The decommissioning of nuclear power plants is sometimes referred to as nuclear decommissioning, to mark the difference between 'conventional' decommissioning and dismantling projects. Nuclear power is any Nuclear technology designed to extract usable Energy from atomic nuclei via controlled Nuclear reactions The main difference to the dismantling of a 'conventional' facility is the possible presence of radioactive or fissile material in a nuclear facility, that requires special precautions. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. In Nuclear engineering, a fissile material is one that is capable of sustaining a Chain reaction of Nuclear fission.
Generally speaking, nuclear plants were designed for a life of about 30 years. Nuclear power is any Nuclear technology designed to extract usable Energy from atomic nuclei via controlled Nuclear reactions Newer plants are designed for a 40 to 60 year operating life.
Decommissioning involves many administrative and technical actions. It includes all clean-up of radioactivity and progressive demolition of the plant. Once a facility is decommissioned, there should no longer be any danger of a radioactive accident or to any persons visiting it. After a facility has been taken out of service it allows its release from regulatory control and relieves the licensee of his responsibility for its nuclear safety.
The International Atomic Energy Agency has defined three options for decommissioning, the definitions of which have been internationally adopted:
A wide range of nuclear facilities has been decommissioned so far. This includes nuclear power plants (NPPs), research reactors, isotope production plants, particle accelerators, uranium mines etc. Nuclear power is any Nuclear technology designed to extract usable Energy from atomic nuclei via controlled Nuclear reactions Research reactors are Nuclear reactors that serve primarily as a Neutron source. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides Uranium mining is the process of extraction of Uranium Ore from the ground There are companies specialized in nuclear decommissioning; the practice of decommissioning has turned into a profitable business. Decommissionning is very expensive; the current estimate by the United Kingdom's Nuclear Decommissioning Authority is that it will cost at least £70 billion to decommission the existing United Kingdom nuclear sites; this takes no account of what will happen in the future. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located The Nuclear Decommissioning Authority (NDA is a Non-departmental public body of the United Kingdom formed by the Energy Act 2004. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Also, due to the latent radioactivity in the reactor core, the decommissioning of a reactor is a slow process which has to take place in stages; the plans of the Nuclear Decommissioning Authority for decommissioning reactors have an average 50 year time frame. The Nuclear Decommissioning Authority (NDA is a Non-departmental public body of the United Kingdom formed by the Energy Act 2004.
In USA many utilities estimates now average $325 million per reactor all-up (1998 $).
In France, decommissioning of Brennilis Nuclear Power Plant, a fairly small 70 MW power plant, already cost 480 millions euros (20x the estimate costs) and is still pending after 20 years. This article is about the country For a topic outline on this subject see List of basic France topics. The Brennilis Nuclear Power Plant is a Decommissioned site located in the Monts d'Arrée in the commune of Brennilis in Finistère Despite the huge investments in securing the dismantlement, radioactive elements such as Plutonium, Cesium-137 and Cobalt-60 leaked out into the surrounding lake. Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 Cobalt (ˈkoʊbɒlt is a hard lustrous silver-grey Metal, a Chemical element with symbol Co.  
In the UK, decommissioning of Windscale Advanced Cooled Reactor (WAGR), a 32 MW power plant, cost 117 millions euros. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located
In Germany, decommissioning of Niederaichbach nuclear power plant, a 100MW power plant, cost about 90 millions euros. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Niederaichbach is a municipality in the district of Landshut in Bavaria in Germany.
In Europe there is considerable concern on the funds necessary to finance final decommissioning. In many countries either the funds do not appear sufficient to pay the financial decommissioning, and in other countries the (substantial) funds are being used (too) freely for activities other then decommissioning, putting the funds at risk, and distorting competition with parties who do not have nuclear decommissioning funds available. 
Currently (2008) the European Commission is looking into this issue.