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Posted by on Mar 26, 2011 in Physical Chemistry | 0 comments

Nuclear Radiation (2): How a Nuclear Reactor Works

In part 1 of this series I described a bit about radioactive processes. In this part 2 I will discuss the how a reactor works.

The fission reactor uses a controlled nuclear fission process to produce thermal energy, which is then converted into electricity by boiling water. With the exception of the reactor core, a nuclear power plant is similar to any other power plant that relies on thermal energy.  The difference is the means by which the thermal energy is produced.

Nuclear fuel is most commonly composed of compounds containing Uranium 235 which makes up less than 1% or uranium ore in natural abundance. Most Uranium is U -238 and we have seen in part 1 that this is radioactive and decays primarily by alpha emission.

Neutron collision produces two fragments and two neutrons

Uranium 235 is fissionable and breaks up when colliding with high energy neutrons.  Each time uranium 235 is split, it produces another neutron.  These go on to produce 4, then 8, then 16 and a chain reaction results.

If the chain reaction gets out of control, then a nuclear meltdown occurs.  This can cause an ecological disaster. In 1979 there was a partial core melt down at Three Mile Island in the USA.   About 140,000 people had to be evacuated to a 7 kilometer (5 mile) radius.  However studies have shown that this accident did not cause any significant increase in radiation or cancer rates in the surroundings.

The Chernobyl disaster in Russia in 1986 was a disastrous melt down.  Although only 57 people directly died, the real figures that include the long term effects or radiation causing cancer may reach up to 400,000 according to Greenpeace. 336,000 people had to be resettled because the surrounding land became uninhabitable. Rivers, lakes and land were contaminated and may remain uninhabitable for several hundred years.

Chernobyl reactor after meltdown

To understand nuclear processes, we have to know how reactors work, and how the nuclear fuel produces heat.

Uranium ore is radioactive.  In most reactors the 1% abundance of the U235 isotope is the material most commonly used.  Since the concentration is not high in ore, the number of Uranium atoms that decay is not great.  This is called sub critical mass.

If the concentration is increased, like in nuclear reactors, then a greater flux of radiation is produced and its energy is used to boil water. Reactors must be maintain the mass close to the critical mass to produce this heat.

The Basic parts of a nuclear reactor

But if the concentration gets too great, then an uncontrollable flux or radiation produces so much heat that a meltdown may occur.  The temperature climbs to over 2000 C which is twice the melting point of steel, and the radiation produced increases exponentially.  Let us look at these masses in more detail and also look at the parts of a nuclear reactor.

The control rods regulate the rate of the nuclear    reaction. This effectively changes the mass of the nuclear material by moving them in and out of the core.

Control rods are made of a material that absorbs neutrons, thereby reducing the number of neutrons available to sustain the reaction at any given rate.

The reactor core with rods and moderators

The reactor core is surrounded by a thick concrete shell that prevents radioactive particles from escaping the core, being instead absorbed by the dense concrete.

The moderator is a substance that fills the reactor core and surrounds the fuel and control rods.  Ideally, it is a substance that will not absorb the neutrons, but instead slow them down.

Common moderators include graphite and heavy water. Often, if heavy water is used as a moderator, it also serves as the reactor’s primary coolant.

The fuel rods are the main components of the reactor.  Each fuel rod is a zirconium or stainless steel tube containing one or more pellets of radioactive fuel (typically enriched uranium). The reactor’s operation is initiated by exposing the fuel rods to a neutron-emitting source.

The reactor’s primary coolant is the fluid that passes through the reactor core.  The primary coolant is heated by the nuclear reaction in the core and transfers the thermal energy from the core to the rest of the reactor.  The primary coolant is always in a closed system to prevent leakage of radioactive materials out of the reactor core.

Reactor's heat exchange: isolated and closed system

The coolant is the part of the reactor that performs the actual work that is converted into electrical energy.  Coolant water passes through a chamber where it is in thermal contact with the heated primary coolant from the reactor core.  The heat from the primary coolant causes the water to turn into steam, which passes through a turbine that is used to produce electrical energy.  It is then condensed back into water to reenter the cycle once again.

In the turbine, steam is forced through a chamber where it rotates a propeller-like arrangement connected to a rotating shaft.  This shaft turns a magnet, which is in a coil of wire, and thereby produces an electric current in the wire.

Primary coolant is isolated from the environment

In the condenser, the steam is cooled and converted into water. This allows the coolant to be reused.

Water from a river or some other large body is drawn into the reactor to absorb heat from the gaseous coolant and thereby condenses it.  The water is then flushed back into the same body of water.  Since the water leaves the reactor at a much higher temperature than it was initially, reactors are often significant sources of thermal pollution.  Because it does not come into direct contact with the primary coolant in the core, no radiation is emitted.

I hope this gives you some useful insight into nuclear reactors.  In the next and final part 3 of this series I will talk about the harmful effects of radiation on our health.

The video above uses clips from the best selling  Chemistry Tutorials and Physics Tutorials from MCH Multimedia: Learn in depth by working through the modules which are presented with clear verbal instructions and descriptions.  There are many places where you can plot results and do simulations. I am sure it will help you learn Chemistry and Physics faster and better!

You can also find similar topics and their explanations, along with  interactive multimedia animations, in the latest edition of Physical Chemistry e-book by Laidler, Meiser, Sanctuary

~ Bryan


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