A Non-Technical Explanation of a Nuclear Meltdown

There is a lot of concern right now about the situation in Japan with the Fukushima Daiichi nuclear plant and the strong possibility of a nuclear catastrophe if the engineers fighting to prevent a meltdown lose control of the reactor.

A few years ago I had a client who is a major player in the nuclear industry, and I was fortunate enough to learn how nuclear power plants are constructed and operated.  I  toured a training facility that had a non-working full scale mock up of what’s called the “Nuclear Island,” the part of any nuclear facility that houses the reactor, coolant pumps and loops, and the steam generator.  I had the opportunity to descend the 160 feet into the heart of the reactor, where the fuel rods would be.

The Japanese earthquake of March 11 and the damage to the Fukushima Daiichi nuclear plant is a scary thing, and people old enough to remember the incidents at Three Mile Island and Chernobyl were no doubt chilled to see news footage of explosions and fires caused by damage the plant sustained in the quake.  We don’t know the extent of the damage yet, but it’s clear the structural integrity of the Nuclear Island has been compromised.

DISCLAIMER:  I’m not a nuclear engineer, nor do I play one on TV, and I didn’t sleep at a Holiday Inn Express last night, but this is my attempt to share my knowledge in a non-technical way.  Errors and omissions are my own.

Nuclear facilities are designed to withstand massive earthquakes, tsunamis, and direct aircraft impacts.  There are multiple layers of redundant safety systems.

The reactor core is protected by 2 layers.  First, the nuclear fuel is encased in metal tubes made primarily of zirconium (the fuel rods  at point #1, above).

The core of the reactor is flooded with coolant water that helps regulate the temperature and steam pressure being generated (point #2).

Lastly, the whole assembly of the reactor coolant pumps, pressure vessel, and steam generators are housed in the containment building (point #3.)  The containment building is a reinforced concrete dome that has a thick steel inner liner and an outer concrete shell.  The walls can be 3 to 6 feet thick.

The basic premise of nuclear plant operation is to raise and lower the fuel rods in the core to heat up the reactor water to generate steam, which is pumped out of the Nuclear Island to the Turbine Island, the power generation part of the facility.  [Scroll to the bottom of this post for a more technical overview of nuclear power generation.] A nuclear plant is safe as long as the heating and cooling balance in the core is maintained.

While nuclear power plants generate electricity, they need electricity to run (DUH!), which comes off the main power grid.  In the event of power failure, the emergency diesel-powered generators kick on and are designed to run the plant until main power is restored, or until the reactor can be safely shut down.

At Fukushima, the power was knocked out when the quake hit, and we don’t know if the generators were able to run, and if so, for how long.  Eventually, that balance of heating and cooling at the core became disrupted, the coolant water levels dropped, and the zirconium cladding of the fuel rods heated up, producing hydrogen, which ignited.  This was likely the cause of the explosion and fire seen in news footage.

A meltdown (“core melt,” in nuclear industry parlance) happens when the nuclear reaction can’t be controlled and enough heat is generated from the core as to begin literally melting the reactor pressure vessel and even the concrete floor around the core.

We can surmise that at this point, all 3 safety layers in the Nuclear Island were breached and radiation began leaking through the breach in the containment dome.

This is a terrifying situation, but under normal circumstances an intact dome with its steel liner should be able to contain a meltdown until it burns itself out.  This, obviously,  is not a normal situation.

The Fukushima reactor appears badly damaged.  I’m sure there are brilliant minds sitting around conference tables all around the world trying to figure this thing out.  There are 50 – 180 engineers – heroes –  inside Fukushima trying to get some control of the reactor, and their dedication will most likely cost them their lives.  All we can do is wait and pray.

How A Nuclear Power Plant Works

In Pressurized Water Reactors (PWR) power plants, ordinary (light) water is utilized to remove the heat produced inside the reactor core by the nuclear fission phenomenon. This water also slows down (or moderates) neutrons (the constituents of atomic nuclei that are released in the nuclear fission process). Slowing down neutrons is necessary to sustain the nuclear reaction (neutrons must be moderated to be able to break down the fissile atomic nuclei). The heat produced inside the reactor core is transferred to the turbine through the steam generators. Only heat is exchanged between the reactor cooling circuit  (primary circuit) and the steam circuit used to feed the turbine (secondary circuit). No exchange of cooling water takes place.

The primary cooling water is pumped through the reactor core and the tubes inside the steam generators, in four parallel closed loops, by coolant pumps powered by electric motors.  Each loop is equipped with a steam generator and a coolant pump.  The reactor operating pressure and temperature are such that the cooling water does not boil in the primary circuit but remains in the liquid state. A pressurizer, connected to one of the coolant loops, is used to control the pressure in the primary circuit.  Feedwater entering the secondary side of the steam generators absorbs the heat transferred from the primary side and evaporates to produce saturated steam.  The steam is mechanically dried inside the steam generators then delivered to the turbine.  After exiting the turbine, the steam is condensed and returned as feedwater to the steam
generators.  A generator, driven by the turbine, generates electricity.

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