Large Fierce Mammal

My first article in this three part series, titled Bullshit Alert – Since 11 March 2011, If You’ve Ever Even Heard of Japan, You’ve Been Exposed dealt with my observations on the ass covering passed off as public information releases that have been streaming out of Japan since the 11 March earthquake and subsequent tsunami shut down backup power systems to a number of nuclear power generating stations. My second article, Radiation – Let’s Get It Straight, dealt with clarifying the facts of “radiation” – what it is, what it isn’t, and how it works. If you haven’t yet read those two pieces I strongly suggest that you take the time to read them both, in the order they were published, before going ahead with this one.

Nuclear reactors are built for a variety of reasons, but the ones affected in Japan exist for the purpose of generating electrical power, so it’s that function we’ll focus our attention on here.

Electrical power generating facilities fall into two categories:

1. Those that capture energy from natural sources existing within the environment, converting it either directly into electrical energy, or initially into mechanical energy by using it to turn a turbine connected to a generator which produces electrical energy; and
2. Those that liberate energy from consumption of a concentrated potential energy source, a fuel, use it to create thermal energy (heat) which is used to turn water into steam which is then converted to mechanical energy by using it to turn a turbine connected to a generator which produces electrical energy.

In the first category we have hydroelectric, tidal, and wind turbine stations, each of which employ means to capture the kinetic energy of moving water or air. In this category we also find photovoltaic generating stations which employ what are generally referred to as “solar cells” to absorb and convert energy radiated by the sun directly into electrical energy.

In the second category we have electrical power plants that burn fossil fuels such as coal, oil, and natural gas; and then of course, we have nuclear power plants which employ the thermal energy released by controlled nuclear fission.

Plants that burn hydrocarbon (fossil) fuels, like those that exploit the energy released by the slow, controlled breakdown of nuclear “fuel”, require that the fuel source be replenished periodically to replace fuel consumed. Like your car, a hydrocarbon burning plant needs its tanks refilled as fuel is burned. Similarly, the “fuel” used in a nuclear reactor will eventually become depleted to the point where it can no longer sustain a fission reaction of sufficient intensity to generate the required thermal energy output. This takes far longer than it would for a hydrocarbon plant of the same electrical output capacity, and hence the allure of nuclear power.

Under normal operating conditions, a nuclear power plant produces no exhaust emissions of any kind, and can be operated for years without needing to be refueled. On the down side, when its fuel does need to be replenished, the old fuel needs to be removed from the reactor and either stored on site, or be removed to a secure “disposal site” where the normal measures are to encapsulate it in containers made of a shielding material, and bury it. The depleted fuel is highly radioactive and will represent a health hazard to any living organism that gets too close to it. For how long? If you’re old enough to be reading this, and you are fortunate enough to live to what passes for a ripe old age, spent fuel that was stored on the day you will born will still be too hot to handle on the day you die.

The first nuclear reactor was built by the people who brought us the fission bombs that made their debut in the skies over Hiroshima and Nagasaki in the summer of 1945. I’m going to start this tutorial with an explanation of how the bomb dropped on Hiroshima worked because it will pave the way to a clearer insight into the fission process. At the end of the day, a nuclear weapon is nothing more than a nuclear reactor with all the stops pulled, and overfueled to the point where a really bad outcome is guaranteed.

“Little Boy” (U.S. Air Force photo by Jeff Fisher)

The bomb dropped on Hiroshima by the bomber Enola Gay on 6 August 1945 was code named “Little Boy”. Inside the bomb was a very simple mechanism – nothing more than a 39 kilogram piece of Uranium 235 with a cylindrical hole in it at one end of a hollow tube, and a 26 kilogram slug, also made of U 235 at the other end. On detonation, a conventional cordite explosive charge behind the larger piece is fired, forcing it down the tube to be impaled on the first piece. BOOM! 65 kilograms of metal is converted into an explosive effect equivalent to 15,000 tons of conventional TNT … that and a lot of other shit we can also do without.

Little Boy’s big day. (U. S. Air Force photo)

So what the hell just happened here? Everything was fine until those two pieces came together. The answer lies in why nuclear fission happens.

Uranium is a naturally occurring element. We dig it out of the ground like iron, tin, aluminum, gold, silver, copper, and all the other ones, in the form of ore. In fact, Uranium is one of the most common elements found in the crust of the earth, being 40 times more common than silver, and 500 times more common than gold.

We all think of oxidation as the rust on our car, caused by a chemical reaction between the iron in its steel structure and oxygen in the air. A traditional fire is also an oxidation reaction. Regardless of its fuel, a fire requires three components to be present – fuel, oxygen, and heat. The result is a persistent and self-sustaining chemical reaction that results in the release of visible light and heat (infrared) energy as well as chemical residues like water vapour and carbon.

Nuclear fission is also a reaction, but one that occurs on an atomic level, and that only requires one component – a sufficient mass of nuclear fuel. Nuclear reactors of the type currently causing the late unpleasantness in Japan are fueled with a metal called Uranium 235, the same one used in Little Boy, usually abbreviated to U235. As it’s dug from the ground, naturally occurring Uranium contains only 0.72%  U 235, with the rest being the more stable U 238. The numbers 235 and 238 are the mass number for an atom of U235 and U238 respectively. The mass number of an atom is the total number of neutrons and protons in its nucleus. We talked about the components of a typical atom in part 2 of this series, so if I’m losing you here, go back and read it again.

For the sake of understanding why Uranium isn’t just Uranium, and why we have more than one flavour of it, think of it this way. There are many varieties of peppers, each with its own flavour and heat – from mildly warming to hellfire and damnation – but they’re still peppers. U 235 is the hot pepper, and the varieties of Uranium are called isotopes.

We mine Uranium, and then put it through a refining process that produces nuclear “fuel”, which is called fissile material. This involves separating the two isotopes in a process made easily possible because U238 with, its larger mass number, is heavier than U235. Unlike its heavier relative, U235 can sustain what is called a nuclear chain reaction. If the reaction is controlled, we can use it to generate heat energy. If it’s uncontrolled, we get a bomb. The U235 used in nuclear reactors does not need to be as pure as that used in so called weapons grade U235 where the size, weight, and efficiency of energy yield are all critical design issues of the weapon.

Nuclear fission.

Nuclear fission occurs when the nucleus of a U235 atom absorbs a free neutron, and splits into fragments. When this occurs, a large amount of energy is released in the form of gamma rays (with an energy level at the top of the electromagnetic spectrum), and also subatomic particles including more neutrons. In the animation at left, we see the blue ball representing a neutron hitting a nucleus which separates into smaller and lighter nuclear masses and additional neutrons. If these neutrons in turn strike other U235 nuclei, the reaction can be repeated. If we engineer some ideal conditions, we can make sure that happens, and the result is called a nuclear chain reaction.

So let’s talk about those ideal conditions. The first and most important thing is the amount of fissile material we bring to the party. That’s important because the more mass we have in one place, the larger the number of U235 nuclei we have to play with, and the more neutrons we have to hit them with. As with most things, there is an ideal amount, and in this case that amount is called critical mass. The critical mass is the amount of fissile material that, when brought together in sufficient proximity, will guarantee a self-sustaining and highly energetic fission reaction.

Obviously we don’t want our nuclear reactor to go boom. We’re going to need to put the brakes on that reaction so we can enjoy the energy released without the unfortunate side effects.

The first thing we’re going to do is use U235 that isn’t quite so highly refined as that used in weapons. That way, if we pull out all the stops, we’ll get a lot of energy but no explosion. The second is to split up our supply of U235 into small, easily handled pieces that we will arrange in the form of rods that are inserted into the reactor so they are in close enough proximity to make a self-sustaining fission reaction possible. The reason for this is so that no individual fuel rod will have sufficient mass to do anything too energetic on its own, and so that all the fuel rods need to be in place for the reactor to operate.

Next we need some means of control. We need to be able to control the energy output – from the maximum the reactor can safely produce without melting itself, to completely shut down.

Interspersed among our fuel rods then, we insert more rods, called control rods, made out of a neutron absorbing material like cadmium or boron. When fully inserted, these absorb so many free neutrons that the fission reaction is brought to a stop. As they are withdrawn, the reaction begins, and when it reaches the point where it is self-sustaining, we say that the reactor has reached criticality. The assembly of fuel rods and control rods make up what is called the reactor core.

When they aren’t being absorbed by the control rods, the neutrons that are so vital to the fission reaction are normally zipping about at levels of energy that make it unlikely they will be absorbed by many of the nuclei they come into contact with. Some designs of nuclear reactor, including the Candu reactors manufactured here in Canada, are able to make use of fuel rods that are less enriched – that is, they have a lower concentration of U235 – because they force free neutrons to hit their target nuclei at lower energy levels that increase the likelihood that they will be absorbed and trigger the fission reaction. They do this by surrounding the fuel rods with a material that slows down those zippy neutrons as they pass through it on the way to collisions in adjacent fuel rods. This material is called a moderator.

In older model reactors, the moderator material was blocks of graphite, what we know as plain old pencil lead. The Candu reactors employ so called “heavy” water, deuterium oxide” as a moderator, which differs chemically from regular old water only  in that instead of hydrogen, the heavy water molecule contains an isotope of hydrogen called deuterium. Never mind why or how, the only thing that’s important about heavy water is that, unlike regular water, it absorbs energy from neutrons passing through it without soaking them up completely which, you can see, would be pretty counterproductive.

Nuclear reactors of the type used at the Japanese power plants that are presently in the news don’t need a moderator. Referred to as fast neutron reactors, or simply fast reactors, they employ more enriched U235 as fuel, and with the heightened concentration of U235 nuclei comes a lessened need to worry about subatomic collisions that don’t end in fission because there are always enough that do. A schematic drawing of a typical fast reactor appears below.

The Japanese reactors employ liquid sodium as a coolant. The radiation emitted by the fission reaction is captured as heat energy and used to generate steam in a closed loop system that is circulated by pumps through to the primary heat exchanger where its heat is transferred to water in the secondary loop. The reason for this is that the water heated directly by the reactor is contaminated with reaction byproducts and is itself radioactive. By having no physical contact with the secondary steam circuit, none of this is transferred to anything that touches the turbines that turn the generator, and the entire system stays cleaner.

When main and backup power systems failed at the Japanese power plants none of the pumps worked. Stories are that the control rods were inserted, but the reactor cores were still at their very high operating temperatures so that, even though no fission was happening, no cooling was going on either so heat was steadily building inside reactor containment vessels with predictable results. When sea water was added in a desperation move to cool them down, chemical reactions ensued that resulted in creation of hydrogen gas which mixed with oxygen and heat with predictably explosive results. If you aren’t familiar with those effects, remember that hydrogen is the gas the Hindenburg was full of.

The jury is still out as to when all this will end, how much of Japan will be cordoned off forever, how many lives have and will be lost, and how much of the world will be irrevocably harmed because man’s reach exceeds his grasp.