Irradiated fuel - the uranium that has been gone through nuclear fissioning in the reactor core - contains hundreds of radioactive isotopes, some of which will be radio-active for millions of years.

When a fuel bundle is removed from the reactor core it is one million times more radioactive than the uranium in the "fresh" fuel bundle before it was placed in the reactor.

The Nature of Nuclear Waste

A public interest information project about nuclear waste burial in Canada.

Know Nuclear Waste 

Nuclear waste from reactors is extremely hazardous now and for hundreds of thousands of years. The radioactive and chemical hazards will outlast the containers and, over time, the wastes - which even the nuclear industry agrees must be strictly isolated from the environment - will be released. 

The wastes from the  next generation of reactors will be even more hazardous according to recent reports, and the challenges of managing the wastes over the long periods of time required will become greater if Canada elects to construct additional reactors that are of a different design.