Naked Science Forum
General Science => General Science => Topic started by: thedoc on 27/03/2012 17:05:13
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What it is about newer plants that make them safer? So what have we learned from the last 40 years of nuclear development in order to make safe plants?
Asked by Jean Shao, Twitter
Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2347/)
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We answered this question on the show...
We’ve learned to make designs that really take advantage of physics. It’s probably misnamed by calling it passive safety, but I would call it better using and understanding the physics. We use gravity, natural convection, those sort of things to help cool the plant after it shuts down. So GE Hitachi’s ESBWR has the ability to remove heat passively without any electricity, without any operator actions for well over 7 days. The PRISM reactor can do that for a very, very long time and if not, forever. So that's where reactor vendors like GE Hitachi have learned “what do we need to do to these designs to make them better and to make them safer?"
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Even the first gen of nuclear reactors are some of the safest machines the human race has ever conceived, certainly its only the word nuclear, a sensationalist media and social memory of the cold war that makes us think they are unsafe.
it is true however that they can be safer, and should be. without employing hyperbole and without making light of sufferings, even the worst nuclear disasters have had very low body counts. so with objective perspective looking at pure risk vrs reward, nuclear power is by far and away the best choice for our future.
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One thing they employ in nuclear reactors are control rods controlled with electromagnets. This is so that, in case of a complete power failure, the control rods will be completely dropped into the reactor core, completely stopping the reaction.
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Here is an alternative view:
Nuclear power stations are not the best energy option at all! If they are evaluated on a dust to dust basis, the first generation actually provided an energy loss in terms of "useful" energy accounting. There are huge costs in carbon-based energy for fuel in mining, refining, transporting, isotope enriching uranium. Then there are other capital costs in terms of concrete and steel for shielding. And finally, there are decommissioning costs after an operating lifetime of only a few decades. The first generation were planned to return only about 150% of the input of useful energy required to build, operate, and decommission them. When they were found to have a lot of problems with down time, and with faster deterioration than expected because of greater physical effects of the high radiation environment on structural components, most had to be decommissioned well before the designed lifetime. And the nuclear waste problem is only partly addressed so far.
It is also the case that until uranium, or preferably thorium breeder technology is sorted out, there is only a small reserve of uranium-235 in exploitable deposits, and the nuclear fuel would be used up in a few decades if it were relied on for a large proportion of our energy requirements.
What is the alternative?
(1) More sensible conservation of energy reserves. A lot of travel is unnecessary, for example, particularly with the revolution in worldwide communications.
(2) Continuing reliance on wind and geothermal power can only ever be a small component of the overall picture.
(3) There is a lot more room for direct exploitation of solar power -- photovoltaic, direct photothermal, and photochemical.
(4) Water power is potentially a large source that is only realised to a small extent. Hydroelectric power is well entrenched, and can only be a minor contributor, but there are developments in exploitation of wave power, tidal power, and ocean current power that are now at the prototype or pilot plant stage. If you think in terms of energy source size, movements of water just have to be much greater than the movements of air that are providing a lot of the "green" power today.
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the control rods will be completely dropped into the reactor core, completely stopping the reaction.
It is true that the control rods will stop the chain reaction within seconds. However there are a lot of short-lived radioisotopes which continue to produce a significant amount of heat even after the chain reaction stops.
It takes an hour or so for the output of a 1 GW reactor to drop to 5% out its output power. However, even at this level, cooling a 50MW thermal output takes a fair bit of power (especially difficult if the electrical grid goes down, as it did in Japan).
That's why passive cooling of reactors is a significant advance in nuclear safety.