Japan has something like 52 nuclear reactors. Two of them are in trouble at this moment – And here’s why:
The reactors were shut down when the earthquakes started up, but you can’t turn off a nuclear reaction, you can only damp it, down to6% in this case. This was done as per normal.
The Reactors are built to survive a 7.9 magnitude earthquake (Not Richter scale – that’s now obsolete, hence ‘Magnitude’), but the Quake that hit was 9.0, the fifth largest in history.
Then the Tsunami hit and caused even more damage. Specifically the emergency power generators that pump water to cool the reactors were off line. An irony because if the power plant could be turned on it could supply it’s own power!
Nuclear reactors work by using fissionable materials – Nuclear Fuel – To produce heat, which boils water into high pressure, superheated steam, which then goes through a turbine, turning it and producing power.
As you damp the reaction down it starts to cool naturally and produce less heat, but this takes several days, during which time, heat is still being generated.
To stop the reactor melting itself under it’s own heat, you just keep pouring water over it. Unfortunately the pumps aren’t working so the water’s just sitting here getting hotter… worse the water is leaking out.
To fix this – Just add more water.
This is one reason nuclear plants are generally placed by rivers and oceans – lots of water to dump your ‘used’ hot water into and in an emergency, get cold water.
So right now, the Japanese are pumping sea water from bay into the cooling system to try and keep Reactor No. 3 cool for long enough that it stops producing heat.
There was an explosion at the NPP (Nuclear Power Plant), which seems like it should be incredibly bad news, but actually isn’t that bad.
The reactor is armoured. It’s designed to withstand 7.9 magnitude earthquakes, being hit by planes, or anything short of… well, a 9.0 earthquake and Tsunami. However it’s also inside a fairly lightweight shed that’s more or less there to keep the rain off and the pigeons and critters out.
When the water level dropped, it let the fuel rods heat up which caused a reaction that produced metal oxides and hydrogen (The water lost it’s oxygen atoms, just leaving hydrogen) which then exploded doing little damage to the well shielded reactor but blew the walls panels off the shed.
The Japanese then checked the radiation levels and found they were decreasing, meaning that it wasn’t the reactor blowing up.
Basically it’s a waiting game. In theory the Japanese just have to keep the reactor cool and name sure that the other reactors aren’t running low on water, for a few more days to avoid any really dramatic problems.
The downside is that the earthquakes are still happening (In the 4-6 mag range!) and though it’s unlikely there’s still a chance of another large enough quake to produce another Tsunami, or that a new problem will be uncovered and will have to be dealt with.
Unlike Chernobyl, the Fukushima reactors do not have graphite cores. Graphite is very useful for containing nuclear reactions but is also flammable at nuclear reaction temperatures, which is what happened at Chernobyl.
The big problem with Fukushima is most likely to be a ‘China Syndrome’ meltdown where the fuel becomes so hot it melts through the floor and digs a pit ‘to China’ (This phrase was coined by the USA after Three Mile Island).
Again this is something that’s planned for and there are safety measures in place to capture molten fuel rods, but it’d be messy, hard to deal with and dangerous as hell.