Your nuclear questions, answered.

The fuel for LDR-50 plants will be sourced from responsible suppliers in friendly nations. Nuclear fuel production is an international industry that includes uranium mining, isotope enrichment, and the manufacture of fuel assemblies. The world’s four largest uranium-producing countries, Kazakhstan, Australia, Namibia, and Canada, account for more than 70% of global output.

After mining, uranium is converted into uranium hexafluoride (UF₆), which is suitable for isotope enrichment. In the enrichment process, the proportion of the fissile isotope U-235 is increased from the natural level of 0.72% to a few percent, with the remainder being U-238. For the LDR-50, the enrichment level is approximately 2.5%.

The enriched uranium is then converted into ceramic uranium oxide, from which small fuel pellets, roughly the size of a fingertip, are manufactured. These pellets are sealed inside metal cladding tubes, which are then assembled into fuel rods and ultimately into larger fuel assemblies. The LDR-50 is designed to use standard pressurized water reactor (PWR) fuel. Such fuel assemblies are manufactured, for example, in the United States, South Korea, France, the United Kingdom, Spain, and Sweden.

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Once removed from the reactor, nuclear fuel becomes high-level radioactive waste. The radioactive materials remain in solid form within the spent fuel assemblies. The LDR-50 core contains 37 assemblies at a time, and the reactor is refueled approximately every two years. Over a 60-year operating lifetime, the reactor would generate just over 400 spent fuel assemblies in total. All of this spent fuel would fit into a single delivery van.

After removal from the reactor core, fuel assemblies are first cooled in the reactor pool for several years. After the spent fuel has cooled down sufficiently, it can either be reprocessed, where fissile materials (i.e., uranium and plutonium) are harvested for new fuel production and the remaining material is processed for storage in a final repository, or it can be stored directly in a final repository without reprocessing.

A single uranium oxide fuel pellet weighs around 10 grams, containing approximately 8.8 grams of uranium (the remainder being oxygen). Based on its energy density, this small pellet produces about 35,000 MJ of energy in a standard reactor — and up to 700,000 MJ in a breeder reactor. To put that into perspective, here’s how it compares to other fuels:

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One LDR-50 reactor produces 50 megawatts (MW) of thermal energy. Multiple LDR-50 reactor units can be installed side by side in the same power plant to meet higher heating demands. This modular approach provides flexibility and redundancy for district heating networks.

In addition to producing electricity, nuclear power plants supply district heating in many countries, including Bulgaria, China, Sweden, Switzerland, Slovakia, Ukraine, Hungary, and Russia.

The idea of a nuclear reactor designed specifically for district heat production is not entirely new. In the 1970s and 1980s, Sweden and Finland jointly developed the SECURE district heating reactor concept, with Helsinki considered as one potential site. The project was ultimately not realized, and coal was chosen as the fuel for heating instead.

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A single LDR-50 reactor module is roughly the size of an upright city bus. The reactor core itself—where the fuel is located—is about the size of a large washing machine. The modules are installed in water pools approximately ten metres in diameter. Each heating plant contains one or more reactor units, arranged in a row inside a shared reactor hall. The control room and the district heating heat exchangers are housed in separate buildings that also contain other technical systems.

Steady Energy’s plants are designed to be built underground, embedded in bedrock. A single plant is roughly the size of a metro station. In total, the facility occupies an area comparable to that of a small or medium-sized industrial lot.

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The main safety functions for nuclear reactors are control of reactivity, cooling of the fuel, and containment of radioactivity. These fundamental issues are common to both small and large reactors. The difference lies in how simple it is to manage these functions, especially the adequate cooling of the fuel, which requires continuous attention even after the reactor has been shut down.

A small reactor means much less decay heat from the fuel to be cooled. And, because of that, sufficient cooling can be implemented in many ways. Instead of complex cooling systems relying on the use of pumps and electrical power, seen with many large reactors, sufficient cooling can be provided with fully passive methods relying only on natural phenomena like gravity.

Modest decay heat power also implies a certain slowness; situations, whatever they are, progress more gradually, which provides more time e.g. for various operator actions. Similarly, it also means long autonomy times, i.e. the ability of the plant to survive on its own for long periods of time with no external help. For example, the reactor pool water in the LDR-50 plant can manage the decay heat removal from a shutdown reactor for several months. And, if, despite everything, an accident at the plant results in damage to the reactor core, a smaller size means more easily manageable accident conditions and less potential for radiological releases and environmental consequences.

Another benefit from the small physical size, especially in the case of LDR-50, is the possibility to place the reactor units underground. The ground above the reactor building provides good protection against many external hazards and threats and, in case of a very unlikely reactor accident, also good protection for the environment.

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The worst possible accident means a so-called severe accident where the cooling of the fuel is not sufficient, resulting in a partial or complete melting of the reactor core. Through the design of the reactor and its safety systems, such accidents can be made highly unlikely, but their possibility cannot be entirely ruled out. Therefore, LDR-50 is provided also with systems and design features intended for severe accident mitigation.  

A key strategy for the management of severe accidents with LDR-50 is in-vessel retention that will prevent the rupture of the reactor pressure vessel (RPV). If the reactor core cannot be cooled due to the loss of coolant (water) from the RPV, fuel will eventually overheat and start melting. The molten core will accumulate at the bottom of the RPV, where it is cooled from the outside with water, preventing the rupture of the RPV. Because of that, the core melt will remain inside the RPV module submerged in the reactor pool full of water.  

The ability to contain the damaged reactor core inside the RPV, where it will eventually solidify, limits significantly the release of radioactive substances from the fuel. Those gaseous and airborne substances that are released from the fuel and the RPV module will be gathered in the reactor hall located underground. It delays the release and reduces, among other things, the activity of noble gas nuclides with short half-lives.  

Eventually, some radioactive substances will inevitably be released into the environment, but the release can be limited to such a small amount that the effects will be confined mainly to the fenced plant area. Therefore, the harmful effects of the release on nearby people and the environment remain minimal. Continuing the operation of the plant, on the other hand, is no longer practically possible after such an accident.    

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No, the phenomena that took place in Chernobyl in 1986 are not possible with a reactor design like LDR-50. Chernobyl reactors are so-called RBMK reactors, i.e. they are graphite-moderated. LDR-50, like e.g. reactors in Olkiluoto and Loviisa, is a light-water reactor moderated and cooled by normal (light) water (H₂O). Such reactors are designed to be inherently safe. That is, any increase in reactor power naturally triggers physical processes that counteract and reduce that power increase. Because of that, the rapid, uncontrollable power surge that destroyed the reactor number 4 in Chernobyl is not physically possible with LDR-50.

Radiation from natural sources is always present in nature. For example, in Finland, statistics from year 2018 show that Finns receive an average annual effective radiation dose of 5.9 millisieverts (mSv). Most of this comes from indoor radon gas, but around 1.1 mSv of the annual radiation dose comes from natural background radiation sources, mainly from cosmic rays from space and from radioactive substances in the soil and building materials. In comparison, the radiation levels in the vicinity of the LDR-50 plant resulting from its operation are practically nonexistent and cannot be distinguished from natural background radiation. To ensure this, radiation levels around the plant are monitored, and effects are assessed regularly.

The radiation effects caused by nuclear facilities must also meet the objectives set forth in legislation. In Finland, the Nuclear Energy Decree states that the constraint for the annual dose of an individual in the population, arising from the normal operation of a nuclear power plant or another type of nuclear facility equipped with a nuclear reactor, is 0.1 millisieverts. In other words, the annual radiation dose received by the most exposed individual must be below 0.1 mSv. In practice, as already demonstrated by the operating nuclear power plants, the radiation doses will amount to only a very small fraction of this.  

District heating water cannot become radioactive. At no point does the water in the district heating network come into contact with the reactor’s primary coolant or the nuclear fuel. The plant uses three physically separate water circuits, each isolated from the others by heat exchangers. Heat is transferred only as heat — not by mixing water or substances between the circuits.

In the event of a disturbance or accident, such as a leak in a heat exchanger, the pressure inside the reactor vessel is lower than the pressure in the intermediate circuit. This means that water would flow into the reactor, not into the intermediate circuit or the district heating network. This same design principle is used in all nuclear power plants, and its safety has been thoroughly demonstrated through extensive operational experience.

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Because heat cannot be transmitted cost-effectively over long distances, a heating plant must be located within the area served by the city’s district heating network. However, the most likely location is not the city centre or any other area heavily used by residents.

A suitable site would typically be an area zoned for industrial activity, where plots are already designated for factory, power plant, or similar uses. Such areas usually also have the necessary infrastructure for building and operating the plant, including road access, electricity connections, and district heating pipelines.

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Building a nuclear energy facility is a multi-stage process that involves several layers of decision-making. In most countries, nuclear projects must demonstrate that they serve the broader public interest, meet strict safety requirements, and comply with national energy and environmental policies.

Typically, high-level political approval is required before a project can move forward. This approval may come from a national government, parliament, or an equivalent authority, and acts as the top-level mandate for constructing the plant.

After this, the project must undergo additional licensing steps, such as construction and operating licences, which are usually granted by the national government or a designated regulatory authority based on the recommendations of an independent nuclear safety regulator.

Ultimately, the host community also plays a central role. Local acceptance and cooperation are essential, and the project cannot proceed without a suitable site and support from the region in which the plant will be located.

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Relying solely on renewable energy sources is challenging because wind and solar power output fluctuates with weather conditions and time of day. Electricity and heat, however, are needed continuously — even when the wind isn’t blowing or the sun isn’t shining. To operate an energy system entirely on renewables, massive storage solutions, extensive backup capacity, and significant overbuilding of generation would be required. With today’s technologies, this is neither economically nor technically realistic on a large scale.

Even biomass, which is classified as renewable, is a limited natural resource. The amount that can be used sustainably is much smaller than its theoretical technical potential. Large-scale biomass combustion would also increase local emissions and air-quality challenges, even if its carbon-neutrality balance appears favourable.

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The VTT study found that the life cycle CO₂ emissions of heat produced with the LDR-50 reactor were just 2.4 grams per kilowatt-hour (gCO₂eq/kWh).

To put that in more context:

Natural gas: ~282 gCO₂eq/kWh

Coal: ~515 gCO₂eq/kWh

Wood chips: ~50 gCO₂eq/kWh

Heat pumps (depending on the system’s electricity mix): 70–200+ gCO₂eq/kWh

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Current legislation in many countries does not prohibit the construction of district heating reactors. However, the regulatory framework is not yet ideally suited for this type of project, because existing licensing processes were originally designed for large, traditional nuclear power plants. For a fleet of multiple identical small reactor units, these processes can be unnecessarily heavy and time-consuming.

Regulatory requirements also do not always fully account for the characteristics of modern small reactors — such as their smaller unit size or the use of passive safety systems. In many parts of the world, governments and regulators are now reviewing and updating their nuclear legislation to better reflect advances in reactor technology and to enable more streamlined licensing for small modular reactors.

One of the most significant recent developments internationally has been the growing recognition that small reactors can be safely sited much closer to communities than conventional large nuclear plants. Updated guidance from several nuclear regulators now allows small reactors to be placed near populated areas, provided that modern safety requirements are met.

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The operation of a nuclear reactor is based on a controlled chain reaction. A neutron is absorbed into the nucleus of a uranium atom, causing it to split into two medium-weight nuclei. This process is called fission. The fission event releases energy and additional neutrons, which sustain the chain reaction.

The reactor’s nuclear fuel is most commonly solid uranium oxide. The fuel pellets are sealed inside metal cladding tubes. These fuel rods are assembled into larger fuel bundles, which together form the reactor’s active region — the core. The reactor coolant flows between the fuel rods and through the core.

A conventional nuclear power plant operates on the same principle as any thermal power plant, with the steam boiler replaced by a heat-producing nuclear reactor. The heat is converted into mechanical energy in the turbine cycle and then into electricity in the generator. A district-heating reactor, however, has no turbine cycle at all; instead, the heat produced by the reactor is transferred through a heat exchanger directly into the district heating network.

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The LDR-50 is a low-temperature, pressurized water–type light-water reactor. The term light-water reactor refers to the use of ordinary “light” water as the reactor coolant. The majority of the world’s nuclear reactors operate on this same principle.

The water flowing through the reactor cools the fuel, but it also performs another essential function. Water acts as a neutron moderator, slowing down neutrons to lower energy levels. Slower neutrons have a higher probability of causing fission in the easily fissionable uranium-235 isotope. A reactor that operates with low-energy (thermal) neutrons can use uranium fuel with only a few percent U-235 content. Without a moderator — such as water — the chain reaction cannot be sustained.

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The power output of a nuclear reactor is typically controlled using control rods, which in most pressurized water reactors are inserted into guide tubes within the fuel assemblies. When the control rods are withdrawn from the core, the reactor power gradually increases. Pushing the rods deeper into the core correspondingly reduces the power.

In conventional pressurized water reactors, power control is also assisted by dissolved boric acid in the coolant, with the concentration adjusted over the course of the fuel cycle. Many small reactor designs, however, omit boron control for the sake of simplicity. The LDR-50 likewise uses only movable control rods for reactivity control.

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Unlike reactors designed for electricity generation, the LDR-50 operates at lower pressure and temperature. This makes it inherently safer and easier to integrate directly into urban district heating systems.