and the pipes for the prevention of corrosion.

4. The solution is chemically stable and has no generation of precipitation for the prevention of an obstruction to the solution’s flow.

Based on the conditions (1) and (2), two aqueous molybdate solutions (aqueous ammonium molybdate and potassium molybdate solutions) were selected as the candidates for the irradiation target among the aqueous solutions of general molybdenum compounds.

The solubilities of ammonium molybdate ((NH4)6Mo7O24 4H2O) and potassium molybdate (K2MoO4) for pure water are 44 g/100 g-H2O and 182.4 g/100 g-H2O respectively, and the molybdenum contents in the solubilities of (NH4)6Mo7O24^4H2O and K2MoO4 are 23.9 g and 73.5 g respectively.

The activation by-product of (NH4)6Mo7O24 4H2O is only 92mNb. The activation by-products of K2MoO4 are 42K and 92mNb. The y-ray energy emitted from 42K is high. However, by using PZC, it is possible to remove 42K and 92mNb from the two aqueous molybdate solutions irradiated with neutrons.

The conditions (3) and (4) were confirmed by tests with the two solutions.