Physics on the Metro

Have you ever spotted how the air moves on metro trains?

On ordinary trains with short carriages there is often a flow of air through the carriage as the train moves along.  It brings blessed relief on those hot summer days!

But something different happens on those trains that are open from one end to the other, and the effect depends strongly on the number of people on the train.

One of those long un-divided trains on Paris metro line 1

If the train is fairly empty, you get a strong rush of air from the front to the back as it accelerates.  Quite often the train accelerates in steps, and you get repeated breezes of different strengths, correlating well with the driver's actions.  Then as the train slows down, the air rushes to the front of the train.

This shows that the mass of air inside the train has to be pushed to get it to speed up and slow down.  The push comes from a pressure gradient which is set up as the train changes velocity.  That gradient is established by filling one end of the train with more air, increasing the local pressure.

I had spotted this happening on a journey on an empty train.  The following day on the same metro line I was expecting to observe the same effect.   However, it happened to a much less noticeable extent.  What was different?  The train was full of people standing.  That meant that the air flow was significantly better damped.

The effect is also much clearer if you sit near the middle of the train than it is at the ends - obviously.  After a week of observations I think this is good(ish) physics.

On the other hand . . . on another journey I watched a roller skater walking down stairs (very carefully) and then rolling along the platform.  As he slowed himself down he spread his feet wide apart, lowering his centre of gravity.  To stop himself he brought them sharply back together, lifting his whole body a few centimetres.  It appeared to me at first sight that he had converted his kinetic energy into potential energy in doing this - but on reflection that is not consistent with conservation of momentum.  This wasn't good physics after all.

Perhaps it was just a flourish in his performance after all.

Isn't physics fun?  It gets everywhere!

Prince Rupert's legacy

Watching a TV programme recently (which is something that I rarely do) I was reminded of the incredible properties of glass.  Can you imagine a glass bulb that will not break when you hit it with a hammer, and yet will explode violently in the event of another tiny intervention.

If not, you will be amazed by Prince Rupert's Drops.  Wikipedia has a good article on the topic (as you would expect), but this video is also spell-binding and explanatory!

200W from The Earth

200 watts!  That is the amount of energy that the Earth/Sun system is emitting as gravitational energy as they orbit each other - according to Einstein's theories of relativity.

When I say they 'orbit each other' of course I meant that the Earth goes around the Sun and the Sun wobbles a bit.  (See more in the Wikipedia article about gravitational waves.)

You could run a few light bulbs on that much power, but that is all.

Only an opinion about nothing . . .

Oxford Skeptics in the Pub had an unusually 'empty' talk this week, entitled "Why nothing matters".  An I'm not saying that the talk didn't happen but that it was full of nothing.

An audience of only about 30 heard Ronald Green speaking about "Why Nothing Matters", hawking his book and clearly expecting to be heckled.  Indeed his style was more-or-less confrontational while he told us that he had spent 5 years studying 'nothing' very seriously.  It is not quite clear where he was when he did that study, nor indeed which universities he has taught at in various continents, or exactly what post-graduate studies he participated in at Oxford University (or indeed which university in Oxford).  But maybe I am too skeptical in wondering about those questions.

Was it obvious that it is true that nothing matters?  You have to ask yourself how deep the following comments are.

"There has to be something around nothing, and there has to be nothing around something."

"Luke says 'nothing is impossible with God' ".

"Zero apples is not the same as zero oranges" and "there are different kinds of nothing".

"Really, nothing is the absence of everything, including ourselves"

Apparently 'silence' might be one kind of nothing [but I don't think the absence of sound is enough to count a real 'nothing']. But he claims that you can never actually have complete silence.  I venture to disagree with this particular claim on grounds of some physics.  If you were in a vacuum there definitely would be silence.  His claim that you hear yourself when it gets quiet enough and that there there is never silence is surely philosophically flawed, because the role of the observer is irrelevant.  If you were not alive and were in a vacuum there definitely would be silence and it doesn't matter a jot that you therefore don't know about it.

More interesting was the tale of the theft of the Mona Lisa in 1911. Apparently far more people queued to see where it had been stolen from than had wanted to see the painting itself. 

But my final point is that I prefer the approach taken by Peter Atkins in his recent talk and Lawrence Krauss in his "A Universe from Nothing".  Green claims that "even Krauss gets terribly mixed up about it".  I think I would ask whether it is overwhelmingly probable that that is true.  Ask who is more mixed up, and I think the answer seems obvious.

On the other hand I saw a review of his book . . .

[Unlike the talk] This is not a book to be dipped into or skimmed over a coffee break. Green writes very clearly and with a great deal of humour, but he is dealing with ideas that perhaps go to the very core of what it means to be human. That he can do so without the nihilistic melancholy of so many of the people he quotes is a tribute to his writing skills. Would I recommend this book? Yes, with the proviso that if you choose to read it you give the time and thought it deserves.

So maybe I misjudge him. 

One foot per nano-second

If anyone ever asks you how fast light travels, you might like to tell them

One foot per nano-second!

Translating that into metric units it is about 30cm in a billionth of a second (where this is a UK billionth - namely one thousand millionth of a second).

Not many people know that!  But for some of us it is actually a useful rule-of-thumb in our daily work.

You might also like to surprise someone with the unit of a micro-century.  Just between you and me, that is just over an hour.  Business meetings should generally limited to a micro-century.  In that time, light manages to travel about 750 million miles.

Icy surprise in the hedge rows

Having mentioned one icy mystery last week, here is another that I have spotted in southern England this week.  We have had a spell of bright but cold weather - around freezing plus or minus a few degrees for a few days.

In the hedgerows by the roads, once in a while you find a surprising and spectacular patch of icicles like this

Icicles in the hedges in UK

or this

More icicles in the hedges in UK

and just a metre or two to each side the hedges are completely dry and free of ice.

It turns out that there is always a puddle of nearly-freezing water on the road and that it is constantly being splashed into the hedge by passing vehicles.  Some of it freezes and forms icicles, and the remains that drip off might form icicles on the ground.

Swedish icy mystery

My friend and colleague, Tom, was visiting family in Sweden recently and on a forest walk he came across this remarkable and so far unexplained icy feature and photographed it.

Surprising sharp edge around a patch of ice. 

At first glance it just looks like an icy puddle, but when you look more carefully it has some very peculiar and surprising features.

Closer view of the upstanding edge - Swedish icy mystery!

Tom described it as follows:

I have never seen or heard of anything even remotely like it before. I probably saw about a thousand other frozen puddles that week, and none of them had any unusual features at all.

Stalks of grass through the mysterious sharp ice feature.

It had snowed about 10-15cm some days before we got there and this had largely thawed by the time of this forest walk, after several days of the temperature being about -2°C overnight and 0 to +2°C during the day.

There are many deer in the forest, and a few dog walkers. Several other puddles showed evidence of the water level having gone down under the ice crust before refreezing. I now see that where some larger grass stalks had lain through the developing ice wall, presumably wiggling in the breeze, they have made round holes in the wall. The little ones are just frozen in position as I described the other day.

Another view - including a reed passing through a melted hole.

Perhaps the explanation for the large green reed having melted a hole is more related to it being a dark colour, and thus collecting more warmth from the sun, but how this sharp edge was formed is still a mystery.

So . . . are there any arctic explorers out there who have an explanation please?

Superfluids

Think of a fluid with no viscosity.  In other words it is perfectly runny and if you stir it to start it rotating in a beaker it will spin for ever.  Literally for ever! 

This same magical fluid can also climb up the walls of the beaker containing it, run over the top and down the outside, to drip off the bottom.  And to cap it all, it transfers heat perfectly well too.

Surely this fluid doesn't exist.

Well, surprisingly it does!  It was well studied by Jack Allen at The University of St Andrews.  It is called a 'superfluid' and the best known example of it is liquid helium (specifically helium 4 rather than the lighter isotope, helium 3).  'Normal' liquid helium has a boiling point 4.2 degrees above absolute zero, or as we describe that temperature on the kelvin scale it is at 4.2K.

However, if you cool this liquid to below 2.2K it changes into a new form.  It still looks like an ordinary colourless liquid (and yes it really is possible to photograph things that are this cold if you do it carefully)  but it now has the amazing properties described above.  The embedded video shows you enough that you might want to investigate the subject further.  I hope you will.

Thank goodness water doesn't have these properties.  Imagine if your nice hot cup of tea could escape from its cup and drip onto your legs.  Ouch!

Small note:  In memory of my friend Bob Mitchell who died last year.  Bob was a key member of the team of cryogenic experts who first filmed the effects of superfluidity for Jack Allen, at St Andrews University.  He also inspired the first part of my career, working with superconducting magnets and cryostats, and as luck would have it, he was the customer on my first cryogenic system installation .

Fill your tyres with nitrogen

Did you know that some people claim that there is a special benefit to be gained by filling your tyres with nitrogen instead of air?

Why could that be?

The gas inside a tyre is only there for one purpose - namely to keep the tyre from going flat (and specifically flat at the bottom as it is rare for a tyre to go flat at the top).  The right pressure ensures that the correct area of rubber is in contact with the road and takes up some of the vibration from the uneven surface. 

Air is 78% nitrogen already, and all but 1% of the remainder is oxygen, so what might be the advantage of going for a gas that you have to pay for instead of free air?

Nitrogen is very slightly lighter than air, so the 'moment of inertia' of the whole wheel might be reduced, but only by an unnoticeable amount.  Nitrogen is dry whereas air contains a little moisture which might condense into a tiny droplet or two of water.  So what?  Certainly corrosion of wheels isn't such a massive problem.  When did you last see a wheel rusted away from the inside.  Why could that possibly be worse than the constant attack of the atmosphere on the outside of the wheel?

Some claim that oxygen molecules are smaller than nitrogen so it diffuses more easily through the rubber allowing the pressure of the tyre to vary from optimal more quickly.  This is plain wrong.  Oxygen is bigger and will diffuse more slowly (albeit not much bigger and not much more slowly).

It is also plain wrong to claim that oxygen expands and contracts more if the tyre temperature changes.  Anyone claiming that is unaware of the 'gas laws' -  basic physics taught to 15 year-olds.

There must be only one explanation for the recommendation to use nitrogen in tyres.

Profit!

Surely it just another way to extract money from the gullible.

Small note: Or have I missed something else?

And yes - we really do spell the word tyre with a y in UK English.  

Thermodynamics of Hell

You sometimes see amusing explanations using science to demonstrate that heaven is actually hotter than hell, or that hell is endothermic or exothermic.  (Don't let those words put you off reading further!)

But what about the concept of the eternal existence of hell?  Is it even a possibility that eternal torment can be maintained?

The second law of thermodynamics seems to suggest that it is not possible. It uses mathematics to describe the way that hot things cool down and that the universe will end its days uniformly cold and lifeless. 

Of course maybe hell is not in this universe but in one where our laws of physics do not operate.  And just in case you get too excited about this, bear in mind that it might take trillions of years to ensure that hell is cold enough to be comfortable.

So that leaves us with one of the usual quandaries. 

Even if hell exists in some non-physical sense, what kind of merciful god would sentence someone to torment for a mere trillion years just because of the sins that they committed during an adult lifetime of a few decades?

Would you worship a god like that?

The small matter of orientation - that's solar panel orientation

I have been thinking about installing some solar panels.  Now obviously if you are installing them on a building, it is easy to see that mounting the panels to face in some directions would be better than others.  In the Northern hemisphere, pointing the panels in a generally southerly direction is obviously better than a northerly direction.  At a particular time of year it might even be possible to find the optimum angle.

However, on a movable structure - namely a boat - it would be much more difficult to choose the best direction.  On any day of the year the boat might be facing to the North, South, East and West at some time of the day.  That could leave you constantly worrying about setting your panels to produce the largest amount of power.  This is something that I don't want to bother with.

The one direction that you can always guarantee to point to, with reasonable accuracy, is UPWARDS!   So I have been wondering whether this is could be the best compromise.  Obviously it is not the very best in ideal conditions, but from the point of view of a pragmatist who wants a tidy roof and the least possible bother, would this be a silly choice?

It seems not.  According to this site, pointing vertically upwards will lose 10% of the optimum energy collection (in the UK at least).  At lower latitudes that loss would obviously be reduced.  A mere 10% loss is much better than pointing the panels in completely the wrong direction for a lot of the time.

Now the only problem is that England is often rainy.  A flat panel will collect water.  How much shall I tilt the panels to keep them clean and relatively dry?

Long live free will!

My friend Steve Zara has recently become involved in an online 'debate' with Jerry Coyne about free will.  As you might remember, I wrote a blog post about the role of Determinism and Chaos in free will, a few months ago.

It seems to me that Jerry Coyne puts up a bit of a straw man when arguing against Jim Al-Khalli's view of free will.  Al-Khalli hardly touches on the question of dualism, at least in the sections quoted by Coyne, and anyway this is all a paper tiger.  (The word is never used in the whole of his article.)

The more serious part of the argument is about how chaos can come to the rescue and actually give us free will without falling into total anarchy.

As I explained in my earlier post, the detailed future state of the universe is not predictable, even in principal.  I gave three reasons to support that idea, namely chaos, Heisenberg's Uncertainty Principle, and quantum fluctuations.

Chaos appears to me to be the strongest factor in this.  The other two might not be significant in everyday life, but over a greater length of time they could build up to make a difference.  It might be possible to compute your way out of the chaos conundrum, but by increasing your computing power you only delay the moment of uncertainty.

However, it is important to note that this unpredictability comes in small doses.  Although we can't predict exactly what we might think or do tomorrow, as new uncertainties approach us, we can at least predict what are the more likely trends.  Without resorting to dualism, our minds have an emergent internal consistency which tends to make us act in a way that is consistent with the way we acted yesterday.

This means that chaos and unpredictability make the future uncertain, but without robbing us of our free will they ensure that future trends are likely to be consistent with what we expect.

Long live free will . . .  even if Christians do often use it as an excuse for the atrocities committed by their God!

Lichtenberg Figures

I recently came across this fascinating image on Facebook.  It shows the interesting pattern left on the green of a golf course after a lightning strike.

A 'Lichtenberg Figure' on a golf course.

Apparently the crazy patterns are called 'Lichtenberg figures', and if you do a Google search for images of these figures you find some other amazing pictures like this one.

Lichtenberg figure on a man's back - struck by lightning!
(from this interesting site)

A quick glance at the relevant Wikipedia page is also interesting.

Which lands first?

Test your knowledge of basic Newtonian mechanics.

If a marksman fires a bullet perfectly horizontally over a level stretch of ground and at the exact same instant drops an identical bullet to the ground, which of them hits the ground first?

Of course we are all tempted to say that the one that has been dropped will land first, and of course we will all be wrong to say that.  Both should hit the ground at the same moment.

The really surprising thing is that the fired bullet has travelled so far in the short time before it lands.

Fusion Fuels part 5 - Tritium.

This is part of a series examining how the fuels for a fusion reactor are likely to be obtained.  In part 1, I described the Isotopes of hydrogen and named them.  In part 2, Mining deuterium, we saw how deuterium can be extracted from ordinary water, and brought up to a concentration of 20 to 25%, and in part 3 we saw how it can be 'vacuum distilled' to produce 'heavy water' with a purity of 99% deuterium (and a 1% impurity of protium).  Part 4 described how that water can be turned into deuterium gas by a process called electrolysis.

The other component of the fuel, namely tritium, is made by a completely different process. 

I mentioned previously that tritium is not found in nature in any significant quantities.  There is a tiny amount in the upper atmosphere where it is produced by high energy cosmic rays interacting with the gases that are present there.  Most of that tritium escapes into space, and the rest decays into ³He (which also then escapes).

It is true that there was a lot more tritium in the atmosphere in the past - particularly in the 1960s when USA and USSR were detonating hydrogen bombs and making a nuisance of themselves to the inhabitants of islands in the Pacific and in desert regions of USA and USSR.  Some people estimate that there may have been more than one tonne of tritium in the atmosphere at the time.  It is still a tiny quantity.

Tritium in air, peaked in 1963

The surprising thing is that all of that tritium was man-made in the arms race, using nuclear fission reactors.  You can look this up for yourself if you like.  I won't dwell on it because it is not the way that tritium is expected to be produced for fusion reactors in the future and therefore I don't care about it very much, except to acknowledge that the tritium used for fusion at the moment comes from a Canadian plant at Darlington in Ontario.  There they run a process to remove the tritium from the cooling water of the CANDU reactors.  In most CANDU reactors, the tritium is an inconvenient by-product.  However, just a few years ago, South Korea proposed building more CANDU reactors specifically in order to make tritium for the ITER project.

The fusion programme has other plans for the long term though.  There is another way to breed tritium and it is one of the greatest benefits of fusion as a future power source.  In principle, you can make your own tritium in your fusion reactor, as long as you get the engineering right.  That means that you will not have to transport the one and only radioactive material that you need to make fusion work.  This is all due to a fortuitous bit of physics.  Since all the physics of the production of deuterium seems to go against us, perhaps it is about time that something went the right way!

In the process of nuclear fusion, the heavy isotopes of hydrogen are forced to combine at high temperature.  As they fuse together to create helium, they have one neutron to spare.  About 80% of the huge amount of nuclear energy released by the reaction is in the kinetic energy of this neutron, and the remainder is in the energy of the helium ion.  Neutrons are not constrained by magnetic fields, so they escape from the hot plasma in the reactor, travelling in all directions.  The doughnut shaped plasma will be surrounded by a 'blanket' which is designed to stop as many of these neutrons as possible and force them to give up their energy as heat.  The heat will generate steam, and the steam will drive turbines to produce electricity.

But the cunning part of the plan is that if this blanket contains a widely available and relatively cheap metal called lithium we get an additional benefit.  If you can slow the neutrons down and 'collect' them with lithium, the metallic element gets turned into two gaseous elements, tritium and helium.  Hey!  Tritium!  As if by magic, the fusion process produces part of its own fuel.  All you have to supply to the system is fresh lithium (and good technology).

Obviously there is an additional step to the process, to separate the tritium from the helium (which is easy) and from the cooling water, which is a little more complex.  However, these aspects of the process are similar to the methods described to separate deuterium from ordinary water.

Each phase in this process has been demonstrated to work, and the ITER tokamak will eventually include a test module to prove the process on an experimental scale.  Industrialisation of the whole process is just one of the steps required to make the concept of fusion power into reality, but it is already well beyond the science-fiction stage.

Now one question remains.  Is it worth all the energy that is used to make the fuel?  Do you get more energy back from nuclear fusion than you put into manufacturing of the fuel.

The answer is affirmative.  The cost of the deuterium is trivially small and the indications are that the processing of lithium ore and production of tritium will cost about the same as the fuel for a fossil-fuel power station.

Now we just need to build a full scale fusion reactor!

Read other articles in this series:
Fusion Fuels: Part 1 - The isotopes of hydrogen
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 3 - Making 'heavy water'
Fusion fuels 4 - Electrolysis of heavy water

Rainbow over La Bastille

Paris - a rainbow over the Bastille monument
(in fact a double rainbow if you look askance)

You might also like to look at Ever thought about rainbows?, a post describing some of the interesting and surprising features of the rainbows that most of us see from time to time.

Fusion fuels 4 - Electrolysis of heavy water

This is part of a series examining how the fuels for a fusion reactor are likely to be obtained.  In part 1, I described the Isotopes of hydrogen and named them.  In part 2, Mining deuterium, we saw how deuterium can be extracted from ordinary water, and brought up to a concentration of 20 to 25%, and in part 3 we saw how it can be 'vacuum distilled' to produce 'heavy water' with a purity of 99% deuterium (and a 1% impurity of protium).  This liquid now has to be turned into deuterium gas by a process called electrolysis.

In chemistry or physics lessons at school you might have electrolysed water.  You probably added a bit of salt to the water so that it conducts electricity, put two electrodes into the water, and connected one end of a battery to each electrode.  You would have seen bubbles rising from the two electrodes and you might remember that one electrode produces bubbles of hydrogen and the other produces bubbles of oxygen.  Collecting the bubbles in a test tube, you probably enjoyed igniting the gas and hearing a loud pop.

In doing that, you are releasing the chemical energy of the hydrogen as it recombines with oxygen and becomes water again.  Hydrogen contains quite a lot of chemical energy, and this is why it is being considered as an alternative, carbon-free, fuel for cars.  But hydrogen contains massively more energy than this if you can release its physical energy by fusing its atoms together to turn them into helium.  This is the power source of the sun and stars.  The heavy form of hydrogen, called deuterium, will be even better for that because it fuses more easily.  Stars contain almost no deuterium at all.  As soon as a deuterium atom is created it fuses with another almost immediately.  This is why we want it as a fuel for fusion reactors.

If we carry out this electrolysis process on the 'heavy water' that we have made by the processes of isotopic exchange and vacuum distillation, we find that something useful happens.  Remember that we want to get rid of the protium and keep the deuterium.

Fortunately, they do not electrolyse at the same rate as each other. 

Unfortunately, as with vacuum distillation, the protium is more easily produced than the deuterium!

But knowing this, you can remove the protium first to leave a higher concentration of deuterium in the water, and then choose when the deuterium concentration is high enough to start collecting it.

Concentrations of 99.9% can be reached by this method.  This is high enough for fusion reactors to use.

At last we have one half of the fuel we need for fusion.  The other half is potentially a little less complicated except for the problem that it is still a little more conceptual. 

Next time I will cover the production of tritium before returning to discuss the energy balance of the production of deuterium.  Is it really worth spending all this energy to make the fuel for fusion?  I hope to convince you that the answer is an emphatic YES!

Next time:  Fusion Fuels part 5 - Tritium

Other articles in this series:

Fusion Fuels: Part 1 - The isotopes of hydrogen
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 3 - Making 'heavy water'

Things Christians say, part 37: Something from nothing?

A weekly series of responses to the things Christians say to atheists, based on the video reproduced here on 30th January 2012.  The aim is to tackle one every weekend, to give both a moderate, polite response to each question ('Piano'), followed by a more forceful rebuttal of the same question ('Forte'). 

It doesn't make any sense - you're saying that something came from nothing?

Piano

Interestingly, it is now quite well established that the things that we perceive as 'something' are rather precisely balanced by anti-somethings.  I'm not talking about matter and anti-matter, but something a little more subtle.

Matter and energy are closely related.  Mass doesn't come in negative quantities but believe it or not energy does.  And it turns out that the negative energy in the universe is rather accurately equal to the positive energy (which includes the energy that is tied up in matter.

In other words, our obvious somethings can indeed come from nothing.

Sometimes common sense doesn't work as our intuitions predict, but that doesn't mean that we are wrong.  Try going on a children's roundabout and throwing a ball to someone on the other side and see whether you can work out how the flight path of the ball makes sense.  With a bit of practise you would find that it did.  You have to do the same thing in physics.

This might be quite tricky physics that is not obvious to the ordinary reader - not that any of my readers are ordinary.    But I have much more trouble with theological arguments about the Trinity than I do about something coming from nothing.

Read some Lawrence Krauss.  You might enjoy it.


***

Forte

It doesn't make sense.  You're saying that God is three and yet one.

Where do arguments like that get us?

Anyway, wherever all the 'somethings' in the universe came from, you still have all your work in front of you to demonstrate that it was your particular God was involved.



Last episode: Faith is evidence of things unseen
Next week:  Aw - that's sad.

2 billion year old nuclear reactor

You might be as surprised as I was to learn that the remains of a 2 billion year old nuclear reactor has been found.  This is not recent news, and no, it is not evidence of a visit by aliens. 

This reactor was created naturally.

The evidence was found in Gabon, in Africa, by French nuclear scientists in 1972.  Although it is the only case that has yet been found, the phenomenon had been predicted as early as 1956.

In that area there are naturally-occurring deposits of uranium, and in a few places it is concentrated enough to reach the critical mass that is needed to start a self-sustaining chain reaction.  But it is not as simple as that.  Nuclear chain reactions will only work properly if you can control the behaviour of the neutrons.  As a uranium atoms split, they spit out high energy neutrons.  High energy neutrons like this might hit another uranium atom but they effectively bounce off without making that atom split.  The reaction is not sustained.

Perhaps counter-intuitively, the trick to make the reaction work properly is to slow down the neutrons.  When slow neutrons hit another atom they are more likely to provoke it to split, producing more neutrons and propagating the reaction.  So how do you slow down a neutron? 

You use water, (or something containing a lot of hydrogen).  You can think of it like this.  If you imagine the neutron as a table tennis (ping pong) ball  bouncing off a large steel ball, you would expect it to bounce back at almost the same speed and have no effect on the steel ball.  However, if it hit another light ball like itself, it might impart as much as half its energy to the other, and slow down in the process.  Neutrons and hydrogen nuclei have almost exactly the same mass, so the neutrons bounce around between the nuclei, slowing down more with each collision.

In this natural nuclear reactor, ground water managed to seep in and slow enough neutrons to get a chain reaction going.  That reaction created heat and turned the water into steam, driving it out and stopping the reaction again until the temperature dropped enough for the water to run back in.

It is calculated that this would turn into a continuous cycle just three hours in length until the uranium was all consumed.

That's pretty surprising, isn't it!


Small note:  You can read more detail here.

Fusion Fuels 3 - Making 'heavy water'

This is part of a series examining how the fuels for a fusion reactor are likely to be obtained.  In part 1 I described the Isotopes of hydrogen and named them.  In part 2, Mining deuterium, we saw how deuterium can be extracted from ordinary water, and brought up to a concentration of 20 to 25%.

In order to get the water to higher concentrations, a process called 'vacuum distillation' is often adopted.  That is the subject of this post.

Water that contains almost all deuterium and almost no protium is often called 'heavy water'.  You might remember that the 1965 movie "Heroes of Telemark" was a dramatisation of the true story of the destruction of a German heavy water plant in occupied Norway during the second world war.  This was necessary because heavy water is useful in some of the techniques used to produce enriched uranium for a nuclear weapon.  (As it turned out, German technology was not going in quite the right direction, but that was not known at the time.)

However, heavy water is also a source of the deuterium that is needed for the entirely peaceful and environmentally friendly fusion reactors of the near future, and it is only for this reason that I care about it enough to write this series.

In the last instalment, we reached the point where we have water with 20 to 25% deuterium atoms and the remainder still containing protium.  You might remember that I explained in part 1 that some lakes around the world (which have no rivers flowing out of them) have slightly higher concentrations of deuterium than 'ordinary' water.  This is because water molecules containing deuterium evaporate slightly less easily than water molecules containing protium.

Now you might be able to imagine a method of using this process on an industrial scale.  If you study the physics of the evaporation of water, you find that you can choose the right conditions of temperature and pressure where the H₂O evaporates preferentially and leaves the HDO and D₂O behind.  In fact it turns out that the best conditions for this are at a temperature slightly above the freezing point of water, and in rather a good vacuum. 

In an ideal world you would like to be able to evaporate the product that you want to keep in preference to the waste product.  After all, the whisky industry in Scotland makes its living by doing exactly this 'distillation'.  They warm a dilute mixture of alcohol in water and the alcohol evaporates preferentially.  The liquid that is distilled contains more alcohol and less water.  They usually repeat this distillation a few times in order to concentrate the alcohol further, taking the product from the first distillation and putting it through the process again and again to increase the purity. 

Unfortunately, in making heavy water the opposite is true.  The part that evaporates first is the part that you want to 'throw away' although in practise it will still contain much more deuterium than most of the water in the world.  It will not be discarded but returned to an earlier stage in the process.

The water that is left behind will be a bit more concentrated than it was before the H₂O was distilled away, and the last water to evaporate will be the most concentrated in deuterium.  Of course the whole process has no clear cut off points where all the H has been removed, leaving all the D behind.  But an iterative approach can yield higher and higher concentrations of deuterium and, in practise, a concentration of 99% can be achieved.  This might be good enough for most applications of heavy water. 

As a fusion fuel, a slightly better concentration might be preferred.

Fortunately the next stage in the process helps further.  It is in this stage that the heavy water is converted into deuterium gas, by a process known as electrolysis.  This gas is one half of the fuel we need for fusion.

More next time: Fusion fuels 4 - Electrolysis of heavy water

Other articles in this series:
Fusion Fuels: Part 1 - The isotopes of hydrogen
Fusion Fuels: Part 2 - 'Mining' deuterium.
Fusion Fuels part 5 - Tritium