Would Radiation isotopes from japan cause our sever weather changes - would it magnify storms and twisters?

No. The radiation that does have an effect on weather comes from the Sun. The amount of radiation coming from Japan is infinitesimally small compared with that from the Sun and has no effect on the weather. Between 1951 and 1992 there were 928 nuclear tests in Nevada with a hundred of them being above ground. If any man-made radiation was going to affect the weather, it would have been seen there. There was no effect on the weather then and there is no effect now.