Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)
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Contribution to Carbon Neutrality by Nuclear Energy

YURUGI KANZAKI*1

JUNICHI NISHITANI*2

TAKAAKI KITAGAWA*2

Nuclear energy is a carbon-free, large-scale, and stable power source and an important
baseload power source from the viewpoint of energy security. Therefore, we recognize that
continuous usage of nuclear energy is essential in order to achieve carbon neutrality in Japan by
2050. Mitsubishi Heavy Industries, Ltd. (MHI) is supporting electric utilities for the restart ofexisting plants in Japan and the early completion of the construction of specialized safety facilities,
along with supporting the early establishment of the nuclear fuel cycle. Furthermore, MHI isdeveloping an advanced light water reactor “SRZ-1200” that will ensure the world's highest levelof safety, and also developing various other types of reactors to meet a diverse set of societal needsin the future. This report describes efforts within our nuclear energy business and associatedefforts to ensure achievement of carbon neutrality by 2050.

|1. Introduction
In October 2020, the Japanese Government announced that the country would aim for“carbon neutral by 2050” to ensure alignment with the global response to climate change. In
ongoing efforts to decarbonize, the recent Russian invasion of Ukraine has led to a sharp rise inresource prices, which has revealed energy security risks worldwide. Also in Japan, the importanceof stable power supply has been highlighted by the nation's first warning of a power supply anddemand crunch issued in March of 2022.
Achieving both decarbonization and a stable energy supply are once again recognized asimportant issues. Renewable energies such as solar and wind cannot maintain a stable power
supply. These forms of renewable power generation depend on seasonal and weather conditions.
Conversely, nuclear energy is a large-scale, stable power source that does not emit CO2 during
operation and is not affected by weather conditions. In addition, it has the ability to work alongsideand complement output fluctuation of renewable energy sources such that a diverse energy
generation mix can be achieved. MHI is working from short-, medium-, and long-term perspectives
to ensure achievement of carbon neutrality and a stable power supply in the future based on ourcultivated nuclear energy technology.

|2. Roadmap for nuclear energy toward achieving carbon neutrality
Nuclear energy is a technologically proven carbon-free, large-scale, and stable power source
and an important baseload power source which ensures energy security for society. Continuous
usage of nuclear power is essential for the achievement of carbon neutrality by 2050.
However, since the Great East Japan Earthquake, the public's confidence in nuclear energy
declined and we recognize that restoring that confidence is one of the most important issue toovercome. As the only one manufacturer for pressurized water reactor (PWR) plants in Japan, we
have proactively supported Japanese electric power companies with restarting of existing domestic
plants and thus far have successfully supported the restart of 10 PWR plants. We will continue toprovide support for all other PWR restarts which occur along with ensuring the successful
completion of specialized safety facilities. Even though we are not the manufacturer for Japan’sboiling water reactor (BWR) plants, we have proactively been working with Japanese electric*1

General Manager, Nuclear Systems Engineering Department, Nuclear Energy Systems, Mitsubishi Heavy Industries, Ltd.

*2

Deputy Director, Nuclear Systems Engineering Department, Nuclear Energy Systems, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 59 No. 4 (December 2022)2

power companies to apply our lessons learned and knowhow to ensure a successful restart of theseplants. These efforts show our commitment to ensure Japan realizes stable electricity supply,
decarbonization, and most importantly restoration of public confidence in our industry.
Additionally, our expectation is that we use nuclear energy sustainably over the long term and toachieve this establishment and integration of a closed nuclear fuel cycle is essential. As a leading
contractor in that effort, we will continue to provide comprehensive support for the establishmentof a closed nuclear fuel cycle.
To contribute to carbon neutrality and stable power supply in the future, we are promoting
the development of an advanced light water reactor “SRZ-1200” that will ensure the world's
highest level of safety. We are working toward achievement of commercialization in the mid-2030s
which will see the introduction of a vast array of safety improvements that take into accountinnovative and proven technologies we have developed. To satisfy a diversified set of societal
needs, we also intend to develop future reactors such as small light water reactors, high temperature
gas-cooled reactors, fast reactors, and micro reactors. Furthermore, we will continue to support andperform additional research to support the development of fusion reactors, a permanent "dreamenergy source", from a long-term perspective after 2050.(Figure 1)

Figure 1

Our nuclear energy business's efforts to achieve carbon neutrality|3. Restart of existing light water reactors and establishment of fuelcycle
To achieve a nuclear power ratio of 20-22%, which is a prerequisite for reducing greenhousegas emissions by 46% by 2030, many existing plants will need to be restarted.
In order to comply with the new regulatory requirements enacted after the Great East Japan
Earthquake, we have made all-out efforts to support electric power companies in carrying out
safety measures for existing plants in Japan and have successfully supported the restart of 10 PWR
plants. We also achieved the restart of Mihama Unit 3 in June 2021, which was the first restart of a
nuclear power plant in Japan that had been in operation for over 40 years. Regarding the
installation of specialized safety facilities for existing PWR plants, as of August 2022, seven
facilities have been completed. Even though we are not the manufacturer for Japan’s BWR plants,
we have proactively been working with Japanese electric power companies to apply our lessons
learned and knowhow to ensure a successful restart of these plants and further restore the public's
confidence in nuclear energy.
For restarted plants, we will continuously conduct evaluations to ensure the proactive
implementation of necessary safety improvements and implement maintenance measures thatincorporate the latest knowledge and technologies. We also understand that we will have tosystematically implement large-scale maintenance work, such as replacement of steam generatorsand reactor internals, to ensure we achieve long-term stable operation over a period of 60 years.
These are just some of the efforts which will contribute to the safe and stable operation of nuclear
power plants over a long period of time.
Reprocessing spent fuel from nuclear power plants and recycling recovered plutonium andother materials, to be used as MOX fuel, will enable, and ensure we effectively utilize uranium

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resources. Additionally, reprocessing will reduce the volume and radiotoxicity of high-level
radioactive waste. As a leading contractor, we are promoting various types of work activities
toward the early completion of the Reprocessing Plant and MOX Fuel Fabrication Plant that Japan
Nuclear Fuel Limited is constructing in Rokkasho Village, Aomori Prefecture, to ultimately ensure
the establishment of a closed nuclear fuel cycle.

|4. Advanced light water reactor “SRZ-1200”
The SRZ-1200*1 is a 1,200 MWe class advanced light water reactor with improved safety byincorporating innovative technologies and global lessons learned, all while ensuring high economic
efficiency based on the culmination of proven technologies. The basic design has been proceededin collaboration with four Japanese PWR utilities*2.
The SRZ-1200 will achieve the world's highest level of safety conforming to Japan’s new
regulatory requirements that incorporates lessons learned from Fukushima Daiichi accident.
Development of the SRZ-1200 is in progress by taking advantage of the new construction thatallows safety measures to be implemented from the design phase. In addition to strengthening its
resistance to natural disasters, new safety mechanisms including passive equipment such asadvanced accumulators and core catchers are introduced. Furthermore, a radioactive material
release prevention system that will reduce the amount of radioactive material released in the eventof a postulated accident is also introduced. (Figure 2)*1*2

The name of SRZ-1200 has the following meaning:
S: Supreme safety, sustainability
R: Resilient light water reactor
Z: Ultimate type (Z) contributing to society with Zero Carbon emission(In Japan, “Z” also has meaning of “ultimate type”)
1200: represents the electrical power output of 1,200MW
Hokkaido Electric Power Co.,Inc., The Kansai Electric Power Co.,Inc.,
Shikoku Electric Power Co.,Inc., Kyushu Electric Power Co.,Inc.

Figure 2

Advanced light water reactor “SRZ-1200”

The main features of the SRZ-1200 are as follows:(1) Enhanced resistance to earthquakes, tsunamis, and other natural disasters
The seismic design includes sufficient margin for severe seismic conditions in Japan,
including an enhanced seismic resistance by embedding the building in solid bedrock. The site
ground level is so high not to be affected by tsunamis (dry site design). Resistance to otherexternal disasters (typhoons, volcanoes, etc.) is significantly enhanced by making the building
more robust and taking measures to prevent volcanic ash intrusion, etc.

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4(2)Enhanced core cooling and containment confinement
Achievement of core cooling and containment confinement is done through adoption of
new safety designs, enhanced redundancy and diversity, etc., in compliance with the new
regulatory requirements in Japan. These initiatives require both rapid response in the earlystages of an accident and rapid accident restoration through the best mix of passive equipmentthat does not require a power source and operates automatically according to plant conditions(e.g., adoption of advanced accumulators) and active equipment that strongly demonstrates
safety functions by being driven by a power source.
(3) Measures against core melt accident
Even in the unlikely event of a core melt (severe accident), the containment vessel, whichis the final barrier preventing radioactive material release, is protected by capturing, cooling,and holding fuel debris in dedicated equipment area (core catcher).
(4) Prevention of radioactive material release
In addition to a filtered containment venting system, a radioactive material releaseprevention system that removes radioactive noble gases from the vent gas is introduced in theunlikely event that venting from the containment vessel is necessary to prevent a large releaseradioactive material, thus limiting the accident impact to the power plant site.
(5) Enhanced security (measure against terrorism)
APC (Airplane Crash) resistance was reinforced by adopting a double containment
structure with a steel containment vessel and toughened external shielding walls, etc. Cyber
security is enhanced with the integration of the latest DX technologies.
(6) Coexistence with renewable energy
Enhanced operational capability, for frequency control and load following, to mitigate
typical issues such as output fluctuations during nighttime and stormy weather, along withpower system instability associated with the expansion of renewable energy. In addition, thesystem can also be utilized for hydrogen production using generated surplus power.

|5. Future reactor and fusion reactor
5.1

Small light water reactor

In recent years, small light water reactors (LWRs) have been attracting attention as a
distributed power source for decarbonization. To meet the diversified needs of the future, MHI isdeveloping a small LWR with an electrical output of about 300 MWe which is targeted to becommercialized around 2040 as a carbon-free power source for small-scale grids. Since the2000s, MHI has been demonstrating elemental technologies specific to small LWRs through thedevelopment of the Integrated Modular Water Reactor (IMR). Additionally, we have beendeveloping innovative small LWRs under the NEXIP (Nuclear Energy x Innovation Promotion)
initiative of the Ministry of Economy, Trade and Industry (METI) from FY2019.
The small LWR adopts an integrated reactor structure in which the main components, such as
the steam generator, are built into the reactor vessel, eliminating the need for primary coolant
piping to connect the main components. It also naturally circulates primary coolant by using thedensity difference between the coolant heated in the core region in the reactor and the coolant
cooled by the steam generator, eliminating the need for a main coolant pump. These features makeit possible, in principle, to eliminate the postulated occurrence of accidents due to loss of coolant
events resulting from the rupture of the primary coolant piping or due to a pump failure. For
dealing with postulated accidents at the reactor, a passive safety system that requires no powersource during an accident is actively adopted, eliminating the need for operation by an operator andfor an external power/water source. In addition, the reactor and buildings containing safety
equipment and other facilities are embedded underground to strengthen resistance to external
hazards such as an airplane crash. Furthermore, even in the unlikely event of a major accident, thedouble containment structure enhances the confinement of radioactive materials. With the
integration of these innovative concepts, we are continuously working toward the achievement of
improvements in safety and reliability, when compared to conventional plants. (Figure 3)

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Figure 3

5.2

Small light water reactor

High temperature gas-cooled reactor

In addition to decarbonization in the power sector, decarbonization in the non-power sectors
such as industry, commercial & residential, and transportation is essential for the achievement ofcarbon neutrality. In the steel and chemical sectors, hydrogen is used as a reductant and raw
material. There are many technological developments underway for hydrogen utilization, alongwith carbon-free methods to generate hydrogen. In order to realize decarbonization of these sectors,
a large and stable hydrogen supply is required.
A high temperature gas-cooled reactor (HTGR) is characterized by the utilization of nuclear
heat at 900°C or higher and is capable of being used to generate large quantities of hydrogen forthese sectors. A HTGR is characterized by the use of highly heat-resistant graphite (moderator) and
SiC ceramics (fuel coating material) for the core and fuel, and the use of chemically stable heliumgas as the coolant to extract nuclear heat. This approach allows the core heat to be naturally
removed outside in the event of an accident. Therefore, in the event of an accident, the heat in thecore can be naturally removed outside, and no core melt will occur.
We have been involved in the development of the HTGR since the 1970s and participated in
the construction of the HTTR (High Temperature engineering Test Reactor) of the Japan Atomic
Energy Agency (JAEA), which is the only HTGR in Japan, as the leading contractor. By utilizing
our knowhow, we have been developing a plant system from 2019 that can produce several
hundred thousand tons of hydrogen per year by using the high temperature heat source (950°C)
from a HTGR with a maximum core output of 600 MWt under the NEXIP initiative of METI,
aiming for commercialization around 2040. (Figure 4)

Figure 4

High temperature gas-cooled reactor (HTGR)In April 2022, we have started a hydrogen production technology demonstration projectutilizing a HTGR in collaboration with JAEA. We plan to connect a new hydrogen production
facility (steam methane reforming method) to the existing HTTR and demonstrate this technology
using the high temperature heat obtained from a HTGR. In collaboration with the National Instituteof Advanced Industrial Science and Technology, we have also started to develop a hightemperature steam electrolysis technology to produce hydrogen by efficiently electrolyzing steam
using the ultra-high temperature nuclear heat of a HTGR as a carbon-free hydrogen production

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technology. By doing this and using a HTGR to generate hydrogen, we will first demonstratehydrogen production using nuclear heat by applying steam methane reforming, which is at a hightechnological maturity level, for early commercialization. Then, in the future, we will demonstratecarbon-free hydrogen production technologies, such as high temperature steam electrolysis andmethane pyrolysis, to ultimately realize large-scale and stable production of hydrogen.

5.3

Fast reactor

The fast reactor is a nuclear reactor that does not use water (moderator) to slow down
neutrons but instead uses the fission reaction of plutonium by neutrons in a fast state. The fast
reactor can also burn long-lived radioactive materials, called minor actinides (MA), which arerecovered after fuel breeding and reprocessing of spent fuel from nuclear power plants. These MAs
can be loaded as fuel again and converted into radioactive waste with short half-lives. Based onthese characteristics of fast reactors, the Japanese government has set a policy to promote thedevelopment of fast reactors as part of its nuclear fuel cycle policy from the viewpoints of effective
use of resources and ability to reduce the volume and radiotoxicity of high-level radioactive waste.
We have participated in national projects such as the development and construction of theexperimental fast reactor "Joyo" and the prototype fast reactor "Monju" since the 1970s. In 2007,
we were selected as a leading contractor for the development of fast reactors in Japan and sincethen we have been promoting the development of MOX-fueled sodium-cooled fast reactors, whichis the most proven type of reactor. Under the NEXIP initiative of METI, we have also started thedevelopment of a compact sodium-cooled fast reactor with enhanced safety and reliability to
expand the diversity of fast reactor applications. (Figure 5) For international cooperation promotedby the Japanese government, we have participated in the development of a tank-type fast reactor as
a leading contractor of Japan's fast reactor development through international cooperation between
Japan and France. In January 2022, we, together with JAEA, etc., agreed upon technologicalcooperation in the development of a demonstration reactor for a sodium-cooled fast reactor
developed by TerraPower Technology Inc. of the United States. Aiming to start operation of thenext-generation fast reactor at an early stage by the middle of the 21st century, we will contributeto the long-term stable supply of clean energy by developing a plant concept unique to Japan thattakes into account the severe seismic conditions in Japan, utilizing knowledge we have obtained
through international cooperation.

Figure 5

5.4

Sodium-cooled fast reactor

Micro reactor

We are developing a multi-purpose modular-type micro reactor for use as a power source inremote islands, remote areas, micro-grids, etc., where power grids are not well developed. Microreactors are portable reactors that are even smaller in power output level and size than small lightwater reactors and can contain the reactor core and power generation systems in a shipping
container. This is accomplished by utilizing graphite-based materials, which have a lower density
than metallic materials, for the core structure to reduce the reactor weight and downsizing the core.
(Figure 6) Furthermore, to realize a long reactor core life that does not require frequent refueling,
HALEU (High-Assy Low-Enriched Uranium) fuel with a maximum uranium enrichment of 20%
will be used.

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In pursuit of safety, the micro reactor adopts the concept of an "all-solid-state reactor" thatuses high thermal conductivity materials instead of liquids or gases as a reactor coolant, thereby
fundamentally eliminating the causes of postulated loss-of-coolant accidents and reducing theradiation impact on the environment. Even in the unlikely event of an accident, the reactor has low
power output and uses graphite materials with high thermal conductivity, which allows for decay
heat to be removed stably only by natural air cooling.

Figure 6

5.5

Micro reactor

Fusion Reactor

Fusion energy has the potential to contribute to solving the world's energy problems and be asolution to combat global warming. We are participating in the International Thermonuclear
Experimental Reactor "ITER" project, which is being promoted through international cooperation
among seven parties (Japan, Europe, the United States, Russia, China, South Korea, and India) by
manufacturing major equipment which was developed from our advanced detailed design andmanufacturing technologies. The ITER project is targeting achievement of the first plasma in 2025.
In January 2020, we completed the first unit of the toroidal field coil, the world's largest
superconducting coil, ahead of the rest of the world. (Figure 7) Other than the toroidal field coil,
we have started fabrication of a divertor, which is an essential device for plasma confinement anddischarge/removal of impurities.
We are also participating in the "Broader Approach" which is an international activity
focused on the early realization of a prototype fusion reactor. This program is being carried out inparallel with the ITER project. The development of a prototype fusion reactor in Japan isproceeding according to the national roadmap. We are also working on the concept consideration ofa plasma confinement vacuum vessel for the prototype reactor and the fabrication of components inequipment for a high-frequency heating system for the experimental fusion superconducting
tokamak device "JT-60SA". These efforts are all based upon the application of design andmanufacturing technologies we have cultivated over the years to support the fusion industry.
Going forward, we will continue to contribute to the realization of fusion energy by actively
supporting the ITER project, which is leading an important effort in the development oftechnologies for global sustainability, and working on the prototype fusion reactor development
program.

Figure 7

Unveiling ceremony of first unit of toroidal field coil for ITER

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8|6. Conclusion
We are a comprehensive nuclear energy manufacturer involved in all areas of nuclear energy
from light water reactors to fuel cycles, and decommissioning. Our company considers it a
privilege to have contributed to the safe and stable operation of nuclear power plants for more than
50 years based on proven records and continuous innovation and remain committed to continue thisachievement expectation for future generations.
Nuclear energy is a carbon-free, large-scale, and stable power source, and it will continue tobe a critical power source for the future from the perspective of energy security in Japan, where
natural resources are scarce. Nuclear power is the trump card for both future decarbonization andstable energy supply, and we will continue to contribute to the achievement of carbon neutrality by
2050 through application of our nuclear technologies.