Energy In The 21st Century

Cheap Energy

Cheap energy is what makes modern industrial society (“western civilization”) go. Modern society started in the Industrial Revolution, and that could not have happened without coal — cheap energy that you just dig out of the ground. We found better forms of fossil fuel: liquid petroleum was much easier to handle (liquids are much easier than solids) and gave us not just cheap energy, but also an amazing source of organic building blocks for “better living through chemistry”.

Without cheap energy we have no technology. We can only invent new technologies with cheap energy.

Cheap energy from fossil fuels is limited, and we will see it end in our lifetimes. Not “run out” as in no more fossil fuel — but it will become more and more expensive as it gets more and more expensive to extract.

Fossil fuels have another huge well-known drawback: they dump carbon dioxide into the environment. This is bad on two fronts: we all know about global warming and climate change; but there is another problem: the CO2 dissolves in the ocean and makes it more acidic, which is no good for most sea life. The oceans are a big part of making a stable ecosphere that humans can prosper in.

We need nothing less than a complete phase-out of all fossil-fuel power plants, especially coal-burning plants. We need to make sure we can develop a source of clean energy while we still have cheap energy.

Alternate/Renewable Energy

Here I'm lumping in solar and wind energy, energy from tides, and hydroelectric power. All these forms have significant and similar problems: they are not enough to power our society, they can only be placed in a few locations, and they all have non-trivial environmental problems. Other than hydroelectric power, they are all intermittent.

Intermittent generation is a problem: if a long-distance transmission line can only carry energy a third of the time, it becomes a significant cost and adds to the cost of the energy it can deliver.

Wind turbines can only be placed in a few areas. Each 500 MW wind turbine requires 10 acres of land, and can be a considerable nuisance to people living within a mile or so — the noise and the strobe effect of the moving shadows of the turbine blades is significant. (See the 2010 documentary “Windfall”, about a New York village's experiences.)

Solar power requires even more land. “No problem,” people say, “we'll put them in deserts.” I find this an offensive attitude. Deserts are not wastelands, they are living ecosystems that are just as valuable and precious as forests are, and they're much more fragile. Furthermore most people don't live in deserts, and the electricity must be moved large distances.

Hydroelectric power destroys the ecosystem they're built on — in the US West we've lost many amazing rivers, canyons and valleys to the their reservoirs. Salmon in the Pacific Northwest are now endangered becuse of all the dams on all the rivers.


Nuclear energy is much-maligned and is a poster-child for environmental problems — and rightly so. We've all seen the coverage of Three Mile Island, Chernobyl, and Fukushima. But nuclear energy is not just one thing: there are many forms of it.

What we think of as “nuclear energy” is in fact just one thing: it's called the Light-Water Reactor (LWR) — it uses a solid fuel and is water-cooled and water-moderated. Why do we have just one way to do it? Well, developing new technology is expensive, and just after WW2 the US Navy — actually Adm Rickover — decided they needed nuclear powered submarines and ships. They had the money to develop the technology, and the LWR is what we came up with. (The plan was to use "fast breeder reactors" to convert ordinary uranium (U-238) into plutonium, which can be used for power and weapons. The breeder program turned out to have major problems and was cancelled.)

There is a slightly different reactor called CANDU from Canada that uses Heavy Water, but it has the same drawbacks and concerns.


The only controlled nuclear reaction we have today is fission, where a large heavy nucleus of an element like uranium (U) or plutonium (Pu) is split into two smaller nuclei when struck by a neutron. This split also gives you two or three more neutrons, which can be used to split other atoms — hence chain reaction. And of course the process releases a lot of energy, in the form of heat. We use the heat to turn water into steam and drive turbines with it, which make electric power.

What is a “moderator”?

The neutrons that come out of a fissioning atom are at high speed (they have a lot of energy). For technical reasons I won't go into, atoms are best split with slow neutrons. This means we need a way to slow down the neutrons that are created in the reactor. This is done by a material called a moderator.

The first reactors (Manhattan Project etc.) used graphite as a moderator. The LWR uses ordinary water as the moderator, and CANDU uses heavy water (water that uses a heavier isotope of hydrogen, called deuterium). Lots of materials are suitable for moderators.

The Pressurised Water Reactor (PWR)

Both the Light-Water Reactor and the Heavy-Water Reactors (like CANDU) use water as a coolant. At ordinary pressures water boils at an inconveniently low temperature, but as the pressure increases, the boiling point rises. These water-cooled reactors work at high pressure to keep the water from boiling. I'm going to lump these designs into one category, the Pressurized Water Reactor (PWR).

Uranium as found on earth has two varieties: about 99.3% is called U-238, and 0.7% is called U-235. Only U-235 can be fissioned in a nuclear reactor, so natural uranium is "enriched" — the proportion of U-235 is increased — to make fuel and bombs.

For the PWR's fuel, uranium oxide pellets are coated in metal and loaded into long sealed cylindrical metal tubes. (Some designs form the solid fuel into multi-coated pellets.) Fuel-rod construction is tricky and expensive. Bundles of these tubes go into the reactor vessel, which is filled with water. The middle of the reactor with the bundle of fuel tubes or heap of fuel pellets is called the core.

The fuel fissions and heats up; the heat is transferred to the water. The neutrons come whizzing out of the fuel, hit the water and slow down, and then go back into the fuel to continue the reaction. The water is pumped out into a heat exchanger, cools down, and is returned to the reactor.

The heat exchanger turns water into steam, which goes to the turbine and generates electricity. It is important to realize that there are two independent cycles (“loops”) of water — the water that goes into the reactor never leaves the reactor/heat-exchanger loop.

Drawbacks of PWRs

The main problems of PWRs result from

Water cooling

Water has lots of nice properties; its biggest drawback is that it boils at the rather low temperature (at atmospheric pressure) of 373 K (212 F). This is far too low; but if we increase the pressure, the boiling point can be high enough to be usable. Therefore the primary loop is kept at a rather high pressure of about a hundred atmospheres which allows the water to stay liquid. The water comes out of the reactor at about 600 K which is sufficient (but not great) for power generation.

What would happen if the coolant pumps failed? The water inside the reactor would keep getting hotter, the pressure would build, and boom! Reactors therefore have multiple levels of redundancy in the cooling system. But if the power fails, ultimately the pumps will stop. If the system is adequately designed there is enough battery power or generator capacity so that when the pumps finally stop, the core is cool enough that it won't meltdown or be damaged.

What would happen if one of the pipes broke? The pressure would drop immediately and all the water to turn almost instantly to steam, a rather poor conductor of heat. The core would probably melt pretty quickly. This very large volume of radioactive steam must not be allowed to escape into the atmosphere, which is why reactor containment buildings are so large — they must trap a huge volume of steam.

Another problem is that the radioactivity splits some of the water into hydrogen and oxygen. (This is called dissociation.) While the reactor is running there are systems to recombine the hydrogen and oxygen into water.

Water at high temperature reacts with the metal of the fuel rods and weakens them. This also generates hydrogen.

At Fukushima the reactors shut down immediately after the earthquake, and the cooling systems kicked in. After a few hours the tsunami then knocked out the diesel generators so the plant lost power. Luckily the interval between the earthquake and the tsunami allowed the core to cool down enough that there was no meltdown. However the core was still hot (both thermally and radioactively), and dissociation was still a problem, so hydrogen built up in the reactor building. Ultimately there were hydrogen explosions in all three buildings; we saw that on TV.

Problems of Solid Fuel

The big problem with solid fuel is that as the reaction proceeds, the byproducts have nowhere to go. Some of these are gases like xenon (Xe) and krypton (Kr), and they build up inside the sealed fuel rods. Furthermore, Xe is a great neutron absorber and inhibits the reaction. After four or five years the fuel rods must be removed from the reactor because they are too physically damaged and Xe-infested, even though only about 4% of the fuel has been used. A PWR is shut down every 18 months or so, and a third of the fuel rods are replaced.

What do we do with “spent fuel”, the fuel rods removed from the reactor? Everyone knows spent fuel is deadly radioactive and must be buried for millennia. Why is it so dangerous? The problem is not usually with the fission products — they are highly radioactive, but that also means their radioactivity is spent after a couple of decades. No, the problem is that sometimes the uranium nucleus doesn't split when it absorbs a neutron, but instead becomes a heavier element like neptunium, plutonium, and americium (these are called transuranics). These elements are moderately radioactive, but also have half-lives of a few thousand years. They are also good for making nuclear weapons... so they need to be saved safely and securely for many millennia.

The other big problem is that solid fuel is a terrible conductor of heat. In the middle of the fuel rod where most of the reactions are occurring, the temperature can be 1500 K, but at the edge where it meets the water it's only 600 K. This is why it's essential that after a reactor is shut down, the coolant must continue to flow, or the fuel rods will all melt.


An engineering issue is that the reactor vessel is a chamber about 20 m (65') high by 10 m (35') wide, made of high-quality steel about 25 cm (9”) thick. There is no way to weld steel that thick; the whole vessel is manufactured in one piece as a forging, and there is exactly one factory in the whole world (in Japan) that can do this.

The Molten Salt Reactor (MSR)

Molten Salt Reactors (MSRs) are a completely different design from the PWRs. They take care of just about all the drawbacks of PWRs. MSRs have been built in labs around the world, but we need a significant investment (about a billion dollars) to make them a practical reality.

There are a couple of different designs of MSRs, but they have a few common features that make them much safer than PWRs.

Molten Salt Coolant

Instead of water, these reactors are cooled with a molten fluoride salt like lithium fluoride. These salts have melting points of about 700 K, but don't boil until 1700 K or so. The nuclear fuel is dissolved in the salt; the “core” is where the moderator is.

Molten salt reactors don't need to be pressurised, since the boiling point of the coolant is well above the highest possible temperature that will be reached even if there are problems. At the same time, the temperature at which the coolant exits the reactor is much higher than for a PWR -- about 1000 K, which allows for much more efficient turbines than are typically used in PWRs and coal-burning plants. This high-temperature low-pressure output is the best thing. (And no need for a special pressure vessel for the reactor core.) The high temperature output can also be used directly by chemical plants, which today burn fossil fuels to reach those temperatures.

The molten salt goes to a heat exchanger where the heat is transferred to the another loop, just as in a PWR.

Fluoride salts are ionic compounds which don't dissociate with radiation like water does.

Liquid Fuel

The liquid form of the fuel has many advantages: for one, it is inherently stable. If the temperature rises because the reaction rate increased, the liquid expands, the fuel atoms get a little further from each other, and the reaction slows down. If the reaction rate drops below the equilibrium point, the fluid contracts, the atoms get closer together, and the reaction rate goes back up.

It is also much easier to cool a hot liquid than a hot solid: you simply move the liquid to a radiator (the heat exchanger), whereas to cool a solid you have to transfer the heat to a coolant first, and then move that to the radiator.

It is also easier to process (remove the fission byproducts from) a liquid than a solid — that's why most of our chemical processes work on liquids. (When we need to process a solid we usually crush it into a powder and disperse it in a liquid.) Gases like krypton and xenon are trivially easy to remove from a liquid.

Passive Safety

The reactor is passively safe — no power or manual action is required to shut things down. At the low point of the molten salt loop, there is a drain that is kept frozen by electric cooling — the salt stays solid in the drain and plugs the drain while the reactor is producing power. If things fail and power is lost, the salt plug melts because it is no longer being cooled. All the molten salt drains into a holding tank, and the reaction stops because the fuel is not next to the moderator. Pretty soon the salt freezes in the tank and safely captures all the radioactive materials. When it's time to restart the reactor, the tank is heated and the molten salt is pumped back into the reactor.

Spent Fuel

There are no fuel rods that deteriorate and must be replaced before the fuel is used up. The fuel stays in the reactor until it is used up; any transuranics stay inside the reactor and are burned. A processing plant inside the reactor continuously removes fission products (including the dreaded xenon!) and adds fuel. The power plant does not have to be shut down to refuel the reactor.

The fission by-products are usually highly radioactive and are stored on-site for a couple of decades; after that they're no longer radioactive and can be disposed of in quite ordinary ways.

There is a much smaller nuclear proliferation threat: any transuranics that might be good for making weapons stay in the reactor and are consumed.

Problems with MSRs

There are still some unresolved problems that must be solved before MSRs can be a mainstream power source. We need a billion or so dollars to figure it all out.

The Chemical Plant

Inside the reactor there is a chemical plant that removes fission products from the molten salt. This is a hard problem: some of the reaction products are noble gases like xenon and krypton which can be removed without much fuss, but the others are more tricky. Pretty much the whole upper half of the periodic table can be expected to be in the mix, most of it highly radioactive. We don't have a lot of experience with this kind of chemical processing which must be done completely automatically.


As mentioned before, the reaction products are pretty much all the heavier elements ("half the periodic table), some of which are corrosive. Additionally, the salt itself is corrosive at the high temperatures involved. The experiments in the 50s at the Oak Ridge labs developed some alloys that are quite promising, but they will only last in the reactor for about 20 years; we need something that will be good for 60-70 years.

Some of the elements created by fission are the "noble" metals like platinum and palladium. These have the inconvenient propery of not staying in solution – instead the get plated onto the cooler parts of the system, the heat exchangers. This can clog the exchangers, making them less effective, and ultimately blocking them completely.


Ordinary lithium is a mixture of lithium-6 and lithium-7. Li-6 absorbs neutrons readily and produces tritium, a radioactive isotope of hydrogen. Hydrogen is a tricky element, and manages to leak through most metals. A tritium leak would be very bad news. We can avoid this by using pure lithium-7 instead, which adds to the cost. But at least it's a one-time cost.

MSR Designs: Uranium vs. Thorium

There are two main companies working on MSRs:

There are still significant engineering issues that need to be solved before either of these designs is viable as a commercial reactor.


Flibe uses thorium, an element that's more common in the earth's crust than many “ordinary” metals like tin. Lemhi Pass in Idaho contains enough Thorium to supply US energy needs for a few thousand years. Many other countries also have vast reserves of thorium. (Byproducts of thorium mining are “rare-earth” metals like neodymium, which are used in super magnets, hybrid cars, etc. In fact thorium is usually considered a nuisance contaminant in rare-earth mining and there are huge quantities in rare-earth mine tailings.)

Thorium itself is not a nuclear fuel (fissile), but when it is irradiated in a reactor it turns into fissile uranium-233. A thorium-fueled reactor needs a small amount of fissile material to get going, but after that it's self-sufficient. Small amounts of thorium are added as it is consumed.

Thorium also has another advantage: it creates about 1% of the transuranics that a uranium PWR does, which is good from a waste standpoint. Also, uranium-233 is suitable for bomb-making; however extracting U-233 from the reactor is tricky and dangerous — thorium becomes U-233 by first becoming protactinium-233, which is a hard-gamma emitter, not just deadly but also easy to detect.

However Flibe does use beryllium, which is very toxic and a controlled substance.

Company website:

Transatomic Power

The TAP design uses uranium like the PWRs, but it can also use spent fuel. We've accumulated huge quantities of spent nuclear fuel in the past few decades: TAP can turn this liability into an asset. The US has enough material lying around to supply the current demand for a couple of hundred years. Let's clean up the radioactive mess the PWRs have made over the last 70 years!

The TAP design can also be modified to run on thorium.

Company website:

New Yorker writeup:









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Last modified: Thu Dec 31 22:40:57 PST 2015