Coal shortage: is nuclear heating the answer?

Zhejiang Haiyan and Shandong Haiyang have completed nuclear energy heating projects, benefiting the people's livelihood. But there are rumors that the heating water heated by nuclear energy is radioactive. What is the truth?

Against the backdrop of carbon emission reduction and carbon neutrality, nuclear heating has officially "entered" our lives.

According to China National Nuclear Corporation, the first nuclear energy heating project in southern my country, the Zhejiang Haiyan Nuclear Energy Heating Demonstration Project (Phase I), was officially put into operation in Haiyan, Zhejiang on December 3, providing heating for an area of ​​460,000 square meters, benefiting nearly 4,000 households. By the end of the 14th Five-Year Plan, the project will be fully completed and will be able to meet the heating needs of about 4 million square meters in Haiyan.

"Nuclear energy heating is a major project for people's livelihood and public opinion." Yu Bing, deputy director of the National Energy Administration, said at the project commissioning ceremony that China National Nuclear Corporation should create a benchmark project, summarize experience, and jointly promote the healthy development of nuclear energy heating; it should make good use of the project, organize "Warm Nuclear" science popularization activities, do a good job in public communication, and create a good environment for nuclear energy heating.

Coincidentally, on November 9, the 4.5 million square meter second phase of the National Energy Nuclear Heating Commercial Demonstration Project was put into operation in Haiyang, Shandong. The heating area covers the entire urban area of ​​Haiyang, benefiting 200,000 residents. Haiyang has become the first "zero-carbon" heating city in the country.

Nuclear heating is an effective way to solve energy and pollution problems. Data shows that in Haiyang, Shandong alone, nuclear heating can save nearly 100,000 tons of raw coal, reduce 180,000 tons of carbon dioxide emissions, and thousands of tons of nitrogen and sulfide gas emissions each year, especially at a time when fossil energy supply is relatively sensitive.

However, many people still fear the word "nuclear". Nuclear heating plants are questioned as unsafe. There are even rumors that the water heated by nuclear energy is radioactive. What is the truth? Is nuclear heating a good way to solve the coal shortage?

▲On December 5, 2021, residents of Nuclear Power New Village, Maple Leaf, and Nuclear Power Nanyuan communities in Haiyan County, Jiaxing, Zhejiang, received centralized heating in their homes (Photo source/Visual China)

Q1: How is nuclear heating achieved?

In the public's impression, "nuclear power" is the most familiar word, and the most direct application of nuclear energy seems to be power generation. In fact, the power generation principle of nuclear power plants, especially shell reactors, is very similar to that of traditional thermal power plants. Simply put, they are both "boiling water". Therefore, nuclear power plants can also achieve cogeneration like thermal power plants, that is, both power generation and heat supply.

One of the most common types of nuclear power plants is the pressurized water reactor, which is also the type used by the Haiyang Nuclear Power Plant. This type of reactor is to install the nuclear reactor in a "pressure cooker". The entire system consists of three loops: one loop is the core of the nuclear power plant, which is equivalent to the boiler of a thermal power plant. After the nuclear fission reaction occurs here, huge heat is generated, which heats the high-pressure water to more than 300℃.

The water in the secondary circuit passes through the steam generator, absorbs the heat of the steam in the primary circuit, and turns itself into saturated steam, which drives the steam turbine to do work and then turns into condensed water. The temperature of this condensed water is basically over 90 degrees, so it needs to be cooled before processing.

If heating is needed, the water in the second circuit will enter another heat exchanger to heat another batch of water, which can also be called heating the water in the third circuit. This operation can continue to the fourth circuit, the fifth circuit or even more. The final output water becomes the "heating water" transmitted to residents' homes.

In addition to shell reactors, there is another type of reactor for nuclear heating called pool reactor. From the name, it seems that energy output is carried out in a pool of water, and in fact it is true - nuclear fuel rods are placed in a pool of purified water, which can be used as a moderator, coolant, reflector and part of the protective layer material. There is also a heavy concrete shielding layer on the side and bottom of the pool to effectively prevent radiation.

If heating is needed, the reactor core is placed deep in a normal pressure water pool, and the static pressure of the water layer is used to increase the water temperature at the core outlet to meet the heating requirements. The heat is transferred to the heating circuit through two-stage exchange, and then transported to thousands of households through the heat network.

It is worth mentioning that shell-type heating reactors are generally not specialized in heat production, but cogeneration, that is, they mainly generate electricity and the remaining energy is used to generate heat. The project in Haiyang, Shandong, uses the waste heat from the Haiyang Nuclear Power Plant.

▲Schematic diagram of Haiyang nuclear energy heating (Photo source: Shandong Nuclear Power Co., Ltd.)

Q2: Why is it said that nuclear heating has no “radiation”?

For the public, the first thing to understand is that nuclear energy heating does not equal nuclear heating. Currently, there is no technology in the world that can directly use nuclear fuel as a heat source. It can only be achieved after layers of medium conversion.

This principle is very similar to the self-heating hot pot we often eat - water is used as a medium to drive the heating pack to heat up, generating hot air to heat the ingredients. During this process, there is no contact between water and ingredients.

Back to the issue of "nuclear energy heating", no matter it is a pool-type heating reactor or a shell-type heating reactor, they all extract steam as a heat source, carry out multi-stage heat exchange through the first heat exchange station within the plant and the heat exchange station of the heating enterprise outside the plant, and finally transfer the heat to the end user through the municipal heating pipeline network.

During the entire process, there are only two modes: "steam heating water" and "water heating water". There are multiple circuits to isolate the nuclear power plant and the heating users. There is only heat transfer between each circuit, and no water exchange, so no radioactive substances will enter the user's heating pipes.

▲Haiyang Nuclear Power Base (Photo source: The Paper)

According to media reports, the nuclear heating in Haiyang, Shandong, extracts some of the steam generated by the nuclear power generators as a heat source, converts it into high-temperature water through the heat exchanger in the plant, and then transports it to the heating company through the pipeline network. The whole process involves as many as 5 circuits.

Another professional engineer said that before the hot water from the first heat exchange station of the nuclear power plant leaves the factory, it will be tested by online monitoring and isolation devices, and a pressure difference design will be adopted between the circuits to ensure safe and reliable nuclear energy heating.

As for residents, after testing, the water used by users was found to have no radioactivity and was consistent with various indicators of normal water use, "you can rest assured, it is very safe."

Q3: Will a nuclear heating station “explode”?

Not only does nuclear heating have no radiation, nuclear heating stations are also very safe.

From a design perspective, the nuclear fuel is wrapped in a special fuel cladding, and the primary circuit system of the shell-type heating reactor is installed in a closed building called a containment. The containment can maximize the protection of the public and the environment from being affected when an accident occurs in a nuclear power plant.

In fact, just like beer cannot burn because its alcohol concentration is too low, the concentration of uranium 235 used as fuel in nuclear reactors is only 3% to 5%, while nuclear weapons require more than 95% or even higher, so the nuclear reactor itself will not explode, and there is no risk of leakage.

Judging from past accidents, there is only one reason for a nuclear power plant explosion - lack of water.

During the power generation process, nuclear power plants must continuously inject cold water into the reactants to achieve circulation. If the circulation stops, the reactants will overheat and melt, causing major accidents.

The Chernobyl Nuclear Power Plant hoped to use steam to drive turbines to generate electricity after a power outage, so it conducted an experiment. However, tragically, an operational error occurred during the experiment, causing the reactor to overheat, and ultimately a meltdown, explosion, and nuclear leak.

▲Third-generation nuclear power plant: Unit 5 of Yangjiang Nuclear Power Plant of China General Nuclear Power Group

The accident at the Fukushima nuclear power plant in Japan is easier to understand. After the earthquake, the nuclear power plant units were shut down. After the backup diesel generators lasted for half an hour, the power system was completely destroyed by the tsunami. Eventually, all four units exploded, causing a serious nuclear leak accident.

If the heating station also "power outages", will the same mistake as Chernobyl happen again? The answer is no, because the newly built nuclear reactors in recent years have all been upgraded to third-generation reactors. Compared with second-generation reactors like Fukushima, their biggest feature is that they have "passive" cooling function - they can be cooled naturally without external power supply, which greatly enhances the safety factor.

As for pool-type reactors, because the nuclear fuel reacts directly in water, the energy generated directly fixes the water temperature at around 100 degrees Celsius, and there is no need for cooling water at all. This simplifies the complexity of the energy conversion equipment system and reduces the loss in the energy conversion process. The safety factor is higher than that of traditional shell-type reactors.

Q4: Will nuclear heating become widely popular in the future?

Compared with other forms of heating such as traditional fossil energy, nuclear energy heating has very obvious environmental advantages: nuclear fission has a high energy density, and compared with coal-fired boilers of the same power, the annual transportation volume of nuclear fuel is only about one hundred thousandth of the amount of coal; nuclear energy heating can also significantly reduce carbon dioxide emissions, which coincides with the needs of "carbon reduction" and "carbon neutrality".

In northern my country, the livelihood demand for centralized heating has become the main obstacle to carbon emission reduction and carbon neutrality. Under this pressure, thermal power plants will not have an easy time in the future. It can be said that the realization of nuclear heating has brought new hope to thermal power companies.

In fact, nuclear heating is not a new concept. It was used in Northern Europe half a century ago, and the Soviet Union also built nuclear heating reactors. my country conducted relevant research as early as the 1980s. Even by the end of 2017, some office buildings of the China National Nuclear Corporation's Institute of Atomic Energy had begun to use its own "Yanlong" swimming pool-type low-temperature heating reactor for heating.

So, what is the reason why nuclear heating has not been popularized so far? Economic factors are a very important reason.

▲Schematic diagram of pool-type nuclear energy heating

Pool-type heating reactors, also known as small reactors, are convenient and flexible, but at the current technical level, they sacrifice heating efficiency. They can only be used in small areas, such as parks, campuses or small towns, and cannot benefit the general public. They do not solve the substantive problem of heating, but instead face great economic pressure.

The other is based on existing medium and large nuclear power plants, using the extraction steam from nuclear power plants to supply heat to the heating network. This model is used in Shandong Haiyang. The extraction steam temperature and pressure are determined by the demand of the heating network and the length of the heat transmission pipeline.

But this model also has disadvantages. Nuclear power plants require a lot of cold water support, which is not suitable for construction in inland areas of my country, especially in the water-scarce northern regions. If they are only built on the coast to supply heat to the inland, not only will it cost a lot to build heat transfer equipment, but the heat loss in the conduction process will also be alarming.

To sum up, it is not difficult to draw the conclusion that although nuclear energy heating is good, it is not easy to popularize it on a large scale.

In fact, nuclear heating is a complex task involving many aspects such as market, technology and policy. The nuclear heating in Haiyan, Zhejiang and Haiyang, Shandong has taken a substantial step forward, which plays an important guiding role in optimizing regional energy structure, regional clean heating and zero-carbon heating. It also points out the direction for the future development and application of nuclear heating technology.

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Written by Zhang Zili (Associate Researcher, Institute of Electrical Engineering, Chinese Academy of Sciences)

Editor/Zhao Tianyu Liu Zhao

New Media Editor/Lv Bingxin

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Science Plus Client

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