Nuclear fuel in a BWR is composed of three parts: uranium oxide pellets (fuel), zirconium alloy tubes (cladding) & structural bits

The fuel is sealed in the 12-15' long tubes, the tubes are arranged in a square grid & held together by a light frame & put in a shroud

The shroud is basically a square sheet metal tube (also made of zirconium alloy) which keeps water flowing up along the fuel.

The fuel is arranged in a roughly circular grid to form a big cylinder inside the reactor vessel.

Cold water enters the side of the vessel, goes down to the bottom & up past the fuel where it's heated to boiling temperatures

(so) the reactor pressure is normally ~1000 psi so boiling temperature is ~565F or so.

The steam water mix goes through a twisty path called a steam dryer to keep water droplets from being carried into the steam lines.

The (dry) steam goes into the high pressure turbine, expands & spins the turbine, then goes into the low pressure side into the condenser.

The condenser is a huge heat exchanger right under the turbine that carries waste heat out of the plant.

Condensed steam is pumped back to the reactor and the whole cycle begins anew. This will thrill the Buddhists in the audience :)

So in a quick shutdown, the control rods enter from the bottom of the core and latch in place so they can't fall out.

Also, if the control rod drive (CRD) pumps fail, you can vent water which sucks the rods in. Very elegant & counterintuitive design.

Why the bottom? (DUDE! GRAVITY!?) The steam dryer is in the way. 2000psi water pushes them up & in.

So shutting down the reactor is pretty easy & foolproof; it has to be. The next thing that happens is shutting the valves to the turbine.

Once you stop the reactor, you stop making steam and you'll start sucking water into the turbine which is Very Very Bad.

How bad? 42" turbine blades spinning at 1800 RPM breaking apart and flying around the turbine building bad. Bye bye turbine.

So when the rods go in, the main steam isolation valves (MSIVs) shut. But now there's no place for the water being pumped in to go.

So at that point you switch to RHR (residual heat removal) which is a safety-related heat exchanger system to remove the decay heat

Recapping from yesterday: Decay heat starts ~10% and drops over the next few days. It's this decay heat that damaged the fuel at Unit 1

The original question was "what is fuel cladding?

This long-winded exposition is getting to that. I've avoided using the term cladding because it's jargon for fuel tube.

What has happened at Fukushima is that after an hour all AC power was lost at the plant and systems like RHR, pumps & valves had no power.

There was one steam-driven pump called RCIC that supplied water to the core but it failed.

So with no electricity & no cooling the water in the reactor began to boil away and pressurized the reactor vessel.

The steam exits through pipes underwater in the big donut-shaped part of the containment. It's released underwater so the steam condenses