Nuclear Energy Fun!

Okay so probably the #1 topic I've followed closely in terms of the recent and terrible Japanese disaster is the issues they are having with their nuclear energy plants. This is a very worrying concern, and I've taken it upon myself to become one of the most knowledgeable around me and my friends so I can put it into perspective just how much this could affect us. And a great learning opportunity too!

Alright so hopefully by now we all know of the 2011 Sendai earthquake/tsunami that has devastated the Japanese country. It's sent them spiraling downwards, and they aren't being given a break. From one of the strongest earthquakes to grace our Earth's crust, to a tsunami that wiped out entire cities and the NE coast of Japan, including aftershocks of magnitude 6.0 and above... they have plenty on their hands to handle.

The explosion seen at Fukushima #1, following major earthquake.

One of the greatest aspects to be known originally was the initial damage caused to the Fukushima #1 plant of Japan. Fukushima is a city that contains 2 nuclear power plants. The first, designated Fukushima #1  (Fukushima Dai-ichi) currently contains 6 nuclear reactors each providing energy, and massive amounts of it. They are each run as a separate entity, and are supplied by different companies. The plant itself is owned and operated by TEPCO (Tokyo Electric Power Company), with GE (General Electronics), Hitachi, and Toshiba being the 3 separate companies supplying all 6 reactors. GE supplies 1, 2, and 6. Toshiba 3, and 5. And Hitachi runs reactor 4. They are all light-water (meaning normal, everyday water) reactors, and more specifically about the function, are boiling water reactors (boiling water being the result of the nuclear fission).

Each of these reactors, in construction, is the same. They are light-water reactors that provide energy through creating steam that powers a turbine. This is the manner many of the world's nuclear plants operate in. The fuel used to power the steam-making is the most complex part of the operation. Essentially, you take uranium pellets that are enriched to have concentrated amounts of uranium. The enriching process of uranium is basically taking mined uranium (natural in the earth), mixing it with other components and getting a specific isotope of uranium to put into reactors. It's a complex process, but not terribly important to the way the plant operates.

Nuclear Plant Fuel - How it's processed - learn more about how uranium pellets are processed/used.

A simple explanation: These reactors will take the uranium, and using it, power the process of nuclear fission. Fission is the breakdown of atoms provided from the uranium and breaking them down into smaller nuclei. This all starts at the reactor core, where the 'fissioning' happens. The core basically just creates such intense amounts of heat, it boils water circulating through the reactor to create steam. This steam is then harbored, and used to turn a turbine with a generator which produces electricity.

I go into more detail below.

1) Reactor Pressure Vessel; 2) Fuel Rods; 3) Control Rods; 6) Steam; 8+9) Turbine; 10) Generator

The reactor core consists mainly of 2 parts. And I'm sure you've heard the terms all along, just not knowing exactly what they are. Well here's a rundown!

So you have your uranium fuel pellets, which are loaded in 'fuel rods' - pellets are inserted into one end of a tube with the other side sealed off. These fuel rods are bundled by the hundreds, and placed in the bottom of the reactor core. At the top of the core, there are 'control rods' - another term I'm sure you're familiar with hearing about now. Basically these control rods are similar to the fuel rods, only instead of uranium they are filled with other substances that capture the neutrons that are created in part of this process, more able to control the chain reaction that happens - but also capturing the radiation that comes from the uranium. These must also be properly stored and controlled as they have radioactive properties.

Control Rod driving mechanism, these will be pushed
up to cover fuel rods

Alright so we've got a basis for the construction, and a general understanding of what all is going on. So we have our reactor core, and it is going to start the process of nuclear fission. If you were just to leave the fuel rods in a nuclear reactor alone, and start the fission process, everything would go hay-wire with far too many fission reactions going on instantaneously. You NEED to be able to control how much fission is happening at any given moment.

That's where the control rods come into play. The fission creates separate neutrons from atoms, creating heat. This heat becomes so intense, it heats up everything around it. The fuel rods, control rods, reactor, and containing facility. If control rods are completely lowered, the materials contained within them will completely halt the fission process. As they are raised, increasing amounts of chain reactions can occur. Thus, controlling how much heat (energy) is created.

To further control the flow of neutrons, and to make sure nothing gets so hot it's unmanageable, there is a coolant used. Luckily for us, ordinary water found from natural sources are an ideal product to use for this. So you can use ordinary water from rivers, lakes, and oceans. And that is why nuclear plants are situated along major water ways. And why Fukushima #1 is on the NE coast of Japan, hardest hit by the earthquake/tsunami.

Water is pumped through the reactor core to carry the heat created from fission away from the control rods, so they don't overheat. The heated water is taken to a turbine, where the actual energy will be created.

Okay so now a quick break to explain the actual plant itself. Generally, a plant will be designed to have the reactor core situated by itself with all required components, and they will pipe away the created steam to a separate room. You will have all the fission/cooling equipment in one area, and the electricity production equipment in a separate area as an added level of security/containment.

Pretty much how Fukushima operates

So you have the reactor room, where a pipe is taking the hot water/steam created from the reactor core to a turbine which will use the steam to power the turbine, sending energy to a generator as is the basis of many power production plants. After the steam passed through a turbine, it is condensed back to regular flowing water, cooled, and sent back through the same process. Depending on the facility, they may add an additional coolant to the water being circulated from a fresh water source to assist in the cooling process.

Now, take everything we've gone over and assemble it to be placed placed within a facility. You've got everything ready to go - fuel is loaded in fuel rods, and control rods are placed over the fuel to prevent any nuclear fission from happening. When everything is given the go-ahead, a reactor that has been down for maintenance comes alive and starts breaking down atoms. It has many systems in place to monitor and control the functions within the reactor. One function is the raising/lowering of the control rods. When you're ready to begin producing energy, a technician will gradually raise the control rods until a self-sustaining chain reaction of nuclear fission takes place. In other words - once enough atoms are being split to cause the steady explosion of surrounding nucleus, it has reached the 'criticality', or operating stages. Then you can start harvesting the steam starting the processes explained above.

A typical nuclear power plant

One last thing to add. Whenever a nuclear reactor is shut down to reload fuel or for maintenance or even in the case of an emergency shutdown that happens automatically, there is still heat contained within the reactor that needs to be dealt with. From what I have read, the Fukushima reactors after shutdown require a cooling process to be handled for another 24 to 48 hours as heat remains (Decay Heat). Decay heat is the energy created from the radioactive elements within the reactor, who in a half life, still create energy. It goes through a steady decrease in amounts of energy created. This is important because if the fuel / control rods are not cooled down properly, they can begin to melt from the intense heat which make them extremely difficult to manage.

This is what a nuclear 'meltdown' is - the literal meltdown of the fuel required for fission inside of a nuclear reactor. If all the uranium puddles together, it creates a near uncontrollable fission process creating massive amounts of uncontained radiation. This is why nuclear situations can have such high risk, and the fuel in and around Fukushima must be closely watched and controlled. Unfortunately, there have been significant problems doing this so far.


Quoting a bit from Wikipedia about decay heat, as it is a very critical measure to take into effect with nuclear reactors.

"An additional 23 MeV of energy are released at some time after fission from the beta decay of fission products. About 10 MeV of the energy released from the beta decay of fission products is in the form of neutrinos, and since neutrinos are very weakly interacting, this 10 MeV of energy will not be deposited in the reactor core. This results in 13 MeV (6.5% of the total fission energy) being deposited in the reactor core after any given fission reaction has occurred.

When a nuclear reactor has been shut down, and nuclear fission is not occurring at a large scale, the major source of heat production will be due to the beta decay of these fission fragments. For this reason, at the moment of reactor shutdown, decay heat will be about 7% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week it will be only 0.2%. The decay heat production rate will continue to slowly decrease over time; the decay curve depends upon the proportions of the various fission products in the core and upon their respective half-lives^[2].  An approximation for the decay heat curve valid from 10 seconds to 100 days after shutdown is"

Essentially, after the initial nuclear fission reaction, a specific amount of energy is created EVERY separation that takes place. And, after the initial fission there is a follow up burst of energy as the radioactive elements reach their half-life. Only a percentage of these (about 1/3) are not 'big' enough to react very strongly. So about 2/3 of the energy created from the radioactive elements reaching half-life is deposited into the reactor core, and needs to be dealt with in the form of water cooling it. Significantly less energy (heat), so fairly easy to deal with and isn't that big of a deal, just extra time to make sure it's properly handled.

Generally, the Fukushima plant needs about 24 to 48 hours to be cooled after shutting down to deal with these issues.

Then, they re-load the fuel rods and do it all over again! It's an amazing process, highly complex (yet simple at the same time!), with a great return but leaves us in an overall disadvantage. Nuclear power is not forever, provides many risks, but for the moment great reward. It will be used until something newer and better is found. It has a much better return compared to other fossil fuels, not ideal, but the risk is worth it to create a functioning and large civilization as others are so innefective. Truly a technological advancement, and there are many different types of reactors. This just explains the ones cooled by water, produce steam, etc. There are nuclear reactors built for the U.S. naval carriers and other large ships, and used in many other applications including similar process for creating nuclear weapons.

The latest nuclear powered supercarrier of the US, CVN-77 named USS George H.W. Bush

Thanks for reading! Any discrepancies in what I've presented and what is the truth would be greatly appreciated if they were pointed out. I wanted to get a rundown of just how nuclear power plants work, giving real life examples and relations to make it a bit easier to comprehend. I couldn't find an explanation enough online that was relative, so I decided to dive in and take it upon myself to dig through all the technical stuff and write out a lengthier, broken down explanation. Hope you learned something! I know I did!


Boiling Water Reactor overview - Wikipedia

Fukushima 1 Nuclear Power Plant overview - Wikipedia

Light Water Reactor overview - Wikiepdia

Uranium Enrichment Process and Products - Argonne National Laboratory

How Stuff Works - Detailed Nuclear Plant workings