Nuclear waste comes from a wide variety of sources. The radioisotopes used in medicine, the nuclear fuel used in atom bombs – even the tiny samples used in smoke alarms all generate waste. Most waste however – about 95% - is produced by nuclear power stations, and may half lives of tens of thousands of years or longer – if we wanted to store it in steel drums, the drums would not stay intact for the lifetime of the radioactivity.
The nuclear industry produces three types of waste.
High Level Waste. This consists of spent fuel rods. High level waste is radioactive, and is hot because of the energy produced during radioactive decay. It has to be carefully stored but is not very radioactive for long. There is not much high level waste produced. All the high level waste in the UK is stored in a pool of water at Sellafield.
Intermediate Level Waste. This is less radioactive than high level waste, but is increasing, since high level waste decays into long lived intermediate level waste. Intermediate level waste is chopped up, mixed with concrete and stored in stainless steel containers. This cannot be a permanent solution.
Low Level Waste. The protective clothing used by workers in the nuclear industry and medical equipment can be slightly radioactive.
Radioactive waste has very little effect on the UK's average background radiation. The danger is for another reason. Nuclear waste may leak into the water supply and subsequently be drunk or taken up by fruit and vegetables and eaten. Any radiation emitted may be absorbed by the body's internal organs.
The UK once dumped nuclear waste at sea. This is no longer done. In fact nuclear waste is currently stored, as mentioned above, not disposed of. In 2004 the British Government set up the Nuclear Decommissioning Authority to deal with nuclear waste generated in the UK. It had a budget of about £1,000 million each year, and it's job was to find safe methods of storing and disposing of waste, and implementing them. Possibilities once discussed include burying in Arctic ice or firing into space. The first is unlikely because the ice is melting, so one day the waste may be exposed, while the second would shower us all with radioactivity if a rocket exploded on the way to space. The best current candidate is to bury it in stable geological structures far from water supplies.