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Hitachi's new reactors addressing diversified needs
―― The innovative light-water reactor HI-ABWR and the small light-water reactor BWRX-300

As the consequences of record-breaking heatwaves, disastrous flooding, and other climate change events are becoming apparent around the world, achieving the decarbonized society is a matter of urgency. There is growing interest in both renewable energy and nuclear energy worldwide with a view to reducing emissions of carbon dioxide, the principal greenhouse gas. Hitachi leverages its Boiling Water Reactor (BWR) technologies and experience accumulated over many years to develop new reactors that will address the needs of customers looking for safety and economic efficiency. We asked Masayoshi Matsuura, Senior Chief Engineer overseeing the technical development of new reactors at Hitachi-GE Nuclear Energy, Ltd. about the key points and social significance of the technology.

Senior Chief Engineer, Hitachi-GE Nuclear Energy, Ltd.
Mr. Masayoshi Matsuura

Reevaluating nuclear energy in terms of safety and energy security

Faced with the increasingly serious issue of climate change, more than 150 countries and regions, including Japan, aim to achieve carbon neutrality*1 by 2050. Since initiatives in the energy field, where greenhouse gas emissions are particularly high, are effective in achieving this goal, expectations are growing for nuclear energy, which, similarly to renewable energy, does not emit carbon dioxide when generating power. In addition to the start in July 2023 of commercial operations of the first new reactor in approximately 30 years in the United States, plans for new reactors are making headway worldwide.

As more renewable energy is introduced towards carbon neutrality, there is growing awareness that nuclear energy, which is capable of supplying stable and large amounts of electricity, is needed as a base load power source to maintain a steady supply of power while taking advantage of renewable energy, which is prone to fluctuations caused by weather conditions.

Masayoshi Matsuura, Senior Chief Engineer, points out that the recent uncertainty around supplies and the skyrocketing prices of natural gas due to the situation in Russia and Ukraine has led to a reevaluation of nuclear energy.

"Compared to fossil fuels, uranium, the fuel for nuclear power generation, is less volatile in price and requires less volume in weight to produce the same amount of electricity, which makes another advantage in terms of transportation cost. In general, once a reactor has been fueled, it can operate for one year without stopping. Since the fuel itself can be used for 4 to 5 years, long-term stable operation is possible. We can say that from the perspective of stronger energy security, the need for nuclear energy is increasing."

In this context, technology providers worldwide are speeding up the development of new reactors, aiming to improve safety and economic efficiency. The lineup at Hitachi-GE Nuclear Energy features two types of new reactors: the 1300 MW class Highly Innovative Advanced BWR (HI-ABWR), which is a large-scale light-water reactor*2, and the 300 MW class BWRX-300, which is a small light-water reactor.

Carbon neutrality
To achieve net zero emissions of carbon dioxide and other greenhouse gases.
Light-water reactor
The most common nuclear reactor. It uses light water (ordinary water) as coolant to absorb heat generated by nuclear fission and as moderator to facilitate a chain reaction that slows down the neutrons generated by nuclear fission inside the reactor.

HI-ABWR pursues safety from the perspective of accident prevention and impact mitigation

What about the performance of the two new reactor types?

The HI-ABWR features the latest safety design, which improves safety from the dual aspects of accident prevention and impact mitigation. The central features of the reactor are reinforced exterior walls, distributed equipment, and a passive safety system. After the Fukushima Daiichi Nuclear Power Plant accident, Hitachi-GE Nuclear Energy established an international standard for ABWR design that reflects lessons learned from the accident and meets new safety regulations in Japan, as well as the criteria for safety reviews in the United Kingdom (UK) and Europe. In 2017, the ABWR design was certified as a standard design for construction in the UK, which is why it is known as the UK ABWR.
The HI-ABWR is an innovative light-water reactor equipped with a new safety mechanism based on the certified UK ABWR and an early commercialization is expected.

Cross-section of the HI-ABWR building

The passive safety system has been adopted from the perspective of preventing severe accidents and mitigating impact. The system consists of several elements, among which the passive reactor cooling system (PRCS) plays the key role in preventing severe accidents. By installing the tank (PRCS tank) that contains the water coolant at a higher position than the reactor pressure vessel*3, which houses the reactor core (where the fuel is located), cooling the reactor by natural circulation even if the power supply to the pumps that circulate coolant is interrupted. Mr. Matsuura explains, "The PRCS tank is prefilled with enough water to cool the reactor for 24 hours, which can be further extended by replenishing the water in the tank. The main goal of this element is to have enough time to execute measures to prevent a severe accident."

We have also installed a passive debris cooling system, which uses gravity to supply cooling water to mitigate the impact of a severe accident. If a core is damaged and molten fuel falls from the reactor pressure vessel into the primary containment vessel, radiation heat from the debris (molten fuel) will activate the fusible plug valve, which connects to the pool in the primary containment vessel (the suppression pool*4). A system that can cool both the reactor core and debris without the use of power enables cooling the reactor safely even if the operator is unable to access the controls.

Reactor pressure vessel (RPV)
A steel vessel containing fuel, the control rods that regulate fuel output, and water as coolant and moderator. A robust structure that can withstand high temperatures and high pressure is required.
Suppression pool
Facility for storing large amounts of water installed at the bottom of the containment vessel. If steam causes the pressure to rise inside the containment vessel in the event of a Loss of Coolant Accident, the steam is cooled by injecting it into the pool to reduce the pressure inside the containment vessel. The pool is also used as a water source for the emergency core cooling system.

Passive reactor cooling system (top right) and the passive debris cooling system (bottom right)

New filters prevent external release of radioactive substances

In case of a severe accident, it is important to prevent the external release of radioactive materials. When the pressure in the primary containment vessel*5 rises because of damage to the reactor pressure vessel, the gases inside must be released by venting to decrease the pressure and prevent damage. Filter vent systems have conventionally been installed to suppress the release of radioactive cesium or other fission products, but Hitachi has developed and will install a new iodine filter for the HI-ABWR to improve the efficiency of radioactive iodine removal.

"Conventional filter vent systems cannot remove radioactive noble gases*6. So, we have developed a noble gas filter that removes the gases by using a membrane that allows steam and hydrogen to pass, but blocks noble gas. As a result, it is possible to remove and contain almost all radioactive substances while ensuring the safety of the nuclear reactor in case of an accident. Currently, we are developing more highly functional membrane material with support from the government, and we plan to apply the development results."

To provide back-up for renewable energy, which is subject to major fluctuations, we are also improving plant performance from the perspective of carbon neutrality by, for example, improving the load following operations, which controls power generation according to demand.

Mr. Matsuura says that the HI-ABWR is attracting a high level of interest because of these special characteristics. "We are receiving high attention in the presentations for electric utility companies and at academic conferences, which gives us more confidence. Existing nuclear reactors are gradually adding safety systems that reflect lessons learned from accidents, but with the HI-ABWR the key point is that we are introducing rational safety mechanisms as a built-in feature from the design stage. We are also seeing positive recognition on our innovative yet feasible approach to utilize the experience of design and licensing in UK as a basis and then incorporate new technologies developed with support from the government."

Primary containment vessel (PCV)
A highly airtight building that houses the reactor pressure vessel, the coolant circulation pumps, and other major equipment. In case radioactive material is released from the reactor pressure vessel in an accident, the primary containment vessel suppresses release to the outside.
Noble gas
A generic term for 6 gaseous elements that are chemically inert. They are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). The English notation has recently been changed from rare gas to noble gas.

The BWRX-300 combines safety and economic efficiency

The history of BWR development

The first BWR was the Dresden Unit 1 power plant (BWR-1), which started operating in the United States in 1960. Since then, we have continued to simplify and improve performance by installing recirculation loops using pipes outside the reactor pressure vessel on the BWR-2 through to the BWR-6, and recirculation pumps directly attached to the reactor on the ABWR. We adopted a system that is capable of cooling the reactor core using only natural circulation for the SBWR (Simplified BWR) and the ESBWR (Economic SBWR). Since the BWRX-300 is the tenth and most recent reactor (BWR-10) counting from the BWR-1, the name includes the X to signify the Roman numeral for 10.

Mr. Matsuura explains the history of developing the BWRX-300. "Generally speaking, the larger the reactor, the more economically efficient it is. However, rising costs caused by delayed construction of large reactors in Europe and the United States have led to a growing demand for small reactors that require a smaller initial investment and aim to shorten the construction period and improve quality by incorporating modular construction techniques. In response to such needs, the BWRX-300 was developed to realize an economically efficient small modular reactor. To improve economic performance, it is necessary to greatly simplify the entire plant system while ensuring safety, which could lead to a trade-off between safety and economic efficiency. The new concept that enables both - safe and economical - is the integrated isolation valves."

The isolation valve, which is typically installed on the main pipe sending steam from the reactor pressure vessel to the power generating turbine, isolates the turbine from the nuclear reactor by closing when an abnormality occurs. The main cause for a Loss of Coolant Accident (LOCA), the critical accident that could occur at nuclear power plants when reactor coolant leaks from the reactor pressure vessel, is damage to the pipe. In conventional nuclear reactor structures, the pipe is welded between the reactor pressure vessel and the isolation valve, so there is a risk of ruptures in the weld zone. To avoid this risk, the BWRX-300 attaches the isolation valve directly to the reactor pressure vessel with a flange*7. As a result, it is possible to mitigate the risk of loss of coolant by closing the isolation valve even if a pipe ruptures. Since this eliminates the need for a water injection system that uses pumps and a suppression pool to condense generated steam and control rising pressure, which has been essential for responding to LOCA, we have successfully simplified the reactor while still ensuring safety."

In the worst case of a severe accident, an isolation condenser will be used to contain the situation. Similar to the HI-ABWR, BWRX-300 is equipped with a coolant pool at a higher position than the nuclear reactor and uses natural circulation to cool the reactor core over a period of 7 days. The cooling period can be extended by injecting water into the pool.
Simplification with the integrated isolation valves reduces the number of measuring and control devices, which, in turn, mitigates the risk of the failure of such devices. It is also possible to downsize the primary containment vessel, which will reduce the volume of the building and the amount of concrete used, thus shortening the period of construction and, ultimately, reducing the cost of construction, operation and maintenance.

A generic term for a flat rim installed around the circumference of the end of a pipe etc. Flanges have excellent sealing performance, strength, and ease of assembly. They are used to connect pipes to each other, or pipes to equipment, and so on.

Integrated isolation valve connected to the reactor pressure vessel with a flange (left)
and Coolant pool installed at a higher position than the reactor (right)

Selected as a candidate reactor for new construction projects around the world

Another feature of the BWRX-300 is that it is a joint development with GE Hitachi Nuclear Energy in the United States, a US partner of Hitachi-GE Nuclear Energy. Most of the technology, including equipment and fuel, has a track record of use in the BWR and the ABWR. Early commercial operation is facilitated by using systems such as the passive safety system adopted for the ESBWR and other technologies that have completed design certification for the United States.

"A total of four BWRX-300 units are now planned for deployment in Ontario, Canada and we aim to complete construction of the first unit in 2028 . We are promoting efficient development by combining the efforts of GE Hitachi Nuclear Energy, which has rich expertise in North American regulations, with Hitachi-GE Nuclear Energy, which has excellent capability in equipment design, manufacturing, and demonstration testing. In addition to the time difference, the COVID-19 pandemic also had an impact on the collaboration between the United States and Japan, but we were able to overcome the obstacles by utilizing online communication tools.
To develop the new technologies, including cooling through natural circulation and integrated isolation valves for nuclear reactors, we worked very hard to verify performance with the actual pressure and temperature at Hitachi-GE Nuclear Energy’s testing facility that is full-scale and one of the largest in the world (Hitachi Utility Steam Test Leading Facility (HUTSLE))." Mr. Matsuura continues with a smile, "However, we were able to achieve high level in both safety and economic efficiency, so I believe the effort is paid off."

In addition to Canada, the BWRX-300 is also in the process of getting licensed for the United States, and it has been selected as a candidate for new reactors in Estonia and Poland. Hitachi has applied for Generic Design Assessment (GDA) which will be required for construction in the United Kingdom. In March 2023, companies in Canada, the United States, and Poland announced joint investment in developing a standard BWRX-300 design for universal use, and in detailed designs for key components.

"It seems to be the past track records, proven fuel and safety that earned evaluation in the Canadian selection as advantages over other reactor types. The United States, Estonia, and Poland also gave BWRX-300 good recognition for its high level of completion and the short lead-up to commercial operation."

Cross-section of the BWRX-300 building

Meeting diverse needs with large and small reactors

Although they are both built to a high safety standard, the HI-ABWR and the BWRX-300 deliver different value. What are the advantages of a lineup with reactors of different sizes?

"Needs for nuclear energy vary depending on the situation of the country, the region, the electric utility company are facing. Since there are many comparatively smaller private electric utility companies in Europe and North America, we are seeing a rapid increase in demand for small reactors to keep the initial investment down. On the other hand, there are also countries where large-scale state-owned companies supply most of the power, and countries where large-scale power plants are needed to meet a rapid increase in energy demand due to economic development. As manufacturers, we believe it is important to be well-equipped with the ability to accommodate diverse needs, having a lineup with both large and small reactors."

Mr. Matsuura also says that he would like to be prepared with more options for customers in Japan as the demand for energy is expected to vary by region in the future. "The advantage of the HI-ABWR, as a large-scale reactor is that it is possible to reduce the site area per unit of power generation. At the moment, it is difficult to find new sites in Japan, but we anticipate the future needs for large reactors in response to the increase in demand for electricity to operate data centers and electric cars as Japan aims to achieve carbon neutrality by 2050. The advantage of the BWRX-300 is that the main buildings will be small enough to fit inside an area with the size of a football field, so less land is needed for construction. It would also be possible to rebuild by making effective use of vacant space on sites where reactors are in the process of decommissioning. I believe that an optimal mix of large and small reactors is needed to respond flexibly to power demands under the limited site conditions."

Contribute to the carbon-free society by social implementation of innovative new reactors

In addition to the HI-ABWR and the BWRX-300 introduced in this article, Hitachi-GE Nuclear Energy is also developing a light-water-cooled fast reactor RBWR (Resource-renewable BWR), which facilitates the reuse of plutonium in spent fuel to establish a fuel cycle, and the small modular sodium-cooled fast reactor PRISM (Power Reactor Innovative Small Module) in collaboration with GE Hitachi Nuclear Energy.

Mr. Matsuura shows his enthusiasm by saying, "As well as the experience of completing design certification for the UK ABWR, Hitachi-GE Nuclear Energy has the technology and expertise to implement designs that comply with safety and regulatory requirements in Japan and overseas. We can also work on development in collaboration with our US partner, GE Hitachi Nuclear Energy. By leveraging the strengths built up over half a century of experience of designing and building BWR in Japan, we can accommodate the needs as well as regulatory requirements in Japan and overseas, enabling us to develop new reactors with the most advanced technologies in the world. Nuclear energy is being reevaluated from the perspective of carbon neutrality and stable energy supply, and it is the mission of Hitachi-GE Nuclear Energy to offer various solutions to meet the society needs and to contribute to a carbon-free society."

Mr. Matsuura has been involved with technology development in the field of nuclear energy for many years. As Chief Engineer, he is now responsible for the overall development of new reactors. To conclude, we asked him his thoughts on working with technology development amid significant changes to the social environment around energy in Japan and overseas.

"Nuclear energy is produced by nuclear fission. As described by Einstein's famous equation E=mc² (energy equals mass times the speed of light squared), the mechanism uses a fundamental principle of the universe, which is that a small mass changes into an enormous amount of energy. Both nuclear fission and nuclear fusion, which is the next goal, are rational systems from the viewpoint of carbon neutrality because they leverage this natural principle. It is not research for the sake of research, but for the sake of practical application. I would like to broaden the awareness through social implementation of safe and economical new nuclear reactors. It is vital to carry through and develop nuclear energy technology to decommission the Fukushima Daiichi Nuclear Power Station. I will continue to engage with research and development of new technologies for the future while passing on what I have learned to the younger generation."

Rendering of BWRX-300