The true direction of steam-punk

 

In a world where liquid fossil fuels never took off;

A place with such a high baseline radioactivity that no one cares for radiation shielding;

I give you…

Making a nuclear reactor doesn’t need to be difficult. There actually isn’t much to it, the difficulty comes in making it smaller.

Much like how super large rockets can be efficient while being super simple, if they are just big enough, you can get access to the energy locked in uranium ore simply by gathering enough of it and adding a moderator. The moderator is simply a material that will slow the neutrons down sufficiently that they can be captured by the nuclei of other uranium atoms.

This isn’t practical. The size required to get the neutron flux required to produce anything useful is prohibitive. As a result, the ore needs to be concentrated isotopically which really is a challenging process and is the step that stops nations from proliferating.

In the absence of isotropic concentration technology but with the will and a lack of necessity for secrecy, one could build a large enough reactor to cause a high enough electron flux. In fact in “Atomic Awakening: A New Look at the History and Future of Nuclear Power” it’s mentioned that a source of Uranium from a particular mine in Niger (The ex-colony where France gets most of its uranium) has a balance of isotopes of uranium that looked as if it had already been through a nuclear reactor. You can imagine the panic that ensued when this was first discovered. It was later found that the structure of the rock formation including the ore was such that groundwater would seep into veins in the rock in just such a way to act as an effective moderator inducing a nuclear reaction to take place, using up the uranium-235 lowering the nature abundance (which really isn’t abundance in the first place).

The reason this reaction didn’t cause the rock to melt with the heat released was that when the temperature reach well over 100 degrees the water that had seeped into the veins in the rock would be vapourised and therefore would no longer act as an effective moderator. That caused the reaction to slow to a standstill until more water filled the veins and the process could repeat.

A naturally occurring nuclear reactor, one that because it has no need for the heat to be captured in electrical generation could have built-in safety measures from a water moderator alone.

In the most common kind of reactor we use today, pressurised water reactor (PWR), we too use water as the moderator along with graphite control rods that are effective at blocking the neutrons and therefore reducing the neutron flux of the reactor. These control rods act as a throttle for the reaction.

In this design, we are trying to extract energy from the nuclear decay and so we need to capture the flow of energy from the intense heat in the reactor to the not-that-hot outside.

This reaction is more efficient the bigger the gap between the hot bit and the not-so-hot bit.

Such that 100 degrees are far too cold.

So that we can run the reactor at much more than 100 degrees while still being able to use water as the moderator we need to keep it under intense pressure: 500 psi.

This is inherently dangerous because there is enough pressure to be an effective explosion independent of any added explosives.

Moreover, if the reaction is left so that the temperature increases, the pressure will only increase. This is in part what happened in both the Fukushima and Chernobyl disasters. In Fukushima, the pump that pumps the water between the reactor core and the heat exchange got broken by the tsunami and the build-up of pressure in the water caused an explosion.

The explosion then caused radioactive material to be released into the atmosphere.

The explosion at Chernobyl was also caused by the water getting too hot but this was due to the on-site engineers thinking the control rods had been lowered when they had in fact got stuck. Even though the pump wasn’t initially compromised the heat ran away and the pressure got too high.

Molten salt reactors try to fix this by using low-pressure molten sodium to transfer the heat to a heat exchanger. A separate moderator is then introduced in the reaction chamber, perhaps a graphite-based one. The sodium-based system can have a similar safety system to the aforementioned natural reactor.

The sodium isn’t under much pressure and so a so-called “Freeze plug” can be introduced to the system that creates a seal in one section of the pipe containing the liquid sodium.

If the sodium gets too hot, the seal is designed to melt, this causes the sodium to exit through this plug.

The plug is designed to be at the bottom of the piping system so that the liquid sodium is drawn down to a separate safety chamber by gravity. No tsunami can turn that off.

There are some other advantages with a liquid metal design that has more to do with including the fuel in this initial coolant loop but first, there are a number of reasons we haven’t used this mechanism yet.

Sodium is highly corrosive, it reacts with water in an explosive way. It will also corrode the pipes and so it’s hard to build the piping not to mention the pumps.

It’s also not liquid at room temperature, which is fine in the reactor coolant loop which is operating at 100s of degrees but it does mean that if the heat stops for any period of time then it will freeze in the pipes. Water will do this too but it’s much easier to keep the pipes above 0 degrees when things go wrong than above 400.

Some of the other benefits of the liquid system, if you have the fuel dissolved in the liquid, is that gasses produced in the fission can be extracted easily as they will naturally bubble out.

Gasses are produced as some of the products of the fission cycle. Fuel rods have to be changed and the reactor shut down while they are because otherwise the walls will become brittle and the build-up of air pockets in the fuel can reduce the neutron flux density.

The fuel can also be cycled without having to shut the reactor down, the non-gaseous products can be chemically separated as part of the cycle and more dissolved fuel added in.

This doesn’t, however, mean the reactor can keep going indefinitely. One issue with radioactive containment is that even if you don’t care about the radiation yourself (Which outside of our steampunk fantasy you very much should be) the walls and piping of the reactor are going to be affected by the neutron flux.

Sometimes this will just make them low level radioactive but often it will cause similar issues to the solid fuel rods in that gasses will build up inside them or they will turn into materials that just have different properties to the alloy you were using.

This chemical change of the structure of the reactor is as of yet an unsolved problem. Current solutions include using materials that can be modelled to be tolerant of neuron bombardment, such as being able to accept more neutrons without inducing radioactive decay; particularly beta emission which is an issue for even light elements which aren’t set to fall apart in the way the unstable rare earths can, converting the original element into an entirely new one to the right on the periodic table.

When decommissioning a nuclear reactor it is this low-level waste, the structure itself, bombarded with neutrons over the 20-40 year life of the reactor, which makes up over 90% of the nuclear waste. 

So yay nuclear power for which the energy cost based only on the fuel costs are 2 orders of magnitude less than current energy prices; just £0.002kWh vs today’s £0.20/kWh

If only we could make backyard reactors we’d figure out how to get the price down.

No! That is a terrible idea, a very very terrible idea.