Frequently Asked Questions

The development of LDR-50 started in the spring of 2020 at VTT Technical Research Centre of Finland, which is a Finnish state-owned research and development company providing research and innovation services for both public and private sectors.

In the first stage of the design, the safety and technical feasibility of the reactor concept were demonstrated by computational models.

Substantial new funding was allocated for the project in January 2023, enabling the development to proceed to the next design phase. The development work is extended from the reactor and its associated systems to the entire heating plant.

In June 2023 a new VTT spin-off company Steady Energy was established to take over the commercialization of LDR technology. The company will also set up an experimental programme for demonstrating the reactor cooling functions in practice.

VTT continues the technical development in the frame of a project agreement made with the Steady Energy, with parties from the Finnish nuclear sector, industrial companies and end users of energy.

There are currently some 30–40 technical experts involved in the LDR development process, with expertise ranging from reactor physics and radiation safety to fluid dynamics and material technology. In total, VTT hosts a pool of almost 2000 experts specialized in various fields of technology. The founders of the Steady Energy company, responsible of commercializing the LDR technology, have several decades of experience on nuclear energy projects in Finland.

The technical development has already been expanded beyond VTT. The Finnish national competence in the nuclear field extends from licensing, operation and maintenance of nuclear power plants to final disposal of nuclear waste. Many industrial and engineering companies have recent experience from the previous NPP projects in Finland.

LDR represents somewhat conservative light water reactor technology, which is well known in Finland. This was one of the main reasons for selecting the reactor type.

The reactor is designed for the purpose of district heating. The process does not include electricity production or any turbine cycle. The heating plant can be connected directly to an existing district heating network.

The goal is to complete the first LDR demonstration plant by the end of the decade. The technology could be in large-scale commercial use in the 2030’s. Steady Energy has signed letters of intent with municipal energy companies on the construction of up to 15 LDR reactor units.

A single LDR-50 reactor module produces 50 megawatts of heat, which is fed through an intermediate circuit to the district heating network at a temperature of 65–120 degrees. The heating plant may consist of one or several reactor units, which allows scaling the production according to demand.

The production from a single 50 MW reactor would be sufficient to cover the annual heating needs of 10,000 – 20,000 Finnish households. It should be noted, however, that the demand varies significantly depending on the latitude. A nuclear district heating plant comprised of 2–4 reactor units would be suitable for a small or medium-sized European city. Larger cities could accommodate several heating plants.

Nuclear power plants have been used for the co-generation of heat and power in many countries where district heating is commonly used. There is previous experience from Bulgaria, China, Hungary, Russia, Slovakia, Sweden, Switzerland and Ukraine.

The idea of developing a nuclear reactor exclusively for the purpose district heating is not new either. A 200 megawatt district heating reactor called SECURE was developed in Sweden and Finland in the 1970’s and 1980’s. Helsinki was one of the proposed sites. The project was eventually canceled, and coal chosen as the heating fuel. District heating reactor technology is also being developed in China.

Similar to solar, wind, and hydro power, nuclear energy does not generate any direct greenhouse gas emissions. Energy production, however, is never completely free of emissions. Construction, operation, maintenance and decommissioning of the production units consumes energy and fossil fuels. In the case of nuclear energy, the carbon footprint also includes uranium mining, isotopic enrichment and disposal of spent fuel.

When all contributing factors are included, the CO2 emissions from conventional electricity-producing nuclear power plants are comparable to renewables. Compared to fossil fuels, such as coal or natural gas, the emissions from nuclear energy production are very low.

Similar life cycle analyses have not yet been carried out for district heating reactors, but since the fuel cycle of LDR-50 does not differ from conventional nuclear power plants, the emissions can also be estimated to be of the same order in magnitude.

In addition to nuclear energy, low-carbon heat can be produced with geothermal energy, direct electric heating, heat pumps and bioenergy.

The possibilities of geothermal energy outside volcanically active regions like Iceland are very limited. Heat has to be pumped from deep inside the bedrock. Small-scale experiments carried out in Finland have so far not been able to meet the expectations.

A heat pump is based on the same operating principle as a refrigerator, where heat is transferred from a cooler space to a warmer one using mechanical work. The device consumes electricity, but produces more thermal output compared to direct electric heating. The optimal performance is achieved when used with a low-quality source of heat. If the heat is extracted directly from ground, air or water, the efficiency of the production remains low, especially on cold winter days. In order to cut CO2 emissions with heat pumps or direct electric heating, the consumed electricity must also be from a low-carbon source.

The carbon footprint of bioenergy is small compared to fossil fuels, but not negligible. In large-scale use, additional limitations arise from requirements related to the preservation of biodiversity. In future fossil-free energy systems, biomass also plays a significant role as an industrial raw material. Burning a valuable natural resource in heating boilers is hardly the best use for the limited feedstock.

Nuclear energy offers a solution parallel to other heating methods, enabling more valuable use for electricity and natural resources. Fluctuations in the production of wind or solar power do not affect the operation of the nuclear district heating plant, and the cost of heat is not coupled to the market price of electricity. Reactor fuel can be storaged for years to come, which is a significant advantage for security of supply.

Commercial nuclear power plants have been built since the 1950’s, and over the years there have been both successful and failed projects. The experiences from the past decades have shown that the construction of a modern nuclear power plant is a complex and time-consuming process, which involves financial and scheduling risks. Small modular reactor (SMR) technology aims to solve these issues that have plagued the construction of large power plants.

Compared to other heating options, nuclear district heating plants are expensive to construct but cheap to run. The project requires long-term investments, and resources especially during the construction phase. Nuclear power plants are typically designed for a service life of at least 60 years. The most significant financial benefits are achieved over a relatively long period of time.

The use of nuclear energy also involves a lot of responsibilities. The operator must be committed to maintaining a high level of safety and management of nuclear wastes. Legislation in most countries also requires that the level of safety must be continuously improved, taking into account technical development and improved understanding of potential risks.

Topics related to safety and nuclear waste are discussed in more detail in later sections of this page.

The first LDR heating plant will demonstrate the licensing process, supply chains of reactor components, and manufacturing techniques used in serial production. The site for the pilot plant has not been chosen yet, but preliminary discussions have already taken place with municipal energy companies in Finland.

Before the construction of the pilot heating plant, the cooling cycle and safety functions of LDR-50 will be demonstrated with large-scale thermohydraulic experiments. Heat produced by the reactor core is simulated using electrical heating elements.

Before starting the LDR project in 2020, VTT carried out general-level techno-economic feasibility studies, where the cost of nuclear district heating was compared to other low-carbon alternatives. It was concluded that nuclear energy is a competitive option that could be available for large-scale commercial use in the 2030’s.

Preliminary estimates of the construction and capital costs of an LDR-50 reactor plant place the price tag below a target level set to 1.5 €/W. The evaluation of operating costs, however, requires more detailed information on how the plants are operated and how much staff is required on site. Unlike electricity, heat is both produced and consumed locally, which means that each district heating network forms its own unique market. Profitability therefore also depends on where the plant is built.

More information on the economics becomes available as the project proceeds. In general, it can be stated that nuclear power plants are expensive to build but cheap to operate. The market price of uranium, for example, has very limited impact on the cost of energy.

There are currently 413 reactor units operating in 31 countries around the world, and 58 reactors under construction. Most of the reactor fleet is relatively old. This is because the pace of construction slowed down considerably in the West in the decades that followed the accidents in Three-Mile-Island (1979) and Chernobyl (1986). In the 2000’s the majority of new reactors have been built in China.

However, interest in the possibilities of nuclear energy is again on the rise. New reactors are under construction in France, Great Britain, the United States and South Korea. The Olkiluoto-3 power plant in Finland started regular electricity production in April 2023. There are on-going discussions about resuming construction of nuclear power plants in Sweden and Japan.

The nuclear energy sector is currently in a state of transition. Hot topics include next-generation technologies and small modular reactors (SMRs). The term refers to nuclear power plants that produce electricity from a few tens to a few hundred megawatts. New applications, such as district heating, extend the potential beyond electricity production. SMR projects are being prepared also in countries like Estonia and Poland, where nuclear is not a part the current energy mix.

The operation of a nuclear reactor is based on a self-sustained neutron-induced chain reaction. A neutron is absorbed into the nucleus of a uranium atom, causing it to split into two intermediate-mass fragments. This reaction is called nuclear fission. The reaction releases energy and new neutrons, which continue the reaction chain.

Reactor fuel is most typically fabricated in the form of solid uranium oxide pellets. The fuel pellets are encapsulated inside metallic fuel rods, which are further collected into larger assemblies. Multiple fuel assemblies put together form the active part of the reactor, i.e. the reactor core. Coolant flows through the core, extracting the heat produced in the nuclear fuel.

A conventional nuclear power plant is essentially a steam power plant, where the heating boiler is replaced by a nuclear reactor. Thermal energy is converted into mechanical energy in the turbine, and further into electricity in the generator. The LDR reactor plant, however, includes no turbine cycle. Heat produced in the reactor is transferred to the district heating network via heat exchangers and an intermediate water loop.

Uranium is a radioactive, metallic element that belongs to the actinide series in the periodic table elements. Uranium is found everywhere on the Earth’s crust, but the concentrations vary significantly by region. The largest uranium producing countries are Kazakhstan, Namibia, Australia and Canada.

Uranium consists of two isotopes: U235 and U238, which differ in the number of neutrons in the nucleus. The structure of the atomic nucleus also determines its nuclear physical properties. Only isotope U235 undergoes fission with high probability when hit by a neutron. The atomic fraction of U235 in natural uranium is only 0.72%, which is not sufficient for maintaining the chain reaction in most reactor types. To increase the fission probability, uranium used as reactor fuel undergoes an enrichment process, in which the fraction of U235 is increased to at least a few percent.

Even though uranium is a radioactive element, it does not in itself pose a significant radiation risk. The dangerously radioactive isotopes are produced in the nuclear fuel after the reactor is started.

LDR is essentially a pressurized water type light water reactor operating at low temperature. The term “light water reactor” refers to the fact that ordinary “light” water is used for cooling the reactor fuel. Most of the world’s nuclear reactors work on the same principle. A less common reactor type is the heavy water reactor, in which the hydrogen nuclei of the water molecule have been replaced by deuterium – a heavier isotope of hydrogen. Other reactor types include high-temperature gas-cooled reactors, molten salt reactors and sodium-cooled fast reactors.

Water flowing through the reactor core removes heat from the fuel, but it also serves another important role for reactor operation. It acts as the neutron moderator. Slowing the neutrons down to low energy increases their probability of hitting fissile U235 atoms. A reactor designed to operate on low-energy neutrons can use low-enriched uranium as the fuel. Without the moderator, the chain reaction cannot reach a self-sustained state.

Reactor power is regulated through the operating state of the chain reaction. Most reactors use control rods containing a neutron absorber for adjusting the neutron balance. In pressurized water reactors the rods are typically inserted into guide tubes built into the fuel assemblies. Boiling water reactors use control rods inserted between the assemblies. When control rods are pulled out of the core, reactor power begins to slowly rise. The opposite effect can be achieved by pushing the rods deeper into the core.

Conventional pressurized water reactors also use boric acid dissolved into the coolant for long-term reactivity control. To simplify the reactor control systems, many SMR designs have abandoned the use of soluble boron altogether. LDR-50 is also designed for boron-free operation.

The major structural difference between a nuclear district heating plant and a conventional nuclear power plant is that instead of a turbine cycle, heat produced by the reactor is transferred to the district heating network.

The plant process involves two separate water loops. The water that cools the reactor fuel flows inside the reactor vessel, and transports heat from the core to the main heat exchangers. The circulation is maintained by natural convection without any electric pumps.

The heat exchanger is comprised of a large number of thin-walled tubes, in which heat from the primary side is extracted to the secondary side. At the other end of the secondary loop there is another heat exchanger, operating with the same principle, that transfers the heat to the district heating network. Since the connection is made through an intermediate water loop, water flowing in the district heating network is never in contact with reactor structures.

In order for a nuclear power plant to produce electricity cost-effectively, steam temperature in the turbine cycle must be raised to almost 300 degrees. This means that the reactor has to be operated at high pressure. The typical operating pressure of a boiling water reactor is 70 bar. Pressurized water reactors operate at even higher 140-150 bar pressures.

District heating does not require similar high temperatures. The feed temperature of the network varies between 65 and 120 degrees, which allows the reactor to be operated below 10 bar pressure. The operating pressure of LDR-50 is closer to a bottle of champagne or an espresso machine than to a conventional nuclear reactor designed for electricity production.

The moderate operating conditions simplify the plant design and manufacturing of components. Reactor pressure vessels of conventional nuclear power plants can be made of 25 cm thick steel. The wall thickness of similar components for LDR-50 is only a few centimeters. The plant process is also simplified by the fact that the turbine cycle and generator can be completely left out.

Innovative alternatives to conventional light water reactors are being actively developed around the world. The molten salt reactor operating on liquid fuel is one of the most discussed next-generation reactor types. But even though the expectations are high, it should be noted that the commercialization of these advanced reactor technologies still requires considerable amount of research and experimental work.

There are also several common misconceptions associated with new technologies. Molten salt reactors are often claimed to be inherently safe, because a runaway chain reaction is prevented by design. In reality, the same applies to virtually all other reactor types as well. The stability of the neutron chain reaction is one of the fundamental requirements of nuclear safety, and something that is also taken into account in the design of LDR-50.

The most significant actual difference between next-generation and conventional reactor technologies is that some other heat transfer medium than water flows in the primary cooling system. This enables increasing the output temperature by hundreds of degrees, while still operating at low pressure. Temperature in water-cooled reactors is limited to approximately 300 degrees. Molten salt reactors can reach temperatures of up to 700 degrees, which opens new possibilities, for example, for the production of hydrogen or industrial heat.

LDR-50, however, is specifically designed for low-temperature district heat production. For this specific purpose, a reactor based on well-known light water technology is well sufficient. The reactor is cooled by water, but the low operating pressure brings similar advantages to the design as in many next generation reactor types.

The footprint of the heating plant is determined by the space required by the reactor unit, the number of units, and the space requirements of the systems required for heat production and supporting operations.

A single LDR-50 reactor module is roughly the size of a city bus, lifted in the upright position. The modules are submerged in reactor pools, which are approximately ten meters long and wide. The heating plant consists of one or multiple independent reactor units, which are placed in a row inside a common reactor hall. The control room and district heating network heat exchangers are located in separate buildings, which also have room for other shared systems.

In total, the heating plant takes up the space comparable to a small or medium-sized industrial site.

Since it is not cost-effective to transfer heat over long distances, the heating plant must be located within the area covered by the district heating network. Even so, the most likely location for the plant is not the city center or any other area that is actively used by the residents. In urban planning, central locations are usually reserved for more valuable use.

A suitable location for the plant would be, for example, an area reserved for industrial activity, which already has factories, power plants or other similar use. Such areas are usually also equipped with good roads, and electricity and district heating infrastructures needed for the construction and operation of the plant.

Placing the reactor units in an underground rock cavern is currently considered the primary siting option. One of the advantages of underground siting is its natural protection against aircraft collisions, which reduces the amount of concrete needed in construction. Also in this option part of the systems would be placed on top of the underground space.

LDR-50 is primarily developed for the Finnish needs, but the technology also has considerable export potential. In the European Union alone there are some 3,500 district heating networks serving 60 million people. 75% of the current production is covered by fossil fuels. There is considerable pressure to phase out of coal for climate reasons. Natural gas was previously considered a viable interim solution, but the Russian war against Ukraine has completely changed the situation.

Outside of Finland, the most potential market lies in countries with both a large number of existing district heating networks and a positive public opinion on nuclear energy. In Europe, such countries include, for example, Czech Republic, Estonia, Lithuania, Poland and Sweden.

There is so much shielding around the active parts that the direct radiation effect does not extend outside the reactor building. The highly radioactive isotopes produced during reactor operation are formed inside the solid fuel rods. Smaller amounts of radioactive substances are produced in the coolant that flows through the reactor core. This primary water, however, flows in a closed circuit inside the reactor vessel, and is not released into the environment.

The primary cooling circuit is isolated also from the district heating network by heat exchangers and an intermediate water loop. Heat is transferred from one loop to another, but the water volumes are not in direct contact with each other. The district heating network operates at a higher pressure compared to the reactor and the intermediate loop, which ensures that water flowing through the core does not reach the district heating network even if there is a simultaneous leak in both heat exchangers.

Contrary to popular belief, radioactivity is not transmitted with radiation. Even though the water flowing on the reactor side of the main heat exchangers is emitting radiation, water flowing on the other side does not become radioactive by being subjected to that radiation.

During normal operation, the highly radioactive isotopes remain in the reactor fuel. However, all nuclear power plants release small amounts of radioactive substances into the environment. Emissions mainly consist of radioactive noble gases, which are released into the air, for example, through the water purification systems of the primary circuit.

There are strict emission limits for nuclear power plants defined in the legislation. The radiation exposure infilcted upon the population closest to the site must remain insignificantly small compared to natural background. In Finland, the average annual radiation dose from radon and other natural sources is around five millisieverts. The additional dose resulting from the use of nuclear energy must remain below 0.01 millisieverts per year.

Nuclear reactors are able withstand the conditions during normal operation, as well as most unexpected operating states. A violent increase in core temperature, however, can lead to fuel damage and release of radioactive material from inside the fuel rods. Such event can result from increase in reactor power, or failure of the cooling system.

Reactor safety design aims to prevent accidents before they happen. One of the most important safety criteria is the stability of the neutron chain reaction. This stability results from the physical characteristics of the reactor, and is not dependent on safety automation or operator action. The changes in the power level are slow and easily controlled, and the reactor settles itself to an operating state where heat production and removal are in mutual balance. The requirement for reactor stability is also written in the regulatory guidelines.

Instead of risks related to a runaway chain reaction, the major challenges in reactor safety design are related to ensuring sufficient cooling for the fuel in all operating states. The decay of radioactive isotopes accumulated in the fuel during reactor operation releases considerable mount of energy, which is converted to heat. In reactor technology, this is referred to as residual or decay heat.

If necessary, the fission power of the reactor can be brought down in a matter of seconds, but because of decay heat, the heat production in the fuel does not immediately drop to zero. If the coolant flow to the reactor core is interrupted for an extended period of time, the fuel can overhear or melt. Since decay heat cannot be switched off, it has to be considered in every aspect of reactor design. For the same reason the reliability of emergency cooling systems is emphasized in the safety requirements.

Fuel damage or even core melt-down does not immediately cause a release of radioactive isotopes into the environment. In addition to preventing accidents, nuclear power plants are designed to withstand accident conditions by isolating the consequences inside the containment building.

The safety design of nuclear power plants is based on the so-called defense-in-depth principle. Structurally, this means that highly radioactive materials are enclosed inside several independent release barriers. From the inside out, the release barriers for most reactor designs are the solid state of the uranium oxide pellets, the metallic cladding of the fuel rods, the closed primary circuit of the plant, and the gas-tight containment building that encloses the reactor and the primary circuit.

The function of the release barriers is supported by various safety systems, the most important of which are related to core cooling. Accidents can be prevented altogether by ensuring sufficient cooling for the fuel rods in all conditions. The cooling systems are traditionally based on electric pumps. Since these are active mechanical components, their failure must also be taken into account in the safety design. Cooling functions must not be compromised, even if only some of the pumps are able to perform their intended task. Similar fault tolerance criteria are applied to the the electrical systems supplying power to the pumps.

In addition to accident prevention, the defense-in-depth principle also includes requirements to prepare for core damage. The outer release barriers must be able to withstand the conditions in severe accidents. The same principle also includes mitigating actions, such as filtration systems for emissions and evacuation measures for the area closest to the plant site.

Passive safety design most commonly refers to reactor emergency cooling systems being based on the natural circulation of water. When there are no pumps involved, the safety-critical cooling functions become independent of power supply, automation and operator action.

Passive systems enable fulfilling the high safety standards set for nuclear power plants with simpler technology, i.e. without complicated and multiply redundant active (electric) systems. Passive systems have been designed for large power reactors, but in SMRs they are more the rule than the exception.

The safety design of LDR-50 also relies on passive technology. The reactor module consists of two nested pressure vessels. The inner reactor vessel encloses the reactor core, the main heat exchangers, and the primary cooling circuit. The containment vessel surrounding the reactor vessel acts as the outer defense-in-depth release barrier.

The reactor module is submerged in a pool of water, which acts as a heat sink. Heat generated in the reactor core has a natural tendency to be transferred through the pressure vessel walls and the intermediate space into the pool. In normal operation, heat is extracted through the main heat exchangers into the secondary circuit, which also closes the transfer path to the pool. If the operation of the main heat exchangers is compromised, the reactor falls back to its natural state, in which no separate cooling is needed.

Passive heat transfer from the reactor to the pool is based on the boiling of the water in the containment space and the condensation of steam against the cool outer wall. The system does not include any pumps, valves or other mechanical moving parts. The capacity of the reactor pool is sufficient for receiving heat for several weeks without any cooling. In practice, however, the cooling functions can be maintained indefinitely, because the water inventory can be replenished by injection through purposely built emergency injection lines.

The safety design of LDR-50 benefits from the moderate operating conditions. Because of low operating temperature and natural circulation, any disruptions in normal operation progress slowly, and accidents are not associated with similar high-pressure phenomena as in conventional nuclear power plants.

The Chernobyl accident in Ukraine in 1986 was caused by a runaway chain reaction, which lead to an explosion that produced a very large radioactive release into the environment. The reactor was a graphite-moderated channel-type boiling water reactor (RBMK), which suffered from several problematic design features when it comes to reactor safety. The risks associated with the reactor type, as well as previous near-miss events, were not known to the operating staff, so human factors also played a major role in the accident sequence.

The fundamental problem with the RBMK is that the reactor is inherently unstable when it comes to coolant boiling. As the temperature rises, also the power of the reactor starts to increase, which in turn further increases the temperature. The result can be a self-feeding cycle that eventually leads to uncontrollable power excursion. Without being informed of the risks, the operating staff inadvertently drove the reactor into a very unstable state over the 24 hours preceding the accident. The accident sequence was initiated by a routine turbine test performed during reactor shutdown.

Conventional pressurized and boiling water reactors behave in the exact opposite way when it comes to coolant boiling. The increase in temperature leads to decrease in power, which stabilizes the chain reaction. If the temperature increases high enough, the reactor shuts itself down. This inherent stability excludes the most significant factors that led to the explosion in Chernobyl, which is why similar accident sequences are not considered possible in other reactor types. The same also applies to LDR-50. The stability of the neutron chain reaction is one of the fundamental safety requirements of nuclear reactors, and also included in the regulatory guidelines.

The Fukushima accident in Japan in 2011 was triggered by a natural disaster. The power lines connecting the plant to the national electricity grid were cut off by an earthquake. The operating reactors were automatically shut down, and power supply switched to emergency diesel generators. An hour later, a massive tsunami wave hit the plant site, destroyed the backup power supply systems, and left the plant completely without electricity.

At this stage, none of the reactors was producing fission power, but radioactive decay in the fuel of the recently shut-down reactors was still producing tens of megawatts of heat. The reactor cooling systems were based on electric pumps. After the loss of emergency diesel generators, the decay heat could no longer be extracted from the reactor cores, which over the course of following days lead to melt-down accidents in three reactor units.

The causes behind the Fukushima accident are both technical and institutional. Design of safety-critical systems includes requirements for diversity and physical separation, which means that any single initiating event should not compromise all safety functions simultaneously. In Fukushima, the emergency diesel generator were located at the basement level, which was completely flooded by the tsunami. The design basis failed completely in this respect.

The legislation in many countries with nuclear power plants includes a principle of continuous improvement, which means that technical development and new information on potential risks must be taken into account also in the design of older reactor units. Similar practice was not applied in Japan at the time of Fukushima. The plants were commissioned in the 1970’s, and the safety design was based on the risks recognized at that period of time. If the requirements applied to modern nuclear power plants had been applied in Fukushima, the accident would most likely have been avoided.

After the accident, more emphasis has been put to passive cooling systems, which require no electric power. Such systems have been designed especially for SMRs. Decay heat removal in LDR-50 is also based on natural circulation of water, without electric pumps or other mechanical moving parts. The cooling functions are not compromised, even if the plant is cut off from the power grid for an extended period of time.

Conventional large nuclear power plants are surrounded by inner precautionary action zones (PAZ) and outer emergency planning zones (EPZ), inside which various restrictions are placed on land use and urban planning. For the Finnish nuclear power plants in Loviisa and Olkiluoto the outer limit of the EPZ extends 20 kilometers from the site.

Similar practice cannot be applied to a nuclear district heating plant, which has to be constructed within the area covered by the district heating network. This naturally raises questions on whether reducing the EPZs also means compromising safety?

Emergency preparadness can be understood as part of the defense-in-depth principle. In an accident scenario where all release barriers preventing a radioactive emission have failed, the consequences are still limited by the fact that there are no large population centers nearby, and the inhabitants can be evacuated to safety without delay.

Overall safety, however, is the sum of multiple factors. If the emergency planning zones are to be reduced, more effort has to be devoted to the inner levels of defense-in-depth. In practice this means reducing the probability of accidents and limiting the resulting emissions.

LDR-50 has several favorable characteristics that support a high level of safety. Low power density, low operating pressure and passive cooling are all factors that reduce the probability of core damage. The time scales associated with accidents are long, which provides considerable time for the operating staff to bring the reactor back to a safe state. This does not require, for example, a working connection to the electric grid. The unit size of the reactor is also small. The amount of radioactive materials in the core is a fraction of the core inventory in large conventional nuclear power plants.

Although nuclear power plants are usually located far from large population centers, research reactors with unit size comparable to LDR-50 have been built, for example, on university campuses.

The regulatory requirements for emergency arrangements of Finnish nuclear power plants where revised in the spring of 2024. The fundamental safety goals remain unchanged, but instead of using fixed, kilometer-based limits, the size of precautionary action zones and emergency planning zones will be determined on a case-by-case basis. This enables constructing SMR plants within urban and industrial areas, as long as the safety requirements are fulfilled.

Nuclear power plants must be designed to withstand the impact of a large passenger jet. The reactor and all safety-critical systems must be protected against the direct effects of the impact, as well as the resulting fire. This is most commonly accomplished by designing the concrete walls of the reactor building to withstand the mechanical and thermal loads. Similar protection could also be provided by locating the reactors underground, or covering the reactor building with a thick layer of earth or gravel.

Earthquakes must also be accounted for in the safety design. The requirements depend on the the local seismic conditions. The granite bedrock in Finland is stable, and the probability of a powerful earthquake is very low. The seismic design of LDR-50, however, also has to consider exporting the technology abroad. In such case, the seismic design will be based on local requirements.

Protection against aircraft impacts and natural disasters also provides some protection against military attacks. The largest risks for conventional nuclear power plants are related to damage inflicted upon the emergency power supply systems. LDR-50 relies on passive cooling, which requires no electric power. However, no nuclear power plant can withstand a deliberate attack intended to invoke serious damage.

The war in Ukraine and Russian offensive against the Zaporizhzhia nuclear power plant have shown that the safety of nuclear power plants in a war zone cannot be guaranteed by international agreements alone. It is likely that there will be discussion on whether the safety requirements should be upgraded to include military threats. Possible new requirements will also be taken into account in LDR design.

Manufacturing of nuclear fuel is an international business. The supply chain includes uranium mining, isotopic enrichment, and fabrication of fuel assemblies loaded into the reactor core. The four largest uranium producing countries in the world are Kazakhstan, Australia, Namibia and Canada, whose combined supply covers more than 70% of the market. The other stages of the supply chain are divided between different countries.

After mining, uranium is converted into uranium hexafluoride – a compound with low evaporation temperature and suitable for isotopic enrichment. In the enrichment process, the atomic fraction of isotope U235 is increased from the 0.72% level in natural uranium to a few percent. The remaining fraction consists of isotope U238. Fuel enrichment in LDR-50 is around 2.5%. Enrichment is most commonly based on the gas centrifuge method, in which uranium hexafluoride is spun in narrow cylinders at high speed, causing the isotopes with different masses to separate by centrifugal force.

Enrichment plants are operated in China, France, Germany, Great Britain, the Netherlands, Russia and the United States. Before the war in Ukraine, the Russian market share in enrichment technology was almost 50%, but the situation is changing as other suppliers are increasing their capacity.

Enriched uranium is converted into ceramic uranium oxide, which is fabricated into pellets about the size of a fingertip. These pellets are encapsulated inside metallic cladding tubes. The fuel rods are further collected into larger fuel assemblies. LDR-50 is designed to operate on conventional pressurized water reactor fuel. PWR assemblies are manufactured, for example, in USA, South Korea, France, Great Britain, Spain and Sweden.

Radioactive waste is classified based on the level of activity into low-, intermediate-, and high-level wastes. Low-level waste includes, for example, protective clothing contaminated during reactor maintenance work. Filters used in primary circuit purification systems collect activated corrosion products from water, and become intermediate-level waste after being discarded. When it comes to low- and intermediate-level wastes, there is no major difference between LDR and conventional nuclear power plants.

After fuel assemblies are removed from the reactor core, they become high-active nuclear waste. Per mass, spent nuclear fuel contains more than a million times the activity inventory of low-active wastes. The radioactive isotopes are deposited inside the solid fuel rods. The core of LDR-50 consists of 37 fuel assemblies. The reactor operates approximately two years with the same fuel loading, after which 13 oldest assemblies in the core are replaced with new ones. If the reactor is designed for 60 years of operation, it produces some 400 spent fuel assemblies during its lifetime. If these assemblies were placed next to each other, they would fit inside two regular parking spaces.

Compatibility with the Finnish nuclear waste management solution has been one of the most important design criteria for LDR-50 early on. The solution is based on direct geological disposal. The spent fuel assemblies are enclosed inside copper capsules, and deposited in tunnels excavated 400–500 meters deep into the granite bedrock. Finland is a pioneer in final disposal technology. The concept has been studied and developed since the 1980’s, and the deposition of spent fuel assemblies from the first Finnish power reactors is scheduled to begin by the end of this decade.

The fuel used in LDR-50 is no different from the fuels used in the other Finnish nuclear power plants, which is why there is no need to develop new solutions for waste management either. However, detailed plans on spent fuel management have not yet been made. Open questions include, for example, the arrangement of a centralized interim waste storage for the reactor fleet.

The safety of geological disposal is based on the use of multiple successive barriers between the radioactive isotopes and living environment. The majority of radioactive inventory in discharged fuel assemblies is contained inside the ceramic uranium oxide pellets. The pellets are encapsulated inside a gas-tight metallic cladding tube. The fuel assemblies are packaged into reinforced copper canisters, and the space surrounding the canisters in the final disposal tunnel is sealed with bentonite clay.

The purpose of the multi-barrier principle is to prevent and limit the release of radioactive substances, and to slow down their migration from the disposal tunnel into the environment. As long as the fuel assemblies remain isolated inside the copper capsules, no radioactive material can escape either. The copper capsule, however, is only one of the successive barriers. Another important barrier is the bentonite buffer, which practically stops the flow of water in the disposal tunnel.

It is estimated that it will take at least 10,000 years for the copper capsule to corrode through. When the water finally comes into contact with spent fuel, the most high-active isotopes have already vanished by radioactive decay. Dissolution of the remaining isotopes is a slow process. Plutonium and other long-lived actinides in particular have very poor solubility to water. The same release barriers that slowed down the flow of water from the disposal tunnel into the surface of the copper capsule also slow down the migration of radionuclides in the opposite direction. Concentrations are diluted as the radioactive substances make their way through the disposal tunnel into surface waters over the millennia.

The long-term safety of final disposal is evaluated based on the radiation exposure inflicted upon population living close to the disposal site. The overall dose resulting from the waste repository must through all times remain insignificantly small compared to background radiation from radon and other natural sources. The reference group selected for the analyses may consist of, for example, a small self-reliant village community, who has settled on top of the repository after the next ice age, and is relying on a local well for their drinking water.

Since the time spans related to final disposal are extremely long, there are also considerable uncertainties associated with the analyses. However, even unknown factors can be taken into account by various conservative assumptions. The parameters used in the computational models are chosen in such way, that the flow of water, corrosion rate of copper capsules, dissolution of radioactive substances and other factors affecting the radiation doses are deliberately estimated to the upper limits of the uncertainty intervals. The estimates produced by the computational models therefore overestimate the actual doses with high probability. This means that there would be no adverse radiation effects on the population living on top of the repository, even if the models contained gross errors at multiple levels.

There are several next-generation reactor types based on a closed fuel cycle being developed around the world. The basic principle is that the fuel assemblies removed from the reactor core are reprocessed, and usable fissile material is separated from the waste stream for the manufacturing of a new batch of reactor fuel. A closed fuel cycle can be based on either plutonium produced from uranium, or uranium produced from thorium. A reactor capable of continuously renewing its own fuel inventory is also called a breeder reactor.

Next-generation reactor types are associated with high expectations. A breeder reactor operating on uranium or thorium cycle is capable of utilizing natural resources more efficiently compared to any conventional reactor type. Historically, the transition to breeder technology has been justified by concerns related to limited uranium resources. According to current estimates, however, there is enough uranium in the identified deposits alone to last for hundreds of years. In the time scale relevant for climate change, for example, the availability of uranium is not a limiting factor for the large-scale utilization of nuclear energy.

In recent decades, advanced fuel cycles have also been developed for the transmutation of nuclear waste. If spent fuel is reprocessed and recycled back into the reactor core, the waste stream to final disposal is correspondingly reduced. However, a closed fuel cycle does not eliminate the need for final disposal. The process involves mainly uranium and plutonium, while fission products and other radioactive isotopes still need to be disposed. Recycling of plutonium would have close to zero impact on the long-term safety of geological disposal. Plutonium does have long-lived isotopes, but due to their low mobility, they have very limited contribution on radiation doses above ground.

A closed fuel cycle also requires much more complicated reactor technology compared to the current once-through cycle. Efficient recycling of plutonium is possible only in so-called in fast-spectrum reactors. Chemical reprocessing of spent fuel is an expensive and technically challenging process, which also involves political concerns related to nuclear proliferation. Industrial-scale reprocessing technology for thorium fuel has not yet been put to practice.

Despite the active discussion on advanced fuel cycles, these technologies are not ready to be deployed on the commercial market. When it comes to fight against the climate change, breeder reactors operating on uranium or thorium cycle are not even necessary technology. All of the most significant advantages of nuclear energy can be achieved with current and near-term deployable reactor types.

Constructing a nuclear weapon requires uranium or plutonium. Uranium is also used as nuclear fuel, and plutonium is produced from uranium during reactor operation. For the same reason, nuclear technology always involves a non-zero risk of proliferation, i.e. the dispersion of nuclear weapons technology. However, the significance of this risk depends on technology and the fuel cycle by which the reactors are operated. The use, import and export of nuclear materials is regulated by international agreements.

In weapons-grade uranium or plutonium, the atomic fraction of fissile isotope (U233, U235 or Pu239) is very high. Fuel enrichment (the atomic fraction of U235 isotope) in civilian applications, on the other hand, is practically limited to 20%. Almost all light water reactors use even lower enriched fuel, with less than 5% of U235. This applies to both conventional large power reactors and new SMRs. Fuel enrichment in LDR-50 is approximately 2.5%. Building a nuclear weapon from this type of uranium is not possible, even in theory.

During reactor operation, plutonium is also inevitably produced in the nuclear fuel. However, the production of weapons-grade plutonium with high Pu239 content requires a reactor specially designed for this purpose. The operating cycle of power reactors is so long that also heavier plutonium isotopes begin to accumulate in the fuel. For the same reason, proliferation risk associated with spent reactor fuel is considered low. The direct fuel cycle involves no chemical reprocessing, which means that all produced plutonium remains inside the solid fuel rods, together with the other isotopes, when the assemblies are taken to geological disposal.

Most countries in the world are committed to the international nuclear non-proliferation treaty, which also includes obligations related to nuclear material safeguards. The International Atomic Energy Agency IAEA conducts e.g. camera surveillance and inspection visits to operating sites. These obligations are also taken into account in the design of LDR-50.