How Does a Nuclear Power Plant Work? Japan's Meltdown Crisis
Suite101 / Mia Carter
13-Mar-2011 (2 comments)

Learn how nuclear reactors generate electricity, why meltdowns occur at power plants and the efforts to prevent a nuclear meltdown in Fukushima, Japan.

The March 11, 2011 8.9-magnitude earthquake and tsunami in eastern Japan has triggered concerns of a meltdown at the Fukushima nuclear power plant, situated approximately 50 miles north of Tokyo.

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Just a little more observation is need

by acopier101 on

The process of shutting down a nuclear reactor is a bit more complicated than what is said in the above article.  In a nuclear power plant reactor, water is used to thermalize (to change the effective speed of a particle to a thermal value) fast neutrons.  A fast neutron is a neutron that is not in thermal equilibrium with the surrounding medium, especially one produced by fission.   Water contains hydrogen, and since hydrogen is a light element it is one of the best elements for removing the energy of neutrons.  This process is called neutron moderation.  In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.  A thermal neutron is a neutron in thermal equilibrium with the surrounding medium, especially one produced by fission and slowed by a neutron moderator.

How do we control the fission rate (power output)?  The output of a nuclear power plant reactor is usually controlled by neutron moderation using water as the medium, and with control rods.  Control rods are made of materials that can scavenge and absorb neutrons (also called poison), and by doing so reduce rate of fission.  For this reason, in case of emergencies these rods are dropped fully in the reactor fuel and by absorbing most of the neutrons it can shut down the reactor in a few seconds.  This process is done automatically and plant operators in control rooms have no control over it.

You may ask; if a reactor is shut down why there is still a danger?  Well, a nuclear reactor is a good source of neutrons, and even after scramming a reactor (an emergency shutdown of a nuclear reactor by dropping control rods, some neutrons still exist, which keeps on creating fission; and that is why cooling the core is an ongoing process even when a reactor is shut down for refueling.  If there are no neutrons, there is no fission.

Along this process, there is a precise water chemistry that controls the water PH, which must be maintained within a certain range to maintain water chemistry quality.

So, what would happen in case of a Large scale LOCA (loss-of-Coolant Accident)?  This happens when the coolant is compromised.  If all fails, the containment building is flooded with water from a containment spray system at the top (to avoid a hydrogen blast), with continuous recirculation using high pressure pumps and passing it through heat exchangers to keep the reactor core cool and under control to avoid meltdown.

Why did explosions happen in the Japanese nuclear reactors?  Obviously all safety measures have failed and gas build up inside the containment building had no place to go but to find a way out.

The question to ask is this; has radioactive byproducts introduced to the environment?  The answer is yes.  The radioactive gases and byproduct have been introduced into the environment, since sea water is used over and on a partially exposed core.


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Just a little Observation

by darius on

The process is a bit more complicated that is said. Water in a reactors

is used to thermalize the fast neutrons. Water contain hydrogen and

since it is a light element  it is best to remove the energy of neutrons,   this process is called moderation.

How do we control the fission rate ( power output)? 

The output of the reactor is controlled by the water moderation and control rods.

These control rods are made of materials that can scavenges and

absorbs  neutrons( also called poison) and by doing so reduce the rate of fission.

For this reason, in case of emergency these rods are dropped fully in reactor fuel and by absorbing most neutrons can shut down the

reactor  in seconds.( this is done automatically and operator in control room has no control)

 You may ask if the reactor is shut down why still there is danger?

Well a reactor is a good source of neutrons and scramming the reactor( dropping control rods) , some neutrons still exist and

keep creating fission  and that is why the cooling the core is  an ongoing process even when the reactor is shut down for refueling.

If there is no neutrons there is no fission. 

Along with this process , there is a water chemistry which is a very precise process , it controlls the water PH and must be maintained within certain range to maintain water chemistry quality.

So, what happens in case of   Large  LOCA? This is when the 

coolant is compromised, if all fails, the containment is flooded with

water along  a spray at the top of containment( to avoid hydrogen blast). continous recirculation  with  high pressure pumps and passing through heat exchangers to keep the reactor core is kept

cold and under control to avoid meltdown. 

Why did explosion happened?Obviously all the safte gaurd has failed and the gas build up inside the containment  has no place to go but to find a way out .

The question to ask is radioactive by products is introduced to the

environment ?

The answer is  yes,the radioactive gases into atmposphere and by

product into environment  since the sea water is used over and on

a partially exposed core.