Readers of this blog know of my advocacy of nuclear power—in particular
fission power. Fission power still faces several hurdles including those of
financing and long-term storage of spent fuel. On more than one occasion I have
been asked if instead of pushing forward with fission power we should wait for
fusion power, which is making substantial advances. Indeed, the past few years
have brought exciting breakthroughs in fusion energy, and fusion power relies
on abundant elements (hydrogen, helium, and boron) and does not produce
long-lived radioactive waste. In this post, I will review these advances and summarize
the remaining technical challenges. Those interested in a more thorough
discussion may want to refer to this ADL report.
First though, a brief recap of the physics of fusion processes. Light
nuclei have less binding energy per nucleon than mid-sized nuclei (like helium
or iron). When small nuclei fuse, some mass is converted into energy via
Einstein’s equation, E=mc2, releasing vast amounts of energy Nuclear
fusion is the process where two light atomic nuclei (typically isotopes of
hydrogen, like deuterium (D) and tritium (T)) combine to form a heavier nucleus
(like helium), releasing tremendous amounts of energy in the process.
D + T → 4He + n, or
D + 3He → 4He
+ 2n
What makes the process difficult to achieve is that the atoms must be
brought very close to each other. Normally, the repulsive coulombic forces
exerted by negatively charged electron clouds surrounding the nucleus prevent
atoms from getting closer than a few picometers (10-12 m). The
strong nuclear forces which hold the protons and neutrons together become
operative at very close distances, femtometer (10-15 m) scale. If
the atoms are heated to very high temperatures, millions of degrees, the
electrons are stripped away from the nuclei producing a plasma—essentially,
ionized gas. At these high temperatures, the nuclei have sufficient kinetic
energy to overcome the even higher repulsion as they approach each other closer
and the collide to produce fusion. This is the process that powers the sun and
stars.
On earth, this was first achieved by raising the temperature of fuel
hydrogen to millions of degrees from the heat of a plutonium bomb to trigger
the fusion of hydrogen. Although we can release energy of fusion in the H-bomb,
releasing fusion energy in a controlled manner for producing power would
require confining and sustaining a plasma while continuously feeding in fuel
and extracting the released energy.
Two main approaches of confining plasma are (a) magnetic and (b)
inertial. The former uses magnetic coils (Tokomak and Stellerator) arranged in
a donut shape to steer the plasma away from the walls and circulating within it
at a high speed. There are designs in which the plasma is accelerated down a
tube Using several different heating techniques such as ohmic, RF heating, or
injecting neutral particles, the plasma is heated to 100 million degrees
(Kelvin) to achieve fusion. For reference, the temperature in the core of the
sun is around 15 million K. The magnets keep the plasma away from the walls of
the containing vessel. Inertial confinement relies on instantly heating a
pellet of fuel gases with multiple lasers.
The National Ignition Facility (NIF) uses the inertial confinement approach.
It dumped 2.05 MJ of energy from 192 lasers firing simultaneously on a gold
cylinder containing D and T to achieve fusion. The experiment produced in 3.15
MJ demonstrating a major scientific breakthrough—a net energy gain with Q = 1.5. This
announcement unleashed a frenzy of activity to develop fusion power. Private
companies like Commonwealth Fusion Systems (CFS) are racing ahead, with SPARC—a compact, high-field tokamak—set to
demonstrate net energy gain by 2027. Meanwhile, Helion Energy signed a power purchase agreement with Microsoft, betting on
fusion-powered electricity by 2028.
Achieving Q > 1, is a remarkable feat! The NIF reported getting 50%
more energy than dumped into the fuel pellet by the lasers. For steady power
generation, the desired value of Q should be 50 or more. Further, this
calculation does not include the 500 MJ of electricity that went into powering the
lasers. Overall process is far from being energy neutral, let alone a power
producer. Among the many remaining challenges is figuring out the process for
replacing the fuel-filled cylinders one after another in the precise location
and be able to fire the lasers at repeatedly. At present there is no pathway
for this approach to be used for power production.
Magnetic confinement approach offers pathways for continuous fuel feeding
and energy extraction, but it has yet to demonstrate net energy gain. Another
issue that will need addressing is the supply of tritium. Tritium is an
unstable isotope with a half-life of 12 years. There are no deposits of tritium
that can be mined or extracted; it must be produced prior to use either by
neutron activation of 6Li or in nuclear reactors that use heavy
water as a moderator.
Most of fusion processes are accompanied by the release of fast-moving neutrons
that cannot be confined by magnetic fields, and they can damage the containing
vessel. There is another scheme for fusion that relies on a stable isotope of boron, 11B, for fuel
and (TAE Technologies) and does not involve release of fast-moving neutrons. It
involves bombarding boron nuclei with protons and the fusion process results in
three alpha particles:
11B +p→ 3 4He
The higher nuclear charge of boron (six protons) means that the incoming
proton has a substantially higher barrier of coulombic repulsion before fusion can
occur. The required temperature for this process is 3 billion degrees!
Despite recent progress, commercial fusion remains decades away. Even
optimistic forecasts place the first grid-scale plants in the 2040s. The
engineering hurdles—plasma confinement, tritium breeding, and material
durability—are immense. So, while we chase the "holy grail" of fusion,
we can’t afford to ignore the proven, low-carbon power source we already have,
namely nuclear fission. Modern reactors are safer, more efficient, and
essential for decarbonization. Countries like France show how fission can deliver
70% of electricity with near-zero emissions. Small Modular Reactors (SMRs) with
lower startup costs offer faster deployment and flexibility. With the World
Bank now reversing its policy against financing nuclear power project there is
a real opportunity to rapidly expand global nuclear power generation.
