Are fusion scientists crazy?

Preamble

I was just about to write another article (1, 2, 3) about the irrelevance of nuclear fusion to the challenges of climate change.

But before I sharpened my pen, I thought I would look again to see if I could understand why a new breed of fusion scientists, engineers and entrepreneurs seem to think so differently. 

Having now listened to two and a half hours of lectures – links at the bottom of the page – I have to say, I am no longer so sure of myself.

I still think that the mainstream routes to fusion should be shut down immediately.

But the scientists and engineers advocating the new “smaller faster” technology make a fair case that they could conceivably have a relevant contribution to make. 

I am still sceptical. The operating conditions are so extreme that it is likely that there will be unanticipated engineering difficulties that could easily prove fatal.

But I now think their proposals should be considered seriously, because they might just work.

Let me explain…

JET and ITER

Deriving usable energy from nuclear fusion has been a goal for nuclear researchers for the past 60 years.

After a decade or two, scientists and engineers concluded (correctly) that deriving energy from nuclear fusion was going to be extraordinarily difficult.

But using a series of experiments culminating in JET – the Joint European Torus, fusion scientists identified a pathway to create a device that could release fusion energy and proceeded to build ITER, the International Thermonuclear Experimental Reactor.

ITER is a massive project with lots of smart people, but I am unable to see it as anything other than a $20 billion dead end – a colossal and historic error. 

Image of ITER from Wikipedia modified to show cost and human being. Click for larger view.

In addition to its cost, the ITER behemoth is slow. Construction was approved in 2007 but first tests are only expected to begin in 2025; first fusion is expected in 2035; and the study would be complete in 2045.

I don’t think anyone really doubts that ITER will “work”: the physics is well understood.

But even if everything proceeds according to plan, and even if the follow-up DEMO reactor was built in 2050 – and even if it also worked perfectly, it would be a clear 40 years or so from now before fusion began to contribute low carbon electricity. This is just too late to be relevant to the problem of tackling climate change. I think the analysis in my previous three articles still applies to ITER.

I would recommend we stop spending money on ITER right now and leave it’s rusting carcass as a testament to our folly. The problem is not that it won’t ‘work’. The problem is that it just doesn’t matter whether it works or not.

But it turns out that ITER is no longer the only credible route to fusion energy generation.

High Temperature Superconductors

While ITER was lumbering onwards, science and technology advanced around it.

Back in 1986 people discovered high-temperature superconductors (HTS). The excitement around this discovery was intense. I remember making a sample of YBCO at Bristol University that summer and calling up the inestimable Balázs Győrffy near to midnight to ask him to come in to the lab and witness the Meissner effect – an effect which hitherto had been understood, but rarely seen.

But dreams of new superconducting technologies never materialised. And YBCO and related compounds became scientific curiosities with just a few niche applications.

But after 30 years of development, engineers have found practical ways to exploit them to make stronger electromagnets. 

The key property of HTS that makes them relevant to fusion engineering is not specifically the high temperature at which they became superconducting. Instead it is their ability – when cooled to well below their transition temperature – to remain superconducting in extremely high magnetic fields.

Magnets and fusion

As Zach Hartwig explains at length (video below) the only practical route to fusion energy generation involves heating a mixture of deuterium and tritium gases to immensely high temperatures and confining the resulting plasma with magnetic fields.

Stronger electromagnets allow the ‘burning’ plasma to be more strongly confined, and the fusion power density in the burning plasma varies as the fourth power of the magnetic field strength. 

In the implementation imagined by Hartwig, the HTS technology enables magnetic fields 1.74 times stronger, which allows an increase in power density by a factor 1.74 x 1.74 x 1.74 x 1.74 ≈ 9. 

Or alternatively, the apparatus could be made roughly 9 times smaller. So using no new physics, it has become feasible to make a fusion reactor which is much smaller than ITER. 

A smaller reactor can be built quicker and cheaper. The cost is expected to scale roughly as the size cubed – so the cost would be around 9 x 9 x 9 ~ 700 times lower – still expensive but no longer in the billions.

[Note added on 8/2/2021: I think this large factor is justified: see my response to the comment from Dr Brian VonHerzen for an explanation]

And crucially it would take just a few years to build rather than a few decades. 

And that gives engineers a chance to try out a few designs and optimise them. All of fusion’s eggs would no longer be in one basket.

The engineering vision

Dennis Whyte’s talk (link below) outlines the engineering vision driving the modern fusion ‘industry’.

A fusion power station would consist of small modular reactors each one generating perhaps only 200 kW of electrical power. The reactors could be produced on a production line which could lower their production costs substantially.

This would allow a power station to begin generating electricity and revenue after the first small reactor was built. This would shorten the time to payback after the initial investment and make the build out of the putative new technology more feasible from both a financial and an engineering perspective.

The reactors would be linked in clusters so that a single reactor could come on-line for extra generation and be taken off-line for maintenance. Each reactor would be built so that the key components could be replaced every year or so. This reduces the demands on the materials used in the construction. 

Each reactor would sit in a cooling flow of molten salt containing lithium that when irradiated would ‘breed’ the tritium required for operation and simultaneously remove the heat to drive a conventional steam turbine.

You can listen to Dennis Whyte’s lecture below for more details.

But…

Dennis Whyte and Zach Hartwig seem to me to be highly credible. But while I appreciate their ingenuity and engineering insight, I am still sceptical.

• Perhaps operating a reactor with 500 MW of thermal power in a volume of a just 10 cubic metres or so at 100 million kelvin might prove possible for seconds, minutes or hours or even days. But it might still prove impossible to operate 90% of the time for extended periods. 
• Perhaps the unproven energy harvesting and tritium production system might not work.
• Perhaps the superconductor so critical to the new technology would be damaged by years of neutron irradiation

Or perhaps any one of a large number of complexities inconceivable in advance might prove fatal.

But on the other hand it might just work.

So I now understand why fusion scientists are doing what they are doing. And if their ideas did come to fruition on the 10-year timescale they envision, then fusion might yet still have a contribution to make towards solving the defining challenge of our age.

I wish them luck!

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Videos

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Video#1: Pathway to fusion

Zach Hartwig goes clearly through the MIT plan to make a fusion reactor.

Timeline of Zach Hartwig’s talk

• 2:20: Start
• 2:52: The societal importance of energy
• 3:30: Societal progress has been at the expense of CO2 emissions
• 3:51: Fusion is an attractive alternative in principle. – but how to compare techniques?
• 8:00: 3 Questions
• 8:10: Question 1: What are viable fusion fuels
• 18:00 Answer to Q1: Deuterium-Tritium is optimal fuel.
• 18:40: Question 2: Physical Conditions
  • Density, Temperature, Energy confinement
• 20:00 Plots of Lawson Criterion versus Temperature.
  • Shows contours of energy ration Q
  • Regions of the plot divided into Pointless, possible, and achieved
• 22:35: Question 3: Confinement Methods compared on Lawson Criterion/Temperature plots
  1. Cold Fusion 
  2. Gravity
  3. Hydrogen Bombs
  4. Inertial Confinement by Laser
  5. Particle accelerator
  6. Electrostatic well
  7. Magnetic field: Mirrors
  8. Magnetic field: Magnetized Targets or Pinches
  9. Magnetic field: Torus of Mirrors
  10. Magnetic field: Spheromaks
  11. Magnetic field: Stellerator
  12. Magnetic field: Tokamak
• 39:35 Summary
• 40:00 ITER
• 42:00 Answer to Question 3: Tokamak is better than all other approaches.
• 43:21 Combining previous answers: 
  • Tokamak is better than all other approaches.
• 43:21 The existing pathway JET to ITER is logical, but too big, too slow, too complex: 
• 46:46 The importance of magnetic field: Power density proportional to B^4. 
• 48:00 Use of higher magnetic fields reduces size of reactor
• 50:10 High Temperature Superconductors enable larger fields
• 52:10 Concept ARC reactor
  • 3.2 m versus 6.2 m for ITER
  • B = 9.2 T versus 5.3 T for ITER: (9.2/5.3)^4 = 9.1
  • Could actually power an electrical generator
• 52:40 SPARC = Smallest Possible ARC
• 54:40 End: A viable pathway to fusion.

Video#2: The Affordable, Robust, Compact (ARC) Reactor: and engineering approach to fusion.

Dennis Whyte explains how improved magnets have made fusion energy feasible on a more rapid timescale.

Timeline of Dennis Whyte’s talk

• 4:40: Start and Summary
  • New Magnets
  • Smaller Sizes
  • Entrepreneurially accessible
• 7:30: Fusion Principles
• 8:30: Fuel Cycle
• 10:00: Fusion Advantages
• 11:20: Lessons from the scalability and growth of nuclear fission
• 12:10 Climate change is happening now. No time to waste.
• 12:40 Science of Fusion:
  • Gain
  • Power Density
  • Temperature
• 13:45 Toroidal Magnet Field Confinement:
• 15:20: Key formulae
  • Gain 10 bar-s
  • Power Density ∝ pressure squared = 10 MW/m^3
• 17:20 JET – 10 MW but no energy gain
• 18:20 Progress in fusion beat Moore’s Law in the 1990’s but the science stalled as the devices needed to be too big.
• 19:30 ITER Energy gain Q = 10, P = 3 Bar, no tritium breeding, no electricity generation.
• 20:30 ITER is too big and slow
• 22:10 Magnetic Field Breakthrough
  • Energy gain ∝ B^3 and ∝ R^1.3 
  • Power Density ∝ B^4 and ∝ R 
  • Cost ∝ R^3 
• 24:30 Why ITER is so large
• 26:26 Superconducting Tape
• 28:19 Affordable, Robust, Compact (ARC) Reactor. 
  • 500 MW thermal
  • 200 MW electrical
  • R = 3.2 m – the same as JET but with B^4 scaling 
• 30:30 HTS Tape and Coils.
• 37:00 High fields stabilise plasma which leads to low science risks
• 40:00 ARC Modularity and Repairability
  • De-mountable coils 
  • Liquid Blanket Concept
  • FLiBe 
  • Tritium Breeding with gain = 1.14
  • 3-D Printed components
• 50:00 Electrical cost versus manufacturing cost.
• 53:37 Accessibility to ‘Start-up” entrepreneurial attitude.
• 54:40 SP ARC – Soomest Possible / Smallest Practical ARC to Demonstart fusion
• 59:00 Summary & Questions

Tags: Nuclear Fusion