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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Anti-glare glasses are useless at night

I noted your comments on glare from headlights. Anti-glare glasses are useless in this scenario. Night vision is mainly processed by the rods in the retina. They are the receptors that respond to low-intensity light. Unfortunately they also stay refractory for 30 minutes after exposure, ie, they take 30 minutes to recharge before being able to fire again at low intensity.

Rods also do not respond to light in the red/orange wavelengths but cones do and they have a short refractory period but require higher intensity light to activate. That is why submarines and night cockpits are illuminated with red light to allow the rods to respond to other low intensity (night) light sources.

Yellow driving glasses sit on the edge wavelength of rod responsiveness, ie, headlights will just begin to depolarise the receptors but will not make them as refractory for as long as white light and will make it easier to see detail in the situation where there are oncoming headlights. Unfortunately, they will also change the perceived colour of stop lights etc.

This is known to car manufacturers as they used to make yellow fog lights to illuminate the side of the road and not blind other drivers, unlike nowadays when fog lights are used as a status symbol, often illegally.

Early cataract formation will further refract light and make it harder to see in the context of oncoming headlights but protecting the responsiveness of your rods will lessen the risk of an accident due to temporary blinding.

Name & address supplied but withheld at writer’s request.

No need to wait for Thorium reactors

Luke Biddle (Mailbag, May 2011) is right to propose the use of Thorium in new designs of nuclear reactors but his enthusiasm might need some tempering.

As Luke says, there is no expensive isotopic separation required. The only isotope of Thorium to occur naturally is Thorium-232. It is slightly radio-

active with a half-life of 14.05 billion years; much longer than that of the naturally occurring isotopes of Uranium, which may account for its greater abundance. The mantles in portable gas camping lamps are made from Thorium dioxide, a ceramic that glows brilliant white when heated and with the highest melting point of all oxides: 3300°C.

Many websites advocating Thorium as a nuclear fuel do not make it clear that the various reactor designs are still very much at the research stage. They also do not indicate clearly that Thorium, unlike that well-known isotope of Uranium, U-235, is not itself fissile, an essential property for an element to become a nuclear fuel. An atom of Thorium-232 must first absorb a neutron to become the Uranium isotope U-233, which is fissile.

www.ga.gov.au/minerals/mineral-resources/Thorium.html offers a good summary of the situation. It also confirms that there are large amounts in Australia in the form of Monazite sands in Queensland, NSW and WA.

To kick-start a nuclear reactor containing Thorium, the Thorium has to be placed in the reactor as a blanket around a fuel load of a fissile isotope, such as U-235 or U-233 and remain there for some months during normal reactor operation so that the absorption of the neutrons and conversion from Th-232 to U-233 can occur. Then, if it is in a current design of water-cooled reactor, the blanket material has to be removed, the newly-created U-233 is separated from the unconverted Thorium, then the U-233 is fabricated into new fuel rods to then be used as nuclear fuel.

Note that U-233, like U-235 and Plutonium-239, can be used to make a nuclear weapon. Luke’s claim in that respect is not correct.

Thorium advocates concentrate on the possibility of the use of liquid-fuelled, high-temperature reactor designs where the Thorium is present as a fluoride salt. This design is known as the Liquid Fluoride Thorium Reactor (LFTR). As with the above design, the reactor still requires a start-up fuel load of fissile Uranium, usually U-233, present in columns as fuel in the form of Uranium fluoride within the Thorium fluoride salt blanket.

All of this material, both fuel and blanket, becomes liquid at an operational temperature of some 600°C. The fuel and blanket do not mix, being contained in separate ceramic tubes.

The Thorium converted to U-233 is subsequently extracted from the blanket by a jet of fluorine gas, then cleaned up outside the core and re-injected. Again, as with present reactor designs, there is the possibility of diversion of the extracted U-233 into weapons production. On the matter of fission product waste production: while the mix of fission products may have somewhat shorter half-lives than that extracted from conventional U-235/U-238 fuel, it is still a nasty mix that has to be properly processed and stored as nuclear waste.

I am not sure what Luke means where he says “radioactive waste can be fed back into the process to fuel the system . . .”. The only thing that is fed back in is the absolutely necessary U-233 fissile fuel. It is indeed radioactive but is not “radioactive waste” in the usually accepted sense.

The claim that “One tonne of [Thorium] produces as much power as 200 tonnes of Uranium . . .” is only correct in the sense that current US policy is not to reprocess spent fuel. Only a small part of the U-238 in the spent fuel is used as fuel on the first pass through a conventional reactor, the rest being either still present as U-238 or converted to various isotopes of Plutonium, all of which are useful as fissile fuels but by a US policy decision are not recycled.

This point becomes important when it is realised that there are a number of so-called Generation IV reactor designs, all based on the same high temperature/non-pressurised operation model as the LFTR, that do indeed consume the U-238 as fuel. The Integral Fast Reactor (IFR) design, for example, which can use any of the nuclear fuels, including Thorium, is also similarly supposedly incapable of a meltdown and is able to extract all the energy of U235/U238.

If the IFR is commercialised, then the oft-quoted one tonne versus 200 tonnes advantage of Thorium disappears because the U238 is then used up rather than removed as spent fuel.

In pointing out these errors, I am not attempting to bury Luke’s enthusiasm for the Thorium reactor; quite the contrary. By all means, advocate a particular reactor design – and the Thorium concept has a great deal going for it – but do not potentially destroy a very good concept in its infancy with statements that are wrong.

For example, to suggest that the LFTR cannot be used to produce weapons is simply wrong – this reactor design, while not very efficient at it, can also produce Plutonium as well as U-233. Both are used in nuclear weapons.

But let’s be clear in our thinking. The current models of pressurised water reactors (PWRs) are very safe and intrinsically safer than the old Fukushima plant design. They are far less polluting than coal-fired plants and deliver real, base-load power. Unlike Generation IV designs, of which Luke’s favoured LFTR reactor is but one, they are commercially available now, as opposed to at least 20-30 years into the future.

Resources:

(1) www.world-nuclear.org/info/inf62.html is an authoritative source.

(2) There is a very good article and excellent discussion, contributed to by some of the world’s leading nuclear power experts, on the pros and cons of the Thorium reactor at http://bravenewclimate.com/2009/12/17/lftr-in-australia/ There are also many excellent blogs on this site on the IFR, other reactor designs and energy issues in general.

(3) Kirk Sorensen’s YouTube video at http://www.youtube.com/watch?

v=N2vzotsvvkw provides both an ex-

cellent perspective on mankind’s need for energy as well as his own enthusiastic support for the Thorium reactor concept.

Paul Miskelly
Mittagong, NSW.

Phone line polarity checker does work

I read your article regarding the line polarity and the tester (SILICON CHIP, May 2011) and thought, “Yeah right!” The devices should comply with Telstra Service Interface Specification (TSIS) for ADSL Access.

Then recently I was remote testing a DSL install (from the DSLAM) and the service would not meet our minimum SNR Margin of 8dB. Before messing with line profiles or logging a fault with Telstra, I got our field engineer to swap the A and B wires.

Wow! It now gave 14dB SNR margin, I’m a convert! By the way, the modem was a Siemens 4200.

Rod Beech,
Fernvale, Qld.

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