Whenever you talk about Nuclear Power, many people think of Nuclear bombs and the like. Alas, the mere word nuclear is enough to send some people into anxiety. I love it in the movies when someone is beside a nuclear reactor and screams that the reactor is going critical and everyone runs around screaming in panic.
The other thing I hear a lot is how clean electricity is when it is produced by Nuclear Power stations. Like most regular power stations, there is a boiler to produce steam, and in a nuclear power station the decay of radioactive atoms in fuel rods inside a closed system stream of circulating water heats a boiler of water that produces the steam to run a turbine to produce electricity (just like any FF source). Is the electricity clean? Only if you narrow the focus down to the single fact that there are no polluting emissions when you heat the boiler. The problem, however, arises from only using that one fact, and not seeing the whole system of generating the electricity – this is also true for renewables, but that is for another post. For most regular power stations using a steam turbine, the boiler needs to be heated by something. In nuclear it is water molecules being hit by radioactive decay particles from the fuel rods that excite the water molecules making them hotter. And curiously, the process for this heating is only efficient when the fuel rod is in a critical state releasing enough particles to do the job. Otherwise it would just be a toxic fuel rod. This criticality is what the power controllers look at consistently by damping or increasing the heating through the lowering or raising of carbon control rods that surround the fuel rods. The before and after process is the story most people ignore.
The fuel rods contain Uranium Oxide as the fuel where the natural decay of uranium by radioactive fission heats water. The rods ability to heat water naturally lessens over time – usually about a 6 year lifespan per fuel rod. Now that Uranium has to be mined from the ground. As I have said before, that is an energetic process that generates immense amounts of waste and pollution. The usual methods of mining can be used (e.g. open pit, underground shaft), but also leach mining where acids are injected into piles of ore on the surface (Heap leaching), or even deep in-situ into specific rock strata (yellow cake). Once the uranium has been extracted form the ground it must then be processed to enrich it. Good Ore level uranium is about 0.7%, but must be enriched to about 5% to use in a fuel rod. This is done through multi-level energetic centrifugation process before the more concentrated ore is further treated to uranium oxide ready for a fuel rod. As a comparison, however, a 1000 MW coal fired power plant requires about a half million tons of coal a year as opposed to about 30 tons of uranium oxide per year. We do have the technology to recycle and reuse Nuclear fuel, but it is about as expensive and polluting to do this as dig new ore, but at least this does cater to the one major problem of nuclear fuel – disposal and storage.
The spent fuel rods are extremely toxic and highly radioactive. This means that they cannot be simply discarded but must be carefully stored on-site at the power plant in large cooling tanks (like large swimming pools). Since there is no place to permanently dispose of these spent fuel rods – there has been a prepared spot in Neveda (Yucca Mountain) but transporting the fuel rods there and maintaining security are on-going problems – you don’t want the rods getting into the wrong hands. After all it is nuclear fuel that can be enriched further for military purposes.
Nuclear Power Problems
The fuel rods have to be kept at a critical state, but what happens if the rods get to a supercritical state – that’s when you don’t want to be anywhere around. Super criticality is what is needed to allow a nuclear fuel source to attain a chain reaction if triggered by a specific detonation – booom!! Fortunately, nuclear power fuel rods don’t become nuclear bombs since there no detonation devices associated with them. What they can do is become quite problematic when power plants don’t go as expected. A popular term in the 1980s was ‘The China Syndrome’ meant to describe how a nuclear reactor behaves when it gets super critical and starts to melt. People perceived that the extreme heat of the melted core would allow it to simply melt its way in the ground and hence keep melting down through planet all the way to the other side. Of course, that can’t happen because the melted core would eventually reach the inner magma layers and simply blend into the magma. The reality however, is that the melted core would melt down until it reached ground water layers in the crust. Then a catastrophic steam explosion would occur and the steam blasting out of the ground would be highly radioactive and move with winds around the planet until it finally settled out or got diluted enough to no longer be a threat. Notably, nuclear fuel has long half-life’s (the time required for a quantity to reduce to half its initial value) of about 30 years. Some estimates are that the fuel rods can become non-toxic after 159,000 years. (As a comparison, plutonium has a half-life of 24,000 years).
Of course, in a nuclear power plant there are many safeguards, but the Homer Simpsons apparently do exist. Three-mile island was a case of shutting of the water surrounding the core. Once that happened the core started heating up. Fortunately, it was caught in time and the water turned back on, but not before the top of the core had already melted requiring the permanent shut down of that core. Rods are designed to be pulled out of place by robotic arms. The melted mess presented a massive logistical problem. The Fermi 1 reactor near Detroit suffered a similar situation (that one never really made the news for some reason, just as many other safety ‘near misses’ were never really reported). Now Chernobyl was more spectacular and widespread. An unplanned emergency coolant shut off was done, but it coincided with a stream turbine failure resulting in a large fire. The cores were heating and no one could get the coolant back quick enough. In an emergency the reactor can be shut down using graphite balls, and as a final desperate measure to completely shut the reactor down, graphite dust. In most reactors, the core is chambered in a closed housing, but at Chernobyl the containment was apparently open to the sky. Once the graphite dust had been dumped into the core, the dust was ignited by the nearby burning turbine, and as you might imagine, the reactor core was not only melting but the fuel rods were also burning. The resulting smoke with nuclear fuel via winds patterns made its way around north-western Europe until heroic efforts by Ukrainian firefighters managed to eventually put out the fire and the cores could be flooded with more graphite dust. Then when Mother Earth does unexpected attacks like the Tsunami at Fukushima plant in Japan, nuclear core problems become everyone’s problem and for a long time to come.
There are plans for new types of nuclear reactors that are potentially walk-away safe, but the costs involved in planning, building, and maintaining nuclear power stations are extremely prohibitive, especially when compared to renewable options that can provide the same electric power, safely and for a fraction of the investment cost with minimal problems.