Power Generation

By Ian Parker, freelancer, ianparkerwriter@aol.com

The world needs new, sustainable sources of energy – wind, solar, wave and tide energy might not be enough. Nuclear fission is well established but presents considerable environmental risks. Nuclear fusion has long held the promise of unlimited power from hydrogen, but it is technically very difficult to achieve and sustain. 

However, last December a major step forward was achieved with a fusion reaction at the Lawrence Livermore National Laboratory (LLNL) in the USA – it yielded more energy than was required to start it. 

Fission reactions involve heavy metals (typically uranium or plutonium) in which large atoms break apart, releasing particles and energy. This happens spontaneously and steps have to be taken to control it. In a conventional nuclear power station, control rods – made of neutron-absorbing materials, such as boron, hafnium and cadmium – in the nuclear pile control the fission, making the energy release manageable. 

However, fusion involves light elements (usually hydrogen atoms or their isotopes) that join to form helium, which also releases energy. This is not spontaneous and has to be driven by huge temperatures and pressures. It’s what happens in the cores of stars and keeps them shining. 

Energy release from nuclear fusion has been achieved on Earth since the early 1950s in hydrogen bombs. So, the nuclear chemistry is well understood. However, this fusion is catastrophic in its energy release – hence its use as a weapon. The fusion part of the bomb is set off by a fission bomb. For the provision of electricity, the fusion reaction will need to be slower and controlled. This is the big challenge for fusion power. 

The two extensively investigated approaches to fusion are Magnetic Confinement Fusion (MCF) and laser driven inertial confinement fusion (laser fusion). One of the questions over MCF is, how do you feed fresh fuel into a reaction chamber? This will require injection into the magnetic bottle or a pulsed system in which the fuel is replenished, and the magnetic bottle switched on again. Laser fusion uses fuel pellets, which are crushed and heated using lasers – producing x-rays in the fixture that holds the pellets.

The LLNL’s National Ignition Facility (NIF) in California is roughly the size of a football stadium and focusses 192 laser beams on the target material. The NIF is using laser fusion to compress a spherical shell of deuterium and tritium, which are hydrogen isotopes and easier to handle and react than hydrogen. The most common hydrogen, protium, has no neutrons; deuterium has one in the nucleus and tritium has two. 

The target materials are held in a small fixing called a hohlraum. In radiation thermodynamics, a hohlraum (a non-specific German word for a ‘hollow space’ or ‘cavity’) is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealised cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material.

Up to last December, experiments in both kinds of fusion had always required more energy to start them than was obtained from the reaction. In an experiment on 5^th December at the NIF, 3.15 megajoules (MJ) of fusion energy was produced while the lasers made an input of 2.05MJ of energy into the target chamber. For the first time, a controlled fusion reaction had yielded more energy than was required to start it. 

To do so, the beams are switched on for just a few billionths of a second and the target implodes inwards on itself at over 1 million miles per hour to heat and compress the fusion fuel to replicate the conditions at the centre of the sun. This is where the fusion reaction starts and releases both neutrons and alpha particles. The neutrons escape, but the alpha particles deposit their energy in the dense fuel, heating it even more. This causes a domino effect between fusion reactions, and more heating, and more fusion reactions, and so on. For a few tens of picoseconds (1 trillionth of a second), the fuel’s own inertia holds it in place while the fuel burns, giving rise to the name inertial confinement fusion.

The NIF laser fusion research is an international collaboration that includes the UK’s Central Laser Facility. Dr Robbie Scott, senior plasma physicist, Science and Technology Facilities Council at the Central Laser Facility and chair of the UK Inertial Fusion Consortium, is a member of the research team working on this project. He said: “…what a huge breakthrough it is for laser fusion research. More importantly, however, is the fact that it paves the way for rapid development of laser inertial fusion energy – power generation by laser fusion.”

UK Science Minister George Freeman said of the breakthrough: “This is a fantastic result that proves the exceptional potential of fusion power, and the National Ignition Facility team should be congratulated on their outstanding achievement. I’m proud that the Department of Business, Energy and Industrial Strategy funded Central Laser Facility was able to play a part in supporting the endeavour. Though there is still some way to go to deliver fusion power generation at scale, results like this illustrate that there is a viable route to commercial fusion energy ahead, and the UK is in pole position to build on this work towards a clean energy future.”

Better than fission

All current nuclear power stations derive their energy from the breaking apart of heavy elements, usually uranium. The fuel is expensive and dangerous to handle and when spent, it remains dangerous as a contaminant. Dealing with spent fuel remains a problem; it has to be stored for a hugely long time before its radioactivity falls to a safe level. 

With fusion, there is no such problem. The fuel is isotopes of hydrogen, which is the most common element in the universe. However, to make it fuse, incredible temperatures and pressures are required. In a fission reactor, control rods slow the reaction to the required level, but there is always a fear that the rod system may be damaged and the core will go out of control and become a ‘melt down’. 

That’s one reason there is such concern about the fission reactors in Ukraine because of the war with Russia. Only a few people will also not know the name ‘Chernobyl’ where a fission reactor exploded in 1986. Fusion reactors have none of these problems. Even though they replicate the reactions at the core of stars (including our own sun), they stop at the slightest opportunity. The big challenge is to keep the reactions going.

Fuel feed in a practical reactor

As part of the process, NIF is using pellets of deuterium and tritium (DT pellets). Deuterium constitutes a tiny fraction of natural hydrogen, only 0.0153%, and can be extracted inexpensively from seawater. Tritium can be made with lithium, which is also abundant in nature. The amount of deuterium present in one litre of water can in theory produce as much energy as the combustion of 300 litres of oil. This means that there is enough deuterium in the oceans to meet human energy needs for millions of years.

Deuterium can be distilled from all forms of water. It is a widely available, harmless, and virtually inexhaustible resource. In every cubic metre of seawater, for example, there are 33 grams of deuterium. Deuterium is routinely produced for scientific and industrial applications. Tritium is a fast-decaying radioelement of hydrogen, which occurs only in trace quantities in nature. It can be produced during the fusion reaction through contact with lithium – it is produced or ‘bred’ when neutrons escaping the plasma interact with lithium.

One of the biggest challenges is how the fuel in a practical reactor will be delivered to the reaction chamber and how will it be secured while the laser impinge on it? Will it be a continuous flow system or will the fuel be delivered in pellets for a pulsed system? Either way, the fixing system will be a big challenge. 

Patti Koning, public information officer in the Office of Strategic Communication at LLNL, explains: “One approach to an Inertial Fusion Energy (IFE) reactor, using Indirect Drive design (lasers shine into a hohlraum that get turned into x-rays to drive the capsule) would use a target assembly that consists of a fuel pellet (with the DT) sitting inside a hohlraum, where one of these fuel target assemblies would get dropped into the chamber 10 times a second. Both the hohlraum and pellet would get expended with each shot.

 “There are also approaches where there is not a hohlraum, and the lasers shine directly on the capsule, which get dropped in at 10 times a second. This is called Direct Drive. However, even if there is no hohlraum, the fuel pellet likely needs some kind of protecting shell as it is dropped into the chamber. An advantage of IFE (compared to magnetic fusion) is that it is a pulsed system, and you would not have a continuous supply of deuterium/tritium. This reduces the tritium inventory needed within the reactor system significantly.”

Whether a practical system uses hohlraums or some other containment, a huge number will be needed, if they are consumed at 10 per second. It would represent a big opportunity for manufacturers of such fixings – to keep the DT pellet in place momentarily while the lasers impinge on them. 

The hohlraums being used by the NIF today are peanut-sized, gold-plated, open-ended cylinders with a peppercorn-sized pellet containing deuterium and tritium. Then, they fire a laser – which splits into 192 finely tuned beams that, in turn, enter the hohlraum from both ends and strike its inside wall. “We don’t just smack the target with all of the laser energy all at once,” points out Annie Kritcher, a scientist at NIF. “We divide very specific powers at very specific times to achieve the desired conditions.”

As the chamber heats up to millions of degrees under the laser barrage, it starts producing a cascade of x-rays that violently crush the fuel pellet. They shear off the pellet’s carbon outer shell and begin to compress the hydrogen isotopes inside – heating them to hundreds of millions of degrees – squeezing and crushing the atoms into pressures and densities higher than the centre of the sun.

When NIF launched in 2009, the fusion world record belonged to the Joint European Torus (JET) in the United Kingdom. In 1997, using a magnet-based method called a tokamak, scientists at JET produced 67 percent of the energy they put in. That record stood for over two decades until late 2021, when the NIF reaching 70 percent. In its wake, many laser-watchers whispered the obvious question – could NIF reach 100 percent? 

But fusion is a notoriously delicate science, and the results of a given fusion experiment are difficult to predict. Tiny, accidental differences in the set-up – from the angles of the laser beams to slight flaws in the pellet shape – can make immense differences in a reactions outcome. It’s for that reason that each NIF test, which takes about a billionth of a second, involves months of meticulous planning.

“All that work led up to a moment just after 01:00am on Monday 5^th December, when we took a shot… and as the data started to come in, we saw the first indications that we’d produced more fusion energy than the laser input,” said NIF Scientist Alex Zylstra.  

“To be honest…we’re not surprised,” said Mike Donladson, a systems engineer at General Fusion, a Canadian-based private firm that aims to build a commercially viable fusion plant by the 2030s. “I’d say this is right on track. It’s really a culmination of lots of years of incremental progress, and I think it’s fantastic.”

These numbers only account for the energy delivered by the laser – omitting the fact that this laser, one of the largest and most intricate on the planet, needed about 300 megajoules from California’s electric grid to power on in the first place.

“The laser wasn’t designed to be efficient,” said LLNL Scientist Mark Hermann.  “The laser was designed to give as much juice as possible.” Balancing that energy-hungry laser may seem daunting, but researchers are optimistic. The laser was built on late 20^th century technology, and NIF leaders say they do see a pathway to making it more efficient and even more powerful. Even if they do that, experts need to work out how to fire repeated shots that gain energy. That’s another massive challenge, but it’s a key step toward making this a viable base for a power plant.

“Scientific results like today’s are fantastic,” states Donaldson. “We also need to focus on all the other challenges that are required to make fusion commercialisable.”

Whilst inertial fusion seems to have taken a lead over MCF, whichever form of inertial fusion becomes viable, it will need hohlraums or holders in their millions – particularly if they are consumed at 10 per second. This could represent a huge opportunity for the fixings business, as this will be the limit rather than the availability of fuel. The shovel becomes the focus rather than the coal and one hopes it would be ‘goodbye’ to coal – and the other fossil fuels – forever.