Like many of you, I’ve been closely following the developments at the Fukushima reactor complex. Below is a set of links to articles I’ve written for the Stranger, as the events have unfolded.

3/12/2011
Explosion at Fukushima Nuclear Plant, Cesium Detected

3/14/2011
Don’t Panic

Geiger Counter Readings Rise in Tokyo

3/15/2011
What’s on Fire at the Fukushima Reactor?

Will Radioactive Particles from the Leaking Reactor Reach Washington State?

The Fukushima Fifty

3/16/2011
“We believe that radiation levels are extremely high” (A discussion of acute radiation injury)

3/17/2011
Video from a Helicopter Flyover of the Fukushima Plant

The Health Effects of Radioactive Isotopes from Fukushima

3/20/2011:
Radiation from Fukushima, in Seattle

3/24/2011:
How Radiation Is Measured

3/27/2011:
Radiation From Fukushima, in Seattle, Tells the Story

A landmark Energy Department project to bury carbon dioxide produced by humans has begun as workers sunk a huge drill bit into Illinois ground this week, signaling continued support for a climate change mitigation strategy that has fallen out of favor in many circles.

The start of drilling marks the launch a geological sequestration project that will deposit a million metric tons of carbon dioxide into the ground by 2012.

While that’s nothing compared to the several billion tons of CO2 that humans emit yearly, it’s the geology of the site that makes the development exciting. The CO2 will be piped into a geological formation that underlies parts of Illinois, Indiana and Kentucky that could eventually hold more than 100 billion tons of CO2.

While I find the term ‘clean coal‘ to be absurd, I still think this sort of technical investment is critical for the future health of the climate. Thanks to years of foot-dragging on alternatives, the entire world has gone on a fossil-fueled power plant building spree. Carbon sequestration may never pan out. It’s, sadly, one of our few remaining shots at averting environmental catastrophe.

Take Shell’s move today, as a portent:

Shell will no longer invest in renewable technologies such as wind, solar and hydro power because they are not economic, the Anglo-Dutch oil company said today. It plans to invest more in biofuels which environmental groups blame for driving up food prices and deforestation.
….
The company said it would concentrate on developing other cleaner ways of using fossil fuels, such as carbon capture and sequestration (CCS) technology. It hoped to use CCS to reduce emissions from Shell’s controversial and energy-intensive oil sands projects in northern Canada.

Well, what of the alternatives? Wind is going to be a challenge, particularly in the context of climate change. Biofuels–at least fuels from bioengineered organisms–are intriguing, but we’ll have to get around our discomfort of genetic modification of organisms.

And then, there is nuclear power. (For a primer, I suggest my series on nuclear power, written a bit ago.) The Obama administration paused work on the Yucca mountain waste repository, exacerbating the waste problem (perhaps in a good way, for the long term.)

A growing consensus of scientists, however, are recognizing nuclear power as one of our better shots out of this mess:

Nuclear power is safe, affordable, and the waste problems are much more manageable than the public realizes. That was the take-home message from this year’s American Association for the Advancement of Science meeting in Chicago, where a group of experts from the US and EU participated in a session called “Keeping the Lights On: The Revival of Nuclear Energy for Our Future.”

My personal impression is slightly less rosy–with a deeper concern about waste management–but I still believe we should be investing massively in nuclear technologies.

We’re well past the point of being able to consider only the most pleasant energy sources. Looking at the number of people on the planet, and the increasingly dire reports of damage caused by the burning of fossil fuels, we need to be realistic. These steps, by the scientific community and the Obama administration, are heartening steps in what seems the right direction.

Wilkins Ice Shelf, a broad plate of floating ice south of South America on the Antarctic Peninsula, is connected to two islands, Charcot and Latady. In February 2008, an area of about 400 km² broke off from the ice shelf, narrowing the connection down to a 6 km strip; this latest event in May has further reduced the strip to just 2.7 km.

This animation, comprised of images acquired by Envisat’s Advanced Synthetic Aperture Radar (ASAR) between 30 May and 9 June, highlights the rapidly dwindling strip of ice that is protecting thousands of kilometres of the ice shelf from further break-up…

Wilkins Ice Shelf has experienced further break-up with an area of about 160 km² breaking off from 30 May to 31 May 2008. ESA’s Envisat satellite captured the event – the first ever-documented episode to occur in winter.

Excellent! The jury might be coming back on climate change. Perhaps this would be a good time to remind you of my posts and introduce you to a new podcast on nuclear power .

Nuclear power plants were first proposed at the dawn of the cold war. It was assumed the best fuels–enriched the most for atoms releasing the most neutrons per fissioning–would forever be reserved for military use. We had bombs to build. Hundreds, thousands, millions–enough to scare the Soviets (and the Soviets to scare us.) Military first, commercial power second. So, the plants were designed around using minimally enriched fuels with moderators to keep the scant neutrons around. Hence the collection of plants in operation today, almost all based around mildly enriched Uranium, moderated and cooled by heavily pressurized water. By far not the safest or most efficient design, but doable, particularly if you are limited to mediocre quality fuel.

Two funny things happened along the way.

The development of the hydrogen bomb vastly increased the efficiency of atomic bombs. Effectively limitless amounts of explosive force can be created using relatively small amounts of highly enriched Uranium or Plutonium, some heavy hydrogen and lithium and a whole lotta unenriched Uranium. About the time the US and the Soviets could destroy the entire surface of the planet three times over, interest in building more bombs started to wane.

Then the US won the cold war. While more and more states can field nuclear bombs, most have realized all you need are a few–just enough to pose a solid deterrent. You can’t eat atomic bombs. Nor can you really use them in war. Atomic bombs have always been a sort of military-industrial masturbation, a show of prowess rather than a practical weapon, a solid way of saying “don’t mess with me.”

The result? Our present world is awash in enriched Uranium-235 and plutonium–fine for making bombs or much better nuclear power plants. With this sort of high quality fuel, we have neutrons to spare. Let’s do some interesting things with them.

The first thing we can chuck from our reactor? The moderator. If we have enough neutrons, we can keep the chain reaction going even if most are flying away before meeting another fuel atom–a fast neutron reactor. No more graphite or water. Now we can use something more forgiving as a coolant, like molten metal or an inert gas. Wait, wasn’t one of the safety features of the water-moderated, water-cooled reactor that the loss of water would shut down the chain reaction? Fine. We’ll load the fuel a matrix that physically expands as it gets hotter. If it gets hot enough, the fuel will get too far away to keep the chain reaction going. Great! We’re back to the lose-coolant, lose-chain reaction situation.

What can we do with all those extra neutrons? Why not use them to smash up all those pesky radioactive decay products! Now, instead of having to periodically remove the fuel rods and chemically remove all the neutron-absorbing, non-fissioning, and highly radioactive waste atoms, we’ll just burn them off right in our reactor.

A traditional pressurized water reactor can only fission about 3-5% of the Uranium in it’s fuel before it has to be removed an reprocessed. About 270,000 metric tons of radioactive waste are scattered around the world, predominantly stored in pools or casks right next to the power plants thanks to this inefficiency. If we use the extra neutrons in our fast neutron reactor to burn off the radioactive waste, we can boost this efficiency to above 90%. With some relatively simple fuel-reprocessing on site, we can boost this to 99%.

Nuclear waste is the overwhelmingly major problem with nuclear power plants today. There is no plan, no strategy beyond burying it someplace for at least a million years. No technology exists that matches the problem. Fast neutron plants, that eat their own waste and potentially the waste of others, are an overwhelmingly better solution than Yucca mountain.

Where are these plants? The ideas here aren’t new ones. A pilot project, the Integral Fast Reactor (IFR) was to build a liquid sodium metal cooled, plutonium and U-235 fueled fast neutron reactor with an on-site waste processing center. The project’s budget was cut in 1994 by President Clinton‘s energy secretary and thus languished before the project could be completed. The ideas from this project have been rejuvenated, with plans for a liquid sodium, liquid lead and gas cooled reactor variants based around the same general principles, called generation IV reactors, to be ready for commercial operation in 2030.

The designs are, individually, brilliant. The lead-cooled variant is designed to be modular. The reactor is small, easily installed and removed and works for about fifteen to twenty years without having to be opened or refueled. Perfect for countries or remote areas with no interest in or infrastructure for refining nuclear fuels. The gas-cooled variant can operate safely at huge temperatures and is incredibly efficient at minimizing waste products in a relatively simple manner. The sodium-cooled design is the dreamiest to me. Such a reactor complex could not only operate at tremendous efficiencies, but also eat up the waste of the older pressurized water reactors. Keen!

2030 is too far away. If we were smart, we would throw resources at these fourth generation technologies, pushing to have the pilot reactors and designs finalized within ten years. None of these are perfect. No source of power is without risk or environmental injury. None. Our planet hosts nearly seven billion people. Fossil fuel reserves are dwindling. The atmosphere and oceans are buckling under the carbon strain. Nuclear power, particularly responsibly applied with standardized plant designs and a real plan for dealing with the waste, remains our best hope. The physics and technology is available. We just need to do it. Now.

Let’s talk about Chernobyl. We enter our time machine, and roll ourselves back to the start of the cold war. We’re nuclear engineers in the Soviet Union charged with getting as many reactors operating as soon as possible. Every bit of enriched Uranium is going to bomb manufacturing, as is all the available heavy water. So, all we have is a bunch of unenriched Uranium and regular water to build our reactors. Hmmm.

Let’s think back to our earlier reactor design talk:

Hey, something nifty! Water is both a good coolant and moderator! No moderator, no chain reaction, right? So, if you use water as your coolant and moderator, your reactor has an intrinsic safety feature. If you lose coolant, you lose moderation and the chain reaction stops. We all live! Thus, almost all nuclear reactors in operation today use water as a coolant and moderator….
But what water, and what fuel? Here’s the trade-off: The better the moderator, the crappier your fuel can be… The rarer versions of fuel (U-235 is better than the common U-238) or moderators (heavy water is better than regular water) tend to be the better. Building a reactor typically requires enriching for either the better moderator or the better fuel.

But, we don’t have enriched Uranium or heavy water. Well, we aren’t required to use water as both our moderator and coolant. What if we stick with water as our coolant, but use something else to moderate the neutrons? Graphite will do the job and it’s cheap and plentiful. So, we’ll build our reactor with unenriched Uranium (or even better, depleted Uranium coming out of the atomic bomb plants) as the fuel, regular water as the coolant and graphite as the moderator. What do we have to lose?

In this design, when the water coolant is lost, the graphite moderator stays and the chain reaction continues. In fact, it speeds up, as the regular water isn’t around to absorb some of the neutrons anymore. Added bonuses? The graphite will tend to chemically explode when it heats up enough. So, lose coolant, the chain reaction speeds up, the reactor quickly overheats and explodes with both chemical and nuclear power as energy sources. Neat. Even better? If the fuel is old, and thus filled with all sorts of highly radioactive waste atoms, the massive explosion will be sure to spread all these atoms far and wide–a sort of gigantic dirty bomb. The resulting mess will result in far far far more radioactivity than dropping an atomic bomb. At least in the bomb, most of the radioactive atoms are consumed to produce the explosive force.

I’d like to imagine the following exchange, between a middle manager in the Soviet Union and us, some plucky nuclear engineers, when planning these plants:

Middle manager: “You have my plant design?”
Us: “Yes, but it is incredibly dangerous!”
MM: “But it will work without any Plutonium, enriched Uranium or heavy water?”
Us: “Yes. In fact, it produces Plutonium as a waste product!”
MM: (Claps hands) “Excellent. We shall have such nice dachas when I tell everyone of this plan.”
Us: “It is far to dangerous to build. I refuse to do it!”
MM: (Laughs. Then pauses.) “Oh. You’re serious.”
MM: (Considers his boss, probably some one-eyed, one-armed veteran of Zhukov’s Berlin campaign in the Great Patriotic War, who won’t be sympathetic to concerns about hoards of irradiated civilians after asking why his reactor isn’t operating yet.)
MM: (Points to us.) “Guards, shoot this man.”
Us: (Shot in the head)
MM: (Turns to our assistant) “So, ready to build the reactors?”
Assistant: “Let’s just pick some places in Ukraine, Romania and other shitholes to build ‘em, yes?”

I have no doubt similar disagreements happened in the United States, when faced with similar shortages. American middle managers couldn’t resolve disputes with engineers by the bullet-to-the-head trick, probably tipping the debate outcome.

Chernobyl was this sort of reactor, a reaktor bolshoy moshchnosti kanalniy (aka RBMK, aka insane). The inevitable eventually happened. Coolant was lost. Does it really matter why? Ok, fine. This particular fiasco wasn’t caused by normal operations. They were messing with the plant, trying an “experiment.”

Like most reactors, these must be shut down periodically to swap out the spent fuel rods for fresh ones. So, shutting down the plant, by putting in all the control rods, is pretty typical. During one of these routine shutdowns, the plant operators got an idea. The water pumps, supplying coolant to the reactor, are powered by the reactor’s own turbine. Backup power is available from on-site diesel generators, but these take 40 seconds to power up. The plant operators asked themselves, if we shut off the steam to the reactor’s turbine, will the turbine keep spinning long and fast enough to keep the water pumps operating until the diesel backup gets up to speed?

Great question, guys! I’ll give you a hint to the answer: No. Kaboom!

Why on earth would they think to try something so nuts, to shut off coolant to a reactor that will run out of control upon the loss of coolant? Most were conscripts from coal power plants elsewhere in the empire. They didn’t know better.

What exactly happened? The wikipedia article on the Chernobyl disaster does an admirable job breaking it down clearly. In short, the coolant ran out, the reaction sped even more out of control, melting the reactor and exploding the graphite. Huge amounts of highly radioactive waste was spread all over Europe.

Something to help you sleep at night? Reactors of this design, admittedly modified to be a little safer, are still operating in the former Soviet Union.

When it comes to “disasters” at nuclear power plants, this is only one deserving the title of disaster. Compared to Chernobyl, the Three Mile Island is a pathetic also-ran. In this contest, the US takes a distinct silver to the Soviets’ uncontested gold.

The reactor at Three Mile Island was far more typical, using pressurized light water as both a moderator and coolant, and slightly enriched Uranium as a fuel. What happened? Again, the Wikipedia article on the TMI accident is superb. The short of this one?

Poorly designed controls and displays for a reactor made it virtually impossible for the plant operators to respond properly when a coolant pump failed. There was no reliable way to see how much high-pressure water remained in the reactor, no reliable way to tell if emergency valves were opened or closed, no way to tell when things were running out of control. The operators made poor decisions based on poor information, resulting in the high pressure water eventually all flashing to steam and the reactor core melting. Still, even with all these bad things in a row, only a teeny, tiny amount of radiation escaped into the environment.

Why on earth would such poorly designed reactor controls allowed? Because, brilliantly, almost every reactor in the United States is different from every other one. Thank the lowest-bid culture, in which almost every commercial power plant in the US was guaranteed to be different from any other built before. With such a system, one cannot even train experts to work around shoddy design choices. Each must be discovered individually at a particular plant. Urk!

Not every nuclear power plant system is run this way. The French, in contrast, have almost all of their reactors one of three basic designs. Yes, they have quirks. But at least the quirks are shared among all the reactors of the same type. The net result? The French get about three quarters of their power from these nuclear plants and have had no accident even approaching the severity of Three Mile Island.

The possibility of a nuclear plant accident, however prominent a role it plays in the public imagination, should be a secondary concern to the problem of nuclear waste. Chernobyl, without a doubt, was a fantastic failure. While there will inevitably be future accidents at plants, they are vastly more likely to be more Three Mile Islands rather than Chernobyls.

We’ve got our reactor up and humming. Our fuel is fissioning, splitting into smaller atoms and releasing neutrons. Our moderator is slowing down the neutrons, keeping them around long enough to fission the next fuel molecule. Our control rods are absorbing enough neutrons to keep the chain reaction in check. The coolant is transferring the heat out of the reactor.

Over time we have to pull our neutron-absorbing control rods out farther and farther just to keep the chain reaction going. Why?

When we loaded our reactor, the fuel was chemically fairly pure. Recall, however, that nuclear decay typically results in new chemicals being created–whether by alpha or beta decay or by fissioning. As our reactor operates, these new atoms build up. Most are radioactive themselves, also undergoing various decays. Most of these atoms are neutron hoarders–gleefully absorbing our precious neutrons, while offering up few when they themselves decay. So, as these new atoms build up, we lose more and more neutrons. Eventually there are too few free neutrons left to keep the chain reaction going, even if we completely remove the control rods. Such fuel, still containing a bunch of Uranium but now contaminated various highly radioactive but non-chain reacting atoms, is called spent. It’s hideously radioactive, more radioactive than when we put the fuel in the reactor, but useless as fuel.

Welcome to the trickiest problem of nuclear power, the waste. What can we do?

Pull the spent fuel rods out of the reactor and replace them with fresh ones. We’ve only used a tiny percent of the Uranium up, but on the positive our reactor will start working again. On the negative, we have a whole bunch of really radioactive former-fuel that is useless.

Consider these unwelcome new atoms. The more radioactive the atom–the more often the atom decays–the quicker it uses itself up and becomes something else. So, the most dangerous to health atoms disappear relatively quickly–within years or maybe decades. The problem is, even the less radioactive waste–that will last for hundreds of thousands of years–is still radioactive enough to be a threat to health.

We could just store the spent fuel rods, for millions of years, until it is only minimally radioactive. Water is a pretty fantastic shield against radiation. So, we build a whole bunch of swimming pools in the reactor containment buildings and then sink the radioactive waste to the bottom, waiting for the most radioactive atoms to decay into other more manageable things. That will be fine for a few years at least. When the pools get filled, we’ll next build some metal and concrete casks outside of the reactor building, next to the plant and store the rods there. Finally, we’ll hope that someone, somewhere, finds a suitable mountain to carve out and drop the waste into, just to get it off our property.

But wait, you say. The new atoms, ruining our chain reaction, are only a teeny percentage of the spent rods. We could chemically break down the fuel, purify for Uranium and create new fuel from the old. Great! We’ve now reduced the freakishly radioactive waste we cannot use to a much smaller mass and recycled the fuel. Neat. We stopped doing this in the 1970′s. Why? While the leftover atoms are useless for commercial power plants, they would make great starting material for a dirty bomb. Ouch.

So, we’re back to the pool, cask, prayer plan for the waste. While there are proposals for more clever ways of dealing with the waste, right now nearly three hundred thousand tons of highly radioactive spent fuel rods are scattered all over the world. Typically in casks next to the power plant, they represent by far the biggest environmental and health risk from nuclear power. And we really don’t have anything better planned in the near future, right as we’re embarking upon a plant building binge.

Unlike waste from chemical plants, or carbon pouring out of smokestacks and tailpipes everywhere, this waste is a uniquely human creation. While sitting in a reading of The World Without Us, about what a world after the end of humanity would look like, I realized that radioactive waste would be the longest lasting legacy of humanity.

After all the bridges and towers crumble, after all the seawalls give up, after all the dams burst, after all the plastic garbage is eaten by some newly evolved bug, after our bones are recycled into every imaginable different thing, our radioactive waste–made of atoms not seen on the planet since its birth–will persist.

Unhappy atoms break up in a few different ways, all releasing energy. We can break it down into three categories:

1. Throw out a nucleon The atom flings off some combination of neutrons and/or protons. Most commonly, this is a helium nucleus comprised of two protons and two neutrons called an alpha decay. Nuclear fission is also in this category and what we care about the most for our chain reaction.

Since we’ve already talked about fission, let’s go through an example of an alpha decay. Uranium-238 goes through alpha decay, loses two protons and two neutrons to become Thorium-234. (Get it? 238 to 234, because four nuclear particles are lost. Uranium has 92 protons, thorium 90.)

2. beta decay: Throw out a nearly mass-less charged particle, an electron or positron So far, I’ve ignored the electrons. Electrons are for chemists. Those losers can’t change atoms; they can only rearrange atoms. We’re nuclear physicists right now.

The most common beta decay, beta negative, involves an atom splitting a neutron into a proton and an electron. The proton sticks around in the nucleus, the electron goes flying outward. Because the proton sticks around, the atomic number of the atom undergoing beta negative decay goes up. Neat.

The Thorium-234 made by an alpha decay of U-238? It’s also unstable, but tends to go through a beta negative decay, gaining one atomic number to 91 and thus becomes protactinium-234. The overall weight doesn’t change, as neutrons and protons remain about the same, but the atomic number does.

3. Spit out energy as a high energy electromagnetic gamma ray. This is similar to glow in the dark stickers, just with far more powerful waves. The atom doesn’t change at all, in a chemical sense, staying with the same weight and atomic number. Therefore, gamma radiation often accompanies another decay, an alpha or beta. The energy released by the atom during the other decay gets stored temporarily in the atom. Its release is what causes the gamma ray.

The huge differences in mass between alpha, beta and gamma radiation really matter. If we think of beta particles as being the size of a bullet, alpha particles are more like cannon balls. At the same energy, the beta particle will be going about forty times faster than an alpha.

Imagine we’re shooting at someone through a wall. The cannon ball, flying 40x slower than the bullet but much heavier, will smash the into the wall, causing huge amounts of damage to the wall (the first thing it hit). But, the person behind the wall will probably be ok, so long as they’re far enough behind the wall. The bullet, much lighter and faster, is far more likely to go straight through the wall–losing some speed, but retaining more on the far side of the wall than the cannon ball. The bullet has a far better chance of hitting the person.

Alpha particles, the cannon balls, can be stopped by a single sheet of paper. Smash! Likewise, the dead outer layer of skin does a damn good job of protecting your living cells from alpha particles. Beta particles, the bullets, go right through paper. A thin sheet of aluminum, or something of similar density and substance, will gobble these up.

Gamma radiation is trickier. Gamma radiation is just a freakishly high energy version of light, with almost no substance. Just like light can pass right through your hand, gamma radiation can pass through all but the heaviest and densest of metals, wreaking havoc deep into the body.

Gamma radiation is the most likely to cause your body misery. Eating an alpha emitter? Not so smart, as your gut takes the big hit rather than the dead layer of skin cells. Beta emitters can cause quite a bit of damage. But it’s gamma rays, passing right into your depths with ease, that really cause misery.

What does radiation do to the body? Mostly, the problem comes from gamma rays shredding DNA and RNA, destroying the cookbooks and scrap paper of the cells, rendering them unable to divide or function properly. At the lowest fatal doses of gamma irradiation, the DNA in blood-forming cells is scrambled to the point these cells stop dividing. Within a few weeks one starts to become anemic from the loss of replacement red blood cells. After a few more weeks, the numbers of white blood cells also drop, as they are also not being replaced anymore. One perishes from opportunistic infections, like one does from AIDS.

At yet higher doses of radiation, the cells in the gut stop dividing–again, thanks to scrambled DNA. The walls of the gut thin, eventually allowing the massive collection of bacteria in each of our guts to enter the body and devour us alive. Even higher doses? So many brain cells are destroyed that the whole brain starts to swell, eventually stopping breathing. Even higher? The cells of the body are literally shot to pieces by all the gamma radiation and fast moving alpha and beta particles. Too many of the cells in the body simply burst and die.

It actually takes quite a bit of radiation to get anywhere close to fatal. Our bodies are adapted to handle quite a bit of radiation, as the world we live in is quite radioactive. Gamma rays are constantly raining down on us from the sun. Water is radioactive. Most metals are radioactive. At least some variants of the atoms that make up most of us–carbon, oxygen, nitrogen and so on–are radioactive. Our cells handle persistent, low-level irradiation with some skill. It’s sudden, massive, bursts that cause big problems.

Lest you believe radiation is only a problem of nuclear power, it’s also worth noting that almost all fossil fuel is contaminated with radioactive metals and gases; carbon isn’t the only thing streaming out of smokestacks.

Coolant is the easiest to grasp. The fission chain reaction in the nuclear reactor will produce heat. It’s the goal! We need some way to transfer the heat away from the reactor and put it to use. Water is about perfect for this task. Yes, the boiling point of water is pretty low at standard atmospheric pressure. No worries. Just make the reactor pressurized, raising the boiling point of the water, and you have the task done. Most reactors use two circuits, plus a heat exchanger. The high-pressure loop of water goes right into the nuclear reactor. The heat is exchanged to a low-pressure loop of water that boils to steam. The steam is then used to run a turbine, and then an electrical generator. Only the water from the first, high pressure loop, tends to get radioactive.

Ok, fuel. Specifically? An atom with large nuclei (a really big party), unstable enough to be shattered when hit by a neutron (joined by someone screaming away from another party that just broke up) that also releases some neutrons when it splits. The problem is, most of these atoms absorb more neutrons than they release when doing so. To build our chain-reaction, we need something that is always releasing more neutrons than it eats. A handful of elements fit the bill, most commonly Uranium and Plutonium–Uranium is about your best bet, for naturally-occurring atoms. The problem is, it comes in a few versions. U-238 is by far the most common, and it also releases the fewest neutrons when breaking apart; the rarer U-235 and U-234 release far more. Better nuclear fuels release more neutrons when they fission. The more neutrons the atom releases when breaking up, the better the chance one will hit another fuel atom.

Time to mix even more metaphors! We can think about our reactor as a giant pool table. The neutrons are our cue balls. The fuel nuclei are sets of pool balls we want to break—to fission. When our fuel fissions, the sets of balls split roughly into halves, with a few cue balls racing outwards. The problem is, the nuclei are so excited to split up, the neutrons are going way too fast. Just like a cue ball flying off the table before a successful break, the neutrons are gone from our reactor before we can do anything useful. We need some way to slow the neutrons down, keeping them around long enough to actually hit the next fuel nucleus and keep the chain going. What we need are a whole bunch of little rubber balls on the table, bouncing around, for the cue balls to bounce off of–bumper pool!

Enter the moderator—any atom or nucleus off which neutrons can bounce without getting stuck too often. Even if the neutrons—our cue balls—are moving way faster than the moderator, just by bouncing off of the moderator molecules so often, the speed of the two will be eventually matched.

Now our slowed-down neutrons will have a vastly increased chance of hitting another fuel nucleus before leaving our reactor. Ideal moderators are very slippery to neutrons, absorbing none. Regular old distilled water–comprised of two hydrogen atoms (basically a proton) attached to an oxygen–makes a decent moderator. The biggest problem with water as a moderator is all those lonely hydrogen nuclei, tending to stick to our neutrons before they can help fission the next fuel nucleus. Ah hah! If you enrich the water for its heavy variant—in which the hydrogen nuclei are already carrying a neutron, heavy water—things start going far better. Heavy water is an excellent moderator, as there are few spaces for a neutron to stick.

Hey, something nifty! Water is both a good coolant and moderator! No moderator, no chain reaction, right? So, if you use water as your coolant and moderator, your reactor has an intrinsic safety feature. If you lose coolant, you lose moderation and the chain reaction stops. We all live! Thus, almost all nuclear reactors in operation today use water as a coolant and moderator.

Controlling the reactor is all about absorbing up the neutrons before they can continue the chain reaction. If you can insert and remove rods made of some neutron absorbing material, you can control the chain reaction.

But what water, and what fuel? Here’s the trade-off: The better the moderator, the crappier your fuel can be. If you’re using a so-so moderator, your fuel must release many neutrons when fissioning, to account for those lost by sticking to your crappy moderator. Likewise, a really good moderator can make up for a crappy fuel, by keeping around more neutrons. The rarer versions of fuel (U-235 is better than the common U-238) or moderators (heavy water is better than regular water) tend to be the better. Building a reactor typically requires enriching for either the better moderator or the better fuel. With a good enough fuel, you don’t need a moderator at all.

The sensible, Canadian, choice is to enrich the water for the heavy variant–to use one of the best moderators available. Yes, it’s painful and expensive. But water is minimally radioactive (thanks to the very, very rare Tritium-containing super heavy water, where at least one hydrogen is a radioactive proton and two neutrons) and doesn’t explode when piled too high. With heavy water as a moderator, unenriched Uranium, mostly U-238, works just fine.

The American way is to enrich the Uranium for U-235. Yes, even unenriched Uranium is hideously radioactive. Yes, even unenriched Uranium will explode, if you pile up more than the critical mass at which an uncontrollable chain fission reaction will start. A chunk of Japan was nearly destroyed by this sort of accident. On the positive, regular old water is perfectly fine as a moderator and coolant for this sort of fuel. Most reactors around the world are of this design: Uranium enriched for U-235 with pressurized regular water as both a moderator and coolant. USA! USA!

Why? Meet the nucleus: Protons and neutrons in an uneasy alliance. Neutrons, conveniently enough, are neutral in electrical charge. No problem rubbing two of them together. Protons, however, are positively charged. Remember, like charged objects don’t like sitting next to one another, thanks to electrostatic forces. Holds true for protons. So, how does the nucleus of an atom hold together? Nuclear force! At really short distances, this attractive force between neutrons and protons overwhelms the electrical forces trying to fling the protons apart.

Think of a nucleus as a party. The protons are like the type A, cliquish people at the party–constantly talking bad about one another when afar, but all lovey-dovey when up and close. The neutrons are the type-B’s, pretty nonpartisan about people from afar, but agreeable enough (if a bit dull) when up close. There is an optimum mix of these two kinds of people, the perfect party mixing the right number of type A’s and type B’s. Too small of a party? Boring! Too big? Unwieldy! Having too many type A’s, with too few type B’s to smooth things out? Disaster. Too many type B’s? Dull disaster. You’ve been at a bad party that suddenly got better, either by someone coming or going. The sense of relief, the release of nervous energy, is palpable.

Iron is that perfect party, combining the ideal total number along with the perfect mix of protons and neutrons.

The energy released in a nuclear reaction is the relief of a small nucleus as it gains some protons and neutrons or the relief of a huge nucleus as it splits into two smaller nuclei–in either case, getting closer to the ideal.

Every nuclear power plant in operation today works by capturing the energy release when a really unhappy large nucleus breaks up into two smaller and more successful get-togethers–atomic fissioning. When these cranky huge parties break up, a few neutrons typically get flung out at high speeds–think of these as a few type-B’s from the party screaming away in tears. If these neutrons hit another large nucleus, teetering towards breaking up already, they can smash the party to pieces, sending yet more neutrons out.

So, you can imagine a game where you place enough of these large nuclei next to one another, such that the neutrons from one breaking up shortly cause a neighboring large nucleus to break up, sending more neutrons out to break up more nuclei… creating a chain reaction.

Building a nuclear reactor is all about getting the right density of heavy, unhappy, nuclei next to one another, and successfully creating a chain reaction. The heat produced can be captured and used to boil water, and then turn a turbine creating electricity. Disco!

Nuclear power, long reviled as a dangerous source of energy, is on the verge of a comeback. That’s because a growing body of scientists, politicians and environmental activists see atomic energy as part of the solution for global warming and our ever-growing dependence on foreign oil, much of it from nations that, if not downright hostile toward us, certainly don’t share our values.

(Washington Post)

It’s time we talked about nukes. For most, the opinions run deeper than knowledge. I’m going to attempt a brief series on the physics of nuclear power plants, the parts needed to build a reactor, the biological effects of radiation, nuclear waste, the two biggest fiascoes in commercial nuclear power and finally talk about what a new nuclear power plant should look like.

Read on!