Nuclear Energy

Pebble Bed Reactors

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* The pebble bed reactor (PBR) is an advanced type of nuclear reactor
* It was first developed in Germany
* China has the only operating pebble bed reactor
* One is currently being built in South Africa.
* One can produce enough energy to sustain 30,000 average sized homes.
* Ten modules can be controlled by one control center.

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* The fuel for the Pebble Bed Reactor are 60mm, 210 gram (9 grams of uranium fuel) spheres made up of 15,000 particles.
* Each particle contains 4 layers. First a layer of porous carbon, then a layer of pyroltic carbon, next a layer of silicon-carbide and finally another layer of pyrolytic carbon.
* The layers act as insulators to contain radioactive decay particles from the fuel core.
* The 15,000 particles are mixed with graphite powder and shaped into spheres. Then the sphere is covered in a layer of pure carbon, heated and hardened.

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* A Pebble Bed Reactor contains a reactor core made of steel, the walls are lined with graphite bricks, to control heat transfer. The core holds 440,000 pebbles, 330,000 are fuel pebbles the remainder are graphite.
* The graphite pebbles are located in the center of the core and the outside, which limits the maximum temperature the core can reach.
* Helium is pumped in to the top of the core at 500 C. It travels through until it reaches the bottom at which point it has been heated to 900 C.
* The helium then travels through three turbines. Two turbines drive compressors, that compress and cool helium before it is pumped back into the core and the third is used to drive an electrical generator, which produces power .
* After the helium is cooled, compressed and heated, it is pumped back into the core.

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Safety:

* The helium, which is used to cool the reaction, is inert: It cannot combine with other chemicals; it is non-combustible, and non-radioactive which means it cannot carry radiation outside of the core.
* The reactor in a very short time will cool down naturally on its own because the increase in temperature makes the chain reaction less efficient (Doppler Broadening) and it therefore will not generate power.
* The size of the reactor's core has a high surface area to volume ratio. Therefore, the heat it loses through its surface is more than the heat generated in the core. So the reactor can never reach the temperature at which a meltdown would occur.
* If the helium gas duct was some how ruptured, it would take nine hours for natural air to circulate through the core.
* If the natural air did circulate into the core it would lead to less than 10-6 (one millionth) of the radioactivity in the core being released per day.
* A 10,000 times greater amount of radioactivity would have to released a day to require any emergency action taken in the surrounding area.


Efficiency:

* Other reactors need to be shut down for periods of time to refuel( money is lost when a reactor is shut down because no energy is being produced) But the pebble bed reactor continuously cycles the pebbles through the core so refueling time is eliminated.
* Other reactors are about 33% efficient (33% of the energy generated can be used for electrical energy, the rest is lost as heat) but the Pebble Bed Reactors are designed to be 40% efficient, and possibly 50% with estimated improvements to the design.