Depends on the particular substance, they have different half lives.
I actually walked on top of a nuclear reactor core a few years back, it was a Magnox reactor, in the process of being decommissioned. Interestingly enough, the same day I visited C.A.T. as well (Centre for Alternative Technology) which was only a few miles away.
The nuclear power station was unique in the UK, as it was the only one NOT on the sea - it used an inland lake for it's cooling - anyone want a 15 foot trout with legs and tentacles?