There is a large number of nuclear power facilities worldwide that were constructed in the second half of the last century. After expiration of validity of license for operation, these facilities have to be shut down and consequently decommissioned. Beginnings of program of nuclear facilities decommissioning in the world go back to the 80-ies, when research works on small pilot projects started, the aim of which was to verify disassembly and decontamination techniques of decommissioning.

Decommissioning of individual nuclear facilities is preceded in most of cases by a demanding legislation process of acquiring of the permit for decommissioning from the respective supervisory body. In most of countries the supervisory body is the governmental institution dealing with nuclear safety, e.g. CNSC (Canadian Nuclear Safety Commission) in Canada, KINS (Korea Institute of Nuclear Safety) in Southern Korea, etc. The operator has to submit, sufficiently early, all the necessary documentation, project and reports on influence of decommissioning on environment to supervisory bodies.

The basic variant of decommissioning, considered at computations and planning, is the variant of decommissioning in green meadow. Permit for decommissioning covers all the activities till the end of decommissioning process, which, in general, makes the locality free for unlimited utilization. It is supposed in some countries, e.g. in Japan, that locality of the power plant being decommissioned, will be used continually for location of new NPPs.

There exists an international consensus that there are, in principle, three different strategies for decommissioning of nuclear facilities:

> immediate decommissioning in time interval of 10 years,
> postponed decommissioning in time interval approx. 30–50 years, after construction of protective storage,
> reactor's safe enclosure.

Operator will choose one of these strategies on the base of decommissioning plan, with regard to time, technical and financial possibilities.

Financing of the decommissioning process, expenses on treatment and storage of originated RAW is carried out in many countries by means of special fund constituted for this purpose. Operators of nuclear power plants are obliged to create financial reserves by means of this fund, such that they will cover financing of expenses on decommissioning of NEF in compliance with proposed variant of decommissioning in the future.

Decommissioning time schedule depends on selected variant, strategy and complexity of the decommissioning project. Decommissioning itself follows after stage of shutdown of the facility, taking away the spent fuel and processing of operational waste. Simultaneously with these activities, specific studies and research works are carried out with the purpose of technologies improvement, optimizing the decommissioning procedure and its timing and with the purpose of expenses reduction.

Practical experience shows that decommissioning of nuclear power plants lasts for more than 10 years. Within the strategy of protective storage, it is necessary to add the time, during which the nuclear facility will be enclosured, but still under surveillance.

This strategy of decommissioning was chosen as the basic one by France in the eighties of the last century. Overall disassembly of facility is executed only after 50 years, when a substantial decrease of radioactivity of ⁶⁰Co comes about.

During the protective storage, nuclear facility is, in fact, a store of facilities, still contained in it, and they are under permanent surveillance. After this stage nuclear facility is completely dismounted.

Decommissioning of NPP requires usage of specific technologies for dismounting. Companies dealing with decommissioning propose and implement various decontamination processes (chemical, electrochemical and mechanical) and various disassemble techniques (cutting, demolition and splitting techniques).

Within the process of NPP decommissioning, large amount of radioactive materials with different activity originate from disassembled technology. These materials (except high active ones) are modified into the form for their storage, most frequently in storage of low and middle active RAW. Storages of low active RAW are designed and constructed with the aim to assure protection of population and environment by usage of multibarrier system for several years. In numerous countries, as for example in Spain and Sweden, population is directly engaged in the decision making process about construction of storages.

High active waste — spent nuclear fuel, reactor's pressure vessel and reactor's inner parts are in relatively small amounts in comparison to low and middle active waste. In many countries intensive research and designing of underground storage of SNF as well as high active RAW is performed. Research and designing of underground storage is taking place in France, Belgium, Germany, Sweden, USA, Japan, countries of the former USSR and numerous other countries.

The alternative solution is construction of surface centralized intermediate stores, which would be capable to store spent fuel and other waste till the time when final storage of this waste is available. According to plan of SKB (Swedish organization for RAW treatment) operation of deep geological repository for spent fuel in depth approximately of 500 m is planned beyond 2020.