Monthly Archives: March 2015

anti-matter as rocket fuel?

easy peasy

easy peasy

This post is in response to a request, I’m delighted to report.

I remember learning first about anti-matter back in about 1980 or 81, when I first started reading science magazines, particularly Scientific American. I learned that matter and anti-matter were created in the big bang, but more matter was created than anti-matter. If not for that I suppose we wouldn’t be here, unless we could be made from anti-matter. I’m not sure where that would leave anti-theists, but let’s not get too confused. We’re here, and so is anti-matter. Presumably there are plenty of other universes consisting mostly of anti-matter, though whether that excludes life, or anti-life, I’ve no idea. Confusion again. If you’re curious about why there’s this lack of symmetry, check out baryogenesis, which will feed without satisfying your curiosity – just what the doctor ordered.

The next time I found myself thinking about anti-matter was in reading, again in Scientific American, about positron-emission tomography (PET), a technology for scanning the brain. As the name implies, it involves the emission of positrons, which are anti-electrons, to somehow provide a map of the brain. I was quite amazed to find, from this barely comprehensible concept, that anti-matter was far from being theoretical, that it could be manipulated and put into harness. But can it be used as energy, or as a form of fuel? Due to anti-matter’s antagonism to matter, I wondered if this was feasible, to which my 12-year-old patron replied with one word – magnets.

The physicist Hans Georg Dehmelt received a Nobel Prize for his role in the development of ion traps, devices which capture particles of different kinds and charges, including antiparticles, within magnetic and electrical fields, so clearly my patron was onto something and it’s not just science-fiction (as I initially thought). It’s obvious from a glance through the physics of this field – using ion traps to analyse the properties and behaviour of charged subatomic particles – that it’s incredibly arcane and complex, but also of immense importance for our understanding of the basic stuff of the universe. I won’t be able to do more here than scratch the surface, if there is a surface.

The idea is that antimatter might be used some time in the future as rocket fuel for space travel – though considering the energy released by matter-antimatter annihilation, it could also have domestic use as a source of electricity. To make this possible we’d have to find some way of isolating and storing it. And what kind of antimatter would be best for this purpose? The sources I’m reading mostly take antiprotons and also anti-electrons (positrons) as examples. The potential is enormous because the energy density of proton-antiproton annihilation is very many times that of equivalent fission reactions. However, experts say that the enormous cost of creating antimatter for terrestrial purposes is prohibitive at the moment. Better to think of it for rocket propulsion because only a tiny amount would be required.

Three types of antimatter rocket have already been proposed: one that uses matter-antimatter annihilation directly as a form of propulsion; another that uses the annihilation to heat an intermediate material, such as a fluid, and a third that generates electricity from the annihilation, to feed an electric spacecraft propulsion system. Wikipedia puts it this way:

The propulsion concepts that employ these mechanisms generally fall into four categories: solid core, gaseous core, plasma core, and beamed core configurations. The alternatives to direct antimatter annihilation propulsion offer the possibility of feasible vehicles with, in some cases, vastly smaller amounts of antimatter but require a lot more matter propellant. Then there are hybrid solutions using antimatter to catalyze fission/fusion reactions for propulsion.

A direct or pure anti-matter rocket may use antiproton annihilation or positron annihilation. Antiproton annihilation produces charged and uncharged pions, or pi mesons – unstable particles consisting of a pair of quarks – as well as neutrinos and gamma rays (high energy photons). The ‘pion rocket’ channels this released energy by means of a magnetic nozzle, but because of the complex mix of energy products, not all of which can be harnessed, the technology currently lacks energy efficiency. Positron annihilation, on the other hand, only produces gamma rays. To use gamma rays as a form of propulsive energy has proved problematic, though it’s known that photon energy can be partially transferred to electrons under certain conditions. This is called Compton scattering, and was an early proof of the particulate nature of light. Recent research has found that intense laser beams can produce positrons when fired at high atomic number elements such as gold. This could produce energy on an ongoing basis, eliminating the need for storage.

The more indirect types are called thermal antimatter rockets. As mentioned, these are divided into solid, gaseous and plasma core systems. It would be beyond my capacity to explain these technologies, but the finding so far is that, though plasma and gas systems may have some operational advantages over a solid system, the solid core concept is much more energy efficient, due to the shorter mean free path between energy-generating impacts.

It’s fairly clear even from my minuscule research on the subject that antimatter rocketry and fuel are in their early, speculative stages, though already involving mind-numbing mathematical formulae. The major difficulties are antimatter creation and, where necessary, storage. Current estimates around the technology are that it would take 10 grams of antimatter to get to Mars in a month. So far, storage, involving freezing of antihydrogen pellets (cooled and bound antiprotons and positrons) and maintaining them in ion traps, has only been achieved at the level of single atoms. Upscaling such a system is theoretically possible, though at this stage prohibitively expensive – requiring a storage system billions of times larger than what has so far been achieved.  There are many other problems with the technology too, including high levels of waste heat and extreme radiation. There are relativistic problems too, as the products of annihilation move at relativistic velocities.

All in all, it’s clear that antimatter rockets are not going to be with us for a long time, if ever, but I suspect that the technical issues involved and the solutions that might be nutted out will fascinate physicists and mathematicians for decades to come.

Advertisements

wind power in South Australia

Starfish Hill wind farm, near Cape Jervis, SA

Starfish Hill wind farm, near Cape Jervis, SA

I was unaware, until I recently listened to a forum panel on renewables broadcast by The Science Show, that wind power has really taken off in SA, where I live. Mea culpa. By August last year 27% of the state’s electricity production was from wind, and it’s now well over 30%, thanks to a new facility outside Snowtown, which came on stream in November. That’s half of Australia’s installed capacity, and it compares favourably with wind production in European countries such as Denmark (20%), Spain and Portugal (16%), Ireland (15%) and Germany (7%). It’s one of the great successes of the Mandatory Renewable Energy Target, introduced in a modest form by the conservative federal government in 2001 and expanded under the Labor government in 2009. The RET, like those in other countries, mandates that electricity retailers source a proportion of energy from renewables. South Australia’s renewable energy developers, under the longest-serving Labor government in the country, have been provided with tax incentives and a supportive regulatory framework to build wind farms throughout the state, to take advantage of the powerful Roaring Forties blowing in from the west.

The first wind turbine in SA was a small affair at Coober Pedy, but from 2004 onwards this form of energy generation has taken off here. The Snowtown wind farm mentioned above is the second in the region, and SA’s largest, with 90 turbines giving it an installed capacity of 270MW. We now have some 16 wind farms strategically located around the state, with an installed capacity of almost 1500MW. As far as I’m aware, we’re in fact the world leader in wind power – always remembering that, in population terms, we would be one of the smallest countries in the world, if we were a country.

The direct beneficiaries of these new farms are, of course, regional South Australians. An example is the 46 MW, 23-turbine Canunda wind farm near Millicent in the state’s south-east, which opened in 2005. The farm provides clean electricity generation to the region and has increased the viability of agricultural production. The facility has generated enough interest from the local community for tours to be undertaken.

Of course, one of the principle purposes of utilising renewable energy – apart from the obvious fact that it’s renewable – is the reduction of greenhouse gas emissions. And South Australia’s emissions have indeed declined in spite of increased electricity demand, due to the high penetration of wind power into the market.

This development has of course had its critics, and these are pretty well summed up on Wikipedia – linked to above:

There has been some controversy with respect to the impact of the rising share of wind power and other renewables such as solar on retail electricity prices in South Australia. A 2012 report by The Energy Users Association of Australia claimed that retail electricity prices in South Australia were then the third highest in the developed world behind Germany and Denmark, with prices likely to rise to become the most expensive in the near future.[24] The then South Australian Opposition Leader, Isobel Redmond, linked the state’s high retail prices for electricity to the Government’s policy of promoting development of renewable energy, noting that Germany and Denmark had followed similar policies. On the other hand, it has been noted that the impact of wind power on the merit order effect, where relatively low cost wind power is purchased by retailers before higher cost sources of power, has been credited for a decline in the wholesale electricity price in South Australia. Data compiled by the Australian Energy Market Operator (AEMO) shows South Australian wholesale electricity prices are the 3rd-highest out of Australia’s five mainland states, with the 2013 South Australian Electricity Report noting that increases in prices were “largely driven by transmission and distribution network price increases”.

The issue of cost to the consumer (of energy in general) is without doubt extremely important (and complex), and I’ll try to wade into it, I hope, in another post, but for now I want to look just at the costs for wind, and whether there are any further developments in the offing.

According to this site, which is informative but perhaps not as regularly updated as it could be in such a changing energy environment, SA’s Premier last year renewed his government’s pledge to have 50% of the state’s annual power supplied by renewable energy by 2025, a very realistic target considering that, according to the same site, wind and solar were already at 38% of annual supply, as of December 2013. However he pointed out that this would be difficult if the federal government reduced its RET target, then at 41TWh by 2020. In October federal industry minister Ian Macfarlane and environment minister Greg Hunt proposed a reduction of the RET to 27TWh.

A more recent article on the Renew Economy website argues that, though the government appears to have upped the proposed figure to around 31 or 32TWh, it may be targeting large-scale wind power projects by trying to incorporate rooftop solar, which has been taken up rapidly in recent years, into the large-scale target. The initial target was 45TWh overall, with a projected rooftop solar take-up of 4TWh, leaving 41TWh for large-scale renewable energy projects. We’re currently at 7TWh for rooftop solar, and the Warburton Review expects this to double by 2020. Hints by the government ministers that the take-up of rooftop solar should be reflected in the renewed target are adding to uncertainty in the industry, which is said to be in limbo at present. It may take a change of government to resolve the situation. Meanwhile however, South Australia leads the way with wind, and if the graph on the Renew Economy website is to be believed, we’ve already passed our 50% target for renewables (though the graph appears to fluctuate from moment to moment). The graph shows that we’re currently generating 710MW from wind, 527MW from natural gas and 179MW from brown coal. That makes just on 50% from wind alone. Compare this with Victoria, a much more populous state, which generates almost as much from wind – 592MW. However, that’s only about a tenth of what it currently generates from brown coal, its principle energy source (5670MW).

A new wind farm has been approved for Stony Gap, near Burra, but there may be delays in the project due to industry uncertainty about the RET and the federal government’s plans. Energy Australia, the project’s developers say ominously: We are now re-assessing the project based on current market conditions as well as government policy and legislation.  

And the cost? This is hard to gauge. As with solar, the cost of wind power has come down markedly in recent times. Basically the cost is for initial capital rather than running costs, but some argue that, because wind farms require back-up, presumably from fossil fuels, for those windless days, this should be incorporated into the cost.

nuclear power, part 2 – how it works

PressurizedWaterReactor

There are many tricky questions around nuclear power, and perhaps the most head-scratching one is, why did the most earth-quake prone country in the world embrace this technology so readily? The well-known environmental scientist Amory Lovins was just one to state the bleeding obvious with this remark: “An earthquake-and-tsunami zone crowded with 127 million people is an un-wise place for 54 reactors”. Combine this with a secretive governmental and industry approach to energy production in a cash-strapped economy, and disaster was almost inevitable. There were a number of earthquake-related shut-downs and cover-ups before the Fukushima disaster essentially blew the whistle on the whole industry, turning the majority of Japan’s population against nuclear power almost overnight. After Fukushima, the generation of nuclear power worldwide fell dramatically largely due to the shut-down of Japan’s 48 other nuclear power plants, though facilities in other countries were also affected by the publicity.

Yet it’s reasonable to ask whether other countries, such as Australia, should reject nuclear power outright because of Japan’s bad example. Australia rarely suffers serious earthquakes – South Australia almost never. And there may be safer ways to utilise nuclear fission as energy – now or in the near future – than has been employed in Japan or other countries since the sixties. So, just how do we generate nuclear power, how do we get rid of waste material, and are there any developments in the pipeline that will make generation and storage safer in the future?

How’s the energy produced?

Much of the following comes from How Stuff Works, but for my sake I’m putting it mostly into my own words. We derive energy from nuclear fission in the same way that we derive energy from coal-fired power stations – by turning water into pressurised steam, which drives a turbine generator. The difference, of course, is the source of the heat – uranium rather than carbon-emitting coal. Nuclear reactors create a chain reaction which splits uranium nuclei into radioactive elements, releasing energy in the process. A thorium fuel cycle rather than a uranium one is also possible, though with limited market potential at this point.

Uranium, in the form of isotope U-235, can undergo induced fission relatively easily. However, naturally occurring uranium is over 99% U-238, so the required uranium has to be enriched so that the U-235 content, which is naturally at around 0.7%, is increased to around 3% (weapons-grade uranium enrichment requires over 90% U-235). The enriched uranium is formed into pellets, each about 2.5 cms long and less than 2cms in diameter. These are arranged into bundles of long rods which are immersed in a pressure vessel of water. This is to prevent overheating and melting. Neutron-absorbing control rods are added to or subtracted from the uranium bundle, by raising or lowering, and these control the rate of fission. Completely lowering the control rods into the bundle will shut the reaction down.

The fissioning uranium bundle turns the water into steam, and then it’s just the technology of steam driving the turbine which drives the generator. But then there’s the matter of radio-activity…

Before we get into that, though, I should mention there are different kinds of reactors, which use different systems and different cooling agents. I’ve been rather cursorily describing a Light Water Reactor, the most common type. They use normal or regular water, and there are three varieties: pressurised water reactors, as described; boiling water reactors, and supercritical water reactors. There are also heavy water reactors which use water loaded with more of the heavier hydrogen isotope called deuterium. But whatever is used as a coolant and/or a neutron moderator (a medium that moderates the speed of neutrons, enabling them to sustain a chain reaction), the issue of radio-activity needs to be dealt with.

What are the safeguards against radioactive decay? 

What I previously termed ‘induced fission’ involves firing neutrons at U-235 nuclei. The nucleus absorbs the neutron and then becomes unstable and immediately splits, releasing a great deal of heat and gamma radiation from high energy photons. Among the products of the split are fissile neutrons, which then go on to split more nuclei, a chain reaction which can be controlled with the manipulation of control rods as described above. Uranium 235 and Plutonium 239 are among the very few fissile nuclei – those that lend themselves readily to nuclear chain reactions – that we know of.

The trouble with induced fission is that the products of the reaction are vastly more radioactive than the fissioned material, U-235, and their radioactive properties are long-lasting, leading to the obvious problems of safeguard, storage and elimination.

In standard light water reactors, the pressure vessel is housed in concrete, which is in turn housed in a steel containment vessel to protect the reactor core. Refuelling and maintenance equipment is housed within this vessel. Surrounding this we have a concrete building, a secondary containment structure to prevent leakage and to protect against earthquakes or other natural (or man-made) disasters. There was no such secondary structure at Chernobyl. The nuclear industry argues that, when these safeguards are properly maintained and monitored, a nuclear power plant releases less radioactivity into the atmosphere than a coal-fired power plant.

Even if this wins some people over, there are the really big issues of mining and transportation of uranium and nuclear fuel and storage of radioactive waste. According to the USA’s Nuclear Energy Institute, 2000 metric tons of high-level radioactive waste are produced annually by the world’s nuclear reactors, which is hazardous to all life forms and can’t be easily contained. This radioactive material takes tens of thousands of years to decay. Low-level waste, which contaminates nuclear plants and equipment, can take centuries to reach safe levels.

Storage, or possible recycling, of waste is probably the major issue for the nuclear power industry’s future, in spite of all the understandable current attention given to melt-downs. The How Stuff Works website summarises the present situation:

Currently, the nuclear industry lets waste cool for years before mixing it with glass and storing it in massive cooled, concrete structures. This waste has to be maintained, monitored and guarded to prevent the materials from falling into the wrong hands. All of these services and added materials cost money — on top of the high costs required to build a plant.

In my next, and hopefully last, post on this subject (for a while at least), I’ll focus more on this storage issue, and on other developments in nuclear fuel, such as they are. I’ll be relying particularly on the MIT interdisciplinary study ‘The Future of the Nuclear Fuel Cycle’, which came out in 2011 – just when the Fukushima-Daiichi disaster hit the headlines…