Scientists at a laboratory in England have shattered the record for the
amount of energy produced during a controlled, sustained fusion reaction.
The production of 59 megajoules of energy over five seconds at the Joint
European Torus – or JET – experiment in England has been called "a
breakthrough" by some news outlets and caused quite a lot of excitement
among physicists.
But a common line regarding fusion electricity production is that it is
"always 20 years away."
We are a nuclear physicist and a nuclear engineer who study how to develop
controlled nuclear fusion for the purpose of generating electricity.
The JET result demonstrates remarkable advancements in the understanding of
the physics of fusion. But just as importantly, it shows that the new
materials used to construct the inner walls of the fusion reactor worked as
intended.
The fact that the new wall construction performed as well as it did is what
separates these results from previous milestones and elevates magnetic
fusion from a dream toward a reality.
Fusing particles together
Nuclear fusion is the merging of two atomic nuclei into one compound
nucleus. This nucleus then breaks apart and releases energy in the form of
new atoms and particles that speed away from the reaction. A fusion power
plant would capture the escaping particles and use their energy to generate
electricity.
There are a few different ways to safely control fusion on Earth. Our
research focuses on the approach taken by JET – using powerful magnetic
fields to confine atoms until they are heated to a high enough temperature
for them to fuse.
The fuel for current and future reactors are two different isotopes of
hydrogen – meaning they have the one proton, but different numbers of
neutrons – called deuterium and tritium. Normal hydrogen has one proton and
no neutrons in its nucleus. Deuterium has one proton and one neutron while
tritium has one proton and two neutrons.
For a fusion reaction to be successful, the fuel atoms must first become so
hot that the electrons break free from the nuclei. This creates plasma – a
collection of positive ions and electrons.
You then need to keep heating that plasma until it reaches a temperature
over 200 million degrees Fahrenheit (100 million Celsius). This plasma must
then be kept in a confined space at high densities for a long enough period
of time for the fuel atoms to collide into each other and fuse together.
To control fusion on Earth, researchers developed donut-shaped devices –
called tokamaks – which use magnetic fields to contain the plasma. Magnetic
field lines wrapping around the inside of the donut act like train tracks
that the ions and electrons follow.
By injecting energy into the plasma and heating it up, it is possible to
accelerate the fuel particles to such high speeds that when they collide,
instead of bouncing off each other, the fuel nuclei fuse together. When this
happens, they release energy, primarily in the form of fast-moving neutrons.
During the fusion process, fuel particles gradually drift away from the hot,
dense core and eventually collide with the inner wall of the fusion vessel.
To prevent the walls from degrading due to these collisions – which in turn
also contaminates the fusion fuel – reactors are built so that they channel
the wayward particles toward a heavily armored chamber called the divertor.
This pumps out the diverted particles and removes any excess heat to protect
the tokamak.
The walls are important
A major limitation of past reactors has been the fact that divertors can't
survive the constant particle bombardment for more than a few seconds. To
make fusion power work commercially, engineers need to build a tokamak
vessel that will survive for years of use under the conditions necessary for
fusion.
The divertor wall is the first consideration. Though the fuel particles are
much cooler when they reach the divertor, they still have enough energy to
knock atoms loose from the wall material of the divertor when they collide
with it.
Previously, JET's divertor had a wall made of graphite, but graphite absorbs
and traps too much of the fuel for practical use.
Around 2011, engineers at JET upgraded the divertor and inner vessel walls
to tungsten. Tungsten was chosen in part because it has the highest melting
point of any metal – an extremely important trait when the divertor is
likely to experience heat loads nearly 10 times higher than the nose cone of
a space shuttle reentering the Earth's atmosphere.
The inner vessel wall of the tokamak was upgraded from graphite to
beryllium. Beryllium has excellent thermal and mechanical properties for a
fusion reactor – it absorbs less fuel than graphite but can still withstand
the high temperatures.
The energy JET produced was what made the headlines, but we'd argue it is in
fact the use of the new wall materials which make the experiment truly
impressive because future devices will need these more robust walls to
operate at high power for even longer periods of time.
JET is a successful proof of concept for how to build the next generation of
fusion reactors.
The next fusion reactors
The JET tokamak is the largest and most advanced magnetic fusion reactor
currently operating. But the next generation of reactors is already in the
works, most notably the ITER experiment, set to begin operations in 2027.
ITER – which is Latin for "the way" – is under construction in France and
funded and directed by an international organization that includes the US.
ITER is going to put to use many of the material advances JET showed to be
viable. But there are also some key differences. First, ITER is massive. The
fusion chamber is 37 feet (11.4 meters) tall and 63 feet (19.4 meters)
around – more than eight times larger than JET.
In addition, ITER will utilize superconducting magnets capable of producing
stronger magnetic fields for longer periods of time compared to JET's
magnets. With these upgrades, ITER is expected to smash JET's fusion records
– both for energy output and how long the reaction will run.
ITER is also expected to do something central to the idea of a fusion
powerplant: produce more energy than it takes to heat the fuel. Models
predict that ITER will produce around 500 megawatts of power continuously
for 400 seconds while only consuming 50 MW of energy to heat the fuel.
This means the reactor produces 10 times more energy than it consumes – a
huge improvement over JET, which required roughly three times more energy to
heat the fuel than it produced for its recent 59 megajoule record.
JET's recent record has shown that years of research in plasma physics and
materials science have paid off and brought scientists to the doorstep of
harnessing fusion for power generation. ITER will provide an enormous leap
forward toward the goal of industrial scale fusion power plants.
is it safe ?
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