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China to Pioneer Melt-Down Proof Reactors
pease1 writes "FT.com reports China is poised to develop the world's first
commercially operated "pebble bed" nuclear reactor. If successfully commercialized, the pebble bed reactor would be the first radically new reactor design for several decades. It would push China to the forefront of development of a technology that researchers claim offers a new "meltdown-proof" alternative to standard water-cooled nuclear power stations." This was mentioned in September of last year but now looks as though the plan is moving forward.
Actually, the design of these reactors is nothing short of ingenious.
The reactive elements are spherical pebbles, each with just a tiny amount of radioactive material inside.
Individually, they do not have enough material to go critical.
when you put them all together inside the reactor, the shape of them puts its nearest neighbour just in range to react.
If the reaction begins to cascade, the elements heat up and expand. This automatically seperates them and cools the stack back down.
You can pour new elements into the top, and extract the lowest from the bottom in a relatively safe manner.
liqbase
I think the CANDU reactor is inherently more meltdown-proof than this design. The CANDU reactors use heavy water as both the moderator and the coolant - if you lose the coolant, you also lose the moderator, so the reaction stops. The only bad situation would be if the coolant pumps stopped moving the coolant, but then you could dump the heavy water manually, or just wait for it to flash to steam and get sucked into the vacuum building that sits beside the containment building. Either way, the reaction stops before it gets to a "meltdown" point.
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Fortunately I just read about the term "unsinkable" as it was applied to the Titanic. The boat-maker never used the term. The dock-workers never used the term. The buyers never used the term. The only one who used it was a marketer for a travel agency booking berths aboard the Titanic. No one, the captain included, thought the ship was unsinkable. The very idea is ridiculous. Pour enough water into the ship, and it will sink.
On the other hand, pebble-bed reactors do not rely on making it difficult to meltdown, they rely on the fact that the natural state of the reactor bed is a "safe" condition. (No, that doesn't mean you can stick your head in it, just that it will not maintain a chain reaction.) So, in the case of a pebble-bed reactor, if you take away all the coolant, the reactor shuts itself down. The coolant (or more accurately the heat-transfer media, since it's used to move heat from the reactor core to the heat exchangers to make steam to turn generators) is integral to the design of the reactor.
To have a sustained reaction, there must be coolant present. If the coolant is present, then the reactor cannot melt down, because it's covered in coolant. If the coolant were to be allowed to boil off, then the reaction cannot be sustained and the reactor shuts off. So, Coolant=no meltdown, no coolant=no meltdown. Please find the way to make the reactor meltdown in the above scenario...
Give up? That's the difference between engineering and physical law. I can engineer a damn tough ship, but physical law says that if I add enough weight, it'll displace more weight than an equal volume of water, and it will sink. On the other hand, if I have a pebble and it releases X number of neutrons, nothing I can do will increase that number of neutrons or moderate them in such a way as to cause a chain-reaction, except adding a moderator, which, in-turn controls the chain-reaction. It's like claiming that I can make a light bulb that's hot get hotter and melt-down by turning off the switch.
Pebble-beds have been built and tested in the harshest ways, and no reaction can be sustained when the pebbles were "exposed" without the sustaining material. The only way to make a pebble-bed melt down is to take the pebbles, grind them down, extract the fissile material and make a regular nuclear reactor out of them.
And that's the whole point.
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When people talk about breeder reactors as "producing more fuel than they burn", what they mean is that the reactor is run on either U-235 or Pu-239. It produces heat energy which is converted into electricity.
At the same time, excess neutrons from the reaction are reacted with an otherwise inert blanket of U-238 around the reactor, converting the U-238 into Pu-239 which can then be used to run the same reactor, or other reactors. It turns out that Pu-239 production is faster than Pu-239 or U-235 consumption.
It is relatively easy to use chemical methods to separate the produced Pu-239 from the leftover U-238 in the blanket, certainly MUCH easier than separating U-235 from natural uranium.
So it's not a perpetual motion machine because a resource is used up, i.e. the natural U-238, but that resource is plentiful and the overall process is easier than the conventional method of getting fissile fuel.
The reason that breeder reactors aren't widely used is partly technical, because they're fairly complex things to design and operate, but mostly political because the Pu-239 produced can relatively easily be used in bombs.
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