Welcome to IEEE-SEECS

The sun, like all other stars is powered by nuclear

The sun, like all other stars is powered by nuclear [1]

By Behlol Nawaz

Nuclear fusion, can be counted as one of the awe-inspiring displays of nature’s power. It has been powering the stars for billions of years and is considered responsible for the creation of most of the natural chemical elements.

With its potential advantages over current systems, it was once hailed as the ultimate solution for our growing energy needs. Control over fusion has always appeared to be within reach since the 1950s, but has eluded humans continuously. So much so, that now there is a common quip, “fusion is always a few decades away!”. Still, a major hope for the future, it has taken a back seat in most of today’s energy related debates.

So what is nuclear fusion? The technology devised to generate power from it? What are the advantages that make it a dream source to some? What are the challenges in making it a viable energy source? And what paths are being taken to try and make nuclear fusion a feasible power source?

The science behind nuclear fusion

The basic science behind the fusion reaction itself is almost a century old now. In nuclear fusion, the nuclei of two atoms get fused together to form a larger nucleus. During this process, some of the mass of the fusing nuclei gets converted into energy. This energy is released as heat and high velocity neutrons.

Two nuclei fuse to give daughter nucleus, a high energy neutron and heat. [2]

Two nuclei fuse to give daughter nucleus, a high energy neutron and heat. [2]

For the nuclear forces to overcome electrostatic force of repulsion between positively charged nuclei, the nuclei must be closer than 10-15m.

In the sun and other stars, massive gravitational pressure and high temperatures make the nuclei come close enough for fusion. However on earth, in the absence of astronomical pressure, even higher temperatures have to be reached to make the nucleons (protons and neutrons) go fast enough to get close to each other for fusion. On sun, it is around 15 million K. On earth the temperatures have to be about 150 million! About 10 times hotter than the sun’s core! The fuel has long become plasma before the fusion is initiated.

The fuel

The nuclei most feasible to fuse together with current technology are two isotopes of hydrogen, deuterium and tritium. Normal hydrogen (protium) has no neutrons, deuterium has one, tritium has two.

The three isotopes of hydrogen.[3]

The three isotopes of hydrogen.[3]

Deuterium is very rare compared to protium. But with the amount of water on the planet, it could still be supplied for a very long time (millions of years!).

Tritium is radioactive (with a half-life of 12 years) and even more scarce. Only found in trace amounts. But other elements like lithium can be converted to tritium through nuclear reactions in fission reactors or even during the operation of a fusion reactor itself.

The Reactor

Confinement

A range of techniques have been devised and for starting fusion and confining the plasma.

Magnetic Confinement aims to do so by using combinations of magnetic fields. Two well known magnetic confinement designs are “Tokamak” and “Stellarator”. In the magnetic confinement designs, the fuel is heated to the required temperatures by microwaves, inducing current in the plasma itself and injecting high speed ions into the plasma.

Tokamak configuration. the coils (red) and magnets (green) create themagnetic fields that confine the plasma. [4]

Tokamak configuration. the coils (red) and magnets (green) create themagnetic fields that confine the plasma. [4]

Inertial Confinement fusion is a technique in which a fuel pellet is heated by lasers, electrons or ions, causing it to implode. This produces the compression required to initiate fusion. Or through magnetic pinching, the current through the plasma is used to make it “pinch”, due to the magnetic forces of the current.

  This cylinder (called a holhraum) holds the fusion fuel pellet for a laser based inertial confinement system. [5]

This cylinder (called a holhraum) holds the fusion fuel pellet for a laser based inertial confinement system. [5]

Absorbing energy and removing Helium

The energy released from the fusion reaction in the form of heat and high speed neutrons is then absorbed by a “blanket”, slowing them down and converting kinetic energy into heat. The heat is then be absorbed by a coolant, which drives steam turbines like conventional power plants.

At the same time, a part of the reactor called “divertor” removes the helium formed during the reaction, so that the reaction may continue without impurities.

Advantages of fusion power

The biggest advantage of fusion power over power from fossil fuel, is zero carbon emissions. The fuel is easier to source and in the long run, it would be much cheaper.

Unlike nuclear fission, where radioactive waste is a huge problem, the fuel, products and parts of a fusion reactor are much less radioactive and with shorter half-lives.

Fusion reactors are safer than fission reactors. Unlike fission reactors, fusion reactors can be shut down immediately. And due to the way they operate, malfunctions would simply stop the reaction, rather than an out of control chain reaction.

Fusion reactors also contribute much lesser than to weapons development than fission power research

Challenges

Generating power from fusion has some of the most challenging problems in science and engineering.

Deuterium-Tritium fusion releases high energy neutrons, which cannot be controlled due to its lack of charge. Bombardment from neutrons can make many materials radioactive. Making the parts of the reactor from material that can survive neutron bombardment without long term radioactivity and also be efficient in their functions, is a challenge.

Similarly, the internal parts of the reactor not only have to survive the heat during the reaction but also the plasma eventually has to come in contact with the reactor interior (e.g, with the divertor, for helium removal). So the reliability and maintenance of the divertor and other internal parts is also an issue, considering the fact that they also become radioactive with neutron bombardment.

Fusion reactors also use superconducting magnets, which have to be cooled to near 0 K (around -270 oC). That requires cryogenic coolants like Helium in a very large quantity. Preparing, keeping and transporting cryogenic coolants is difficult and very expensive.

Due to problems like these, fusion cannot be sustained in a reactor for long. The record time is in seconds. So although fusion has been performed in laboratories for decades, there is yet to to be a controlled fusion reaction that yields more energy than was used to initiate it.

Some fusion research projects

  • ITER

“International Thermonuclear Experimental Reactor”, (ITER) is an international fusion power research project. It is funded and run by EU, Japan, China, USA, Russia, South Korea and India, with the aim of demonstrating 10 times more output power than input. It is one of the  largest fusion projects in physical scale and economics.

The formal agreement was signed in 2006 and construction work started in 2008. It is expected to start Deuterium-Tritium fusion by 2027.

  • NIF

The “National Ignition Facility” of the Lawrence Livermore National Laboratory, California, USA, has one of the most powerful lasers in the world. They are among the world leaders in research on inertial confinement fusion. They are the first to demonstrate an experiment in which the fuel pellet released more energy from fusion than it absorbed. (NOT more than what was applied in the experiment).

The target chamber for the NIF laser. The large rod to the right is the target positioner, which holds the fuel pellet. [6]

The target chamber for the NIF laser. The large rod to the right is the target positioner, which holds the fuel pellet. [6]

  • HiPER

High Power laser Energy Research facility (HiPER) is a proposed inertial confinement fusion experiment. This European project is currently in the design phase.

It is being designed to study a new approach for initiating nuclear fusion, which may give a higher output to input ratio than previous inertial confinement designs and reduce costs at the same time.

  • MAST

The Mega Ampere Spherical Tokamak (MAST) in Britain, is a further development on a previous experiment “START” (Small Tight Aspect Ration Tokamak), which gave some unexpectedly good results.

Unlike the conventional Tokamak designs where the plasma’s confinement area is toroidal in shape, spherical Tokamaks like the MAST have a more spherically shaped confinement area.

Glow of plasma in a spherically shaped Tokamak. [7]

Glow of plasma in a spherically shaped Tokamak. [7]

  • Wendelstein 7-X

The Wendestein 7-X is an experimental stellarator, being constructed by “Max-Planck-Institut für Plasmaphysik” in Greifswald Germany. The stellarator design, in general, is complicated and difficult to construct. Not much attention was paid to it until recently and so they are not experimentally explored as well as the Tokamak.

The Wendelstein 7-X is under construction and the first experimental run is scheduled for 2015.

The configuration of the electric coils in a stellarator (Wendelstein 7-X). Note that there is just one coil enforcing a field, unlike the Tokamak. [8]

The configuration of the electric coils in a stellarator (Wendelstein 7-X). Note that there is just one coil enforcing a field, unlike the Tokamak. [8]

  • Z-machine

The Z-Machine is an X-Ray generator operated by the Sandia National ٰLaboratories, USA. It is used for research in different areas, including nuclear weapons. More recently its studies have been focused on inertial confinement fusion, utilizing the magnetic pinch.

As research in nuclear fusion is extremely expensive there hasn’t been any notable work on it in Pakistan. However, there is a small tokamak type fusion reactor “GLAST”, in National Centre for Physics, installed by the Pakistan Atomic Energy Commission in 2008. It is used mainly for plasma physics research, with the aim to acquire the technical know-how of fusion technology and build capacity for future research in the area.

Still 30 years away?

Fusion power has appeared to be within reach ever since the first thermonuclear weapons were tested. But with breakthroughs, newer challenges also appeared.

Many consider its research an overly-expensive path, that may not lead to results worth the expenses. It has absorbed plenty of money and hasn’t offered any significant results on those investments.

But proponents of its research argue that its estimate is always “a few decades away” because the required amount has never been spent when needed. And although it is very expensive and has taken in a significant amount of funding, the end result will be worth the costs. Others even argue that given potential, fusion power is a path that must be fully explored before being rejected, for the sake of knowledge.

 

Bibliography:

A review of current state of nuclear power.

Culham Centre for Fusion Energy’s site.

Official page of the ITER project.

An Institute of Physics report about nuclear power research.

An NAE report about energy from fusion.

Official site of National Ignition facility

 

References:

  1. http://sdo.gsfc.nasa.gov/assets/img/browse/2010/08/19/20100819_003221_4096_0304.jpg
  2. http://cdn.eteknix.com/wp-content/uploads/2013/10/Nuclear_Fusion.jpg
  3. http://upload.wikimedia.org/wikipedia/commons/6/6c/Protium_deuterium_tritium.jpg
  4. http://upload.wikimedia.org/wikipedia/commons/6/6c/Protium_deuterium_tritium.jpg
  5. https://lasers.llnl.gov/content/assets/images/media/photo-gallery/medium/nif-1002-05557.jpg
  6. https://lasers.llnl.gov/content/assets/images/media/photo-gallery/medium/nif-0806-12609_red.jpg
  7. http://www.ccfe.ac.uk/assets/Images/Research/MAST%20plasma%20web.jpg
  8. http://upload.wikimedia.org/wikipedia/commons/b/b3/W7X-Spulen_Plasma_blau_gelb.jpg
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: