To the best of my knowledge, there are no laws in the lower 48 concerning hazardous cargo transport through inclement weather.  This is usually left to the discretion of the driver, who may or may not be trying to get the load through early for a bonus, or a quicker turnaround home.  Many companies have their own regulations concerning operations in bad weather.  I have spent time hauling CO2 on the eastern seaboard, and can remember seeing a lot of guys pulled into the rest areas when I went on down the road.  As an O/O, the only rule you follow is to outrun the weather, or drive through it; otherwise go hungry.  Now, I do know from other life experiences that there are considerably more and stricter DOT regulations when it comes to radioactive waste; however the waste is divided into (3 I think) levels, corresponding to the potential for danger.  With each level, the licensing and transport regulations get tighter.  There may also be some OSHA rules concerning this area.

The only consolation for us is probably the incredible designs of the waste containers.  I can't say much more than...trust me, they are designed to withstand anything.