DEMO (Demonstration Power Plant)

DEMO (DEMOnstration Power Plant) is the next-generation fusion device designed to demonstrate the technical and economic feasibility of fusion power generation. While ITER aims for scientific demonstration of fusion energy, DEMO will actually deliver electricity to the power grid, serving as a bridge to commercial fusion power plants.

Fusion research has a history of over 60 years, but no fusion device has ever produced electricity. DEMO carries the historic mission of demonstrating to the world that fusion energy can be a practical energy source.

Position Between ITER and Commercial Reactors

Hierarchical Structure of Fusion Development

The development toward fusion power generation progresses in stages from scientific demonstration to economic realization. Each stage builds upon the achievements of the previous one and aims to solve new technical challenges.

Stage	Device	Primary Objective	Q Value	Target Timeline
Experimental Reactor	ITER	Achieve Q=10, Demonstrate Burning Plasma	10	2035~
Prototype Reactor	DEMO	Power Generation Demonstration, Tritium Self-Sufficiency	25-40	2040s-2050s
Demonstration Reactor	-	Economic and Reliability Demonstration	40+	2050s~
Commercial Reactor	-	Practical Power Generation	40+	2060s~

In the fusion field, the distinction between prototype reactor and demonstration reactor differs from fission power generation. From the perspective of fusion output, the difference between prototype and demonstration reactors is small, and since the prototype reactor often fulfills the functions of the demonstration reactor as well, they are collectively referred to as DEMO.

Technical Leap from ITER

Several decisive technical leaps are required between ITER and DEMO.

$$\text{Technology Maturity Index} = f(\text{Q value}, \text{Availability}, \text{TBR}, \text{Net Electric Output})$$

Comparing the required specifications of ITER and DEMO reveals the magnitude of this leap.

Parameter	ITER	DEMO	Leap Factor
Fusion Power $P_\mathrm{fus}$	500 MW	2000-3000 MW	4-6x
Q Value	10	25-40	2.5-4x
Pulse Length	400-600 seconds	Steady-state or 2+ hours	10x+
Availability	Few % (experimental device)	30-50%	10x+
TBR (Tritium Breeding Ratio)	N/A	1.05+	New requirement
Net Electric Output	None	300-500 MW	New requirement

The Q value (energy multiplication factor) is defined as the ratio of fusion power to heating input:

$$Q = \frac{P_\mathrm{fus}}{P_\mathrm{heat}}$$

ITER’s target of Q=10 is sufficient for scientific demonstration of fusion energy, but for the power generation system to be viable, the net electric output, accounting for auxiliary power and thermal losses, must be positive.

Requirements for Power Generation System Viability

For DEMO to be viable as a power generation system, the net electric output $P_\mathrm{net}$ must be positive:

$$P_\mathrm{net} = \eta_\mathrm{th} \cdot P_\mathrm{th} - P_\mathrm{aux} - P_\mathrm{recirc}$$

Where:

• $\eta_\mathrm{th}$: Thermal conversion efficiency (typically 30-40%)
• $P_\mathrm{th}$: Thermal output (fusion power plus neutron multiplication and heating in structural materials)
• $P_\mathrm{aux}$: Auxiliary power (cooling systems, control systems, etc.)
• $P_\mathrm{recirc}$: Recirculating power (plasma heating, current drive, etc.)

The relationship between fusion power $P_\mathrm{fus}$ and thermal output $P_\mathrm{th}$ is:

$$P_\mathrm{th} = P_\mathrm{fus} \times M$$

Here, $M$ is the energy multiplication factor, typically around 1.1-1.3 due to neutron multiplication reactions in the blanket.

The recirculating power fraction $f_\mathrm{recirc}$ is an important indicator of power generation efficiency:

$$f_\mathrm{recirc} = \frac{P_\mathrm{recirc}}{P_\mathrm{gross}}$$

For commercial reactor viability, $f_\mathrm{recirc} < 0.2$ is desirable, which is the primary reason for requiring high-Q operation.

Bridge to Commercial Reactors

As a “prototype” of commercial reactors, DEMO must demonstrate the following functions:

1. Achievement of net electric output (hundreds of MW class)
2. Tritium fuel self-sufficiency
3. High availability operation (30% or more)
4. Component replacement through remote maintenance
5. Demonstration of licensable safety

These functional demonstrations will establish the technological foundation for commercial reactor design.

DEMO Plans by Region

Multiple countries and regions, centered on ITER participants, are advancing their own DEMO plans. Each plan has distinctive characteristics in terms of technical approach and development schedule.

EU-DEMO (Europe)

Development Organization and History

The European DEMO plan led by EUROfusion is the world’s most systematically advanced prototype reactor development program. Conceptual studies progressed under Horizon 2020 starting in 2014, and the formal Pre-Conceptual Design Phase began in 2021. Conceptual design is scheduled for completion by 2027, with operation start targeted for 2051.

EUROfusion is a consortium of fusion research institutions from over 30 European countries, advancing DEMO development with an annual budget of approximately 600 million euros.

Design Parameters

EU-DEMO’s design parameters are set from the perspective of balancing power generation demonstration and technology transfer to commercial reactors.

Parameter	Value	Notes
Major Radius $R_0$	9.0 m	1.45x ITER (6.2m)
Minor Radius $a$	2.9 m
Aspect Ratio $A = R_0/a$	3.1
Plasma Current $I_p$	18-20 MA
Toroidal Field $B_T$	5.7-5.9 T (on-axis)
Fusion Power $P_\mathrm{fus}$	2000 MW
Q Value	40-50
Net Electric Output $P_\mathrm{net}$	300-500 MW
Thermal Conversion Efficiency	33-36%
TBR	> 1.05
Pulse Length	2+ hours	Steady-state operation also in view

Plasma performance is characterized by normalized beta $\beta_N$ and Greenwald density fraction $n/n_G$:

$$\beta_N = \frac{\beta}{I_p/(aB_T)} \times 100 \approx 2.5-3.0$$ $$\frac{n}{n_G} \approx 0.8-1.0$$

The Greenwald density limit is:

$$n_G = \frac{I_p}{\pi a^2} \times 10^{20} \, \mathrm{m}^{-3}$$

Blanket Concepts

Two blanket concepts are being studied in parallel for EU-DEMO.

Helium Cooled Pebble Bed (HCPB):

• Breeder: Lithium ceramic (Li₄SiO₄ or Li₂TiO₃) pebbles
• Multiplier: Beryllium pebbles
• Coolant: High-pressure helium (8 MPa, inlet 300°C / outlet 500°C)
• Structural material: EUROFER97 (reduced activation ferritic steel)

Water Cooled Lithium Lead (WCLL):

• Breeder/Multiplier: Liquid lithium-lead (Pb-15.7Li)
• Coolant: Pressurized water (15.5 MPa, inlet 295°C / outlet 328°C)
• Structural material: EUROFER97

TBR evaluation for both concepts is performed by Monte Carlo neutron transport calculations:

$$\mathrm{TBR} = \int \sigma_{\mathrm{^6Li}(n,\alpha)t} \cdot \phi_n \cdot N_{\mathrm{^6Li}} \, dV + \int \sigma_{\mathrm{^7Li}(n,n'\alpha)t} \cdot \phi_n \cdot N_{\mathrm{^7Li}} \, dV$$

Here, $\sigma$ is the reaction cross-section, $\phi_n$ is the neutron flux, and $N$ is the nuclide density.

Divertor Design

EU-DEMO’s divertor heat load will be more severe than ITER:

$$q_\mathrm{div} = \frac{P_\mathrm{SOL}}{A_\mathrm{wet}} \approx 10-20 \, \mathrm{MW/m^2}$$

Here, $P_\mathrm{SOL}$ is the heat flux to the scrape-off layer, and $A_\mathrm{wet}$ is the wetted area of the divertor.

The scrape-off layer width $\lambda_q$ is estimated by Eich scaling:

$$\lambda_q \propto B_\mathrm{pol}^{-1.19}$$

For EU-DEMO, the following divertor concepts are being studied:

• Water-cooled tungsten single-null (extension of ITER type)
• Liquid metal divertor (tin or lithium)
• Advanced divertor configurations (Super-X, Snowflake)

JA-DEMO (Japan)

Development Organization and History

Japan’s prototype reactor plan is centered on the National Institutes for Quantum Science and Technology (QST) and the National Institute for Fusion Science (NIFS), utilizing the results of BA (Broader Approach) activities conducted in parallel with ITER.

In 2019, the basic concept was clarified by the Ministry of Education, Culture, Sports, Science and Technology’s Prototype Reactor Development Comprehensive Strategy Task Force. In April 2023, the Cabinet Office formulated the “Fusion Energy Innovation Strategy.” Under this strategy, efforts toward power generation demonstration in the 2030s are accelerating.

Design Parameters

JA-DEMO is based on a design philosophy emphasizing steady-state operation.

Parameter	Value	Notes
Major Radius $R_0$	8.5 m
Minor Radius $a$	2.4 m
Aspect Ratio $A$	3.5	Higher than EU-DEMO
Plasma Current $I_p$	12.3 MA	Suppressed for steady-state operation
Toroidal Field $B_T$	6.0 T (on-axis)
Fusion Power $P_\mathrm{fus}$	1500 MW
Q Value	25-40
Gross Electric Output	640 MW
Net Electric Output	300 MW
TBR	> 1.05
Operating Mode	Steady-state or 2-hour pulses

A distinctive feature of JA-DEMO is the pursuit of high bootstrap current fraction $f_\mathrm{BS}$:

$$f_\mathrm{BS} = \frac{I_\mathrm{BS}}{I_p} \approx 0.7-0.8$$

The bootstrap current is a current spontaneously driven by pressure gradients:

$$I_\mathrm{BS} \propto -\frac{1}{B_\theta} \frac{\partial p}{\partial r}$$

High $f_\mathrm{BS}$ reduces the burden of external current drive and lowers recirculating power.

Coordination with BA Activities

JA-DEMO development is closely coordinated with the following BA activity facilities:

JT-60SA (Superconducting Tokamak):

• Demonstration of steady-state high-beta plasma
• Development of advanced operating scenarios
• Establishment of disruption mitigation techniques

IFMIF-DONES (Fusion Neutron Source):

• Irradiation testing of DEMO materials
• Damage evaluation by fast neutrons (14 MeV)
• Accelerated irradiation of 20-50 dpa per year

IFERC (International Fusion Energy Research Centre):

• Coordination of DEMO design activities
• Simulations using supercomputers
• Development of remote experimentation technology

Schedule

JA-DEMO milestones:

• Around 2025: Transition to engineering design
• 2027: JT-60SA deuterium experiments begin
• Early 2030s: Construction start decision
• 2035: ITER burning experiments begin (data acquisition)
• Mid-2040s: Operation start

An advancement of about 5 years from the original target of around 2050 is being considered.

CFETR (China)

Development Organization and History

The China Fusion Engineering Test Reactor (CFETR) is China’s own plan aiming to bridge ITER and commercial reactors. Led by the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP), conceptual design was completed in 2017 and engineering design in 2020.

China has the world’s largest number of fusion researchers (approximately 3,000) and is channeling technology accumulated from predecessor devices such as EAST and HL-2M into CFETR.

Design Parameters and Phased Operation

CFETR is characterized by a two-phase operation plan.

Parameter	Phase I	Phase II
Major Radius $R_0$	7.2 m	7.2 m
Minor Radius $a$	2.2 m	2.2 m
Aspect Ratio $A$	3.3	3.3
Plasma Current $I_p$	10 MA	14 MA
Toroidal Field $B_T$	6.5 T	6.5 T
Fusion Power $P_\mathrm{fus}$	50-200 MW	1000+ MW
Q Value	1-5	10+
TBR	> 1.0	> 1.2

Phase I focuses on engineering testing and technology demonstration, with full-scale power generation demonstration in Phase II.

Technology Development Facilities

Prior to CFETR, technology development is progressing at the following facilities:

CRAFT (Comprehensive Research Facilities for Fusion Technology):

• Superconducting magnet fabrication and testing
• Divertor heat load testing
• Blanket technology development
• Location: Hefei

BEST (Burning plasma Experimental Superconducting Tokamak):

• Spherical tokamak configuration
• Expected completion in 2027
• Research on high-beta plasma
• Open to international collaboration

Schedule

CFETR milestones:

• 2017: Conceptual design completed
• 2020: Engineering design completed
• Late 2020s to around 2030: Construction start
• Mid-2030s: Phase I operation start
• 2040s: Phase II operation start

K-DEMO (South Korea)

Development Organization and History

South Korea is planning K-DEMO based on achievements from KSTAR (Korea Superconducting Tokamak Advanced Research). In 2024, the “Accelerated Fusion Energy Realization Strategy” was announced, with clarification of the development schedule and significant budget increases.

This strategy also added the construction of CPD (Compact Pilot Device) as an intermediate step prior to K-DEMO.

Design Parameters

Parameter	K-DEMO	Notes
Major Radius $R_0$	6.8 m
Minor Radius $a$	2.1 m
Aspect Ratio $A$	3.2
Plasma Current $I_p$	12 MA
Toroidal Field $B_T$	7.4 T	HTS adoption
Fusion Power $P_\mathrm{fus}$	2200 MW
Net Electric Output	500 MW
TBR	> 1.05

A distinctive feature of K-DEMO is the design based on the adoption of High-Temperature Superconducting (HTS) coils. HTS enables higher magnetic fields, and device compactness and improved economics are expected.

CPD (Compact Pilot Device)

CPD is planned as an intermediate step prior to K-DEMO:

Parameter	CPD
Major Radius $R_0$	4.5-5.0 m
Fusion Power $P_\mathrm{fus}$	200-400 MW
Objective	Steady-state operation technology demonstration
Operation Start	Early 2040s

Schedule

K-DEMO milestones:

• 2030s: CPD construction and operation
• 2040s: K-DEMO construction start
• After 2050: K-DEMO operation start

International Comparison of Design Parameters

A unified comparison of DEMO plans from each region.

Parameter	EU-DEMO	JA-DEMO	CFETR-II	K-DEMO
Major Radius $R_0$ [m]	9.0	8.5	7.2	6.8
Minor Radius $a$ [m]	2.9	2.4	2.2	2.1
Aspect Ratio $A$	3.1	3.5	3.3	3.2
Plasma Current $I_p$ [MA]	18-20	12.3	14	12
Toroidal Field $B_T$ [T]	5.7-5.9	6.0	6.5	7.4
Fusion Power $P_\mathrm{fus}$ [MW]	2000	1500	1000	2200
Q Value	40-50	25-40	10	30-40
Net Electric Output [MW]	300-500	300	-	500
Operation Start Target	2051	2045	2035-40	After 2050

For comparing plasma performance, the fusion triple product is useful:

$$n_i T_i \tau_E \geq 3 \times 10^{21} \, \mathrm{keV \cdot s \cdot m^{-3}}$$

Triple product margin in each DEMO design:

$$M_\mathrm{ignition} = \frac{(n_i T_i \tau_E)_\mathrm{design}}{(n_i T_i \tau_E)_\mathrm{Lawson}}$$

Key Technical Challenges

Realizing DEMO requires solving technical challenges that cannot be sufficiently verified in ITER. These challenges are interrelated, and integrated solutions are required.

Tritium Self-Sufficiency

Need for Tritium Fuel Cycle

The D-T fusion reaction consumes tritium (T) as fuel:

$$\mathrm{D} + \mathrm{T} \rightarrow \alpha (3.5 \, \mathrm{MeV}) + n (14.1 \, \mathrm{MeV})$$

The tritium consumption rate is proportional to fusion power:

$$\dot{m}_T = \frac{P_\mathrm{fus}}{17.6 \, \mathrm{MeV}} \times m_T \approx 55.8 \, \mathrm{kg/GW \cdot year}$$

A fusion power of 1 GW consumes approximately 56 kg of tritium per year.

However, tritium is a radioactive isotope with a half-life of 12.3 years and exists only in trace amounts in nature. Current world supply is about 25 kg (mainly recovered from Canadian CANDU reactors), and tritium production (breeding) within the reactor is essential to sustain fusion power generation beyond DEMO.

Tritium Breeding Ratio (TBR)

Tritium production in the blanket is achieved through nuclear reactions between lithium and neutrons:

$$\mathrm{^6Li} + n \rightarrow \mathrm{T} + \alpha + 4.78 \, \mathrm{MeV}$$ $$\mathrm{^7Li} + n \rightarrow \mathrm{T} + \alpha + n' - 2.47 \, \mathrm{MeV}$$

The ⁶Li reaction is highly efficient with thermal neutrons, while the ⁷Li reaction requires fast neutrons and is endothermic.

The Tritium Breeding Ratio (TBR) is:

$$\mathrm{TBR} = \frac{\text{Tritium produced}}{\text{Tritium consumed}}$$

DEMO requires TBR > 1.05. This margin compensates for the following losses:

• Tritium decay losses (half-life 12.3 years)
• Losses during fuel cycle (recovery efficiency < 100%)
• Accumulation as startup inventory
• Supply to new reactors

Design Requirements for TBR Achievement

The following design innovations are needed to increase TBR:

Use of neutron multiplier materials:

$$\mathrm{Be} + n \rightarrow 2n + \mathrm{Be^*}$$ $$\mathrm{Pb} + n \rightarrow 2n + \mathrm{Pb^*}$$

By using beryllium or lead as multiplier materials, multiple neutrons can be generated from a single 14.1 MeV neutron, increasing tritium production reactions.

⁶Li enrichment: Natural lithium has a composition of 7.5% ⁶Li and 92.5% ⁷Li, but TBR can be improved by enriching ⁶Li (30-90%).

Blanket coverage: Minimizing neutron leakage through ports and gaps is also important. If the coverage fraction is $f_\mathrm{cov}$:

$$\mathrm{TBR_{actual}} \approx \mathrm{TBR_{ideal}} \times f_\mathrm{cov}$$

Typically $f_\mathrm{cov} \approx 0.85-0.90$, resulting in a 10-15% reduction from ideal TBR.

Tritium Fuel Cycle

DEMO’s tritium fuel cycle consists of the following subsystems:

1. Fuel supply system: Fuel injection by pellet injection or gas puff
2. Exhaust processing system: Exhaust gas processing from divertor
3. Isotope separation system: Separation and purification of H/D/T
4. Tritium recovery system: Tritium extraction from blanket
5. Storage and accounting system: Tritium inventory management
6. Confinement and safety system: Confinement by multiple barriers

Burn fraction $\eta_b$ (burn rate relative to injected fuel) is typically low at 1-5%:

$$\eta_b = \frac{\text{T burned}}{\text{T injected}} \approx 0.01-0.05$$

Unburned tritium is recovered and reused, but this low burn fraction increases tritium inventory.

Achieving High Burn Fraction

Q Value and Recirculating Power

For a fusion reactor to be viable as a power generation system, a sufficiently high Q value is necessary.

The main components of recirculating power $P_\mathrm{recirc}$ are:

$$P_\mathrm{recirc} = P_\mathrm{heat} + P_\mathrm{CD} + P_\mathrm{cryo} + P_\mathrm{pump}$$

Where:

• $P_\mathrm{heat}$: Plasma heating power
• $P_\mathrm{CD}$: Current drive power
• $P_\mathrm{cryo}$: Cryogenic refrigeration power
• $P_\mathrm{pump}$: Vacuum and cooling pump power

Net electric output is:

$$P_\mathrm{net} = \eta_\mathrm{th} M P_\mathrm{fus} - P_\mathrm{recirc}$$

The relationship between Q value and net electric output is:

$$P_\mathrm{net} = P_\mathrm{fus} \left( \eta_\mathrm{th} M - \frac{\eta_\mathrm{CD}^{-1}(1-f_\mathrm{BS})}{Q} - \frac{P_\mathrm{other}}{P_\mathrm{fus}} \right)$$

Here, $\eta_\mathrm{CD}$ is current drive efficiency, $f_\mathrm{BS}$ is bootstrap current fraction, and $P_\mathrm{other}$ is auxiliary power other than heating.

The minimum required Q value is derived from the condition $P_\mathrm{net} > 0$. For typical DEMO parameters, Q > 20-30 is required.

Conditions for High-Performance Plasma

Achieving high-Q operation requires the following plasma conditions:

Fusion power density:

$$p_\mathrm{fus} = E_\mathrm{fus} \cdot n_D n_T \langle \sigma v \rangle_{DT}$$

Here, $\langle \sigma v \rangle_{DT}$ is the D-T reaction rate, which is a function of temperature $T$:

$$\langle \sigma v \rangle_{DT} \approx 1.1 \times 10^{-24} T^2 \, \mathrm{m^3/s} \quad (10 \lesssim T \lesssim 20 \, \mathrm{keV})$$

Fusion power scales as:

$$P_\mathrm{fus} \propto \beta^2 B^4 R^3 / A^4$$

Here, $\beta$ is the beta value (ratio of plasma pressure to magnetic pressure):

$$\beta = \frac{2\mu_0 \langle p \rangle}{B^2}$$

High beta and high magnetic field are advantageous for high Q, but are constrained by MHD stability limits and coil technology.

Beta Limit and Stability

Toroidal beta $\beta_t$ is constrained by the Troyon limit:

$$\beta_t [\%] \leq \beta_N \frac{I_p [\mathrm{MA}]}{a [\mathrm{m}] B_T [\mathrm{T}]}$$

The limit value of normalized beta $\beta_N$ is approximately 2.8 in ideal MHD, but 3.5 or higher is possible with resistive wall mode (RWM) stabilization.

DEMO requires stable operation at $\beta_N \approx 2.5-3.0$.

Long-Duration and Steady-State Operation

Pulsed and Steady-State Operation

In the tokamak configuration, pulsed operation is fundamental because plasma current is induced by transformer principles.

The magnetic flux consumption rate is:

$$\frac{d\Psi}{dt} = V_\mathrm{loop} = R_p I_p + L_p \frac{dI_p}{dt}$$

In steady state ($dI_p/dt = 0$):

$$V_\mathrm{loop} = R_p I_p = \frac{\eta_\parallel J_p}{n_e T_e^{3/2}} I_p$$

To maintain steady-state operation, plasma current must be sustained by non-inductive means (non-inductive current drive).

Bootstrap Current

Bootstrap current is a current spontaneously driven by pressure gradients:

$$j_\mathrm{BS} = -\frac{p}{\langle B_\theta \rangle} \frac{dp}{dr} L_{31}$$

Here, $L_{31}$ is the neoclassical transport coefficient.

The bootstrap current fraction is:

$$f_\mathrm{BS} = \frac{I_\mathrm{BS}}{I_p} \propto \sqrt{\epsilon} \beta_p$$

Here, $\epsilon = a/R_0$ is the inverse aspect ratio and $\beta_p$ is the poloidal beta.

Achieving high $f_\mathrm{BS}$ (0.7-0.8) can significantly reduce the burden of external current drive.

External Current Drive

The remaining current ($1 - f_\mathrm{BS}$) is maintained by external current drive:

Neutral Beam Injection (NBI) current drive:

$$\eta_\mathrm{NBI} = \frac{n_{e,20} R I_\mathrm{CD}}{P_\mathrm{NBI}} \approx 0.2-0.4 \times 10^{20} \, \mathrm{A/(W \cdot m^2)}$$

Radio-frequency current drive (ECCD/LHCD):

$$\eta_\mathrm{ECCD} \approx 0.2-0.3 \times 10^{20} \, \mathrm{A/(W \cdot m^2)}$$ $$\eta_\mathrm{LHCD} \approx 0.3-0.5 \times 10^{20} \, \mathrm{A/(W \cdot m^2)}$$

Current drive efficiency depends on density and temperature:

$$\eta_\mathrm{CD} \propto \frac{T_e}{n_e}$$

Efficiency improves at high temperature and low density plasma, but there is a trade-off with fusion power.

Thermal Stability

In steady-state operation, the balance between fusion power and losses is critical:

$$\frac{dW}{dt} = P_\alpha + P_\mathrm{heat} - P_\mathrm{loss}$$

Here, $W$ is plasma stored energy, $P_\alpha$ is alpha particle heating, and $P_\mathrm{loss}$ is transport loss.

In steady state ($dW/dt = 0$):

$$P_\alpha + P_\mathrm{heat} = P_\mathrm{loss} = \frac{W}{\tau_E}$$

In burning plasma, $P_\alpha \gg P_\mathrm{heat}$, and control of alpha particle heating is key to thermal stability.

The thermal instability growth rate is:

$$\gamma_\mathrm{thermal} = \frac{1}{\tau_E} \left( \frac{\partial P_\alpha}{\partial T} - \frac{\partial P_\mathrm{loss}}{\partial T} \right) \frac{T}{P}$$

Under the condition $\partial P_\alpha / \partial T > \partial P_\mathrm{loss} / \partial T$, thermal runaway can occur, requiring stabilization through heating control or profile control.

Remote Maintenance and High Availability

Definition and Requirements of Availability

Availability is an important parameter that determines the economics of a fusion reactor:

$$f_\mathrm{avail} = \frac{t_\mathrm{op}}{t_\mathrm{op} + t_\mathrm{maint}}$$

Here, $t_\mathrm{op}$ is operation time and $t_\mathrm{maint}$ is maintenance time.

Commercial reactors require $f_\mathrm{avail} \geq 0.75$ (9+ months of operation per year). DEMO targets $f_\mathrm{avail} \approx 0.3-0.5$.

Need for Remote Maintenance

After D-T operation, the vacuum vessel interior is subject to strong activation:

$$\dot{D} \approx 10^4-10^6 \, \mathrm{Sv/h}$$

At this dose rate, maintenance by humans is impossible, and all in-vessel components must be replaced and maintained by remote operation.

Blanket replacement time estimate:

• Number of segments: 100-200
• Replacement time per segment: 10-20 hours
• Total replacement time: Several months to 1 year

Reducing this maintenance time is key to achieving high availability.

Maintenance Scenarios

The following maintenance scenarios are being studied for DEMO:

Horizontal port maintenance (EU-DEMO):

• Extract blanket segments through large equatorial ports
• Number of ports: Approximately 16
• Segments are refurbished at factory before reinstallation

Vertical maintenance (JA-DEMO):

• Lift entire blanket from upper ports
• Uses large cranes and hot cells
• High work efficiency due to large replacement units, but requires large-scale facilities

Sector maintenance:

• Replace vacuum vessel sector by sector
• Enables shortest shutdown time
• Cost of spare sector manufacturing is a challenge

Reliability Design

Improving equipment reliability is also important for achieving high availability:

Relationship between Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR):

$$f_\mathrm{avail} = \frac{\mathrm{MTBF}}{\mathrm{MTBF} + \mathrm{MTTR}}$$

For systems consisting of multiple independent components:

$$f_\mathrm{avail,system} = \prod_i f_\mathrm{avail,i}$$

Since fusion reactors consist of thousands of components, high reliability of each element is essential.

Divertor and Heat Exhaust

Heat Load Challenge

The divertor is the component subject to the most severe heat load in a fusion reactor:

$$q_\mathrm{div} = \frac{P_\mathrm{SOL}}{2\pi R_\mathrm{div} \cdot \lambda_q \cdot f_\mathrm{exp}}$$

Where:

• $P_\mathrm{SOL}$: Heat flux to scrape-off layer (approximately 20% of fusion power)
• $R_\mathrm{div}$: Divertor radius
• $\lambda_q$: SOL width (a few mm)
• $f_\mathrm{exp}$: Magnetic flux expansion factor

At DEMO scale, $q_\mathrm{div}$ can easily reach tens to 100 MW/m², which no material can withstand.

Heat Load Mitigation Strategies

Detachment operation: Impurities (N, Ne, Ar, etc.) are injected into the divertor region to increase radiation losses:

$$P_\mathrm{rad} = n_e n_Z L_Z(T_e)$$

Here, $L_Z(T_e)$ is the radiative loss coefficient.

Radiation losses disperse heat flux and reduce direct heat load to the divertor:

$$f_\mathrm{rad} = \frac{P_\mathrm{rad}}{P_\mathrm{SOL}} \gtrsim 0.8-0.9$$

Advanced divertor configurations:

• Super-X divertor: Extends strike point outward to increase $f_\mathrm{exp}$
• Snowflake divertor: Forms additional null points near the X-point
• X-divertor: Maximizes field line expansion

Liquid metal divertor:

• Uses liquid surface of lithium or tin
• Cooling by latent heat of vaporization
• Self-healing capability
• Reduced hydrogen recycling

Materials Challenges

Neutron Irradiation Damage

In DEMO, materials are exposed to neutron fluence far exceeding ITER:

Displacement damage (dpa: displacement per atom):

$$\mathrm{dpa} = \int \phi(E) \sigma_d(E) dE \cdot t$$

Here, $\phi(E)$ is neutron flux, $\sigma_d(E)$ is displacement cross-section, and $t$ is irradiation time.

Device	Target dpa	Notes
ITER	~3 dpa	Experimental device
DEMO	50-100 dpa	Over lifetime
Commercial	150+ dpa	High availability operation

Helium production:

$$(\mathrm{He/dpa}) \approx 10-15 \, \mathrm{appm/dpa}$$

Fusion neutrons (14.1 MeV) have higher energy than fission neutrons, resulting in higher helium production rates through (n,α) reactions. Helium accumulates in materials and causes embrittlement.

Candidate Structural Materials

Reduced Activation Ferritic-Martensitic Steel (RAFM):

• Representative examples: F82H (Japan), EUROFER97 (EU)
• Composition: 8-9% Cr, 1-2% W
• Operating temperature: 300-550°C
• Challenges: High-temperature strength, irradiation embrittlement

ODS Steel (Oxide Dispersion Strengthened):

• Dispersed oxide particles (Y₂O₃, etc.) improve high-temperature strength
• Operating temperature: Extended to ~650°C
• Challenges: Manufacturing cost, joining technology

SiC/SiC Composites:

• Operating temperature: ~1000°C
• Excellent reduced activation characteristics
• Challenges: Manufacturing technology, joining, gas-tightness

Tungsten Alloys:

• Plasma-facing material for divertors
• High melting point (3422°C), high thermal conductivity
• Challenges: Brittleness, recrystallization, activation

Materials Irradiation Testing

Qualification of DEMO materials requires irradiation testing with fusion neutrons:

IFMIF-DONES (International Fusion Materials Irradiation Facility - DEMO Oriented Neutron Source):

• Generates 14 MeV-class neutrons by irradiating liquid lithium with 40 MeV deuterons
• Accelerated irradiation of 20-50 dpa per year possible
• Location: Granada, Spain
• Operation start: 2030s

Significance of Power Generation Demonstration

Establishing Credibility of Fusion Energy

Power generation demonstration by DEMO is a decisive milestone in demonstrating the credibility of fusion energy to society.

In experiments up to ITER, energy extraction from fusion reactions will be demonstrated, but it will not be converted to electricity or delivered to the power grid. DEMO will be the first to demonstrate that “electricity can be generated from fusion.”

This demonstration is expected to:

• Provide justification for continuing fusion development to policymakers
• Prove commercialization potential to investors
• Promote social acceptance of fusion energy

Engineering Integration

DEMO is the first fusion power plant integrating numerous subsystems:

$$P_\mathrm{net} = f(\text{Plasma}, \text{Blanket}, \text{Divertor}, \text{Magnets}, \text{Remote Maintenance}, \text{Tritium}, \text{Power Generation})$$

These subsystems are individually verified in ITER and other test facilities, but verification as an integrated system will first occur in DEMO.

Key integration challenges:

• Interface consistency between subsystems
• Overall optimization of safety systems
• Establishment of operating scenarios
• Integration of instrumentation and control systems

Foundation for Economic Evaluation

Operating experience from DEMO provides essential data for economic evaluation of commercial reactors:

Levelized Cost of Electricity (LCOE):

$$\mathrm{LCOE} = \frac{\mathrm{CAPEX} + \sum_{t} \mathrm{OPEX}_t / (1+r)^t}{\sum_{t} E_t / (1+r)^t}$$

Here, CAPEX is capital cost, OPEX is operating cost, $E_t$ is annual electricity production, and $r$ is discount rate.

Actual data from DEMO will reduce the following uncertainties:

• Actual construction costs
• Actual availability
• Actual maintenance costs
• Actual tritium fuel costs
• Improved accuracy of decommissioning cost estimates

Construction Schedule and Outlook

Timeline Comparison by Region

Milestone	EU-DEMO	JA-DEMO	CFETR	K-DEMO
Conceptual Design Complete	2027	2025	2017	Under study
Engineering Design Complete	2037	2032	2020	Under study
Construction Start	2040s	After 2035	Late 2020s	2040s
Operation Start	2051	2045	2035-40	After 2050
Power Generation Demonstration	2050s	After 2045	2040s	2050s

China’s CFETR shows the earliest schedule. This is due to the policy of advancing development independently without waiting for ITER results.

Coordination with ITER

DEMO development is based on ITER results, but the relationship between the two is complex:

Technology transfer from ITER:

• Burning plasma physics demonstration data
• Superconducting magnet technology
• Divertor technology
• Remote handling technology
• Test Blanket Module results

Development that should proceed independently of ITER:

• Full-scale blanket technology
• Steady-state operation scenarios
• High availability achievement technology
• Material qualification

Each region is taking the stance of “proceeding with development without waiting for ITER, while assuming ITER’s success as a prerequisite.”

Risks and Challenges

Major risks for DEMO realization:

Technical risks:

• Achieving TBR > 1 in blanket
• Maintaining high-beta steady-state plasma
• Long-term material durability
• Remote maintenance system reliability

Schedule risks:

• Design data shortage due to ITER delays
• Material qualification delays (dependent on IFMIF-DONES progress)
• Budget uncertainty
• Prolonged licensing process

Cost risks:

• Cost escalation as first-of-a-kind
• Procurement risks for superconducting materials
• Currency and price fluctuations

Acceleration Since 2023

Since 2023, fusion strategies have been formulated in succession in various countries, with acceleration of development through public-private partnerships.

Major developments:

• Japan: Fusion Energy Innovation Strategy (April 2023)
• USA: Bold Decadal Vision for Commercial Fusion Energy (2022)
• UK: Towards Fusion Energy Strategy (2021)
• South Korea: Accelerated Fusion Energy Realization Strategy (2024)
• EU: EUROfusion Roadmap Update (2023)

These strategies share common pillars of collaboration with private companies, regulatory framework development, and human resource development.

Path to Commercial Reactors

From DEMO to Commercial Reactors

DEMO’s success opens the path to commercial reactor realization, but it cannot be commercialized as-is.

Differences between DEMO and commercial reactors:

Item	DEMO	Commercial
Objective	Technology demonstration	Economic power generation
Availability	30-50%	75%+
Lifetime	20-30 years	40-60 years
Economics	Secondary	Top priority
Scale	Medium	Optimal scale
Number built	1 per region	Many

Commercial Reactor Requirements

Economic viability conditions for commercial fusion reactors are estimated as:

LCOE target:

$$\mathrm{LCOE_{target}} \lesssim 50-100 \, \mathrm{USD/MWh}$$

This is a level competitive with current nuclear power and renewable energy (+ storage).

Capital cost guideline:

$$\mathrm{Overnight \, Cost} \lesssim 5000-8000 \, \mathrm{USD/kW_e}$$

Achieving this requires significant cost reduction from DEMO:

• Design standardization
• Manufacturing technology maturation
• Supply chain establishment
• Economies of scale

Technology Development Paths

Several paths are conceivable for technology development from DEMO to commercial reactors:

Evolutionary development path:

• Gradual improvement of DEMO design
• Economics improvement through scale-up
• Reliable but time-consuming

Revolutionary development path:

• High magnetic field through high-temperature superconductors
• Transition to compact tokamak
• Adoption of new materials (SiC/SiC, etc.)
• Transition to advanced fuels (D-D, D-³He)

Hybrid path:

• Parallel development of multiple technology options
• Selection and integration according to maturity

Rise of Private Fusion Ventures

In the 2020s, private fusion ventures have rapidly emerged. These companies are pursuing commercialization through approaches different from public programs:

Commonwealth Fusion Systems (USA):

• MIT spin-off
• Compact tokamak with high-temperature superconducting magnets
• SPARC (Q > 2 demonstration) → ARC (commercial reactor)
• Over $2 billion in funding raised

TAE Technologies (USA):

• FRC (Field-Reversed Configuration) approach
• p-¹¹B fuel in view
• Over $1.2 billion in funding raised

Tokamak Energy (UK):

• Spherical tokamak
• High-temperature superconducting magnets
• Achieved 100 million degrees in ST40

These ventures are pursuing commercialization through paths different from DEMO, forming multiple routes to fusion energy realization.

Future Vision of Fusion Power

If multiple DEMO-class devices are operating in the 2050s-2060s and power generation demonstration succeeds, fusion power generation may occupy a share of practical power sources in the latter half of the 21st century.

Anticipated scenarios:

• 2050s: Multiple DEMOs operating, power generation demonstration
• 2060s: First commercial reactor begins operation
• 2070s-2080s: Full-scale deployment of fusion power
• End of 21st century: Fusion provides 10-20% of global electricity

Advantages of fusion power:

• Fuel (deuterium, lithium) is virtually inexhaustible
• Does not emit CO₂
• Does not produce high-level radioactive waste
• Excellent safety (no runaway reactions)

Due to these advantages, fusion is expected to contribute to both climate change mitigation and sustainable energy supply.

Physical Background of Design Parameters

Scaling Laws and Design Optimization

DEMO design is optimized based on plasma physics scaling laws.

IPB98(y,2) scaling for energy confinement time:

$$\tau_E = 0.0562 \, H \, I_p^{0.93} \, B_T^{0.15} \, n_{19}^{0.41} \, P^{-0.69} \, R^{1.97} \, \kappa^{0.78} \, \epsilon^{0.58} \, M^{0.19}$$

Where:

• $H$: Confinement enhancement factor (in H-mode, $H \approx 1$)
• $I_p$: Plasma current [MA]
• $B_T$: Toroidal field [T]
• $n_{19}$: Electron density [$10^{19}$ m⁻³]
• $P$: Heating power [MW]
• $R$: Major radius [m]
• $\kappa$: Elongation
• $\epsilon = a/R$: Inverse aspect ratio
• $M$: Ion mass number

Fusion power scales as:

$$P_\mathrm{fus} \propto \beta^2 B_T^4 V$$

Here, $V \propto R^3$ is the plasma volume.

Determining Design Points

DEMO design points are optimized under the following constraints:

MHD stability:

$$\beta_N \leq \beta_{N,\mathrm{max}} \approx 2.5-3.0$$

Density limit:

$$n_e \leq n_G = \frac{I_p}{\pi a^2}$$

Heating and current drive power:

$$P_\mathrm{CD} = \frac{(1-f_\mathrm{BS}) I_p}{\eta_\mathrm{CD} \cdot n_e \cdot R}$$

Divertor heat load:

$$q_\mathrm{div} \leq q_\mathrm{div,max} \approx 10-15 \, \mathrm{MW/m^2}$$

Design points are searched to maximize net electric output $P_\mathrm{net}$ while satisfying these constraints.

System Codes

DEMO design uses system codes that integrate physics and engineering constraints:

Representative system codes:

• PROCESS (EU)
• DEMO-NET (Japan)
• SYCOMORE (EU)

These codes explore the design parameter space and identify feasible and optimal design points.

Objective functions such as power generation cost minimization or device size minimization are set for optimization:

$$\min_{\mathbf{x}} \, \mathrm{LCOE}(\mathbf{x}) \quad \text{subject to} \quad g_i(\mathbf{x}) \leq 0$$

Here, $\mathbf{x}$ is the design variable vector and $g_i$ are constraint conditions.

Summary

DEMO (Demonstration Power Plant) is the most important milestone toward the practical application of fusion energy. Following scientific demonstration of burning plasma in ITER, DEMO will actually generate and transmit electricity, demonstrating to the world that fusion power generation is technically and economically viable.

Multiple DEMO plans are being advanced in parallel, centered on the EU, Japan, China, and South Korea. Each plan has different approaches and timelines, but with multiple DEMOs expected to operate in the 2040s-2050s, the path to commercialization of fusion power is expected to become clear.

Realizing DEMO requires solving technical challenges that cannot be sufficiently verified in ITER, including tritium self-sufficiency, high burn fraction, steady-state operation, remote maintenance, and material durability. The global fusion research community is collaborating to address these challenges.

Since 2023, fusion strategies have been formulated in various countries, and private company participation has followed in succession. With both public programs and private ventures working in tandem, expectations for early realization of fusion energy are rising.

Related Topics

• Future Outlook: Overview - Vision of fusion energy
• Private Fusion Ventures - Development acceleration by startups
• ITER Project - International collaborative experimental reactor
• JT-60SA Project - Japan-EU advanced tokamak
• Tokamak Configuration - Mainstream magnetic confinement approach
• Stellarator/Helical Configuration - Configuration suited for steady-state operation
• Blanket and Tritium Breeding - Fuel self-sufficiency technology
• Plasma-Facing Materials - Materials that withstand harsh environments