DU radiation

Published 10 May 2003

I would like to comment on your excellent article on depleted uranium in Iraq (19 April, p 4).

Depleted uranium emits about 40 per cent fewer alpha particles than natural uranium, due to the removal of most of the uranium-235 and, more importantly, the uranium-234. Immediately after its production, that is the whole story.

However, within a few weeks of production, decay of uranium-238 re-establishes equilibrium quantities of the first two isotopes in the decay chain of uranium-238: thorium-234 and protactinium-234. These are both beta emitters, and once equilibrium is established, DU emits on average two beta particles for every alpha particle. The betas from protactinium-234 are particularly energetic.

These complicate the radiobiology considerably, because beta particles have much longer ranges in tissue, affecting large numbers of cells to a minor (possibly carcinogenic) extent, as opposed to the small number of cells heavily affected (probably killed) by the alpha particles.

It should be noted that the first daughter nucleus of both uranium-235 and uranium-234 is relatively long-lived, so neither contributes significantly to the radioactivity of natural uranium[1]. Thus DU is actually quite as harmful as natural uranium in terms of beta radiation.

Finally, the Pentagon claims that uranium oxide dust is so dense that it quickly settles, and therefore poses a threat only to persons in the vicinity of the target at the time of impact. Having seen television coverage of dust storms in Iraq, I find it hard to believe that fine uranium oxide dust would somehow avoid being blown about during such storms.

[1] Note that this is recently extracted natural uranium metal, not uranium ore. Uranium ore normally contains these daughter products (some may have leached out and continued the decay chain elsewhere), and all the subsequent daughters – some of which take thousands or millions of years to reach equilibrium. At equilibrium, each daughter product produces exactly as many decays as the uranium at the head of the series. 234U is only present because it’s the third daughter of 238U.

Plutonium PR

Published 1 October 2008

Gregg Brunskill writes: “All nuclear-reactor fission nuclides have also been found in the Earth’s geological record, especially in the ancient natural reactor at Oklos in Gabon” (6 September, p 25).

This is a nonsense popular with the nuclear industry’s PR people. It is true that some – very few – such nuclides have been found, but almost all the nuclear-reactor fission nuclides have such short half-lives that no detectable trace of them would be left by now, 1.7 billion years after the reactors are supposed to have been operating.

Their ultimate decay products are nuclides normally present in rocks in quantities so much larger than would be produced in the natural reactors that the reactor products would be untraceable.

A fact sheet issued by the US Department of Energy’s Office of Civilian Radioactive Waste Management in connection with the Yucca Mountain project even claims: “Plutonium has moved less than 10 feet from where it was formed almost 2 billion years ago.”

No isotope of plutonium has a half-life long enough to be present in detectable quantities after 1.7 billion years, and all such isotopes’ decay products are indistinguishable from materials naturally present in very much larger quantities in uranium ores.

Limited improbability

Published 29 October 2008

Tony Budd is right to criticise the kind of thinking that says, “this high-rise block of flats is designed to withstand a wind speed unlikely to be exceeded more than once in 100 years, so since its design life is 60 years, we have got 40 years to spare” (18 October, p 20). But he is wrong to say that “the correct interpretation would be that there is a 1 per cent chance that the maximum wind speed will be exceeded in any given year, or a 60 per cent chance in the building’s lifetime”. That would include the odds of the building blowing down twice or more.

If the risk is 1 per cent in any given year, then the risk in the building’s lifetime is (1 – (99 per cent)60), which is just over 45 per cent. I too find that understanding of this kind of issue is rare, even among engineers. It’s nuclear engineers I worry about.

Overenergiser

Published 10 December 2008

Charlie Robinson pokes fun at concerns about the safety of lithium-ion batteries in cars, pointing out that today’s cars carry a tankful of fuel (5 November, p 21). There are important differences.

A lithium-ion cell can release all its energy immediately in an explosion, because it isn’t dependent on atmospheric oxygen as an oxidiser. A tankful of petrol cannot instantly explode (outside the movies). In the event of an accident, petrol is liable to be spilled and catch fire – nasty enough, but for petrol to cause a major explosion, it has to vaporise and mix with a large volume of air. This can happen after a road accident, but it is quite unusual, and it takes a while, almost always long enough for injured people to be removed from the scene.

Home grid

Published 15 July 2009

Larry Curley complains that UK law requires small domestic wind turbines to shut down in the event of a mains electricity failure (6 June, p 27).

There is actually a good reason for this. If a turbine remains connected to the mains supply during a power cut, it tries to send power into the system. This, of course, is far too much of a load for it to supply so it short circuits. Even if there is an automatic switch to disconnect your household system from the mains supply, your own loads will probably still be more than the turbine can supply.

It is possible to arrange a more sophisticated system, getting the turbine to charge a battery and then converting the battery output to alternating current. This output can either be permanently separate from the mains, or have an isolator switch that disconnects it from the mains in the event of mains failure.

The whole system will continue to run even in the absence of mains power, as long as your loads are not too great for the system.

Such a system is perfectly legal, but of course considerably more expensive than the simple system that shuts down when the mains goes down.

ITERative production

Published 4 November 2009

In his article on the International Thermonuclear Experimental Reactor, Stephen Battersby repeats the claim that nuclear fusion will produce much less radioactive waste than nuclear fission since the only radioactive waste produced is from the materials made radioactive by absorbing the neutrons produced during fusion (10 October, p 40).

However, a fusion reactor needs a supply of deuterium and tritium fuel. Deuterium exists aplenty in seawater, but with a half-life of only 12.33 years, tritium does not occur in significant quantities naturally on Earth. It has to be manufactured, and for that you need a supply of neutrons. There are two possible sources of neutrons in the quantities required: the fusion reactor itself, and fission reactors.

The fusion of one tritium nucleus with one deuterium nucleus produces one neutron, and one neutron can be used to produce one tritium nucleus. So, the neutrons produced in your fusion reactor are needed in the manufacture of tritium, you can’t afford to waste any.

Battersby’s article does a good job of showing how difficult it is to build a working fusion reactor, but does not consider that you must ensure that every neutron produced ends up making new tritium. The loss of neutrons that are absorbed in the reactor vessel or the superconducting magnets means that another source is needed to make up the deficit. In fact, you would probably be doing well to produce half your tritium using neutrons from your fusion reactor, which would mean you’d need to produce the rest in fission reactors.

For a given power output, fission reactors produce fewer neutrons than fusion reactors. However, they produce several times as much energy as a fusion reactor, and of course generate correspondingly large quantities of radioactive waste.

Overall, the fusion reactor and its fission reactor fuel suppliers will produce less radioactive waste than a fission only system, but not very much less.

The editor writes

• ITER intends to test an approach to make tritium by coating the inside of the reactor with tiles made of lithium. It is hoped that neutrons interacting with the lithium will provide tritium for future fuel.

 Yes, but there’s no chance of making enough. see Neutron Reactions in a Lithium Blanket.

Linguodiversity

Published 21 July 2010

Morley asserts that: “if we all spoke the same language, it would be a very positive outcome for humankind”.

I disagree, and not from nostalgia. It is good that we are nearly at a stage where there is one global second language, but lose a language, and you lose a way of thinking and a culture.

The perspectives offered by alternative ways of thinking may mean that a problem considered insoluble becomes easy to solve, or is shown to be of such little importance that no solution is needed. Similarly, it can be hard to see the flaws in one’s own culture, but to someone brought up differently they can be clear. Trying to “fix” flaws in someone else’s culture can be a recipe for disaster, but if someone somewhere can see them there is some hope of people within the culture becoming aware too.

 Understandably, New Scientist abridged my letter somewhat. For what I originally wrote, see A Global Language?.

No, sunshine

Published 15 June 2011

In his letter, Michael Phillips suggested a solar furnace could focus sunlight into a beam to power a plasma-engined rocket (28 May, p 31).

Unfortunately, solar furnaces cannot focus sunlight into a beam, they focus it into an image of the sun at a single focal distance. The sun subtends an angle of 0.5 degrees at the Earth, and the focused image subtends the same angle.

The rocket would rapidly reach a height where the diverging “beam” was much larger than it. This is why you need lasers: the beam remains narrow over a much greater distance.

 Even lasers diverge in the end; whether this kills the whole idea of getting spacecraft up to high velocities this way depends on how much acceleration your craft can stand, how much power you can pack into your laser, how short a wavelength it has, and how wide it is. The best possible angle of divergence of a laser (in radians) is theoretically equal to the wavelength divided by pi times the initial width of the beam – in practice it will be worse than this. How much worse depends on the quality of the engineering...

Shipping forecast

Published 25 April 2012

Jeff Hecht writes that the biggest threats to marine cables in warmer waters come from “fishing trawlers and ships’ anchors”, which are extremely rare in the Arctic. This is true today, but for how much longer?

Diet for the planet

Published 28 November 2012

Fred Pearce says that everyone should put their shoulder to the wheel to double global food production by 2050 (13 October, p 50). Then when challenged, it is made clear this target is not because of the predicted increase in population from 7 to 9 billion, but because of rising demand, especially demand for meat (10 November, p 33).

A far better wheel to put our shoulders to is reducing consumption, especially of meat, by the wealthy. This would not only be good for the global environment, but for health, too. I’m not advocating vegetarianism, merely moderation.

It is rocket science

Published 18 December 2012

Elon Musk says that “with a nuclear thermal rocket, you could definitely reach a tenth of the speed of light” (1 December, p 27). Nonsense.

The speed of light is 300,000 kilometres per second. The most heat-resistant material known (for the combustion chamber[2] of such an engine) has a melting temperature of about 4300 kelvin. The mean velocity of hydrogen ions at about 4300 K is about 10 km/s – so that’s the highest possible exhaust velocity for a thermal rocket, whatever its energy source.

This implies that the initial mass of a thermal rocket must be at least 2(v/10) where v is the final velocity in km/s. This is an inescapable lower bound, and in practice it would be much higher. To reach one-tenth of the speed of light (30,000 km/s), the initial mass of a thermal rocket is therefore at least 23000 times its final mass, or 10900 if you prefer. You could do a lot better, in theory, with a rocket with an ion drive, with its much higher exhaust velocity – but one-tenth of the speed of light would still be cloud cuckoo land.

[2] In a nuclear thermal rocket, it isn’t really a “combustion chamber” – there’s no combustion involved. It’s the chamber in which the working medium (ideally hydrogen, for maximum exhaust velocity) is heated by the nuclear reactor.

Too hot to handle

Published 31 December 2013

Julien Glazier refers to Fergus Gibb’s idea of packaging the hottest nuclear waste into tungsten capsules and letting them melt their way down through the Earth’s crust (7 December, p 32). Surely if our hottest nuclear waste was really as (thermally) hot as that, and would remain so for a protracted period, we wouldn’t call it waste at all. We would put it in those capsules and drive a power station with it.

Waste galore

Published 5 February 2014

In his letter (25 January, p 31), Phillip Graham writes: “Local aboriginal tribes should be the only ones who decide whether to allow and profit from a nuclear waste repository. The site of the first nuclear test on the Australian mainland in South Australia would be a good place for consultations and geological studies.” It is important to remember that the quantity of radioactive fission products generated in a nuclear explosion is less than 1 per cent of what is present in a reactor at any moment, and that it is a very much smaller fraction still of what a reactor produces over its lifetime. Bear in mind also, that many reactors would contribute their waste to a dump.

I hope the local aboriginal tribes know all that.

Hot air

Published 12 March 2014

I find the frequent puffing of the fusion power dream very depressing (15 February, p 11). We get these optimistic announcements of “getting there” at intervals, as we have for decades. Even if one day scientists really manage to make a fusion reactor work – and I have to admit to being very sceptical about even that – where is the tritium going to come from? You might be able to manufacture some of it in a lithium blanket around the reactor, but there is no way that this could produce enough. It would need a supplementary supply from a very large number of fission reactors.

Michael Dittmar, a particle physicist at the Swiss Federal Institute of Technology in Zurich, was quoted as saying: “In this chain reaction, you cannot lose a single neutron, otherwise the reaction stops. The first thing one should do [before building a reactor] is to show that the tritium production can function. It is pretty obvious that this is completely out of the question.”

Fusion? Great fun and lovely salaries for the scientists and engineers involved, but a practical source of energy? No chance.

Fake news has existed for a long time

Published 20 March 2019

You observe that researchers are struggling to find out how people are influenced by disinformation (Leader, 23 February). I would add to this nice piece that fake news is not an internet phenomenon. It has always used whatever medium is available. You have only to pick up a copy of some newspapers to realise that.

Engineering obstacles to electrolysing seawater

Published 9 October 2019

Why can’t we use seawater to make hydrogen, asks Albert Lightfoot (Letters, 21 September). We can, but corrosion of the anode by chloride ions in seawater is a problem. It isn’t insurmountable: you could use anode materials like gold or platinum, but they are expensive. Researchers at Stanford University are working on exactly this issue: see bit.ly/NS-brine.

I wonder whether we could use graphite for the electrodes. I don’t think it would be corroded, but its relatively low conductivity would mean the cells would have to be bigger for a given rate of hydrogen generation.

Lightfoot suggests that there might be useful by-products, but sadly, rare earth metals and cobalt aren’t present in significant amounts in seawater. Lithium only forms 160 parts per billion by weight of seawater and the hydrogen production process wouldn’t help much in its extraction from that water.

More thoughts about metallic hydrogen (2)

Published 22 January 2020

If metallic hydrogen were a room-temperature superconductor, experiments on it might produce useful information. But, given the enormous pressures that seem to be required to keep it metallic, it surely isn’t practically useful.

A thought occurs to me: have any of the teams doing these experiments considered using pure deuterium, an isotope of hydrogen? I suspect that the pressure required to produce metallic deuterium might be significantly lower. It would pretty certainly still be much too high for practical applications, but possibly easier to experiment on.

The End

There won’t be any more of these. I’ve cancelled my subscription, because they’ve been bought by the Daily Mail group, to which I refuse to contribute. A sad day indeed.