CargoOne,
I've just run this passed our DG Department & they agree that short of being a Rocket (Nuclear?) Scientist, with full access to the type of radionuclide properties & activity level it's going to be next to impossible to evaluate. As CR2 says, it's not the weight, it's the activity. If it is indeed UN2912, then it's non fissile or fissile excepted so the explosive characteristic should not be a factor.
Check out 10.5.17.1. Max TI per package is 10 & under 9.3.C for Cargo Aircraft the max per a/c is given (Non-fissile/Fissile - exclusinve use/Non exclusive etc).
As before, good luck!