Today's light water reactors mainly use uranium oxide fuel elements. These are removed from the reactor after use and stored first temporarily and then permanently. Water is used as a coolant and moderator. Some of the alternative reactor concepts will use other fuels, coolants and moderators. This will influence the waste produced. Some of the challenges are presented as examples below.

Fuel
SFRs, LFRs, SCWRs, GFRs and ADSs all use solid fuel elements, just like light water reactors. However, they have a higher burn-up than fuel elements from light water reactors, meaning that more nuclear fission takes place per mass. Due to this higher burn-up, the spent fuel elements from the alternative reactor concepts will, predictably, have a higher radiation level and release more heat. This will complicate handling.

With most concepts for molten salt reactors (MSR), the fuel is not solid but a liquid molten salt. These molten salts are more soluble in water, making final disposal more difficult. For this reason, processes must be developed to treat the waste in such a way that its mobility underground is reduced.

High-temperature reactors (VHTR) utilise so-called TRISO particles. These are small fuel spheres with a diameter of around 1 mm that are surrounded by several thin protective layers. A large number of TRISO fuel particles are then embedded in a graphite matrix. Graphite matrices either spherical (about the size of a tennis ball) or prism-shaped are being discussed. The graphite matrix leads to a significantly higher volume of waste compared to light water reactors. The most likely option is direct final disposal of the graphite matrix and TRISO particles. This would require a suitable conditioning process; cementation or sand backfilling are being investigated here, for example.

Coolant
While water is used as a coolant in light water reactors, other coolants are sometimes used in alternative reactor concepts. These include sodium (SFR) or lead (LFR), for example. They must also be disposed of due to activation or contamination with radionuclides. Sodium, for example, contains a number of activation products such as Na-22 or Co-60 (from the steel structure) as well as contamination from fission products and actinides. Further challenges stem from the chemical properties of the coolants; sodium, for example, is flammable.

Moderator
Most alternative reactor concepts utilise a fast neutron spectrum. This means that no moderator is required to slow down the neutrons from a fast to a thermal neutron spectrum. However, the thermal molten salt reactor and the VHTR use graphite as a moderator. The long-lived and biologically active radioactive isotope carbon-14 is formed in the process. The graphite must therefore also be disposed of. This may be done together with the fuel.

Conclusion
In summary, it can be said that the use of alternative reactor concepts raises new issues for the disposal of the resulting waste, for which no solutions have yet been found. One example is the Molten Salt Reactor Experiment conducted in the US between 1965 and 1969. Since its shutdown, the reactor building has remained in an unchanged state, with the solidified salt melt still in the reactor, as the issue of disposal has not yet been resolved.

Existing studies were analysed to determine how the use of alternative reactor concepts could affect waste quantities. The results show that a significant reduction (up to a factor of 37) in the amount of high-level radioactive waste per amount of energy generated might be possible. However, this effect is mainly due to the fact that uranium 238 - which is the main component of high-level radioactive waste - is separated and assessed as low- and intermediate-level radioactive waste.

The size of a repository, or rather the space required for storage, and, thus, the costs, are largely determined by the total volume of waste and its thermal output. An analysis of existing studies shows that the reduction in waste volumes is significantly lower when using alternative reactor concepts: waste volumes would only be reduced to around one third. The reason for the comparatively smaller reduction is that the heat generated by this waste is significantly higher per mass. The waste can therefore be packed less densely.

If both high-level radioactive as well as low- and intermediate-level radioactive waste are considered, significantly higher waste volumes are produced in all scenarios involving the alternative reactor concepts studied.

Thanks to the Manhattan Project, the US had been a world leader in the development of reactor technologies since the 1950s. However, it was only light water reactors that were successfully commercialised, both in the US and abroad, and not - as originally expected - the other technology lines. The US nuclear power plant technology has been in decline since the 1980s, when orders for the construction of light water reactors were largely cancelled, and not even the Energy Act of 2005 has put a stop to this. The activities to promote both low power light water reactors (SMR concepts) and alternative reactor concepts, which have been observed for about ten years, are an attempt to re-establish US nuclear power plant technology as a global technology leader. No commercial breakthrough is currently in sight.

The initial focus of nuclear development in Russia was on reactors with a fast neutron spectrum (SFR, later also LFR) in conjunction with reprocessing (Mayak, pilot plant and fuel element factory for uranium-plutonium mixed oxide fuels in Zheleznogorsk). This focus was subsequently deepened (BN-600, BN-800). The Russian innovation system regarding alternative reactor concepts is currently in a phase where the research infrastructure is ageing (BOR-60, in operation since 1969) and projects are being postponed (e.g. BN-1200); the BREST-OD-300 is currently being prioritised. Russia continues to pursue its long-term strategy of achieving a so-called closed fuel cycle using reactors with a fast neutron spectrum, while at the same time pressing ahead with the development of light water reactors.

China has used an import strategy to drive forward its nuclear innovation system since the 1960s. Following military developments in the 1950s, progress was made in both light water reactors and alternative reactor concepts. The latter are being developed in parallel with the expansion of light water reactors. China has developed a broad spectrum of technology lines, in particular fast reactors and high-temperature reactors. The projects are currently still at the research and development stage or at the construction and operation stage of prototypes. The high-temperature reactor (Shidao Bay-1) went into commercial operation at the end of 2023. Broad commercial utilisation is not yet foreseeable.

South Korea is one of the leading industrialised countries, and has become one of the few suppliers of reactor technology, originally with the support of the US. South Korea has its own extensive commercial nuclear power programme, which also saw exports in the 2000s. The country maintains particularly close relations with the US in terms of research and development. With regard to alternative reactor concepts, South Korea is intensifying its participation in foreign, particularly American, developments. In addition, the country is continuing its own developments, e.g. reprocessing technologies in conjunction with fast reactors. Commercial utilisation of these reactor concepts is currently not on the horizon.

Having become one of the first countries with commercial nuclear power plant utilisation in the 1950s for historical reasons, Belgium has developed a small national innovation system since this initial phase. Belgium's efforts to develop alternative reactor concepts have focused on the development and internationalisation of the MYRRHA research project, a combination of an accelerator-driven subcritical reactor (ADS) and a lead-bismuth-cooled fast reactor (a variant of the LFR). Initial schedules and cost estimates have been exceeded, and there are difficulties in financing the project.

Poland has been discussing the introduction of commercial nuclear energy for several decades. To date, however, this has not yet been realised. Research into reactor technology has been ongoing on a small scale since the 1950s, particularly at the MARIA research reactor (in operation since 1974). When it comes to alternative reactor concepts, Poland is building up knowledge by involving Polish scientists in European research projects. There is a particular focus on the development of high-temperature reactors, including plans to build a gas-cooled high-temperature research reactor (TeResa).

The authors of the study analysed the extent to which the regulations of the United States, Canada and the United Kingdom can be applied to alternative reactor concepts. The regulations of the following international organisations were also considered in the study:

• the International Atomic Energy Agency (IAEA),
• the Nuclear Energy Agency of the Organisation for Economic Co-operation and Development (OECD/NEA), and
• the Western European Nuclear Regulators Association (WENRA).

In summary, it can be said that the countries analysed do not yet have a set of regulations that is suitable for providing proof of safety for alternative reactor concepts. As a result, the countries and organisations examined are currently updating their regulations. The new regulations are to focus more on target-orientated, technology-neutral specifications.

However, this approach might lead to an additional workload for applicants and approval authorities when preparing and checking the safety case. One reason being the lack of experience from operating the plants. This may result in a longer authorisation procedure.

The following results were obtained by analysing the regulations of the United States, Canada and the United Kingdom:

United States:
In the US, there are currently two procedures for the authorisation of nuclear power plants. Both contain prescriptive requirements that cannot simply be transferred to alternative reactor concepts. The US Nuclear Regulatory Commission (NRC) is, therefore, developing a new set of regulations that will be more goal-orientated and technology-neutral. The regulations are due to be finalised in 2027.

Canada:
The Canadian nuclear regulatory framework is more goal-oriented than prescriptive, which should facilitate the use of alternative reactor concepts. Nevertheless, the responsible licensing authority (Canadian Nuclear Safety Commission - CNSC) has recognised the need for a revision. The aim is to develop a technology-neutral set of rules; a target date has not yet been set.

United Kingdom:
The regulatory authority in the United Kingdom (Office for Nuclear Regulation - ONR) is pursuing a work and research programme to strengthen its expertise in the area of alternative reactor concepts, and to revise requirements for licensing new reactors. In a first step, the procedure for conducting a generic design assessment was modernised. This involves a non-binding preliminary review of the concept by the ONR with the aim of identifying potential problems for the developer at an early stage.

A ONR assessment of basic guidelines with regard to their applicability to alternative reactor concepts is planned. Initial research reports are available, but the ONR still sees a considerable need for future research.