Originally Posted by TurboTim
I wonder how this chinese pebble reactor idea turned out?
"China has licensed the German technology and is actively developing a pebble bed reactor for power generation. The 10 megawatt prototype is called the HTR-10. It is a conventional helium-cooled, helium-turbine design. The program is at Tsinghua University in Beijing. The first 250-MWt plant is scheduled to begin construction in 2009 and commissioning in 2013. There are firm plans for thirty such plants by 2020 (6 gigawatts). "
Originally Posted by jbresee
Just wanted to say thanks for your comments. You have a gift for explaining complex concepts in a way that even I can understand. Appreciate it!
Thanks, Not sure which one you're referring to (I was tired as hell when I wrote the Chernobyl response, so it is massively abridged) but I'm certainly no expert, just a civilian engineer who is fascinated by nuclear power in general.
For those who aren't familiar with plant design, a few notes are probably in order.
First, here's an extremely simplified drawing of a typical nuke plant.
This particular image is of a PWR (Pressurized Water Reactor) which is one of two styles common in the US and the west in general. The other common style is the BWR (Boiling Water Reactor) which is very similar, except that the control rods go in from the bottom, and the pressure inside the reactor is low enough that the water is permitted to boil inside the core, and exit the top as steam.
So, at the far left is the core. This is a giant stainless steel pot in which the uranium fuel is housed. Water is constantly being pumped through this core, and then up into the steam generator (aka, the boiler), as shown by the red lines. This is the primary coolant loop. You'll also note the pressurizer at the top of the primary loop. This is basically a shock-absorber. In a PWR, this is the one and only place where they allow steam to collect, to dampen any pulsation in the system. And it's actually critical that some steam be kept inside here. If the pressure in the system gets too high and all the steam in the pressurizer collapses into liquid water, then you lose your shock absorber and also your ability to fine-tune pressure in the system. So operators are trained to never, ever let this happen.
(BWRs don't have a pressurizer, since they're full of steam anyway. They also typically don't have a boiler or a secondary loop; the steam coming out of the core goes directly to the turbine. So, on the plus side, they are somewhat simpler. On the minus side, the steam coming out of the core and hitting the turbine and condenser (which are outside of the containment building) is radioactive, so there are a lot more potential failure points in the system for a primary loop rupture, release of radioactivity, etc.
At TMI, when the valve on top of the pressurizer stuck open, it started to let down the pressure in the primary loop. All the steam flowed out the top, and the water below expanded. Since the operators didn't want the pressurizer to fill with water completely, they started draining water out of the primary system (thus making matters worse) but couldn't re-establish equilibrium because of the stuck valve.
Ok, that's the primary loop. Next, you have the secondary loop. This goes from the steam generator to the turbine, then the condenser, and then back to the steam generator. This is the loop that ruptured at TMI and set the whole process in motion. Without this loop, no heat was being taken away from the steam generator, and consequently, all the heat built up in the primary system.
What heat? Well, even with the control rods fully inserted and the reactor "shut down" there is still a massive amount of decay heat as the reactor core gradually cools down. You know how after you shut off the engine in a turbo car, the water inside the turbo starts to boil even through there's no more exhaust flow? Same deal. You gotta keep the system running until the reactor has totally cooled off, which takes hours or days.
Last is the tertiary loop, which runs from the condenser off to the right side of the picture. On most reactors, this is the loop which runs out to the huge cooling towers outside. Some reactor (those located near large bodies of water) simply draw in water from the lake / river / ocean and use it as the tertiary loop.
Ok, nuclear physics 101: When a uranium atom splits (fission) it releases, among other things, a couple of neutrons. If those neutrons hit other uranium atoms, then those atoms split and emit more neutrons. This process creates heat.
Ideally, for every atom that splits, the emitted neutrons cause exactly one other atom to also split. This is called a critical reaction. If each fission event causes less than one atom to split, then power goes down. If each fission event causes more than one atom to split, this is a supercritical reaction, and power goes up. (Atomic bombs are massively supercritical.)
There are a couple of factors that govern this reaction. The most obvious are the control rods. These rods are made from a material which absorbs neutrons. So if you want to lower power, you insert more rods, thus capturing atoms and preventing them from hitting other uranium atoms.
There's a second factor as well, one which is in fact far more critical. It may sound odd, but when a uranium atom first splits, the atoms which it ejects are traveling so fast that the chance of them striking another uranium atom and causing a fission is quite small. In order to achieve a stable reaction, you actually need to slow the atoms down. You do this by adding a "moderator" to the core- some substance which slows down neutrons without capturing them.
In a western-style reactor (PWR or BWR), plain ole' water is used as both the moderator and the coolant. This is actually a neat safety feature. Remember earlier where I talked about "negative void coefficients?" Well, since the same water that's cooling the reactor is also moderating it, if that water starts to disappear, then it can't slow down the neutrons. So, if the water goes down, the power level goes down. Essentially, this is what prevented TMI-2 from physically exploding, rather than simply melting down.
Now, in the Chernobyl reactor (the RBMK design which I referred to earlier), water was present in the core as a coolant, however the primary
moderator was a massive matrix of graphite blocks. And due to an oddity of nuclear physics which I don't claim to understand, the water which is present acts more as a neutron absorber than a neutron moderator. So basically, you can take away all the coolant, and because the graphite is still there, not only will moderation continue to occur, but it will in fact increase, as the water is no longer capturing any neutrons.
And that's basically what happened. The operators were idiots, but the basic design of the Chernobyl reactor is what allowed
it to go totally apeshit and blow up.
Some good reading, for those who are interested in a better explanation than mine:
Detailed narrative of the TMI-2 incident
(A hell of a good read)
List of significant events
including TMI, Chernobyl, Windscale, etc
Description of BWR reactors
Description of PWR reactor
Description of RBMK reactor
(such as the one at Chernobyl.)
Narrative of Chernobyl incident
The Nuclear Tourist
Lots of good info.
Originally Posted by sixshooter
Progress Energy is currently building two nuclear reactors on the Gulf coast of Florida just 50 or so miles north of their existing nuclear reactors at Crystal River, FL. Yay!
That's awesome. I had no idea they were moving forward on these units.