As we have started our weekly energy seminars again we have asked all of our speakers to write a blog post on the same topic as their talk. Today we hosted Sam Ha an engineering analyst from the Culham Centre for Fusion Energy. As a graduate of Imperial College London with an MEng in Mechanical with Nuclear Engineering. His work is looking at the development of commercially viable nuclear fusion and this blog post is a companion piece to his talk Fusion: Power source of the indefinite future. You can also download his slides [PDF] from the talk.
Nuclear fusion is the process that powers the sun, and there are attempts here on earth that aim to harness this power for the benefit of humanity. The benefits of generating electricity from fusion are simple to see:
- Carbon neutral electricity generation
- No long lived radioactive waste
- Abundant fuel
What would a fusion power plant look like and what role could fusion play in helping humanity meet its demand for electricity?
Current State of Affairs
International efforts into fusion research are currently focussed squarely on ITER, an enormous machine being built in the south of France with the aims of achieving a controlled, lasting burning plasma, generating more power from fusion than the machine consumes.
No magnetically confined fusion device has demonstrated breakeven yet, and the success of ITER is seen as a necessary achievement on the way to a fusion power plant, for a number of reasons. One key to the success of ITER is the validation of the scaling laws used to design large, powerful tokamaks.
Breakeven is defined as the energy produced by fusion being equal to the energy required to heat the plasma, and is also known as a fusion gain (Q factor) of 1.
A power plant must have a Q factor significantly greater than 1, somewhere in the region of Q>25. To design a fusion reactor that has such a high fusion gain, a fairly simple tool called a ‘scaling law’ is used.
How to Design a Fusion Reactor
Scaling laws are best-fits of data from thousands of experiments across many tokamaks, used to estimate performance of tokamaks with various operating parameters. These scaling laws are empirical tools that don’t account for the mechanisms in action inside of the plasma, but provide a design of new tokamaks by estimating their performance from the key plasma parameters.
A small confinement time signifies that the plasma loses energy (heat) too quickly and fusion is harder to achieve. A larger confinement time is needed to achieve high fusion gain, as the heat produced by the fusion reactions takes longer to leave the plasma. If this piques your interest, have a look into the Lawson Criterion (wiki)
The scaling law used in the design of ITER (ITERH-98(y,2)) predicts that the most significant factor in improving the performance of a tokamak is the major radius of the plasma. So to make a fusion power plant (with a high Q factor), the tokamak must be large.
How large? The current estimates are that a power plant will need a major radius of around 7~9m, or roughly 3 times the size of the Joint European Torus, which is currently the most powerful fusion reactor in the world.
As a direct result of this, the power output of a fusion power plant is inherently going to be large. The design of the European demonstration fusion plant (DEMO) aims for 2GWt (or 500MWe), while Japan designs aim for 2GWe. At these large volumes of power, the aim for fusion power plants becomes clear: a baseload generating power plant.
Playing the Long Game
There is one important distinction here: all of this hinges on our knowledge of plasma physics being in line with the scaling laws used to design ITER. Is it possible that our knowledge may be superseded? Absolutely!
The scaling laws outline the performance of reactors with basic parameters changed (such as plasma size, magnetic field strength, heating power, etc.). What scaling laws don’t help with, is the step improvements in plasma confinement that are entirely different phenomena
Now it might seem fanciful to hope for new discoveries to open the path to new reactor designs, but that’s exactly what has happened in the past. The Lawson criterion (a key measure for the performance of fusion reactors), or the triple product, has seen major, consistent improvements over the last 50 years.
Through operating powerful reactors (such as JT-60U and JET), understanding of plasma physics has improved massively, as evidenced by the improvements in the triple product. The operation of ITER may lead to greater understanding of how to design fusion reactors. Developments of this type open the door to a much wider range of power plants, which may one day include an SMR equivalent of fusion reactor or advanced, hybrid fusion-fission reactors.
All of the developments in fusion research are possible because of the willingness of the international community to collaborate and develop this power source for the future. Without the international collaboration in fusion research, the road to fusion power would be far more difficult.