Corrosion resistance improving method for channel box

A material less causing β rays and backward scattering is used for a member in adjacent with a channel box. The distance between the channel box and the adjacent member is made greater than the maximum flying distance of β rays. In addition, the plate thickness of a member in adjacent with the channel box is determined to such a value as not causing saturation back scattering of β rays. Namely, a material made of an element having low atomic number and less causing β rays and smaller saturation backward scattering, namely, a saturation back scattering coefficient at an average energy of β rays of less than 1.5 is used for the constitutional members. Alternatively, the plate thickness of the constitutional members is determined to a value 1/10 or less of the maximum flying distance at an average energy of β rays. Alternatively, the distance between each of the constitutional members is determined to at least twice the maximum flying distance of β rays in order not to cause backward scattering even taking the occurrence of β rays of the members into consideration, and the effects are exhibited substantially by the combination of them. (N.H.)