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Current Fusion Technology

 - By Duncan Williams -

Nuclear fusion has long been sought after as an alternative to nuclear fission as a means for generating electricity.  This is because fusion technology provides several advantages over fission.  For example, the fuels used in nuclear fusion cost less than fission fuels.  One of the fuels used in fusion reactors is deuterium, which can be readily extracted from seawater.  Another fusion fuel, tritium, can be produced from lithium, which is readily obtainable from the earth’s crust.

In contrast, fuels used in the fission process, like plutonium and uranium, are rare and must undergo a costly enrichment process before it can be used in fission reactors.  Another advantage of fusion technology is that it is safer than fission because a different set of nuclear reactions occur in fusion reactors that create far less radiation.  Additionally, fusion reactors produce a smaller volume of high-level radioactive waste, and are therefore more environmentally friendly.

So, based on these advantages, why aren’t we building more nuclear fusion reactors instead of fission reactors?  Mainly because the amount of energy required to simply operate a fusion reactor is greater than the amount of energy it can produce.  To date, no fusion reactor has output more energy than is input.  Indeed, the highest power output by a fusion reactor was accomplished by the Joint European Torus (JET) fusion reactor, which produced 70% of its input power.

The sheer complexity of a fusion reactor creates a barrier for its commercial use as a viable power source.  For example, in order to create the conditions required for fusion, plasma inside a fusion reactor must be heated up to 10 times the temperature of the sun, which requires an enormous amount of electricity.  In order to control the fusion reaction, super-cooled electromagnets are needed to produce magnetic fields, which also requires an enormous amount of electricity.  Aside from these massive electrical requirements, a section of the reactor that is hotter than the sun must be placed adjacent to super-cooled magnets, whose temperature is near absolute zero.  This greatly convolutes the operation of a fusion reactor given our current inventory of known materials.  As complex as fusion reactors may seem, many research facilities are currently studying how to increase the energy output in order to make fusion reactors competitive with fission reactors.

The nuclear fusion reaction is the exact opposite of the nuclear fission reaction.  Nuclear fission reactors generate power by breaking down larger atoms into smaller atoms.  In fission reactors, neutrons split large uranium atoms, resulting in the release of energy and the creation of smaller elements.  In contrast, nuclear fusion reactors generate power by combining smaller atoms to form larger ones.  In fusion reactors, hydrogen isotopes come together to form helium atoms, neutrons, and energy.  Typically, the reaction involves combining the hydrogen isotopes known as deuterium and tritium.

An isotope of an element has the same number of protons, but a different number of neutrons.  Hydrogen is the lightest and most abundant element, and has only one proton and no neutrons.  Deuterium exists naturally in seawater at a concentration of 154 ppm, and consists of one proton and one neutron.  Tritium is slightly radioactive and consists of one proton and two neutrons.  Fusion involving deuterium and tritium results in the formation of a helium atom and the release of a neutron.

Since the neutron produced during fusion has no electrical charge, it is not affected by the magnetic fields in a fusion reactor and will react with the walls.  This reaction causes the walls of the reactor to heat up, which also heats up the coolant circulating within the walls.  As the coolant turns to steam, it can then be circulated to turbine generators which can produce electricity.  After exiting the turbine generators, the coolant condenses to a liquid and returns to the walls of the fusion reactor to begin the cycle again.

But producing electricity from fission reactors in such a manner is complicated by the fusion process itself.  Deuterium and tritium will only combine under extremely high temperatures and pressures.  There are two methods of achieving these conditions.  The first method uses laser beams or ion beams to heat fuel in the form of a pellet.  This approach is currently being studied at the National Ignition Facility at the Lawrence Livermore Laboratory in California.  The second method is to use magnetic and electric fields to heat and confine the hydrogen isotopes, transforming them into a circulating plasma.

Although there are several types of devices that use magnetics, the most studied device is a donut-shaped machine known as a tokamak.  A consortium from the United States, Russia, Europe and Japan is currently building the largest tokamak fusion reactor in the world known as the International Experimental Reactor (ITER) in Cadarache, France.

Although none of the buildings have yet been built at ITER, the goal is to manufacture a fusion reactor that produces 50 times more power than its input.  In order to produce the high temperatures necessary for fusion, the plasma will be heated to close to 150 million degrees Celsius.  The superconducting magnets will then be cooled to near -269 Celsius by supercritical helium in order to produce the necessary magnetic fields.

The complex system of magnetic fields generated inside the inner cavity of the donut-shaped tokamak will accelerate the deuterium-tritium plasma, which will never actually touch the inner walls in order to prevent them from melting.  Construction of the ITER is currently scheduled to be completed in 2017.

Instead of attempting to derive electricity from pure fusion, inventors at the University of Texas have been focusing on using fusion in combination with fission in order to eliminate the nuclear waste produced by conventional nuclear power plants.  The invention could eventually eliminate up to 99 percent of the nuclear waste found in spent nuclear fuel that has been removed from conventional reactors.

U.S. Patent Publication No. 20100063344, published March 11, 2010, and currently assigned to the Department of Energy, describes the fission-fusion hybrid reactor originally developed at the University of Texas.

As can be seen in the diagram, the heart of the hybrid reactor is the toroidal chamber (160) that contains the plasma.  Magnetic coils (120, 140, 190, 210, 220) produce a complex system of magnetic fields that confine and guide the flow of plasma.  The plasma undergoes fusion and produces highly energetic neutrons which travel out of the toroidal chamber (160).

The neutrons then bombard a layer of fissionable material (150), which may include radioactive waste from conventional nuclear reactors.  These neutrons cause the waste to undergo a nuclear fission reaction that will reduce the radiotoxicity of the material.  To prevent neutrons from escaping the reactor, circular lead sections (290) are placed at the top and bottom of the reactor.  Additionally, a lead sheath (110) surrounds the outer walls of the reactor.  It should be noted that the fissionable material (150) is located outside the magnetic coils (120, 140, 190, 210, 220).  Previous fission-fusion hybrid designs locates the fissionable material (150) inside the magnetic coils.

This new design allows the magnetic coils to act as an electromagnetic shield against any sudden plasma transients, ensuring that the fissionable material will not come into contact with the extremely hot plasma.

This diagram from the patent publication shows a more detailed side-view of the hybrid reactor design.  As can be seen in the diagram, the plasma (160) is confined within the closed magnetic field (180), which is itself some distance from the inner wall of the toroidal chamber (170) of the reactor.  The magnetic fields (180, 260) are produced by the magnetic coils (280, 220, 120, 140, 190, 210) that are located close to the toroidal chamber (170).  The layer of fissionable material (150) is embedded in the lead sheath (110), which is located outside the magnetic fields.

One of the novel developments in this invention is the divertor plates (130, 200), whose general purpose in fusion reactors is to divert heat, energy, and particles away from the core of the plasma (160).  Divertors in fusion reactors are typically placed at the intersection of magnetic field lines where high-energy plasma particles strike the components, subjecting them to intense heat.

The new divertor, known as the Super-X divertor, uses a new magnetic geometry that is capable of withstanding the enormous heat fluxes anticipated during the operation of the hybrid reactor.

The Super-X divertor stretches the plasma channel to a larger circulating radius where the heat flux naturally decreases and spreads out over a larger area.  This causes the plasma to cool before it reaches the Super-X divertor so that it is subjected to less heat stress than standard divertors.  This geometry also serves to shield the Super-X divertor from neutrons, protecting it from deformation due to neutron bombardment over its lifetime.

The development of the Super-X divertor will theoretically allow for more compact fusion reactors, since it can be located more closely to the plasma spiraling in the toroidal chamber of a fusion reactor without being subjected to heat that would normally destroy standard divertors.  This redesign of the standard divertor may also result in a five-fold increase in the core power density of toroidal fusion devices, making this design highly attractive.  Several groups are researching whether to incorporate the Super-X divertor in their machines, such as the Meg Amp Spherical Tokamak (MAST) in the United Kingdom, as well as the DIII-D program, operated by General Atomics, and the NSTX program, operated by Princeton University.

It is very likely that the development of new materials capable of withstanding the magnitude of contrasting temperatures inherent in a fusion-type device will soon intensify research and development in this area.  Until recently, the Nuclear Regulatory Commission (NRC) has not asserted regulatory control over commercial fusion energy devices. 

However, last year, in a memorandum dated July 16, 2009, the NRC stated that it would assert regulatory control over fusion energy devices that relate “the common defense or security.”  However, the NRC went on to say that it “will wait until commercial deployment of fusion technology is more predictable… before expending significant resources to develop a regulatory framework for fusion technology.”

So it seems that commercial fusion energy research and development can presently proceed unregulated by the NRC.  But developments like the Super-X divertor that focuses on a hybrid fusion-fission process brings the commercial viability of fusion energy devices closer to a reality, and closer to ultimate regulation of the field.

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About Duncan Williams
Duncan Williams graduated from the University of Florida in 1994 with a B.S. in Physics, and a minor in mathematics.  Upon graduation, he was commissioned  in the U.S. Navy where he completed training in the Navy’s Nuclear Propulsion program.  He then served onboard an aircraft carrier, the USS Theodore Roosevelt, as a reactor control division officer.  Onboard, he was responsible for the operation and maintenance of the electrical and mechanical components that make up the reactor control systems.  This includes the control rod drive mechanisms, the reactor safety and emergency systems, the reactor coolant pump systems, and the ion exchangers.  He also developed and implemented ship-wide reactor safety drills in order to educate sailors in reactor safety.

Duncan then transferred to the U.S. Naval Academy, where he served as a senior instructor teaching Thermodynamics to senior cadets.  While serving as an instructor at the Naval Academy, Duncan attended night law school at the George Washington University Law School.  After receiving his J.D. in 2004, he resigned his commission and began working as an intellectual property associate with Kenyon & Kenyon LLP.  While at Kenyon & Kenyon, he drafted numerous patents relating to medical devices, electronic devices, telecommunications, as well as other technologies.  He also has experience in all stages of patent litigation, and has represented numerous Fortune 500 companies in protecting their intellectual property rights.  Duncan is currently an intellectual property associate at Blank Rome LLP.

If you have questions, comments, or know of a patent that you think Duncan should review E-mail Duncan Williams>> duncan@nuclearstreet.com