No harm: Why nuclear is considered a source of clean energy

In the early 2000s, Germany abandoned the construction of new nuclear power plants. This was the result of a series of unsuccessful initiatives, including the introduction of a retrospective tax of about 50 billion German marks, which hit the participants in the energy generation market and limited the service life of existing plants. However, in 2010, the service life of power units built before 1980 was extended by another 8 years, and newer ones were allowed to operate for another 14 years after the expiration of their service life.

Then, in 2011, after the disaster at the Fukushima nuclear power plant, the discussion about abandoning nuclear energy was renewed. Irrational fear, negative information background, and insufficient public awareness put Germany back on the path to abandoning nuclear energy. As a result, the country closed seven of the oldest plants, and by 2023 stopped the operation of all the others. Most of the research reactors also stopped working — at the moment, instead of 30, only a few are operating.

Other European countries are in no hurry to abandon nuclear power plants. The Swedish government initiated the repeal of the 1980 law on the phase-out of nuclear energy. In 2022, two reactors at the Ringhals NPP resumed operation, and there are plans to upgrade existing plants in the future. In Belgium, the planned closure of nuclear power plants is postponed. Poland and the Netherlands are planning to expand nuclear power. France, where nuclear power plants are responsible for generating approximately 70 percent of all electricity, does not plan to reduce the share of capacity generated at nuclear power plants — it remains the most important exporter of electricity to Germany.

International organizations believe that to comply with the Paris Agreement by 2050, countries need to increase the share of nuclear energy in electricity generation. In 2022, the European Parliament voted to include nuclear and gas energy in the list of environmentally sustainable types of energy generation.

Does not emit radiation

Nuclear power plant waste, unlike waste from other energy sources, is radioactive. However, nuclear energy is still considered one of the most environmentally friendly. For example, according to the Nuclear Energy Institute (NEI), in 2020, 471 million tons of carbon dioxide emissions were avoided thanks to the use of nuclear power plants.

Gaseous waste generated during the operation of nuclear power plants consists mainly of noble gases — chemically inert substances that are not involved in biological processes. Most of this waste is captured by filters or decomposes without harming the environment. One of the main markers that allows tracking the impact of nuclear power plants on the environment is the long-lived noble gas krypton (85Kr), which is released both during reactor operation and during the reprocessing of nuclear fuel. In 1970, the atmosphere contained 0.4 Bq / m3. According to scientists, by 2030 the concentration may increase to 3 Bq/m3.

In addition to gaseous waste, liquid and solid radioactive waste (RW) are also generated during the operation of nuclear power plants. Most of the radioactive isotopes contained in them have a short half-life and do not pose a danger. Therefore, RW can be safely recycled and disposed of. I will talk about this in more detail below.

In normal operation, the share of emissions from nuclear power plants is so small that the radiation background around nuclear power plants is indistinguishable from natural radiation — their contribution to the increase in background radiation does not exceed 0.01 percent. At the same time, the plants are designed in such a way that when going beyond the normal operating limits, there is a large reserve for returning them to operational limits. And only in the rare case when this is impossible, the risk of an emergency may arise. In 2021, Rosatom did not record a single level “1” event (an abnormal situation going beyond the permissible limits during operation) on the International Nuclear Event Scale (INES).

How does the INES scale work?

The International Atomic Energy Agency (IAEA) proposes to assess such incidents using a unified scale. Events are classified into seven levels:

1. Levels 1–3. The situation does not require additional measures and is under the control of the nuclear facility personnel

2. Levels 4–7. The consequences of the events extend beyond the nuclear facility

Events that are not significant from a safety point of view are assigned a rating of “below scale/level 0”.

Level 3 does not require measures related to the release of radioactivity, contamination, or irradiation of people outside the plant. Level 4 is assigned to an event that resulted in the death of people as a result of irradiation. Level 7, the maximum, is assigned to an event that resulted in a radiologically equivalent release into the atmosphere of more than several tens of thousands of terabecquerels of the radioactive isotope iodine-131 (131I).

The IAEA scale was initially used only to classify events at nuclear power plants but was later expanded and refined to allow it to be used to assess any events involving the transport, storage, and use of radioactive materials and radiation sources. More information about the scale can be found here.

If we compare nuclear power and other energy sources in terms of their impact on the environment, the comparison will not be in favor of the latter. For example, thermal power plants (TPPs) that use coal to generate energy emit toxic compounds of lead, thorium, uranium, and other heavy metals into the atmosphere, which can accumulate in the body. Scientists consider TPP emissions to be more dangerous than other sources of fine particulate pollution, and associate them with increased mortality.

In addition, fossil fuel extraction itself carries risks of radiation contamination. The main threat to human health comes from the use of isotopic sources and the increase in concentrations of the naturally occurring radioactive gas radon. It is one of the leading causes of lung cancer, accounting for 3 to 14 percent of all cases of the disease.

Lots of heat

According to the second law of thermodynamics, the conversion rate of heat in a nuclear power plant reactor into electrical power is about 30–40 percent. Therefore, 60–70 percent of the low-grade heat generated by the reactor must be utilized through a cooling pond or special cooling towers. The systems that remove heat from the environment do not come into contact with radioactive fuel. In total, a nuclear power plant can have one or two, and in some cases, three closed systems of pipes (circuits) through which the coolant circulates.

The first circuit is necessary for transferring heat from the reactor to the steam generators: it heats the coolant (water) in the second circuit to a state of steam, which pushes the turbine blades and sets the electric generator in motion. The energy from the rotation is converted into electricity. Then the exhaust steam condenses, giving off the remaining heat to the environment, and again enters the steam generator.

The secondary coolant inevitably contains some amount of dissolved gases. In the most common type of water-cooled reactor, their sources may be additives required to maintain optimal coolant characteristics and processes that change its composition: metals carried away by droplets from turbine blades, corrosion of structures, radiolysis, and induction of radioactivity. Diffusion of gases formed as a result of nuclear fission also plays a role.

The release of large amounts of heat into the atmosphere can affect the thermodynamic balance of ecosystems. In particular, an increase in water temperature reduces the solubility of oxygen and can affect the biodiversity of nearby water bodies. Therefore, in the cooling ponds of various NPPs, including Kola, Kalinin, Balakovo, and Kursk, measures are taken to stock fish and trout farms are organized to combat the effects of thermal pollution.

Cooling reservoir of Balakovo NPP

Reprocessing and disposal

After use in the reactor, the fuel turns into radioactive material. Spent nuclear fuel (SNF) is stored for several years on the territory of the nuclear power plant in cooling pools. When the radioactivity and temperature of the SNF decrease, and the residual energy release no longer requires forced heat removal, it is sent for reprocessing.

Most countries with nuclear power have implemented the so-called open fuel cycle (OTC): spent fuel does not participate in the production of new fuel assemblies (FA) and is sent for long-term storage or buried if it is impossible or impractical to separate the nuclides remaining in the fuel.

Russia, on the contrary, is investing in the reprocessing of nuclear materials and is currently the leader in the use of this technology. After the separation of fissile nuclides from the SNF, new fuel assemblies are formed for further use. They partially consist of uranium and plutonium oxides, such fuel is called MOX (Mixed Oxide Fuel).

The fast neutron reactor BN-800 of the Beloyarsk NPP has now completely switched to MOX fuel. They are also testing fuel that, unlike MOX, consists of nitrides — it is planned to use it in the BREST-OD-300 reactor under construction in Seversk. It belongs to the IV generation of reactors, the design of which is aimed at increasing the reliability and efficiency of nuclear energy.

Today, the fast neutron reactors of the Beloyarsk NPP are the largest industrial power units operating on fast neutrons. The peculiarity of the fuel cycle of these reactors is that the amount of fissile material loaded into the core is less than the amount unloaded. This means that such reactors use fast neutrons more efficiently, causing the transmutation of isotopes in the fuel and creating more plutonium atoms than the fission of uranium-235 atoms.

Recycling fissile nuclides from spent fuel is the first step towards a closed nuclear fuel cycle (CNFC), which involves reprocessing the fuel and reusing “unburned” fissile nuclides. The most valuable component for nuclear power is considered to be unused uranium-235, as well as fissile isotopes of plutonium-239 and plutonium-241.

However, it is not yet possible to fully utilize fissile nuclides in nuclear fuel. The fact is that fission products capable of capturing free neutrons accumulate inside the reactor. This reduces the likelihood of fission of uranium-235, as well as plutonium isotopes, which are formed in the fuel and create difficulties in maintaining the neutron balance. In a WWER reactor, the proportion of unused uranium-235 is about 50 percent, and the amount of produced plutonium is about 50 percent of the burnt uranium, that is, about one percent of plutonium-239 and plutonium-241 ends up in the fuel.

Spent nuclear fuel can be used for more than just MOX fuel. For example, it can be used to produce components that are used in medicine. For example, isotopes of iodine-131, samarium-153, molybdenum-99, and others are needed in the diagnostics and therapy of oncological diseases. In addition, radionuclides, such as americium, can serve as fuel for potential interplanetary nuclear engines, and can also be used in RTGs that are installed in spacecraft such as the Voyager 2 probe and the Curiosity rover.

Radioactive materials, the use of which is not considered appropriate or is associated with unjustified risks to humans and the environment, are simply called waste.

There are various approaches to classifying radioactive waste (RW), but the main criteria are considered to be specific radioactivity and the heat released. By activity, RW is divided into high-level (HLW), intermediate-level (ILW), and low-level waste (LLW). There is also a classification by aggregate state: solid (SW) and liquid (LW). Sometimes, gaseous waste (GW) is also distinguished, which, after settling on filters, passes into the category of solid waste.

Handling of liquid and solid radioactive waste of medium and low activity is generally straightforward. Its volume is reduced by evaporation and, where possible, conditioning or incineration. The waste is then vitrified or cemented, placed in containers, and disposed of. Most often, this is done in a closed repository hundreds of meters underground, which is designed to prevent the disposed waste from coming into contact with groundwater and the surface. Programs for deep geological disposal of spent nuclear fuel are being developed, for example, in Canada, the United States, and Japan.

Things are more complicated with high-level waste. If it consists mainly of short-lived isotopes, it is kept until it can be transferred to the category of ILW or even RW. Waste from long-lived isotopes is kept until the energy release decreases so much that it no longer requires cooling. Then it is also vitrified or cemented and sent to underground burial sites.

Future plans

There are several options for the safe handling of nuclear fuel, spent nuclear fuel, and radioactive waste in the future:

• Firstly, launching more fast reactors. This type of reactor is compatible with reprocessed fuel and produces more plutonium, which can be used to create fuel for slow reactors of the WWER type. In the long term, this will reduce the exploitation of existing uranium deposits and burn fuel more efficiently, which means reducing the volume of highly active radioactive waste and reducing the potential number of deep burial sites.

• Secondly, maximum use of fuel with minimal reprocessing in reactors with heavy water as a moderator. The most suitable for this are CANDU reactors. This approach allows for a significant increase in the depth of fuel burnup using spent fuel from pressurized water reactors (PWR, WWER), and an increase in the production of plutonium, which can be returned to the cycle after reprocessing of spent nuclear fuel.

Electricity may be generated using so-called “utilization reactors”. They use molten salts as fuel, including elements of spent fuel from nuclear power plants: thorium, plutonium, and others. Such developments are carried out, for example, by the company Terrestrial Energy.

In addition, some projects use the traveling wave effect. This is a fundamentally new technology that in the future may allow the utilization of depleted uranium (tails obtained as a result of fuel enrichment). A traveling wave reactor does not use up the entire volume of fuel. Energy is generated by the gradual conversion of uranium-238 into plutonium, which is divided with the release of heat and converts the remaining uranium into plutonium until the entire volume of fuel is used up.

Power engineers are considering thorium fuel as a long-term replacement for uranium-235. Thorium is more common and is not a fissile nuclide in itself, but can turn into the isotope uranium-233. A reactor for the implementation of a thorium nuclear fuel cycle, which should reduce the amount of long-lived highly active elements, is being developed in India.

Finally, states and private investors are investing heavily in the development of thermonuclear fusion, even though this technology has not been mastered for over 50 years. It does not produce harmful emissions and is capable of generating four times more energy per kilogram of fuel. Russia is participating in the International Thermonuclear Experimental Reactor (ITER) project. Its goal is to obtain more energy than is needed to start a thermonuclear reaction. In 2022, scientists from the Lawrence Livermore National Laboratory managed to do this. However, at $3.5 billion, the energy was only enough to boil a kettle.

The efforts of nuclear scientists around the world are aimed at reducing the amount of existing waste and spent nuclear fuel through their reuse, recycling, and disposal. Work is also underway to modernize fuel cycles and nuclear technologies that contribute to the development of waste-free nuclear energy. All this makes it possible to reduce the already small impact of nuclear power plants as a carbon-free source of heat and electricity on the population and the environment.

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