This indicator monitors the quantity of spent nuclear fuel, which provides a good estimate of the situation in the field of radioactive waste. Furthermore, it contains a projection of the production of electricity from nuclear power, as it is a rough indication of the quantity of spent nuclear fuel in the future. It also monitors the volume of intermediate and low-level waste.

The measurement unit for the quantity of spent nuclear fuel is tHM (tonnes of heavy metal). Nuclear electricity generation is calculated in terawatt hours (TWh). Low and intermediate-level waste is stated in the number of drums or other units or in terms of volume (m³).

Nuclear waste is classified as follows:
• High-level waste (HLW) – highly radioactive, generates a considerable amount of heat and long-lived radioactivity;
• Intermediate-level waste (ILW) – higher amounts of radioactivity; further classified into beta-gamma emitters (low and intermediate-level of radiotoxicity, negligible heat) and alpha emitters (intermediate-level of radiotoxicity, low heat);
• Low-level waste (LLW) – small to negligible amounts of radioactivity and, as is the case with intermediate-level waste, classified into beta-gamma emitters and alpha emitters (Agency for Radwaste Management, 2007).

Data on the annual production and storage of high, intermediate and low-level waste enable a rough estimate of the potential health and environmental risks. Radiation is harmful to living organisms, so nuclear waste has to be managed in a manner preventing excessive exposure to radiation. Precautions have to be taken against nuclear waste causing direct radiation and the environment being polluted by radionuclides. The first can be achieved by storing nuclear waste behind appropriately thick biological shields, such as concrete, soil or water. The release of radioactive substances into the biosphere is prevented by storing waste as dry and solid matter and preventing water contamination by installing several barriers. Water is namely a substance that can “dissolve" nuclear waste and radioactive substances can be passed through plants into animals and into human food (Agency for Radwaste Management, 2005).

Spent nuclear fuel (SNF) is termed high-level waste. It decays rapidly at first and after 40 years the level of radioactivity typically drops to 1/1000th of the initial value. But it takes around 1000 years to drop to the level of the original uranium ore which was needed to produce that quantity of spent fuel. In Slovenia, spent nuclear fuel is currently being stored in the Krško Nuclear Power Plant, in the spent fuel pool. The quantity of spent nuclear fuel provides a good indicator of the production of nuclear waste. In addition to electricity production, which is by far the largest source of waste, research, medical and industrial activities, also generate nuclear waste. These sources generate intermediate and low-level waste.

The Krško Nuclear Power Plant is by far the largest source of nuclear waste in Slovenia. Spent nuclear fuel is considered high-level waste, while overhauls and operations also generate intermediate and low-level waste. All the waste generated by the nuclear power plant is stored in the plant itself. Spent nuclear fuel is stored in the spent fuel pool and intermediate and low-level waste in the repository. Waste from other sources (industry, research organisations and medical activities) is stored in the central repository in Brinje.

Ownership of the Krško Nuclear Power Plant is divided between Slovenia and Croatia. All issues regarding the exploitation of the power plant and its decommissioning between Slovenia and Croatia are regulated by the Act Ratifying the Treaty between the Government of the Republic of Slovenia and the Government of the Republic of Croatia on the regulation of the status and other legal relations regarding investment, exploitation and decommissioning of the Krško Nuclear Plant and the Joint Declaration at the time of signature of the Treaty between the Government of the Republic of Slovenia and the Government of the Republic of Croatia on the regulation of the status and other legal relations regarding investment, exploitation and decommissioning of the Krško Nuclear Plant (Zakon o ratifikaciji Pogodbe med Vlado Republike Slovenije in Vlado Republike Hrvaške o ureditvi statusnih in drugih pravnih razmerij, povezanih z vlaganjem v Nuklearno elektrarno Krško, njenim izkoriščanjem in razgradnjo in Skupne izjave ob podpisu Pogodbe med Vlado Republike Slovenije in Vlado Republike Hrvaške o ureditvi statusnih in drugih pravnih razmerij, povezanih z vlaganjem v Nuklearno elektrarno Krško, njenim izkoriščanjem in razgradnjo (BHRNEK), Official Gazette of the Republic of Slovenia, No. 23/2003).

The annual quantity of spent nuclear fuel sharply fluctuates and depends on the operation of the nuclear power plant. In 1991, no fuel was removed. In 1994 an overhaul began in December and concluded in January of the following year. In 2004 an 18-month fuel cycle was implemented so there were no overhauls in 2005 or 2008. Throughout the entire period, the quantity of stored spent nuclear fuel grew at an average rate of 6 % annually. In 2003, total storage capacity was increased to 577 tHM and as such it suffices for the entire period of the nuclear plant’s operation (until 2023). By 2008, 52 % of storage capacities had been used.

In addition to the nuclear power plant, there is also the TRIGA MARK II research reactor operating in Slovenia within the framework of the Jožef Stefan Institute. Despite almost 40 years of operation, the research reactor currently has no spent nuclear fuel, as in 1999 spent nuclear fuel was returned to the United States within the framework of a special programme. The storage capacity of the research reactor is 0.8 tHM.

The Krško Nuclear Power Plant is expected to remain in operation until the year 2023. If its operation is not prolonged, decommissioning of the plant is planned which will substantially increase the quantity of nuclear waste. The Resolution on National Development Projects for the 2007-2023 Period as well as the Green Paper for the National Energy Programme include a project for constructing a second nuclear power plant in Slovenia by the year 2017. If this project is realised, the quantity of nuclear waste will additionally increase, as its power output is planned to be stronger than that of the existing plant in Krško. The implementation of this project is currently uncertain and was not included in the projection of electricity production from nuclear energy. The research reactor is planned to be operational until 2015 or until May 2016 at the latest (Agency for Radwaste Management, 2005) which will be followed by decommissioning.

The projection of electricity production foresees the Krško Nuclear Power Plant closing in 2023. Up to that point, constant electricity generation of 18 TWh with an 18-month fuel cycle is projected allowing us to estimate that the quantity of spent nuclear fuel in storage will increase linearly.

The nuclear power plant generates the majority of intermediate and low-level waste in Slovenia. In 2008, there were 2189 m³ of waste at the nuclear plant's repository (Slovenian Nuclear Safety Administration, 2009). As evident from the figure, several attempts have been made at reducing the waste volume since 1990: waste compression in 1995, waste incineration in 1998, 2002 and 2005 (in Sweden) and super-compaction since 2005.

Radioactive waste generated by research, medical and industrial activities is stored in the Brinje central repository for radioactive waste. These are:
• contaminated objects, such as protective clothing, gloves, cleaning materials, glass containers, filters and similar that are stored in storage drums;
• special waste, e.g. contaminated or activated metal tubes, rods and other parts of devices which are too big to store in drums;
• sealed sources of radiation: different measuring devices used in industry, e.g. for measuring thickness, density and humidity, for measuring the level and faults of metal castings and welds.

In 2008, there were 80 m³ of radioactive waste at the Brinje Central Repository (Slovenian Nuclear Safety Administration, 2009).

The fundamental programming document for managing radioactive waste and spent fuel is the Resolution on the 2006-2015 National Programme for Managing Radioactive Waste and Spent Nuclear Fuel (Official Gazette of the Republic of Slovenia, No. 15/2006). The National Programme for Managing Radioactive Waste and Spent Nuclear Fuel determines priorities ensuring a permanent and safe solution for managing radioactive waste and spent nuclear fuel regardless of whether or not Slovenia will remain a nuclear state after the end of the Krško Nuclear Power Plant’s operation. The construction of the LILW repository is seen as a fundamental environmental project, as a timely acquired location and construction of the repository ensure efficient protection of the natural environment from uncontrolled releases of radioactive matter. The implementation of activities related to the long-term management of radioactive waste and spent nuclear fuel is the responsibility of the Agency for Radwaste Management.

Globally speaking, there is currently still no disposal site for spent nuclear fuel, while some countries (Sweden, Finland, the USA, France, Japan and Great Britain) have managed to develop acceptable concepts of disposal sites that ensure proper protection of human health and the environment. Slovenia is one of the few countries with a nuclear programme that does not have a final repository settled for any type of radioactive waste. The Governmental Decree on the Detailed Plan of National Importance for the Low and Intermediate Level Radioactive Waste Repository in Vrbina, in the Krško Municipality adopted by the Government of the Republic of Slovenia at its session on 30 December 2009 and that came into force on 15 January is undoubtedly a step in the right direction. This represents the final confirmation of the location. The construction of the repository is due to begin in two and a half years and it will place Slovenia among the countries that have arranged for a permanent solution in terms of radioactive waste management.