A Question About Nuclear Fusion

[Jade44]

I freely admit that my knowledge in the area of nuclear fusion is extremely limited, but I do remember one aspect of nuclear fusion that was stressed in an article I read many years ago; and that was that as an energy source it was completely safe.

The article stated that nuclear fusion depended on the existence of temperature in the hundreds of millions of degrees celsius; but that if the fusion reaction ever lost containment all that would happen would be that reaction would simply stop. In other words, unlike a fission reaction nuclear fusion was absolutely safe as once containment was breached the reactor would no longer be able to maintain the necessary temperatures and the reaction would just fizzle.

The reason I am asking this is because a number of authors have featured the failure of fusion reactors in their stories, among them David Drake and Graham Sharp Paul. In their vision of the future the fusion reactors explode with the intensity of thermonuclear bombs. I was just wondering if this event is pure fiction or would fusion reactors actually detonate in such a manner.

 

[chrispenycate]

Absolutely safe is a total nul concept when dealing with energies like this; a windmill isn't totally without risk.

What they probably wanted to say is there's no risk of a runaway chain reaction, a China Syndrome. No vast radioactive clouds drifting downwind. No polluted streams or lands. No (as was feared before the bikini test) hydrogen in seawater beginning to fuse and setting the oceans light, making for a very small (and extremely temporary) star. (Yes, before the test certain scientists thought there was a possibility of setting off a chain reaction in Earth's oceans – but as it was less than a one in a thousand chance, they went ahead and did the test anyway).

But several kilos of matter at several millions of degrees Celsius being released suddenly does give a fairly good simulation of a nuclear explosion, yes. Like Hiroshima, though without the radiation and fallout, and you would presumably build concrete walls and berms to protect your population, but it's essentially just energy release, not poison.

The hypersonic shock wave, even if diverted, will burst eardrums and windows for kilometres arond, and pulp anyone not incinerated in the immediate vicinity. It'll probably cause tsunami, if the facility is on an island, for safety or cooling reasons, and quite possibly change local weather conditions.

Still, in what way is this different from a traditional generator producing the same number of megawatts? The temperature is higher, but the quantity less. It's just enough energy to boil a million kettles, microwave a hundred thousand TV dinners and illuminate most of the Sahara desert escaping in a restricted space in a short interval of time. Hardly a problem for humans who've been farming the slopes of volcanoes for tens of millennia.

 

[Venusian Broon]

Off the top of my head, the simple answer would be no.

Current thermonuclear weapons, or what we called Hydrogen bombs when they came out, use a fission bomb to activate a fusion component, thus increasing the destructive yield. But, it should still be pointed out that the majority of the destructive energy still comes from the fission bomb.

So to make a reactor explode in a thermonuclear fashion, you would have to comparable forces to impact, say, the working tokamak of a fission reactor (if you are using that design and going for magnetic fusion, there are other designs that avoid this i.e. using very high powered lasers and other methods.)

In real life I'd doubt very much (but am ready to be corrected!) that we can even begin to approach the intensity of magnetic field required to create a bomb-like explosion. Plus the fact that there is a limited amount of hydrogen/deutrium in the chamber that puts limits on any destructive energy - even if all of it was to fuse in a flash. Concievably you could 'pump it up' but my guess is that current methods of containment would only really trap a fixed small amount (again I'm guessing here - I don't know how much stuff flies about the current fusion experiments)

Perhaps a Chernobyl style explosion flinging lots of radiactive nuclear compounds about.

Of course in SF land, we can put in thingymabobs, 'new' physics and unobtanium to use to make your fictional reactor go off like a 5 kiloton warhead.

 

[nightdreamer]

Not totally safe, no. Fusion reactions still produce high-energy gamma radiation, and most of the familiar reactions also produce free neutrons. Of the ones that don't produce a lot of free neutrons, many still produce protons, which are probably safer. Regardless, fusion reactors would still need shielding. The way in which they are safer than fission is that they do not produce large quantities of radioactive waste as spent fuel and there should be less of a tendency to make all the structural materials radioactive, especially with an aneutronic reaction and magnetic confinement. I'd think one going "critical" and blowing up is highly unlikely.

BTW: Not a single one of mine has ever blown up.

 

[steve12553]

Gosh Guys, I feel better.

 

[Jade44]

Thanks to those who took the time to answer my questions, but there does not seem to be a definitive answer so far, so I did what I should have done in the first place and that is Google the question. When I did I found this answer at this site. http://www.generalfusion.com/safety.html

To quote:
No risk of meltdown or explosion

Fusion systems cannot melt down or explode since the fusion reaction only acts on a small amount of nuclear fuel at a time and can only occur if suitable conditions can be created and maintained for a sufficient time. If any part of the process does not work perfectly, fusion will not occur. In contrast, in a fission reactor, fuel is added in bulk and the reactor controls the rate at which a chain reaction occurs; if the control mechanism fails, the reaction can run away and a meltdown can occur.

I also found this more detailed article on Wikipedia
https://en.wikipedia.org/wiki/Fusion_power

Once again to quote:
Accident potential

There is no possibility of a catastrophic accident in a fusion reactor resulting in major release of radioactivity to the environment or injury to non-staff, unlike modern fission reactors. The primary reason is that nuclear fusion requires precisely controlled temperature, pressure, and magnetic field parameters to generate net energy. If the reactor were damaged, these parameters would be disrupted and the heat generation in the reactor would rapidly cease.https://en.wikipedia.org/wiki/Fusion_power#cite_note-afraid-39
Fusion reactors are extremely safe in this sense, and it makes them favorable over fission reactors, which, in contrast, continue to generate heat through beta-decay for several months after reactor shut-down, meaning that melting of fuel rods is possible even after the reactor has been stopped due to continued accumulation of heat.https://en.wikipedia.org/wiki/Fusion_power#cite_note-McCrackenStott2012-40
There is also no risk of a runaway reaction in a fusion reactor, since the plasma is normally burnt at optimal conditions, and any significant change will render it unable to produce excess heat. In fusion reactors the reaction process is so delicate that this level of safety is inherent; no elaborate failsafe mechanism is required. Although the plasma in a fusion power plant will have a volume of 1000 cubic meters or more, the density of the plasma is extremely low, and the total amount of fusion fuel in the vessel is very small, typically a few grams.https://en.wikipedia.org/wiki/Fusion_power#cite_note-McCrackenStott2012-40 If the fuel supply is closed, the reaction stops within seconds. In comparison, a fission reactor is typically loaded with enough fuel for several years, and no additional fuel is necessary to keep the reaction going.https://en.wikipedia.org/wiki/Fusion_power#cite_note-Angelo2004-41
In the magnetic approach, strong fields are developed in coils that are held in place mechanically by the reactor structure. Failure of this structure could release this tension and allow the magnet to "explode" outward. The severity of this event would be similar to any other industrial accident or an MRI machine quench/explosion, and could be effectively stopped with a containment building similar to those used in existing (fission) nuclear generators. The laser-driven inertial approach is generally lower-stress. Although failure of the reaction chamber is possible, simply stopping fuel delivery would prevent any sort of catastrophic failure.
Most reactor designs rely on the use of liquid lithium as both a coolant and a method for converting stray neutrons from the reaction into tritium, which is fed back into the reactor as fuel. Lithium is highly flammable, and in the case of a fire it is possible that the lithium stored on-site could be burned up and escape. In this case the tritium contents of the lithium would be released into the atmosphere, posing a radiation risk. However, calculations suggest that at about 1 kg the total amount of tritium and other radioactive gases in a typical power plant would be so small that they would have diluted to legally acceptable limits by the time they blew as far as the plant's perimeter fence.[42]
The likelihood of small industrial accidents including the local release of radioactivity and injury to staff cannot be estimated yet. These would include accidental releases of lithium, tritium, or mis-handling of decommissioned radioactive components of the reactor itself.
So, to sum up it appears that from a scientific viewpoint that an explosion from the fusion reactors featured on the starships and vehicles in novels written by a number of authors is remote to impossible. Good news from the point of view of those who advocate nuclear fusion and good news for humanity if nuclear fusion is ever achieved.

But it is bad news for SF authors who so enjoyed having the fusion reactors in their novels explode with extreme violence.

 

[Mark_Lawrence]

I freely admit that my knowledge in the area of nuclear fusion is extremely limited, but I do remember one aspect of nuclear fusion that was stressed in an article I read many years ago; and that was that as an energy source it was completely safe.

The article stated that nuclear fusion depended on the existence of temperature in the hundreds of millions of degrees celsius; but that if the fusion reaction ever lost containment all that would happen would be that reaction would simply stop. In other words, unlike a fission reaction nuclear fusion was absolutely safe as once containment was breached the reactor would no longer be able to maintain the necessary temperatures and the reaction would just fizzle.

The reason I am asking this is because a number of authors have featured the failure of fusion reactors in their stories, among them David Drake and Graham Sharp Paul. In their vision of the future the fusion reactors explode with the intensity of thermonuclear bombs. I was just wondering if this event is pure fiction or would fusion reactors actually detonate in such a manner.

I worked on laser-driven fusion, spending far more time than I wanted to at the Culham Laboratories which housed the Joint European Torus (not actually on JET which is a magnetic confinement system). There's plenty of potential for localised harm, especially on an industrial scale, but I can't see any accident having great effect beyond the security fence. The mass of material at high temperature at any one time would be very small.

 

[RichF]

I recently visited the JET tokamak (the fusion reactor in Oxfordshire) where they achieved a power input to output ratio of 0.7, which is pretty impressive. According to them the plasma is at a temperature of 150 million degrees but the amount of matter at this temperature is so small and the conditions have to be perfect for the plasma to persist any breach of the vacuum will instantly diffuse it as you say. And as others have pointed out there's no large quantities of fission products to disperse over the environment just small quanitities of tritium which is not the most dangerous nuclear product as it passes through the body extremely quickly. But the question was asked whether a significant explosion was possible and the answer given was a resounding no.

An interesting aside was that apparently a compost heap produces more energy per metre cubed than the sun. Hence why fusion reactors use tritium and deuterium as their reaction produces more energy.

 

[Stephen Palmer]

An interesting aside was that apparently a compost heap produces more energy per metre cubed than the sun. Hence why fusion reactors use tritium and deuterium as their reaction produces more energy.

That's the most interesting fact I've heard this year!

 

[Venusian Broon]

An interesting aside was that apparently a compost heap produces more energy per metre cubed than the sun. Hence why fusion reactors use tritium and deuterium as their reaction produces more energy.

I suppose it makes sense when you think of compost as concentrated, stored and processed star mass/sunlight

 

[nightdreamer]

An interesting aside was that apparently a compost heap produces more energy per metre cubed than the sun. Hence why fusion reactors use tritium and deuterium as their reaction produces more energy.

Quite true, but the way I heard it was that your body produces more heat per unit volume than the sun.

 

[hitmouse]

Quite true, but the way I heard it was that your body produces more heat per unit volume than the sun.

Neither fact (body or compost) remotely true. I would be curious to see the sources for these pieces of information.

 

[RichF]

Neither fact (body or compost) remotely true. I would be curious to see the sources for these pieces of information.

Yes it is, after a quick google: http://www.efda.org/2012/03/more-powerful-than-the-sun/

The Sun's large energy output is because of its size not because it produces large amounts of energy per volume.

 

[hitmouse]

The Sun has a diameter of 1,391,980km
..so a volume of 10 ^ 18m3
power output is 3.826 x 10 ^ 26 Watts (source)

..so that's a power output of 3.8 * 10 ^ 8 Wm-3


If only we could get a compost heap giving out half a gigawatt per metre cubed. Energy crisis solved, forever.

 

[chrispenycate]

The actual bit of the sun (well inside) where conditions are suitable for hydrogen fusion (high enough pressure and temperature – temperatures an order of magnitude higher than we were considering for the tritium/deuterium reaction talked about earlier, which is a major reason for using the rarer isotopes rather than attempting a Bethe solar phoenix catalytic CNO reaction {do you know how difficult it was to Google any information about the Bethe cycle, even knowing how to spell all the words right?}) and generates much more energy per volume than a compost heap, a human or even a Saturn five rocket; but it is a tiny, tiny fraction of the sun's total mass, and due to density gradients an even smaller fraction of the total volume. So it's not worth writing a story about a planet orbiting a very large compost heap.

 

[nightdreamer]

The Sun has a diameter of 1,391,980km
..so a volume of 10 ^ 18m3
power output is 3.826 x 10 ^ 26 Watts (source)

..so that's a power output of 3.8 * 10 ^ 8 Wm-3

You forgot to convert km^3 to m^3. My Mathematica ramblings:

In[18]:= volume=4/3.*Pi*((1391980*1000/2)^3) (* to meters^3*)
Out[18]= 1.4122*10^27
3.826*(10^26)/volume (* to watts/meter^3*)
Out[19]= 0.270924

This is in agreement with RichF's link.

 

[RichF]

The Sun has a diameter of 1,391,980km
..so a volume of 10 ^ 18m3
power output is 3.826 x 10 ^ 26 Watts (source)

..so that's a power output of 3.8 * 10 ^ 8 Wm-3


If only we could get a compost heap giving out half a gigawatt per metre cubed. Energy crisis solved, forever.

Did you not read the article? If you do not trust that source the volume of the sun is 1.4*10^27 metres cubed: http://solarsystem.nasa.gov/planets/profile.cfm?Display=Facts&Object=Sun. So 3.826 x 10 ^ 26 Watts/1.4*10^27 = 0.273 watts per metre cubed.

The estimate is higher in the EFDA article because it only considers the core.

(Sorry didnt see above post)

 

[hitmouse]

OK this is more interesting than I thought initially. I think that my original comment was a bit blunt. And on second look I make the suns volume more like 10 e27

Talking to a some physicist friends I have had the following comments:

The point is that although nuclear fusion can generate stellar amounts of power, it only works at extreme temperature and pressure - which means it's only achieved in the innermost quarter of the Sun; which actually means that about 98% of the volume of the Sun has no part in the energy generation.

and

The sun's fusion happens entirely within a central core which is something like about 2.5% of the total volume. However even allowing for that, a factor of 40 still doesn't make it a very large emitter of energy per unit volume within that core. However when you are size of the sun, and can carry out processes in your interior rather than just on your surface, you can afford to be pretty inefficient with your use of space in comparison to applications confined to the surface of the earth, and still get a truly huge output from it.

Re: Rich F's 08.37

I worked at JET for a summer, and attended the European Summer School on Plasma Physics and Fusion.

JET does indeed produce more power than the Sun per kg of fuel during a run. But they operate at densities much much lower than the Sun, and have a lot less mass. When EFDA manage to make a fusion reactor the mass of the Sun, then they will be able to make that claim. And JET can only sustain burning for 60 seconds or so every 45 minutes.

Also, the Sun burns hydrogen, the most abundant element in the universe, and has fuel for the next 5-10 billion years stored in it. JET and ITER will burn deuterium and tritium, which are much much less abundant, and only contain enough fuel for 90 seconds or so.

 

[Venusian Broon]

which actually means that about 98% of the volume of the Sun has no part in the energy generation

That's not really true - if it wasn't for the mass of the 98% bearing down and putting the core at intense pressures there would be no fusion whatsoever. If you removed that, it would just be a massive Jupiter-type object, or brown dwarf if you were lucky. So all that non fusion volume is essential for a star to shine at the wattage that it does.

 

[hitmouse]

That's not really true - if it wasn't for the mass of the 98% bearing down and putting the core at intense pressures there would be no fusion whatsoever. If you removed that, it would just be a massive Jupiter-type object, or brown dwarf if you were lucky. So all that non fusion volume is essential for a star to shine at the wattage that it does.

Agreed. It is just that the fusion itself happens in a small part of the core. I like to have my preconceptions overturned sometimes.

However, the comparison with the compost heap or the human body, whilst giving a useful mental hook, is a little bit misleading, if one considers heat generation in those systems is due to chemical reactions, whereas energy is produced by the sun through nuclear processes, which are quite different.