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Fusion: Basic Principles and C urrent Progress. Lukáš Maretta. What is Nuclear Fusion?. Nuclear Fusion is the energy-producing process taking place in the core of the Sun and stars

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what is nuclear fusion
What is Nuclear Fusion?
  • Nuclear Fusionis the energy-producing process taking place in the core of the Sun and stars
  • The core temperature of the Sunis about 15 million °C. At these temperatures hydrogennuclei fuse to give Helium and Energy. The energy sustains life on Earth via sunlight
energy released by nuclear reactions
Energy Released by Nuclear Reactions
  • Light nuclei(hydrogen, helium) release energy when they fuse (Nuclear Fusion)
  • The product nucleiweigh less than the parent nuclei
  • Heavy nuclei(Uranium) release energy when they split (Nuclear Fission)
  • The product nucleiweigh lessthan the original nucleus
energy released by nuclear fusion and fission
Energy Released by NuclearFusion and Fission
  • Fusion reactionsrelease much higher energies than Fission reactions
fusion reactions
Fusion Reactions
  • Deuterium– from water(0.02% of all hydrogen is heavy hydrogenor deuterium)
  • Tritium– from lithium(a light metal common in the Earth’s crust)

Deuterium + Lithium → Helium + Energy

This fusion cycle(which has the fastest reaction rate) is of interest for Energy Production

the world particularly developing countries needs a new energy source
The World, particularly developing countries, needs a New Energy Source
  • Growth in world populationand growth in energy demand from increased industrialisation/affluencewill lead to an Energy Gap which will be increasingly difficult to fill with fossil fuels
  • Without improvements in efficiency we will need 80%more energy by 2020
  • Even with efficiency improvements at the limit of technology we would still need 40%more energy
incentives for developing fusion
Incentives for Developing Fusion
  • Fusion powers the Sun and the stars
    • It is now within reach for use on Earth
  • In the fusion process lighter elements are “fused” together, making heavier elements and producing prodigious amounts of energy
  • Fusion offers very attractive features:
    • Sustainable energy source

(for DT cycle; provided that Breeding Blankets are successfully developed)

    • No emission of Greenhouse or other polluting gases
    • No risk of a severe accident
    • No long-lived radioactive waste
  • Fusion energy can be used to produce electricity and hydrogen, and for desalination

Fission (PWR)

Fusion structure


Tritium in fusion

fusion energy disadvantages
Fusion Energy – Disadvantages
  • Fusion reaction is difficult to start!
    • High temperatures (Millions of degrees) in a pure High Vacuum environment are required
    • Technically complexand high capital costreactors are necessary
  • More Research and Development is needed to bring concept to fruition
    • The physics is well advanced but requires sustained development on a long time scale (20 to 40 years)
  • A Plasmais an ionised gas. A mixture of positive ionsand negative electronswith overall charge neutrality
  • Plasmas constitute the 4th state of matter, obtained at temperatures in excess of 100,000 degrees
  • Plasmas conduct electricityand heat
self sustaining or ignited plasmas
Self-Sustaining or ‘Ignited’ Plasmas
  • Deuterium – tritiumfusion reaction:

D + T →4He + n + Energy

The 4He nuclei (‘a’ particles)carry about 20% of the energy and stay in the plasma. The other 80% is carried away by the neutrons and can be used to generate steam.

Plasmas become Self-sustainingor Ignitedwhen there is enough a power to balance losses from the plasma

  • In starsplasma particles (including a’s) are confined mainly by gravityand high plasma densities achieved
  • On Earth:
    • hot dense plasmas can be confined in Magnetic fields (Magnetic Confinement Fusion)
    • superdense plasmas can be obtained by imploding solid deuterium-tritium pellets (Inertial Confinement Fusion)
inertial confinement
Inertial Confinement
  • Laser implosion of small (3mm diameter) solid deuterium–tritium pellets produces fusion conditions
  • Pressure generation
  • Compression

Fuel is compressed by rocket-like blow off

200,000 million atmospheres in core

  • Ignition and burn
    • Peak compression fuel reaches 1000-10000 times liquid density for extremely short time (10–11 seconds)
    • Core is heated and ‘spark ignition’ occurs
magnetic confinement
Magnetic Confinement
  • Magnetic fieldscause charged particles to spiral around field lines. Plasma particles are lost to the vessel walls only by relatively slow diffusion acrossthe field lines
  • Toroidal(ring shaped) system avoids plasma hitting the end of the container
  • The most successful Magnetic Confinement device is the TOKAMAK(Russian for ‘Toroidal Magnetic Chamber’)
how large a device
How Large a Device?
  • For fusion power to ignite a plasma:
    • There has to be sufficient densityof deuterium and tritium ions (ni);
    • The reacting ions have to be hot enough (Ti);
    • The energyfrom the fusion a’s must be confined for long enough (tE).

tE increases with the square of the device size – a large machine is needed.

  • The fusion triple product (niTitE)and the ion temperature (Ti)must both be large enough (below a certain temperature the fusion reaction probability is too small)

pressure (niTi) ≥2 atmospheres

confinement time > 5 seconds

plasma ion temperature ≈100-200 Million °C

jet joint european torus
JET(Joint European Torus)
  • The Joint European Torus(“JET”) is the largest magnetic fusion test device in the world.
  • Situated at Culham, Oxfordshire, JET:
    • was constructed between 1978-1983;
    • has operated 1983 - present;
    • is the largest Project in the European Union’s Fusion programme
  • The participating countries are the 15 EU nations + Switzerland
  • The Project has a capital investment of over £500 Millionand an Annual Budgetof around £53 Million
  • JETis a Tokamakwith:
    • Torus radius 3.1m
    • Vacuum vessel 3.96m high x 2.4m wide
    • Plasma volume 80m3
    • Plasma current up to 5MA
    • Main confining field up to 4 Tesla (recently upgraded from 3.4 Tesla)
progress with magnetic confinement fusion
Progress with MagneticConfinement Fusion
  • JETand the similar large Tokamaks in:
    • USATokamak Fusion Test Reactor (TFTR)Doublet IIIDTokamak (DIIID)
    • JapanJapanese Tokamak – 60U (JT-60U)

Have made significant progress in:

    • Technology of fusion;
    • Approaching the conditions of an Ignited plasma;
    • Predicting the behaviour of a reactor plasma;
    • Controlling impurities which enter the plasma
    • Operating with Tritium fuel
progress towards ignition
Progress towards Ignition
  • The Fusion Triple Product (PitE = niTitE) required to reach ignition can be compared with leading edge performance of the devices year-on-year.
  • The best plasmas now need an improvement of only 6 in performance. This requires a new larger device.
controlling impurities
Controlling Impurities
  • Fuel Impuritiesare a major threat to reactor success
  • Two primary sources of impuritiesexist:
    • Helium “ash”from the fusion reaction
    • Material impurities from plasma-wall interactions
  • Impurities must be controlledsince they:
    • Radiate energy, and reduce the plasma temperature
    • Dilute the fuel, thereby preventing ignition
  • The “Magnetic Divertor”is a device for controlling impurities. This has been tested successfully in JET.

Three different concepts have been compared.

Results agree with code predictions.

progress with magnetic confinement fusion1
Progress with MagneticConfinement Fusion

Pumped Divertor in JET

  • Impurities(C, Be) are produced byion impacton target and are ionisedin the plasma and returned to target
fusion power development
Fusion Power Development

The diagram encompasses :

  • Two pulses with 10% T in D in JET in 1991;
  • A result from the D-T studies on TFTR (1993 to 1997);
  • High fusion power and quasi-steady-state fusion power from the >200 pulses with >40% T in D in the JET D-T experiments of 1997.
iter design main features
ITER Design - Main Features


Central Solenoid

Blanket Module

Vacuum Vessel

Outer Intercoil Structure


Toroidal Field Coil

Port Plug (IC Heating)

Poloidal Field Coil

Machine Gravity Supports

Torus Cryopump

iter objectives
ITER Objectives


  • Demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes.


  • Demonstrate extended burn of DT plasmas, with steady state as the ultimate goal.
  • Integrate and test all essential fusion power reactor technologies and components.
  • Demonstrate safety and environmental acceptability of fusion.
iter parameters
ITER Parameters

Total fusion power 500 MW (700MW)

Q = fusion power/auxiliary heating power ≥10 (inductive)

Average neutron wall loading 0.57 MW/m2 (0.8 MW/m2)

Plasma inductive burn time ≥ 300 s

Plasma major radius 6.2 m

Plasma minor radius 2.0 m

Plasma current (inductive, Ip) 15 MA (17.4 MA)

Vertical elongation @95% flux surface/separatrix 1.70/1.85

Triangularity @95% flux surface/separatrix 0.33/0.49

Safety factor @95% flux surface 3.0

Toroidal field @ 6.2 m radius 5.3 T

Plasma volume 837 m3

Plasma surface 678 m2

Installed auxiliary heating/current drive power 73 MW (100 MW)

iter location
ITER Location

Caradache (France)

Rokkasho (Japan)


Fusion Nuclear Technology (FNT)

Fusion Power & Fuel Cycle Technology

FNT Components from the edge of the Plasma to TF Coils(Reactor “Core”)

1. Blanket Components

2. Plasma Interactive and High Heat Flux Components

a. divertor, limiter

b. rf antennas, launchers, wave guides, etc.

3. Vacuum Vessel & Shield Components

Other Components affected by the Nuclear Environment

4. Tritium Processing Systems

5. Instrumentation and Control Systems

6. Remote Maintenance Components

7. Heat Transport and Power Conversion Systems







Vacuum vessel

First Wall

Coolant for energy conversion


Tritium breeding zone

blanket including first wall
Blanket (including first wall)

Blanket Functions:

  • Power Extraction
    • Convert kinetic energy of neutrons and secondary gamma-rays into heat
    • Absorb plasma radiation on the first wall
    • Extract the heat (at high temperature, for energy conversion)
  • Tritium Breeding
    • Tritium breeding, extraction, and control
    • Must have lithium in some form for tritium breeding
  • Physical Boundary for the Plasma
    • Physical boundary surrounding the plasma, inside the vacuum vessel
    • Provide access for plasma heating, fueling
    • Must be compatible with plasma operation
    • Innovative blanket concepts can improve plasma stability and confinement
  • Radiation Shielding of the Vacuum Vessel
blanket materials
Blanket Materials
  • Tritium Breeding Material (Lithium in some form)

Liquid: Li, LiPb (83Pb 17Li), lithium-containing molten salts

Solid: Li2O, Li4SiO4, Li2TiO3, Li2ZrO3

  • Neutron Multiplier (for most blanket concepts)

Beryllium (Be, Be12Ti)

Lead (in LiPb)

  • Coolant

– Li, LiPb – Molten Salt – Helium – Water

  • Structural Material
    • Ferritic Steel (accepted worldwide as the reference for DEMO)
    • Long-term: Vanadium alloy (compatible only with Li), and SiC/SiC
  • MHD insulators (for concepts with self-cooled liquid metals)
  • Thermal insulators (only in some concepts with dual coolants)
  • Tritium Permeation Barriers (in some concepts)
  • Neutron Attenuators and Reflectors

A Helium-Cooled Li-Ceramic Breeder Concept: Example

  • Material Functions
  • Beryllium (pebble bed) for neutron multiplication
  • Ceramic breeder (Li4SiO4, Li2TiO3, Li2O, etc.) for tritium breeding
  • Helium purge (low pressure) to remove tritium through the “interconnected porosity” in ceramic breeder
  • High pressure Helium cooling in structure (ferritic steel)

Several configurations exist (e.g. wall parallel or “head on” breeder/Be arrangements)

li vanadium blanket concept

Vanadium structure



Secondary Shield


Primary Shield


Breeding Zone

(Li flow)


Primary shield (Tenelon)

Secondary shield



Vanadium structure


Li/Vanadium Blanket Concept
Flows of electrically conducting coolants will experience complicated magnetohydrodynamic (MHD) effects

What is magnetohydrodynamics (MHD)?

  • Motion of a conductor in a magnetic field produces an EMF that can induce current in the liquid. This must be added to Ohm’s law:
  • Any induced current in the liquid results in an additional body force in the liquid that usually opposes the motion. This body force must be included in the Navier-Stokes equation of motion:
  • For liquid metal coolant, this body force can have dramatic impact on the flow: e.g. enormous MHD drag, highly distorted velocity profiles, non-uniform flow distribution, modified or suppressed turbulent fluctuations
large mhd drag results in large mhd pressure drop
Net JxB body force p = cVB2 where c = (tw w)/(a )

For high magnetic field and high speed (self-cooled LM concepts in inboard region) the pressure drop is large

The resulting stresses on the wall exceed the allowable stress for candidate structural materials

Perfect insulators make the net MHD body force zero

But insulator coating crack tolerance is very low (~10-7).

It appears impossible to develop practical insulators under fusion environment conditions with large temperature, stress, and radiation gradients

Self-healing coatings have been proposed but none has yet been found (research is on-going)

Large MHD drag results in large MHD pressure drop

Conducting walls

Insulated wall

Lines of current enter the low resistance wall – leads to very high induced current and high pressure drop

All current must close in the liquid near the wall – net drag from jxB force is zero

dual coolant concept designs from eu and usa
Dual Coolant Concept Designs from EU and USA

Cross section of the breeder region unit cell


  • The D-T Fusion process offers the promise of:
    • Virtually unlimited energy source from cheap abundant fuels;
    • No atmospheric pollution of greenhouse and acid rain gases;
    • Low radioactive burden from waste for future generations.
  • Tremendous Progress has been achieved over the past decades in plasma physics and fusion technology.
  • Fusion R&D involves many challenging areas of physics and technologies and is carried out through extensive international collaboration
  • EU,J, USA, RF, PRC, Korea are about to construct ITERto demonstrate the scientific and technological feasibility of fusion energy (ITER will produce 500MW of fusion power and the project total cost is about $15B)