Comparison of Nuclear Fusion and Fission

Nuclear fusion and fission are both technologies that extract energy through the transformation of atomic nuclei, possessing energy densities orders of magnitude greater than chemical reactions. However, their physical principles, technical characteristics, safety profiles, and environmental impacts are fundamentally different. This chapter comprehensively compares both technologies from scientific, engineering, and societal perspectives, clarifying the position of each technology in addressing humanity’s energy challenges.

Detailed Comparison of Physical Principles

Binding Energy and Mass Defect

The key to understanding nuclear stability lies in “binding energy” and “mass defect.” The sum of the masses of nucleons (protons and neutrons) that constitute an atomic nucleus is greater than the total mass of the nucleus itself. This difference is called the mass defect, which is converted to binding energy according to Einstein’s mass-energy equivalence formula.

$$E = \Delta m \cdot c^2$$

Here, $\Delta m$ is the mass defect and $c$ is the speed of light ($2.998 \times 10^8$ m/s).

Nuclear masses are expressed in atomic mass units (u), where 1 u = 931.5 MeV/c$^2$. For example, calculating the mass defect of a helium-4 nucleus:

Component	Mass (u)
2 protons	2 × 1.007276 = 2.014552
2 neutrons	2 × 1.008665 = 2.017330
Total	4.031882
Helium-4 nucleus	4.001506
Mass defect	0.030376

Converting this mass defect to binding energy:

$$E_B = 0.030376 \times 931.5 = 28.3 \text{ MeV}$$

The binding energy per nucleon is $28.3 / 4 = 7.07$ MeV, indicating the stability of helium-4.

Binding Energy Curve

The curve plotting binding energy per nucleon against mass number is one of the most important diagrams in nuclear physics.

Nucleus	Mass Number A	Binding Energy/Nucleon (MeV)
Deuterium ${}^2\text{H}$	2	1.11
Helium-3 ${}^3\text{He}$	3	2.57
Helium-4 ${}^4\text{He}$	4	7.07
Lithium-7 ${}^7\text{Li}$	7	5.61
Carbon-12 ${}^{12}\text{C}$	12	7.68
Iron-56 ${}^{56}\text{Fe}$	56	8.79
Nickel-62 ${}^{62}\text{Ni}$	62	8.79
Uranium-235 ${}^{235}\text{U}$	235	7.59
Uranium-238 ${}^{238}\text{U}$	238	7.57

From the characteristics of this curve, two energy release mechanisms are derived:

1. Nuclear fusion: Combining light nuclei to create heavier nuclei with greater binding energy
2. Nuclear fission: Splitting heavy nuclei to create medium-mass nuclei with greater binding energy

Since iron (Fe) and nickel (Ni) are the most stable, elements lighter than these can release energy through fusion, while heavier elements can do so through fission.

Bethe-Weizsacker Mass Formula

The binding energy of atomic nuclei can be described by the semi-empirical mass formula (Bethe-Weizsacker formula):

$$B(Z,A) = a_V A - a_S A^{2/3} - a_C \frac{Z(Z-1)}{A^{1/3}} - a_A \frac{(A-2Z)^2}{A} + \delta(A,Z)$$

Where:

• $a_V \approx 15.8$ MeV: Volume term (nuclear force binding)
• $a_S \approx 18.3$ MeV: Surface term (reduced binding energy of surface nucleons)
• $a_C \approx 0.71$ MeV: Coulomb term (proton repulsion)
• $a_A \approx 23.2$ MeV: Asymmetry term (proton-neutron number asymmetry)
• $\delta$: Pairing term (nucleon pair stabilization effect)

From this formula, we can understand that for heavy nuclei (large Z), the Coulomb term becomes large, making fission more likely to occur.

Details of Fusion Reactions

In fusion reactions, the Coulomb barrier between light nuclei must be overcome to bring nucleons into contact and bind them through the nuclear force. The height of the Coulomb barrier is:

$$V_C = \frac{Z_1 Z_2 e^2}{4\pi\epsilon_0 r_0 (A_1^{1/3} + A_2^{1/3})}$$

Here, $r_0 \approx 1.2$ fm. For the D-T reaction:

$$V_C \approx \frac{1 \times 1 \times 1.44 \text{ MeV fm}}{1.2 \times (2^{1/3} + 3^{1/3})} \approx 0.5 \text{ MeV}$$

However, due to quantum mechanical tunneling effects, reactions can occur without completely surmounting this barrier. The tunneling probability is described by the Gamow factor:

$$P \propto \exp\left(-2\pi\eta\right), \quad \eta = \frac{Z_1 Z_2 e^2}{\hbar v}$$

Here, $v$ is the relative velocity.

The main fusion reactions and their energy balances are as follows:

Reaction	Energy (MeV)	Cross-Section Peak	Temperature (keV)
D + T → ${}^4$He + n	17.59	5 barn @ 64 keV	10-20
D + D → ${}^3$He + n	3.27	0.11 barn @ 1 MeV	50-100
D + D → T + p	4.03	0.11 barn @ 1 MeV	50-100
D + ${}^3$He → ${}^4$He + p	18.35	0.9 barn @ 250 keV	50-100
p + ${}^{11}$B → 3${}^4$He	8.68	1.2 barn @ 675 keV	100-300
T + T → ${}^4$He + 2n	11.33	0.16 barn @ 100 keV	20-50

The D-T reaction is the most achievable because its Coulomb barrier is low and its cross-section is large.

Details of the D-T reaction:

$${}^2_1\text{H} + {}^3_1\text{H} \rightarrow {}^4_2\text{He} (3.52 \text{ MeV}) + {}^1_0n (14.06 \text{ MeV})$$

By conservation of momentum, the energy distribution of the products is determined by the inverse ratio of their masses:

$$\frac{E_\alpha}{E_n} = \frac{m_n}{m_\alpha} = \frac{1}{4}$$

Therefore, the neutron carries away about 80% of the total energy (14.06 MeV), while the alpha particle retains about 20% (3.52 MeV) and contributes to plasma self-heating.

Details of Fission Reactions

Fission is a reaction in which heavy nuclei (typically uranium or plutonium) absorb a neutron, become unstable, and split into two medium-mass nuclei (fission products) and several neutrons.

Fission of uranium-235 by thermal neutrons:

$${}^{235}_{92}\text{U} + {}^1_0n \rightarrow [{}^{236}_{92}\text{U}]^* \rightarrow X + Y + \nu \cdot n + \gamma + \beta$$

Here, $X$ and $Y$ are fission products, and $\nu$ is the number of emitted neutrons (average about 2.4).

The mass distribution of fission products is asymmetric, showing a “double-humped” pattern:

Mass Region	Typical Nuclides	Yield (%)
Light peak (A ~ 90-100)	${}^{95}$Zr, ${}^{99}$Mo, ${}^{103}$Ru	3-6 each
Heavy peak (A ~ 130-145)	${}^{133}$Cs, ${}^{137}$Cs, ${}^{140}$Ba	3-6 each
Symmetric fission (A ~ 117)	-	< 0.01

Breakdown of energy released per fission event:

Energy Form	Value (MeV)
Kinetic energy of fission products	167
Prompt neutrons	5
Prompt gamma rays	6
Delayed beta rays	8
Delayed gamma rays	7
Neutrinos (unrecoverable)	10
Total	203
Recoverable energy	~193

Comparison of Reaction Cross-Sections

The likelihood of nuclear reactions is expressed by the reaction cross-section $\sigma$.

The cross-section for fusion (D-T reaction) depends strongly on ion temperature:

$$\langle\sigma v\rangle = \int_0^\infty \sigma(E) v f(E) dE$$

Here, $f(E)$ is the Maxwell-Boltzmann distribution.

Approximate formula for the D-T reaction rate coefficient $\langle\sigma v\rangle$:

$$\langle\sigma v\rangle \approx 1.1 \times 10^{-24} T^2 \exp\left(-\frac{19.94}{T^{1/3}}\right) \text{ m}^3/\text{s}$$

($T$ is in keV)

On the other hand, for fission, the thermal neutron (about 0.025 eV) absorption cross-section is important:

Nuclide	Fission Cross-Section (barn)	Capture Cross-Section (barn)	α Value
${}^{233}$U	529	46	0.09
${}^{235}$U	584	99	0.17
${}^{239}$Pu	748	271	0.36
${}^{241}$Pu	1013	368	0.36

(α value = capture/fission ratio)

Fuel Resource Reserves and Sustainability

Fission Fuel Resources

The reserves and supply outlook for uranium, the main fuel for fission power generation:

Category	Identified Reserves (10,000 tonnes U)	Mining Cost
Reasonably Assured Resources (RAR)	~630	<$130/kg
Inferred Additional Resources (IAR)	~300	<$130/kg
Undiscovered Resources (estimated)	~1,060	<$260/kg
Total	~2,000	-

At current consumption rates (about 60,000 tonnes per year):

• Light water reactors only: About 100 years
• Including fast breeder reactors: Theoretically thousands of years

Major uranium-producing countries:

Country	Production (2023 est., tonnes)	Share
Kazakhstan	22,000	43%
Canada	7,500	15%
Namibia	5,600	11%
Australia	4,100	8%
Uzbekistan	3,500	7%
Others	8,300	16%

Uranium in seawater is also attracting attention as a potential resource:

• Concentration in seawater: About 3.3 ppb (μg/L)
• Total amount: About 4.5 billion tonnes
• Technical recovery cost: Currently $300-600/kg (not economical)

Fusion Fuel Resources

Fusion fuels are virtually inexhaustible:

Deuterium (D)

Parameter	Value
Concentration in seawater	0.0156 mol% (about 33 g/m$^3$)
Total oceanic amount	About 4.6 × 10$^{13}$ tonnes
Annual consumption (10 GWe plant)	About 250 kg
Sustainable years	Billions of years

Tritium (T)

Tritium barely exists in nature and must be produced in the reactor:

$${}^6_3\text{Li} + n \rightarrow {}^4_2\text{He} + {}^3_1\text{T} + 4.78 \text{ MeV}$$ $${}^7_3\text{Li} + n \rightarrow {}^4_2\text{He} + {}^3_1\text{T} + n' - 2.47 \text{ MeV}$$

Lithium resources:

Source	Reserves	Sustainable Years for Fusion Power
Land deposits (confirmed)	About 26 million tonnes	About 10,000 years
In seawater	About 230 billion tonnes	About 100 million years

Major lithium-producing countries (2023 est.):

Country	Production (tonnes Li)	Reserves (10,000 tonnes Li)
Australia	86,000	620
Chile	44,000	920
China	33,000	200
Argentina	12,000	270

Fuel Energy Density Comparison

Fuel	Energy Density (J/kg)	Oil Equivalent
D-T fusion	3.4 × 10$^{14}$	8 million tonnes of oil
Uranium fission	8.2 × 10$^{13}$	2 million tonnes of oil
Coal	2.4 × 10$^{7}$	0.6 tonnes of oil
Oil	4.2 × 10$^{7}$	1
Natural gas	5.5 × 10$^{7}$	1.3 tonnes of oil

Energy generated by 1 gram of fusion fuel:

$$E = \frac{17.6 \text{ MeV} \times 6.02 \times 10^{23}}{5 \text{ g}} = 3.4 \times 10^{14} \text{ J}$$

This is equivalent to the combustion energy of about 8 tonnes of oil.

Detailed Comparison of Radioactive Waste

Types and Characteristics of Fission Waste

Fission power generation produces diverse radioactive wastes:

High-Level Radioactive Waste (HLW)

Spent nuclear fuel or vitrified waste after reprocessing:

Component	Representative Nuclides	Half-Life	Generation (per GWe-year)
Fission products	${}^{137}$Cs	30 years	About 25 kg
	${}^{90}$Sr	29 years	About 25 kg
	${}^{99}$Tc	210,000 years	About 25 kg
	${}^{129}$I	15.7 million years	About 6 kg
Minor actinides	${}^{237}$Np	2.14 million years	About 0.7 kg
	${}^{241}$Am	432 years	About 0.4 kg
	${}^{243}$Am	7,370 years	About 0.1 kg
	${}^{244}$Cm	18 years	About 0.02 kg
Plutonium	${}^{239}$Pu	24,100 years	About 200 kg (if not reprocessed)

Time evolution of spent fuel radioactivity:

$$A(t) = A_0 \sum_i f_i \exp(-\lambda_i t)$$

Cooling Time	Radioactivity Level (relative)	Major Sources
1 day	100	Short-lived fission products
1 year	1	${}^{137}$Cs, ${}^{90}$Sr, etc.
100 years	0.01	${}^{137}$Cs, ${}^{90}$Sr, Am, Cm
1,000 years	0.001	Actinides
100,000 years	0.0001	Long-lived fission products

Low and Intermediate Level Radioactive Waste

Classification	Source	Annual Generation (per GWe)	Disposal Method
Low-level (LLW)	Work clothes, tools, resins	About 50 m$^3$	Near-surface disposal
Intermediate-level (ILW)	Filters, metal parts	About 20 m$^3$	Intermediate-depth disposal
High-level (HLW)	Spent fuel/vitrified waste	About 2 m$^3$	Deep geological disposal

Types and Characteristics of Fusion Waste

Waste from fusion reactors is fundamentally different from fission reactors:

Reaction Products

The direct products of the D-T reaction are helium-4 and neutrons; helium is completely stable and harmless.

Activated Structural Materials

Activation of structural materials by 14.1 MeV neutrons is the main source of waste:

Material	Major Activated Nuclides	Half-Life	Contact Dose Rate (after shutdown)
Stainless steel (316SS)	${}^{54}$Mn	312 days	High (persists long-term)
	${}^{60}$Co	5.27 years	High
	${}^{59}$Ni	76,000 years	Low
Reduced activation ferritic/martensitic steel (RAFM)	${}^{54}$Mn	312 days	Medium
	${}^{55}$Fe	2.7 years	Medium
Vanadium alloys	${}^{49}$V	330 days	Low
SiC/SiC composites	${}^{14}$C	5,730 years	Very low

Radioactivity decay when using low-activation materials:

Cooling Period	Relative Radioactivity	Handling Capability
Immediately after shutdown	100%	Remote handling only
After 1 year	10%	Remote handling
After 10 years	1%	Accessible with heavy shielding
After 50 years	0.1%	Work possible with light shielding
After 100 years	0.01%	Recyclable level

Tritium

Tritium (half-life 12.3 years) is the most abundant radioactive material handled in fusion reactors:

$${}^3_1\text{T} \rightarrow {}^3_2\text{He} + \beta^- + \bar{\nu}_e \quad (E_{\max} = 18.6 \text{ keV})$$

Parameter	Value
Half-life	12.3 years
Maximum beta energy	18.6 keV
Skin penetration depth	About 6 μm (does not reach epidermis)
Biological half-life (water form)	About 10 days
Annual intake limit (general public)	About 10$^9$ Bq

The low-energy beta rays from tritium do not penetrate the skin, and the risk of external exposure is extremely low.

Comparison of Waste Management

Aspect	Fusion	Fission
High-level waste generation	None	Yes (spent fuel)
Ultra-long-lived nuclides	Almost none	Actinides (tens of thousands of years)
Required disposal period	About 100 years	Tens of thousands to 100,000 years
Need for geological disposal	No	Required
Recyclability	High (after 100 years)	Limited
Waste volume	Comparable to slightly more	Baseline
Long-term toxicity	Significantly lower	High

Detailed Safety Comparison

Runaway Reaction and Meltdown Risks

The most fundamental safety difference between fission and fusion reactors is the possibility of runaway reactions.

Criticality in Fission Reactors

Fission reactors operate by maintaining criticality (sustained chain reaction). This is characterized by the effective multiplication factor $k_{eff}$:

$$k_{eff} = \frac{\text{Number of neutrons in next generation}}{\text{Number of neutrons in current generation}}$$

State	$k_{eff}$	Behavior
Subcritical	< 1	Reaction decay
Critical	= 1	Steady operation
Supercritical	> 1	Reaction increase

The existence of delayed neutrons (about 0.65%, average delay time about 13 seconds) allows the reaction to change on controllable time scales during normal operation. However, if prompt criticality is reached ($k_{eff} > 1 + \beta$, where $\beta$ is the delayed neutron fraction), the reaction rapidly increases and becomes uncontrollable.

Reactivity accident scenarios:

Event	Cause	Result
Reactivity insertion accident (RIA)	Rapid control rod withdrawal	Power spike, fuel damage
Loss of coolant accident (LOCA)	Pipe rupture	Core meltdown
Station blackout (SBO)	Earthquake, tsunami, etc.	Loss of cooling function, core meltdown
Recriticality	Reconfiguration of molten fuel	Uncontrolled power generation

Inherent Safety of Fusion Reactors

Chain reactions are fundamentally impossible in fusion reactors:

$$\text{D} + \text{T} \rightarrow {}^4\text{He} + n$$

The neutrons generated in this reaction have no reactivity with helium and do not induce new D-T reactions in the plasma.

Self-limiting mechanisms in fusion plasma:

Perturbation	Effect	Result
Impurity contamination	Increased radiation cooling	Plasma temperature drop, reaction stops
Density increase	Increased heat loss	Plasma cooling
Wall contact	Local cooling	Reaction stops
Power loss	Loss of heating/confinement	Immediate reaction stop

Comparison of in-reactor fuel inventory:

Item	Fusion Reactor (1 GWe)	Fission Reactor (1 GWe)
In-reactor fuel amount	Several grams (D+T)	About 100 tonnes (UO$_2$)
Conditions for sustained reaction	External heating (100 million degrees maintained)	Maintenance of critical configuration
Time to stop reaction	Several seconds (heating stop)	Hours to days (decay heat removal)
Decay heat	Extremely small	About 7% of output immediately after shutdown

The Decay Heat Problem

In fission reactors, decay heat from fission products continues to be generated after shutdown:

$$P_d(t) = 0.066 \cdot P_0 \cdot \left[ (t-t_s)^{-0.2} - (t+T_{op})^{-0.2} \right]$$

Here, $P_0$ is operating power, $t$ is time after shutdown (seconds), $t_s$ is shutdown time, and $T_{op}$ is operating time.

Time After Shutdown	Decay Heat/Operating Power
1 second	About 6%
1 hour	About 1.4%
1 day	About 0.5%
1 week	About 0.2%
1 month	About 0.1%

For a 1 GWe reactor, about 60 MW of decay heat is generated immediately after shutdown, and about 5 MW even after 1 day, requiring continuous cooling.

The decay heat from fusion reactors comes only from activated structural materials, at about 1 MW immediately after shutdown for a 1 GWe reactor, orders of magnitude smaller.

Nuclear Proliferation Risk

Fission Technology and Nuclear Weapons

Fission technology includes materials and knowledge directly relevant to nuclear weapons manufacturing:

Material	Critical Mass for Nuclear Weapons	Relationship to Power Reactors
${}^{235}$U (HEU, >90%)	About 15 kg	Can be separated from low-enriched uranium
${}^{239}$Pu (weapons-grade)	About 4 kg	Can be extracted from spent fuel
${}^{233}$U	About 8 kg	Produced in thorium cycle

From the perspective of non-proliferation, uranium enrichment and reprocessing technologies are strictly controlled.

Fusion Technology and Nuclear Weapons

The proliferation risk of fusion technology has different characteristics from fission:

Aspect	Risk Assessment
Production of fissile materials	None (D-T cycle)
Military use of tritium	Booster for enhanced nuclear weapons
Use as neutron source	Theoretically possible
Technical barriers	Significantly higher than fission weapons

Tritium can be used in boosted fission bombs, but tritium from fusion reactors will be strictly controlled. Manufacturing a pure fusion bomb requires extremely advanced technology separate from fusion reactor technology.

Comparison of Accident Impact Range

Accident Scenario	Fusion Reactor	Fission Reactor
Maximum expected release	Several grams of tritium	Large amounts of radioactive iodine, etc.
Evacuation zone	Site boundary level	Several tens of km
Long-term residence restrictions	None	Years to decades
Land contamination	None	Potentially severe

Economic Comparison

Structure of Power Generation Costs

Power generation costs (LCOE: Levelized Cost of Electricity) are calculated by the following formula:

$$\text{LCOE} = \frac{\sum_t \frac{I_t + M_t + F_t}{(1+r)^t}}{\sum_t \frac{E_t}{(1+r)^t}}$$

Here, $I_t$ is capital cost, $M_t$ is operation and maintenance cost, $F_t$ is fuel cost, $E_t$ is electricity generation, and $r$ is the discount rate.

Costs of Fission Power Generation

Cost structure of current light water reactors (estimated):

Item	Proportion	Cost (yen/kWh)
Capital cost	60-70%	6-8
Operation and maintenance	20-25%	2-3
Fuel cost	10-15%	1-1.5
Decommissioning/waste	5-10%	0.5-1
Total	-	10-13

Capital costs for new construction have risen sharply in recent years:

Project	Output	Construction Cost	Cost per kW
Korean APR1400	1.4 GW	About $6 billion	About $4,300/kW
Chinese Hualong One	1.2 GW	About $5 billion	About $4,200/kW
UK Hinkley Point C	3.2 GW	About $35 billion	About $11,000/kW
French Flamanville 3	1.6 GW	About €20 billion	About $12,000/kW

Cost Projections for Fusion Power

Since fusion power plants do not yet exist, cost projections have significant uncertainty.

DEMO (demonstration reactor) stage projections:

Item	Projected Range
Construction cost	$20-50 billion
Cost per kW	$10,000-30,000/kW
Power generation cost	15-40 yen/kWh

Long-term projections for commercial reactors (nth unit onward):

Item	Optimistic Projection	Conservative Projection
Cost per kW	$4,000/kW	$8,000/kW
Power generation cost	8 yen/kWh	20 yen/kWh
Capacity factor	80%	60%

Cost Reduction Factors

Potential for cost reduction in fusion power:

Factor	Effect
Minimal fuel cost	Fuel cost is less than 1% of power generation cost
Reduced waste disposal costs	No deep geological disposal required
Reduced insurance/liability costs	Extremely low accident risk
High-temperature superconducting magnets	Device compactification
Modularization/mass production	Reduced construction costs

Technology Readiness and Path to Commercialization

Comparison of Technology Readiness Levels (TRL)

Evaluation using NASA-style Technology Readiness Levels:

TRL	Definition	Fission	Fusion
1	Basic principles discovered	Achieved 1938	Achieved 1920s
2	Technology concept formulated	Achieved 1940s	Achieved 1950s
3	Proof of concept	Achieved 1942	Achieved 1990s
4	Validation in laboratory environment	Achieved 1951	Achieved 1997 (JET)
5	Validation in relevant environment	Achieved 1950s	ITER (planned 2035)
6	Demonstration in relevant environment	Achieved 1954	DEMO (planned 2050s)
7	Demonstration in operational environment	Achieved 1956	Planned 2060s
8	System complete and qualified	Achieved 1960s	Planned 2070s
9	Actual operation	1956-present	2080s onward

History of Fission Power

Year	Milestone
1938	Discovery of fission (Hahn, Strassmann)
1942	First controlled chain reaction (Chicago Pile-1)
1951	First power generation (EBR-I, USA)
1954	First commercial reactor operation (Obninsk, USSR)
1956	Large-scale commercial reactor operation begins (Calder Hall, UK)
1979	Three Mile Island accident
1986	Chernobyl accident
2011	Fukushima Daiichi accident
2024	About 440 reactors operating worldwide

Current nuclear power:

Region	Operating Reactors	Installed Capacity	Electricity Share
North America	93	98 GW	About 18%
Europe	106	100 GW	About 22%
Asia	170	165 GW	About 8%
Others	71	47 GW	About 5%
World Total	440	410 GW	About 10%

Development Status of Fusion Power

Year	Milestone
1920s	Theory of fusion as stellar energy source
1951	Tokamak concept proposed (Sakharov, Tamm)
1958	Fusion research made public at Geneva Conference
1968	High-temperature plasma achieved in T-3 tokamak
1983	JET first plasma
1991	First D-T experiment at JET
1997	16.1 MW fusion power at JET
2022	Q > 1 achieved at NIF (laser fusion)
2035	ITER first plasma (planned)
2039	ITER D-T operation begins (planned)
2050s	DEMO operation begins (planned)

Rise of Private Fusion Companies

Investment in private fusion companies has surged since the 2020s:

Company	Country	Technology	Total Funding
Commonwealth Fusion Systems	USA	High-temperature superconducting tokamak	About $2 billion
TAE Technologies	USA	FRC	About $1.2 billion
Helion Energy	USA	FRC (pulsed)	About $600 million
General Fusion	Canada	Magnetized target fusion	About $300 million
Tokamak Energy	UK	Spherical tokamak	About $200 million
Kyoto Fusioneering	Japan	Fusion plant technology	About $100 million

These companies aim for commercial reactors in the 2030s and may accelerate conventional government-led development.

Detailed Comparison of Environmental Impact

Greenhouse Gas Emissions

Lifecycle greenhouse gas emissions (gCO$_2$eq/kWh):

Energy Source	Median	Range
Coal-fired	820	740-910
Natural gas	490	410-520
Solar (residential)	41	18-180
Wind (onshore)	11	7-56
Nuclear (fission)	12	5-55
Fusion (estimated)	5-15	2-30

Lifecycle emission sources for fusion power:

Stage	Emission Source	Contribution
Construction	Concrete, steel manufacturing	About 60%
Operation	Power consumption (initial), maintenance	About 20%
Fuel cycle	Deuterium extraction, lithium mining	About 10%
Decommissioning	Dismantling, waste management	About 10%

Land Use

Land area per GWe:

Energy Source	Area (km$^2$/GWe)	Notes
Nuclear (fission)	1-3	Site area
Fusion (estimated)	1-3	Expected to be similar
Solar	20-50	15% capacity factor assumed
Wind	50-150	25% capacity factor assumed
Coal-fired	2-5	Excluding coal mines
Hydroelectric	50-5000	Including reservoir area

Fusion and fission have high energy density and excellent land use efficiency.

Water Usage

Water consumption per kWh of power generation (L/kWh):

Cooling Method	Fission	Fusion (estimated)
Direct cooling (seawater)	100-200	100-200
Cooling tower (evaporative)	2-3	2-3
Air cooling	0.1	0.1

Since both fusion and fission use steam turbines, water usage is expected to be similar.

Impact on Ecosystems

Impact Category	Fusion	Fission	Coal
Air pollution	None	Very slight	Severe
Water pollution	Very slight	Very slight (normal operation)	Severe
Soil contamination risk	Very slight	High in accidents	Moderate
Mining impact	Very slight	Uranium mining	Severe
Thermal discharge	Yes	Yes	Yes

Historical Development and Future Outlook

Future of Fission Technology

Development of Generation IV reactors is underway:

Reactor Type	Features	Development Status
Sodium-cooled Fast Reactor (SFR)	Breeding, waste reduction	Demonstration reactor stage
Lead-cooled Fast Reactor (LFR)	Inherent safety	Conceptual design
Very High Temperature Reactor (VHTR)	Hydrogen production	Demonstration reactor under construction
Supercritical Water-cooled Reactor (SCWR)	High efficiency	Conceptual design
Molten Salt Reactor (MSR)	Thorium cycle	Experimental reactor stage
Gas-cooled Fast Reactor (GFR)	High efficiency, closed cycle	Conceptual design

Small Modular Reactors (SMRs) are also attracting attention:

Design	Output	Country	Status
NuScale	77 MW	USA	Licensed
BWRX-300	300 MW	USA/Japan	Under review
SMART	100 MW	South Korea	Licensed
HTR-PM	200 MW	China	Operating

Future Scenarios for Fusion Technology

International roadmap (based on IAEA, ITER Organization, etc.):

Decade	Milestone	Key Facilities
2020s	Burning plasma preparation	JT-60SA, KSTAR, EAST
2030s	Burning plasma demonstration	ITER
2040s	Prototype reactor design/construction	DEMO (various countries)
2050s	Prototype reactor operation	DEMO
2060s	Commercial reactor design optimization	-
2070s	Commercial reactor deployment begins	-

Private company acceleration scenarios:

Company	Goal	Target Year
Commonwealth Fusion Systems	SPARC demonstration reactor operation	2025
Helion Energy	Commercial power generation begins	2028
TAE Technologies	Achievement of D-T equivalent conditions	2025
Tokamak Energy	Q > 1 achievement	Early 2030s

Role in the Energy Mix

Position of fusion and fission in 2050 and 2100 energy scenarios:

2050 Scenarios

Scenario	Fission	Fusion	Renewable Energy
IEA Net Zero	10-12%	0% (under development)	60-70%
Nuclear expansion	20-25%	0% (under development)	50-60%
Fusion success	10-15%	2-5% (initial)	50-60%

2100 Scenarios (Long-term Projections)

Scenario	Fission	Fusion	Notes
Fusion as main source	5-10%	30-50%	Fusion becomes primary power source
Coexistence	15-20%	15-25%	Both technologies complement
Renewables-led	5-10%	5-10%	Nuclear is supplementary

Summary Comparison Table

Comparison Item	Fusion	Fission
Reaction principle	Combining light nuclei	Splitting heavy nuclei
Energy source	Mass defect (light→heavy)	Mass defect (heavy→medium)
Typical reaction	D + T → He + n	U-235 + n → FP + n
Energy/reaction	17.6 MeV	200 MeV
Chain reaction	None	Yes
Fuel	D (seawater), Li	U (ore)
Fuel availability	Hundreds of millions of years	100 years (LWR)
Reaction products	He (harmless)	Fission products (radioactive)
Radioactive waste lifetime	~100 years	Tens of thousands of years
Runaway accident risk	None	Yes
Proliferation risk	Low	High
Technology maturity	Under development	Commercialized
Commercialization timing	2050s onward	1950s~
Power generation cost	Undetermined (expected high)	10-15 yen/kWh
Capacity factor	Undetermined	70-90%
CO$_2$ emissions	Very low	Very low
Land use efficiency	High	High

Conclusion

While both fusion and fission utilize nuclear energy, their characteristics are strikingly different.

Fission power generation is a mature technology with over 70 years of operational experience, currently supplying about 10% of the world’s electricity. However, it faces fundamental challenges including the risk of severe accidents, disposal of long-lived radioactive waste, and connections to nuclear proliferation.

Fusion power generation has ideal characteristics as an energy technology: virtually inexhaustible fuel, no chain reaction making runaway physically impossible, and no generation of long-lived high-level waste. However, due to the technical challenge of maintaining ultra-high temperature plasma exceeding 100 million degrees for extended periods, commercialization is expected to require until the 2050s or later.

To meet humanity’s energy demands in the latter half of the 21st century and address climate change, an optimal combination of diverse energy sources, including both technologies, is necessary. Fission technology aims for further safety improvements and waste reduction through improvements to existing technology and development of Generation IV reactors, while fusion technology steadily advances toward commercialization through ITER and private company efforts.

The realization of fusion holds the potential to provide humanity with a virtually inexhaustible, clean energy source, making its achievement one of the greatest scientific and technological challenges of the 21st century.

Related Topics

• What is Nuclear Fusion - Basic principles of fusion reactions
• Energy Problems and Fusion - Global energy challenges and the role of fusion
• Lawson Criterion - Ignition conditions for fusion plasma
• ITER - International Thermonuclear Experimental Reactor project
• Confinement Methods - Technologies for maintaining plasma
• Safety and Environment - Safety and environmental impact of fusion