Evaluation of Life Cycle CO₂ Emissions for the LDR-50 Nuclear District Heating Reactor

Abstract

The LDR-50 low-temperature nuclear reactor is designed for the Finnish and European district heating markets, as an environmentally sustainable heating option for the 2030s. While the carbon footprint of conventional electricity-producing reactors is known to be small, there have been no comprehensive studies on the emission reduction potential when the technology is applied to the heating sector. This paper aims to fill this knowledge gap by means of life cycle assessment (LCA) analysis. The carbon footprint of the LDR-50 heating plant is evaluated, and compared to conventional heating fuels, direct electric heating, and heat pumps. The results of the analysis show that the life cycle CO₂ emissions are low, although there are still significant uncertainties related to the construction phase, due to missing data. In addition to carbon footprint, the analysis is also extended to other adverse environmental impacts. It is concluded that significant reductions in CO₂ emissions can be achieved by replacing fossil heating fuels with nuclear energy. The technology is considered a viable option alongside biofuels and heat pumps. The overall environmental impacts are low, and the production does not compete for low-carbon electricity or scarce natural resources.

1. Introduction

In an effort to mitigate the consequences of climate change and limit global warming to 1.5 °C, the Intergovernmental Panel on Climate Change (IPCC) has set a carbon neutrality target for the year 2050. This goal is extremely ambitious considering the fact that global greenhouse gas emissions have been increasing over the past decades, despite improvements in energy efficiency and large investments in renewable power sources [1].

A considerable fraction of carbon dioxide (CO₂) emissions stem from the energy sector, in particular electricity production, heating, industry, and transportation. The main focus in public discussion is often put on replacing fossil fuels with renewables in electricity production. In reality, however, low-carbon electricity is only a part of the solution. The largest challenges lie in the decarbonization of the other segments of the energy sector. Electricity can be transported over long distances, but when energy is consumed as heat, the production must be brought much closer to consumption. The range of available options is also more limited, as intermittent wind and solar power cannot directly replace the burning of fossil fuels in heating and industrial applications.

Heating of residential, commercial, and industrial buildings is a major source of CO₂ emissions in countries with a cold winter climate. Northern and Eastern Europe, in particular, rely heavily on district heating. As district heating is a centralized solution with large production units, with heavy dependence on fossil fuels and already existing distribution networks, it also offers significant potential for decarbonization.

This paper presents a life cycle assessment (LCA) of a nuclear district heating plant, to evaluate the potential of using nuclear energy for reducing carbon emissions in the heating sector. The selected technology is the Low-temperature District heating Reactor LDR-50 [2], developed by the VTT Technical Research Centre of Finland and the Steady Energy company for small, medium-size, and large district heating networks. The technology aims to become a viable option for the Finnish and European heating markets in the 2030s.

There have been several studies on the carbon footprint of electricity production, in which the life cycle emissions of nuclear energy were found to be comparable to renewables [1,3]. The conclusions from these studies, however, cannot be directly generalized to heating. Some cost and emission estimates have also been performed for nuclear-based heating, but mostly focusing on co-generation of heat and power, and not using the LCA methodology presented here [4,5,6,7,8]. The purpose of the present study is to fill this knowledge gap by evaluating the climate impact of a nuclear reactor developed exclusively for heat production, and comparing the results to other heating options. The main focus is on life cycle greenhouse gas emissions, but other adverse environmental impacts are also included in the comparison.

The structure of the paper is as follows: The context of the study, including an overview of the Finnish energy sector and the LDR-50 reactor technology, are presented in Section 2. The research methodology is covered in Section 3, with the results in Section 4. The final section is left for a summary, conclusions, and discussion.

2. Context

2.1. The Finnish Energy Sector

The European Union (EU) is aiming to decrease its overall greenhouse gas emissions by 55% by 2030, and to become climate neutral by 2050. In Finland, the target for carbon neutrality has been set to 2035, with an additional moratorium for coal in energy production at the end of this decade. In 2022, the energy sector constituted 72% of overall CO₂ emissions [9]. However, when the emissions from the energy sector are further divided into smaller segments, it turns out that the contribution of electricity production is only around 10%.

The low climate impact for electricity is explained by the fact that 94% of domestic production comes from low-carbon sources: nuclear (42%), hydro (19%), wind (18%), and biomass (13%) [10]. The share of fossil fuels was less than 10%, mostly consisting of co-generation of heat and power. The production of nuclear energy increased considerably in 2023 with the commissioning of the Olkiluoto 3 nuclear power plant. The share of wind power has also been consistently on the rise. Furthermore, this already small carbon footprint implies that achieving further reductions in CO₂ emissions requires more than low-carbon electricity, namely the decarbonization of the other segments of the energy sector.

Unlike electricity production, fossil fuels are still widely used in transportation, industry, and heating. The most common heating form in Finland is district heating, which covers some 45% of the market [11]. The specific emissions for district heating are still higher (83 ${\mathrm{gCO}}_{2\mathrm{eq}}$/kWh) compared to electricity (33 ${\mathrm{gCO}}_{2\mathrm{eq}}$/kWh), but there has been a strong downward trend over the past several decades, as fossil-based production has been gradually replaced with renewable biofuels.

Bioenergy, however, also has its drawbacks. The burning of biomass results in biogenic CO₂, particulate, and nitrogen oxide emissions. The energy density is relatively low, and large-scale extraction of raw material has negative impacts on biodiversity and carbon sinks [12]. The war in Ukraine has also brought concerns related to security of supply. Sanctions imposed in 2022 practically stopped wood imports from Russia, which had previously formed a significant supply chain for biofuels. This share has to some extent been replaced by other sources and increased domestic production, but it has become clear that it is difficult to meet the demand without relying on foreign imports [13].

Another challenge faced by the Finnish heating sector is the decoupling of electricity and heat. District heating in Helsinki and other major cities relies heavily on large fossil-fueled co-generation plants. The decarbonization of electricity production and phase-out of coal has led energy companies to seek alternative solutions. Electricity from co-generation units is being replaced by increasing production with wind power, but when the plants are eventually decommissioned, their heat supply will also be lost. Large-scale use of bioenergy was previously considered a viable option, but for the reasons pointed out above, it is no longer seen as a long-term, but rather a transitional solution.

2.2. The LDR-50 Project

Preliminary techno-economical studies carried out at VTT in 2019 suggested that nuclear energy might become an economically viable heating option for Finland in the 2030s [8]. It was also discovered that none of the promising small modular reactor (SMR) concepts under development at the time provided an ideal solution for this use case. Most SMRs were primarily designed for electricity production, and with the energy sector moving towards the decoupling of heat and power, it seemed that a reactor operating in co-generation mode would not be able to compete in both markets. Another identified issue was that consumption was divided between more than 160 local district heating networks. A typical co-generation SMR could supply heat for Helsinki and other major cities, but the capacities of the candidate reactor concepts were generally over-sized for most customers.

The 2019 study was one of the main motivations for starting the development of the LDR-50 reactor one year later [2]. The reactor is designed specifically for district heating applications for the Finnish and European markets. LDR-50 combines conservative light-water reactor technology with innovative passive safety features. The reactor operates without a turbine cycle at low temperature and pressure. The maximum feed temperature for most district heating networks is around 120 °C, which is low compared to the steam temperature in the turbine cycle in a conventional power reactor (∼290 °C). This simplifies the manufacturing of components and the overall plant design. A single reactor unit supplies heat at up to 50 MW power. The heating plant can be connected to an existing district heating network, and may consist of one or multiple independently operated reactor units.

3. Materials and Methods

3.1. LCA Analysis

Analysis of the climate and other environmental impacts of the LDR-50 reactor was conducted with life cycle assessment (LCA). LCA is a commonly used method for quantitatively and systematically evaluating the potential environmental impacts of a product or system throughout its whole life cycle [14]. The analysis not only focuses on the direct input and output flows, but also takes into account the indirect and embodied emissions, energy, wastes, and materials. LCA is a tool used to support decision-making and to avoid a narrow view of environmental problems. Its application also reduces the risk of problem shifting, i.e., situations where an improvement in one stage of the life cycle leads to weakening another.

LCA is based on two ISO standards: ISO 14040 (Environmental management—Life cycle assessment—Principles and framework) and ISO 14044 (Environmental management—Life cycle assessment—Requirements and guidelines) [15,16]. Furthermore, several other more detailed guidelines on the use of LCAs have been created, such as the Greenhouse Gas Protocol’s standard [17]. The methodology used in this study also followed the Product Environmental Footprint Guidance of the European Union.

LCA consists of four different phases: goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and interpretation of the results. As the name suggests, in goal and scope definition, the aims of the study are defined. In the LCI phase, the raw materials and energy required by the studied system are listed, and data are collected on the production and the air, water, and soil emissions of the system. The LCIA phase consists of the evaluation of the potential environmental impacts caused by the system. Finally, in the interpretation phase, each of the previous phases are interpreted. It also highlights the key factors for decision-making regarding the studied system.

Commonly used LCA software includes Simapro [18] and Gabi [19], but calculations are also made, e.g., using Excel. For a more comprehensive review of LCA software and methods, see Hermann and Moltensen (2015) [20], or Iswara et al. (2020) [21]. The calculation tool used for the LCI phase in this study was SULCA [22], which was developed at VTT. The LCIA calculations were performed in Excel.

3.2. Inputs for the LCA Analysis of the LDR-50 Reactor

In this study, LCA was applied to calculate the potential climate and other environmental impacts of the LDR-50 reactor used for district heat production. The analysis considered the construction, operation, and decommissioning of the production units, as well as the nuclear fuel cycle, which consists of uranium mining and milling, isotope enrichment, fuel fabrication, and the interim storage and deep geological disposal of spent fuel.

Since the technology is still at an early stage of development, the availability of input data was also limited. The calculation therefore relied on the nuclear power LCI conducted by the United Nations Economic Commission for Europe (UNECE) in 2021 [3]. The analyses presented in the UNECE report were carried out for a conventional large pressurized water reactor (PWR).

The choice of using the UNECE data was justified by the fact that a large part of the life cycle impacts for nuclear energy typically originate from the fuel cycle [3], which for the LDR-50 is very similar to conventional, electricity-producing plants. The reactor operates on similar low-enriched uranium fuel, and waste management relies on geological final disposal. The fuel cycle material and energy flows from the UNECE report could be adjusted to LDR-specific values, without introducing significant errors.

The main difference with conventional nuclear power plants is that the LDR-50 uses lower fuel enrichment, which also leads to lower discharge burnup. Fuel enrichment in the preliminary equilibrium core design is 2.4% (compared to 3–5% in conventional power reactors). Fuel burnup measures the utilization rate, i.e., the amount of fission energy produced per enriched uranium mass. The LDR-50 achieves an average burnup of 18.5 MWd/kgU (compared to 40–50 MWd/kgU achieved in conventional power reactors).

It should be noted, however, that even though fuel burnup is associated with the fuel utilization rate, a lower burnup does not directly imply less efficient utilization of natural resources. The LDR-50 produces less heat per enriched uranium mass compared to conventional PWRs, but because the level of enrichment is lower, the amount of natural uranium required for manufacturing an equal size fuel batch is also smaller. The most significant difference, however, comes from the end product and the overall efficiency (95% for heat production vs. 34% for electricity). One kilowatt-hour of heat produced in the LDR-50 requires less natural uranium than one kWh of electricity from a conventional PWR.

The largest uncertainties in the applied methodology were associated with the construction of the LDR heating plant. The inputs in the UNECE report were based on a 1000 MW power reactor. The thermal power of the reactor was around 3000 MW, which exceeds the unit size of LDR-50 by a factor of 60. Conventional nuclear power plants are large production units, with massive concrete structures and a very large area footprint. The LDR-50 features modular technology, with multiple reactor units inside the same structure. The plant footprint is reduced by the fact that the process involves no turbine cycle. Further, the nuclear island of the heating plant is designed to be constructed in an underground rock cavern, which significantly reduces the amount of concrete, but also introduces additional material and energy flows associated with the excavation.

Due to these uncertainties, estimates obtained by scaling the UNECE inputs by the ratio of unit size cannot be considered very reliable. However, based on preliminary evaluation of construction costs, it is believed that the error is on the conservative side, i.e., that the environmental impact of constructing the LDR-50 heating plant was more likely over- than under-estimated.

There are also differences in the way the heating plant is operated compared to conventional power reactors. The LDR-50 is designed to produce heat instead of electricity. The production is not subject to thermodynamical losses from the turbine cycle, which considerably increases the overall efficiency (from 34% for the conventional nuclear power plant to about 95% for a nuclear district heating plant). This is directly reflected in the amount of usable energy produced per uranium mass.

Selected parameters used in the LCA analysis of LDR-50 are presented in

Table 1. Selected operating parameters of the LDR-50 reactor. Material flows and other data used as input for SULCA were obtained by combining these parameters with data provided for a conventional nuclear power plant in the UNECE report [3].

Parameter	LDR-50	PWR 1	Unit
Thermal power	50	3000	MWth
Efficiency of the plant	95	34	%
Annual utilization factor 2	75	N/A	%
Life time of the plant	60	60	years
Fuel enrichment	2.4	4.2	wt-% 235U
Natural uranium per enriched uranium mass 3	4.4	8.1	kgU (nat.)/kgU (enr.)
Separative work per enriched uranium mass 4	2.9	6.7	SWU/kgU
Average discharge burnup	18.5	42	MWd/kgU

¹ Reference 1000 MW_e pressurized water reactor used in the UNECE study [3]. ² For LDR-50 this corresponds to heat production for 9 out of 12 months per year (simplification, taking into account the reduced heat demand during the summer months). ³ Calculated based on enrichment, assuming 0.22% tails assay. ⁴ Separative work refers to the effort required in the uranium enrichment process. Different enrichment methods are associated with their characteristic energy consumption per standard separative work unit (SWU).

. Corresponding values for the large power reactor used in the UNECE report are included for comparison.

Direct inputs and outputs of the different life cycle phases were taken from the UNECE report and the life cycle inventory data (i.e., data on the material and energy resources needed to produce those inputs, and the resulting emissions and waste) were mainly taken from the Ecoinvent database v. 3.8 [23].

Life cycle impact assessment (LCIA) was conducted by applying the impact assessment methods recommended in the EU Product Environmental Footprint (PEF) method [24,25]. For normalization, global characterization factors per one person were used. In LCIA, characterization refers to the extent to which each pollutant contributes to different environmental impacts. In normalization, the characterized results are divided by the total impact of a reference region for a certain impact category (e.g., climate change, eutrophication, etc.) in a reference year. In this study, characterization factors followed version EF 3.0 of the EU PEF method, but normalization factors were updated to EF 3.1 presented in Ref. [24].

3.3. Environmental Impacts of Other Heating Options

The life cycle environmental impacts of the LDR-50 reactor were compared to other heating options. The comparison included several biofuels: wood pellets, wood chips, and biogas, as well as the fossil fuels natural gas and hard coal. The reference data were taken directly from the Ecoinvent database [23]. Another common heating fuel in Finland is peat, for which the life cycle emissions were obtained from an LCA study carried out at VTT [26]. For biofuels, the estimates include both biogenic and equivalent CO₂ emissions. The former refers to the direct CO₂ emissions released in the combustion, while the latter includes CH₄ and N₂O from combustion and the life cycle emissions, taking into account, e.g., the harvesting and processing of the raw material.

Heat can also be produced by direct electric heating or using different types of heat pumps. These options were also included in the comparison. Due to lack of data, the estimated life cycle emissions only included the carbon footprint of electricity consumed during the heating process. Heat pumps were assumed to operate with a coefficient of performance (COP) of 4, meaning that 1 kWh of electricity generated 4 kWh of heat. There is large variation in the carbon footprint of electricity production, which was accounted for by including the average specific emissions from several European countries in the comparison (

Table 2. Average specific emissions of electricity production in selected European countries in 2023 [27].

Country	Specific Emissions (gCO2eq/kWh)
Sweden	25
France	53
Finland 1	82
Denmark	149
Estonia	278
Germany	400
Czechia	500
Poland	794

¹ The value differs from that provided in Section 2.1 (33 gCO_2eq/kWh). The source of data was different, with different emission factors used for the primary energy sources. It is also possible that there are differences in the way emissions from co-generation are divided between heat and power.

).

Excluding the material streams and losses means that the results for direct electric heating and heat pumps were, to some extent, under-estimated. Using the annual average emissions of electricity production as the basis for evaluating the carbon footprint of generated heat also fails to account for any seasonal variation. In countries relying heavily on wind and solar power, the momentary emissions may rise significantly on cold and dark winter days when the production is typically low. These emission peaks coincide with high heat consumption, which also means that the annual average emission numbers likely underestimate the climate impact.

4. Results

4.1. Climate Impacts of Heat Produced Using LDR-50

The LCA estimate for the total life cycle greenhouse gas emissions for heat produced using the LDR-50 district heating reactor plant was 2.4 gCO_2eq/kWh. The emissions from the different stages are presented in Figure 1. The largest contributors are uranium mining and milling, and plant construction. Both account for just over one third of the total emissions. The fact that the back-end of the fuel cycle has practically negligible contribution to CO₂ emissions is explained by the way in which the final disposal of spent fuel is implemented. In Finland, the same repository and associated facilities would most likely handle waste from the entire lifetime of the complete LDR reactor fleet, which also reduces the carbon footprint per unit of produced heat.

As discussed in Section 3.2, the carbon footprint for the fuel supply chain is well justified. The LDR-50 operates on a conventional fuel cycle, to which the applied methodology is well suited. The estimates for plant construction, however, contained much higher uncertainties. The inputs were based on down-scaled data for large power reactors, which failed to consider the characteristic features of the LDR-50 heating plant. Since plant construction accounts for a relatively large fraction of the overall emissions, these uncertainties were also reflected in the final outcome.

4.2. Comparison to Other Heating Options

The specific emissions for the LDR-50 are compared to heating fuels in Figure 2. The carbon footprint was more than two orders of magnitude smaller when compared to fossil fuels. This result was somewhat expected, and it clearly shows that replacing fossil production with nuclear energy can be a very efficient means to reduce CO₂ emissions within the heating sector. Differences with biofuels are smaller. When biomass is utilized in a sustainable way (i.e., carbon bound to growing biomass compensates for the emissions from combustion), the life cycle emissions are close to the equivalent values, which are of the same order of magnitude as with nuclear-based district heating.

A comparison with direct electric heating and heat pumps with electricity production corresponding to different European countries is presented in Figure 3. When the carbon footprint of production is low, these heating options compare well with the LDR-50. But for countries like Estonia, Germany, Czechia, and Poland, electricity-based heating is a poor choice when it comes to reducing CO₂ emissions. Significant reductions are difficult to achieve without first decarbonizing the electricity supply.

The reference specific emissions for the heating fuels were obtained from well-established sources [23,26], but the numbers for direct electric heating and heat pumps contain more uncertainties. As pointed out in Section 3.3, the values only account for the carbon footprint of the consumed electricity, and the results are therefore somewhat underestimated.

4.3. Other Life Cycle Environmental Impacts

In addition to life cycle climate impacts (carbon footprint), the LCA analyses also produced estimates for other adverse environmental impacts for the LDR-50 reactor (Figure 4). The results are not discussed here in detail, but there were certain observations that are worth pointing out.

Most of the adverse effects resulted from the initial stages of the nuclear fuel cycle, in particular, uranium mining and milling. This was the highest contributing stage to almost every impact category, such as fossil fuel consumption, freshwater ecotoxicity, eutrophication of marine waters, and ionizing radiation. Potential accidental releases were not included in the radiological effects, but radioactive emissions from normal operation were. The fact that the effects from uranium mining exceeded the effects from the other stages of the life cycle reflects the strict limits imposed on operating nuclear reactors and the final disposal of nuclear waste.

The second important observation is that the adverse environmental impacts for the LDR-50 reactor are low compared to other heating fuels. In fact, there are worse options in every impact category, including for ionizing radiation. This results from the high energy density of uranium compared to combustible (chemical) fuels. A single LDR-50 fuel assembly contains 126 kg of enriched uranium, which produces as much energy as six million liters, or 30,000 barrels of heating oil. The reactor core is comprised of 37 assemblies. The operating cycle is around two years, after which the 13 oldest assemblies are replaced with new ones.

A comparison is presented in Figure 5. Different options stand out in different categories. Hard coal has the greatest impact on freshwater ecotoxicity, and light fuel oil stands out in carcinogenic human toxicity. Renewable wood-based fuels have by far the largest impact on land use.

The comparison shows that the choice of heating options should not be based on carbon footprint alone. But what complicates the assessment is the fact that accounting for the other adverse environmental effects requires some value choices in ranking the different impact categories against each other. In the big picture, few effects compete with global warming when it comes to severity of consequences. Even so, factors like ecotoxicity and irrevocable harm to biodiversity can hardly be ignored either.

5. Summary, Conclusions, and Discussion

In this study, the life cycle climate and other adverse environmental impacts of a nuclear district heating plant were evaluated by means of LCA analyses. The work is related to the on-going development of the LDR-50 reactor, and the motivation was to provide better data for evaluating the potential of the technology for reducing the CO₂ emissions in the Finnish and European district heating markets. Similar studies have been carried out for electricity production, but the results cannot be directly generalized to the heating sector.

Based on the LCA analyses, the estimated life cycle emissions for the LDR-50 district heating plant were 2.4 gCO_2eq/kWh. This result is low, even compared to biofuels, for which the values ranged from 10 to 50 gCO_2eq/kWh. More importantly, it was shown that the carbon footprint of nuclear-based district heating can be more than two orders of magnitude smaller than that of fossil heating fuels. The nuclear option also did well in comparison to direct electric heating and heat pumps, even when the electricity supply came from low-carbon sources.

It was pointed out that the results were also subject to some major uncertainties. While the methodology is considered well suited for evaluating the emissions from the nuclear fuel supply chain, the inputs used for plant construction were obtained from down-scaled values evaluated for a conventional large power reactor. These estimates failed to account for the small unit size and specific features of the LDR-50 heating plant. The error was most likely on the conservative side, but the reliability of the analysis could be improved with the better input data that will become available as the development proceeds.

Even if the results are taken as order-of-magnitude estimates only, the study shows that nuclear energy can become a viable option for replacing fossil fuels in heat production. The largest emission reduction potential lies in countries where energy production still relies heavily on coal and natural gas, and district heating holds a large market share. Such countries include, for example, Estonia, Poland, Czechia, Slovakia, and Ukraine. This description also applies to Denmark and certain parts of Germany, but because of the local political climate, it is not likely that nuclear reactors will be built in these countries within the foreseeable future.

When nuclear energy is compared to other viable low-carbon heating options, i.e., biofuels and heat pumps, there are also other advantages and drawbacks to be considered. Nuclear reactors are subject to complicated licensing procedures, and require commitment to high safety standards and management of radioactive wastes. The initial investment costs for constructing the heating plant are high, even though the operation is relatively cheap. On the positive side, the security of supply for nuclear energy is high, as fuel for several years of operation can be stored on site.

There are also differences that arise from the consumption of energy and natural resources. It is generally accepted that meeting the IPCC climate goals requires a complete overhaul of the energy system. For the industry and transport sectors, this implies electrification, hydrogen economy, and replacing fossil-based hydrocarbons with biomass-based feedstock. The increased demand is likely to lead to competition between different applications, in which case using low-carbon electricity and wood-based fuels to produce low-temperature heat for residential and office buildings may no longer be seen as the most efficient use of limited natural resources. This is where the advantages of the nuclear option also stand out. The environmental impacts are low in all categories, not only for CO₂ emissions, and the production is decoupled from electricity and the upcoming hydrogen markets.

The first LDR-50 units are planned to be built in Finland, where the CO₂ emissions from the heating sector are already declining as fossil-fired plants are switching to biofuels. The long-term sustainability of this option, however, has been questioned, due to adverse effects on biodiversity and carbon sinks. For this reason, the large-scale utilization of wood-based fuels is no longer considered a viable long-term, but rather a transitional solution. It is possible that tightening environmental regulations will put an end to all combustible fuels being used for district heating within the following decades.