Nuclear reactor cores for dummies: (Including overview of "meltdown")

[snagglepuss]

The heart of the reactor is the core which is an assembly of "fuel rods' and control rods. The fuel (U-235, PU, etc) is an unstable isotope (or mix) which decays, releasing energy and particals. The particals smash into other atoms of fuel, releasing energy and particals, which smash into more fuel atoms, releasing more energy and particles.....and so forth: a chain reaction. Left uncontrolled, the fuel will continue to consume itself at an increasing rate. In order to allow control of this process, 'moderators' are used, primarily fuel rods and water. The moderator (a stable substance) intercepts particles of decay and limits the chain reaction. Water removes the heat as well (which is used to generate electricity).

The fuel rods and control rods are designed to fit together in a precise geometry to limit/control the chain reaction ongoing in the fuel. Note: This process cannot be "turned off". The chain reaction continues but is reduced to a low, manageable level by the control rods and water (moderators) and there is still heat produced which must be continuously removed.


Meltdown/partial meltdown:

If part of this assembly is damaged (melted), the precise geometry of a portion of the assembly is destroyed. The control rods' role as a moderator is diminished in this area and precise control of the chain reaction is lost. Depending on the level of damage, the chain reaction can build on itself resulting in further damage to intact parts of the core....and so-forth. Meltdown commences. If the damage is slight, continuing to supply water to the core will cool the damaged part of the core and hopefully moderate the chain reaction. The chain reaction in the damaged part of the core cannot be stopped, only prevented from further damaging the intact parts of the core assembly; a standoff. In this case, boron is being added to the water to increase its effectiveness as a moderator.

While this is a greatly simplified explanation, I hope it helps folks understand the fix these folks are in. This is a 'fire' that cannot be easily extinguished, only, hopefully, prevented from spreading.



- Posted by Ghung - The Oil Drum Ghung on March 13, 2011 - 12:14pm





http://www.theoildrum.com/node/7637#comments_top

[stevedeshazer]

1. Thank you, that is a reasonable explanation.

There's a lot of misinformation out there.

[snagglepuss]

4. Your welcome.

Nice avatar.

[Peace Patriot]

2. Here's an excellent summary of the whole situation...

Posted by LiberalEsto:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=102x4768612

The meltdowns and emergency actions at Fukushima Units 1 and 3 is a very complicated situation. They've had valve and gauge failures, as well as failures of the main freshwater pumping system and the backup battery power system. They've given up on these two plants ever functioning again and are pouring sea water and boron on them, to try to contain what they presume to be meltdowns. (They can't get close in to tell for sure.) Water gauge failures are making it difficult for them to know if enough sea water is being pumped in. One of the units may be leaking water. And there are four other plants with cooling system problems. Another 7.0 earthquake has been predicted in the next couple of days. The infrastructure damage from the 9.0 earthquake and tsunami is already severe, both generally (roads, communications, power) and at some of the plants. (One of the plants blew off its outer containment structure--I'm not sure if by human action, trying to vent the plant of hydrogen buildup, or because there wasn't time to vent it, after the tsunami damaged the water pump system and the meltdown started).

Anyway, it all sounds so simple, in theory (the OP description) but when things start to go wrong...

[Canuckistanian]

3. What's a "partical"?

I've heard of "particles". Oh well, can't expect nuclear physicists to be English majors.