The most important element of the solution irradiation method is the aqueous molybdenum solution as the irradiation target. The solution with a high concentration near the saturation is used for efficient 99Mo production, and the solution always is in contact with the structural materials of the capsule and the pipes in the 99Mo production system under irradiation. Aqueous molybdate solutions are promising candidates for the irradiation target. The effect of the solutions on metals such as the structural materials has been researched, and molybdates are known as corrosion inhibitors (Kurosawa & Fukushima, 1987; Lu et al, 1989; McCune et al, 1982; Saremi et al, 2006). However, the behavior of aqueous molybdenum solutions including the aqueous molybdate solutions under such the conditions is not well understood. Therefore, the following subjects about the fundamental characteristics of the solutions should be investigated:

1. Selection of the aqueous molybdenum solutions as candidates for the irradiation target

2. Compatibility between the solutions and the structural materials

3. Chemical stability of the solutions

4. Effect of у ray and neutron irradiation on the solutions such as the radiolysis, the у heating and the activation by-products.

The some subjects described above had already investigated (Inaba et al., 2009), and the progress of the development made thus far is explained as below: