Explaining the crisis in Japan, Part I: Nuclear reactors

I have not posted in quite awhile. Frankly, I haven’t been all that inspired and have been in a bit of a funk. With that said, my heart goes out to Japan, a country I have very much wanted to visit for a long time since I’m a big fan of Haruki Murakami.  I feel a bit helpless not being able to do anything at the moment.  But I am a physicist and an engineer, and I have the unique qualification of having some background in all three of the disasters that have struck Japan – the earthquake, the tsunami, and the ongoing nuclear problems.  So, at least for now, perhaps I can offer some background on these topics and discuss what happened as a way to get information across in a “spin-free” way, as it were.  I’m going to break this post up into a series of three or four, focusing on one problem at a time (and maybe tying them altogether at the end).  Since the nuclear crisis is on everyone’s mind at the moment because it is still unfolding, I thought I’d start with that.  I’m going to focus some of my comments on how this relates to things here in my neck of the woods in order to put things in perspective (e.g. I live 40 miles from Seabrook Nuclear Power Station).

All elements possess a fixed number of protons in their nucleus (that is what makes an element unique from another element) but nearly all can possess a varying number of neutrons and so they come in different “types.”  We call the various types of a given element, isotopes.  Nearly all elements exhibit some kind of radioactivity in some form since many of the isotopes are unstable.  Unstable isotopes spontaneously decay in one or more of three basic ways: alpha, beta, and gamma radiation.  In alpha and gamma radiation, sometimes free neutrons are formed.  Nuclear power stations are all based on the idea of fission in which a heavy element breaks down into two or more lighter elements after being bombarded by a free neutron (frequently produced by spontaneous decay).  This produces more free neutrons which triggers more fission, and so on, in a chain reaction.  In the process, gamma radiation – very high energy light – is also given off.  This is typically used to heat water into steam which then turns a turbine to generate electricity.  But note that a chain reaction is an exponential process: when a fissile material splits, typically more than one neutron is created.  So imagine you start with a single neutron that splits a heavy element.  This split produces two neutrons, each of which then collides with another atom producing, now, four neutrons.  Since gamma radiation is also being released and gamma radiation causes things to heat up when absorbed, it is imperative to control this chain reaction so that it does not overheat.  This is accomplished by introducing a coolant (like water) and also something to absorb excess neutrons (basically, something that can “grab” some neutrons before they have a chance to cause fission).  In most nuclear power station reactors, the nuclear material (usually uranium, but sometimes plutonium or both) is shaped into rods that are placed near each other.  Some of the fission comes from neutrons from one rod penetrating a neighboring rod.  So another way to control the reaction is to raise barriers between the rods.  Typical nuclear plants use a combination of all of these techniques.

The Fukishima Daiichi plant employs what are called boiling water reactors (BWRs).  BWRs were developed by the Idaho National Laboratory and GE back in the 1950s and are the second most popular type of reactor behind pressurized water reactors (PWRs) that were originally developed for nuclear powered submarines.  Seabrook Nuclear Power Plant in New Hampshire is a PWR facility.

The primary difference between the two is that in a BWR, the water that cycles through the turbine is the same water that comes in direct contact with the radioactive rods.  In a PWR, the water is indirectly heated.  It’s almost a bit like the solar panels on my house.  The panels heat up in the sun, but it’s not water that gets directly heated.  Rather, there’s glycol in the system that is heated, and this glycol flows through piping into my basement and through coils in my hot water heater where the water is heated.  In PWRs this closed loop flow of material requires pressurization and so BWRs have the advantage of operating at lower pressure levels.  A PWR tends to create less heat and can be actively stopped by dropping the rods directly into coolant when the power fails.  In addition, a PWR tends to be extremely stable since it actually produces less power as the temperature rises.  However, both systems require active cooling after shut down.  This means that just because the power goes off doesn’t mean the radioactive decay is going to stop.  So the rods will continue to give off some heat even after they are separated.  It’s this heat that needs to be cooled to prevent a buildup of hydrogen gas that can cause an explosion and/or can cause the rods and their cladding to melt into a gooey mess called “corium” that can very easily melt right through the containment vessel. Explosions can cause damage to the containment vessel which could then lead to the leaking of radioactive material into the ground, water, and air where it can do a great deal of damage.  The Fukishima plant had backup systems designed to cool the plant in the event of a power failure.  First, diesel generators kicked in, but they were subsequently destroyed by the tsunami which was large enough to top a seawall designed to protect the facility.  Batteries were then used for a time, with others shipped in from other sites, but these all have a limited lifespan.  New generators could not be hooked up because these would normally be connected through a switching panel in the basement of the facility, but the basement was flooded by the tsunami.  They then began to pump a combination of seawater and boric acid (to absorb stray neutrons) into the facility to cool it.  This action renders the facility useless and thus is a last-ditch effort.

The problem with nuclear radiation is multifold, but very important to keep in context.  First, gamma radiation is very high energy light (e.g. like x-rays though in some cases even more penetrating).  Gamma rays will pass right through your body, ionizing cells in the process, leading to anything from increased risk of cancer (on the mild end) to nearly instant death (on the more serious end) depending on exposure times and amounts.  Second, ingesting an alpha or beta emitter can produce some gruesome results (not worth going into right now).  However, note that alpha and beta emission cannot penetrate human skin.  So you have to ingest it or breath it in for it to be a problem.  So the immediate threat from the meltdown of a nuclear reactor, if one takes the proper precautions, is gamma radiation (which is essentially only stopped by lead or similar compounds).  The proper precautions to which I refer include staying indoors, using a gas mask or other such filter if you must be outdoors, washing yourself thoroughly and disposing of clothing when you come back indoors.  The longer-term problems are when the alpha and beta emitters get into the soil and water, and thus into the foodchain.  One of the decay products of uranium, for instance, is radon.  There’s quite a bit of radon in the soil in New England and there also happens to be a fair amount of naturally occurring uranium compounds (uraninite and uranophane – I have pieces of it found in the White Mountains when I taught Geology this past summer).  Much of the radon was originally uranium, probably in these compounds.

That’s it in a nutshell.  It’s quite short and doesn’t do the subtleties justice, but should give people some sense of what is happening.  Tomorrow or Friday I will post a description of the tsunami and earthquake.


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