nuclear reactor

A nuclear reactor itself is the heart of a nuclear power plant, much like a car has combustion chambers inside of its engine, which includes many more components for the purpose of getting someone from point A to point B.

A nuclear reactor is much like a chemical reactor except it is a vessel where atomic nuclei are broken apart and formed. When nuclei are broken, the reaction is called fission, and when nuclei are formed, the reaction is called fusion. Like chemical reactions, nuclear reactions can be exothermic (net energy generators) or endothermic (net energy consumers) and like chemical reactions, nuclear reactions can chain react.

nuclear reactor

Function Of Nuclear Reactor-

The sole function of a nuclear reactor is to channel and shepherd the nuclear reaction going on inside it. It does this via control of temperature (like in a chemical reactor), control of radiation (via absorption or generation of the radiation), and control of leakage (via physical reflectors or sometimes electric/magnetic fields)

Nuclear reactors are found in many sectors of industry from the electric power industry to medicine to high energy physics research as they offer the ability to change chemical elements into other chemical elements. This is called transmutation, and it is essentially doing reactions that the alchemists of old wanted to do (turning lead into gold) except for elements with industrial applications.

Also Read:Nuclear Power Plant – Purpose & Working

We currently use nuclear fission reactors for power as those are the reactions that we can safely control. Humanity is working on making nuclear fusion power reactors so that we won’t have to deal with long-lived radioactive elements, however, as of yet, this hasn’t been achieved yet. However, both nuclear fission and fusion are routinely used to produce needed elements in medical labs, doctor’s offices, oil/gas drilling rigs, and spacecraft as means of producing radiation for imaging inside solid objects.

Types Of Nuclear Reactor-

Recall that when a heavy atom absorbs a neutron, it can split into two lighter atoms, releasing energy.  Some of this energy is released in the form of… more neutrons.  These new neutrons might go on to split other atoms, causing a chain reaction.  But they are generally too energetic to do so.  Neutrons released by fission move faster than is ideal to cause more fission.  They are likely to fly right through the fuel and on through the walls of the reactor.  This is why nuclear fuel can’t sustain a chain reaction by itself.  We need something in our reactor that slows neutrons down.  Such a material is called a “moderator”.

nuclear reactor

                                                  Parts of nuclear reactor

 

Graphite Reactors-

Moderated reactors use graphite (like in pencils) to do this.  The very first man-made reactor was of this type.  They are sometimes called “pile reactors” because it was literally a pile of graphite and uranium bricks.
British “magnox” reactors are graphite-moderated, as is the AGR described in the article, and the Soviet RBMK reactors that were made famous by the Chernobyl accident.  There are about 15 graphite-moderated reactors in use around the world today.  No new ones are planned.

Also Read:Nuclear Power Plant – Purpose & Working

CANDU Reactors-

These reactors use heavy water to moderate neutrons instead of graphite.  Heavy water is a more efficient moderator, and this means CANDU reactors can be more compact.  The drawback is that heavy water is very rare — it must be separated from seawater, and this is a very energy-intensive process.  Somewhere around 20 CANDU reactors are running today, mostly in Canada.

LWR Reactors-

LWR stands for Light Water Reactor.  “Light” just means ordinary water, as opposed to the heavy water used in CANDU designs.  This solves the water separation problem but creates a Uranium separation problem.  Natural uranium found in the ground is about 1% U235 and 99% U238.  This mixture of isotopes can sustain a chain reaction with graphite or heavy water, but not regular water.  So LWR fuel must be “enriched” — U238 is removed until the U235 concentration reaches 4%.  The rejected U238 is “depleted Uranium” (depleted of U235).

Also Read:Nuclear Power Plant – Purpose & Working

It turns out that enriching uranium is easier than enriching seawater, and the LWR, pioneered by the U.S. Navy, is the most popular type of reactor in the world today.  There are two main varieties: BWR and PWR.  B stands for Boiling, P stands for Pressurized.  In a PWR, water is kept at high pressure so it can’t boil.  PWRs are the more popular of the two types, probably because they can make a bit more electricity from the same amount of fuel.  Both types have an important safety advantage over graphite or CANDU reactors: they use a single substance as both moderator and coolant.

In an LWR, if the reaction rate increases for some reason, the reactor heats up and the water expands.  This means there are fewer water molecules around the fuel to moderate neutrons, and that causes the reaction rate to decrease again.  This feedback works like an automatic control system.  If all the water suddenly leaves the reactor, the chain reaction stops by itself.  In graphite and CANDU reactors, a loss of cooling water does not mean a loss of moderator — their reactivity actually goes up.

Fast-spectrum reactors-

Fast-spectrum reactors are interesting because they use no moderator at all.  As the name implies, they run on fast neutrons.  To start up, they need large quantities of nuclear fuel, enriched to 10% or more U235 (or Pu239).  The point is to transmute the balance of the fuel (U238) into Pu239 during operation.  This is called “breeding” fuel and so these reactors are often called “fast breeders”.  The idea is to be able to use a much greater portion of the natural uranium we find in the ground.  While LWRs can transmute some U238 to Pu239 and burn it, they have limited ability to do so.

Also Read:Nuclear Power Plant – Purpose & Working

If you put gas in your car and drive until your car stops, you know the gas tank is empty and that all the gas has been burned.  But when a nuclear reactor stops, not all of the heavy atoms in its fuel have necessarily been split.  The portion that has been split is called the reactor’s “fuel burnup”.

Graphite, CANDU, and LWR reactors are capable of burning a percentage of their fuel roughly equal to the percentage of fissile (U235 or Pu239) in the fuel at startup. Their spent fuel “waste” can be reprocessed (re-enriched) and burned again, but this is a messy and expensive process.  Fast reactors were supposed to deliver very high fuel burnup, but they have proven difficult to operate in practice.  The U.S., France, and Russia have built power stations around fast reactors, but today only Russia has them in service.

There are a few other kinds of reactors out there, known collectively as research reactors.  They are used in laboratories and are not licensed to make electricity.

Parts Of Nuclear Reactor-

NUCLEAR FUEL

It is a material that can be fissioned by neutrons. A certain mass of the nuclear fuel can be taken in the form of rods, tightly sealed in aluminum containers. The rods are placed in the core of the nuclear reactor and are separated by the moderator. They can be U-235, Th – 232, et cetera.

nuclear reactor

MODERATOR

In the fission of Uranium, fast neutrons are released which have a very high energy and velocities. These fast neutrons have more tendency to escape, instead of triggering another nuclear fission reaction. Slow neutrons are efficient in triggering nuclear fission reactions. Thus, moderators like heavy water, graphite, Beryllium Oxide are used to slow down the fast neutrons to thermal velocities.

nuclear reactor

CONTROL RODS

To start, stop or control the chain reaction, rods of neutron absorbing material like Cd, B are inserted into the core of the reactor. The rate of neutron production is controlled by adjusting the depth of the control rods.

nuclear reactor

COOLANT-

It is a material with high boiling point and high specific heat which is used to cool the fuel rods and the moderator. It can take away a large amount of heat produced in the fission process. It then transfers this heat to the working liquid like water and produces steam which is then used to drive the turbine and switch on the generator. Heavy water, sodium can be used as coolants.

SHIELDING

The intense neutrons and the gamma rays thus produced are highly harmful to the human body. Thus, the reactor core is surrounded by a reactor shield which protects the workers. This reactor shield is a thick concrete wall.

Working Of A Nuclear Reactor-

Initially, some alpha particles are used to produce fast neutrons, by their action on Be or Po. These fast neutrons are slowed down and then used to start fission in U-235 nuclei. Fast neutrons are released which are slowed down using moderators. The slow neutrons are again used to cause fission in the U – 235 nuclei. Thus, a chain reaction builds up. By raising or lowering the control rods, the chain reaction is suitably controlled.

nuclear reactor

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