One that is used to start and organize

One of the
nuclear reactions is called nuclear fission. Nuclear fission is basically a
reaction that happens when a nucleus of a massive atom is divided into a few
lighter parts (usually two parts, often called binary fission) when this
massive nucleus is hit by a particle such as a neutron. That procedure produces
free neutrons and a huge amount of energy (Santics, Monti and Ripani, 2016). In
1938, Hahn and Strabmann unexpectedly discovered nuclear fission in the German
capital city, Berlin (Krappe and Pomorski, 2012).

A nuclear
fission reaction cannot take place without four main components:

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1-    
 Reactor: to take place a nuclear fission
reaction requires a reactor which is a device that is used to start and
organize a nuclear reaction. There are many different types of nuclear
reactors. Some of these reactors are Magnox, AGR, PWR, BWR, CANDU and RBMK. In
the UK, AGR and PWR are mainly used to generate power (Nuclear Reactor Types,
2005). The UK has 15 reactors generating about 21% of its
electricity but almost half of this capacity is to be retired by 2025. These reactors are usually called thermal reactors because they utilise
slow or thermal neutrons. There is also a different type of reactors that is
called fast-neutron reactor or simply fast reactor which uses originally fast
neutrons to initiate the fission chain reaction (Nuclear Reactor Types, 2005).

 

2-    
Moderator:
a moderator is substance that is utilised to slow down the neutrons released in
fission, therefore, that would lead to produce more fissions. A moderator is a
component in all thermal reactors but not the fast reactor. (Krappe and
Pomorski, 2012).

 

3-    
Fuel:
it consists of substances that are potentially fissile such as 233U,
235U, 239Pu, 241Pu. The fuel is the medium
where fission reactions and most of the energy transformation of fission energy
into thermal energy happens. In general, uranium is the basic fuel in most of
the operating reactors (Krappe and Pomorski, 2012).

 

 

4-    
Coolant:
It is a liquid or gas surrounding the nuclear reactor core used to restrict its
temperature. The coolant could be water,
heavy-water, liquid Na, He gas, CO2, liquid Pb or a liquid
lead-bismuth eutectic mixture. In water reactors, the
moderator works also as coolant (Krappe and Pomorski, 2012).

Figure
1 is a summary of the different types of thermal reactors with their moderators,
fuels and coolants:

Figure
1: Summary of the Different Types of Thermal Reactors with their Moderators,
Fuels and Coolants (Source: Nuclear Electric, 2005)

 

Reactions are implemented
in thermal or fast reactors. The main difference between them is that thermal reactors slow down neutrons as
quickly as possible so that less neutrons are wasted to 238U. To
avoid that, a moderator is inserted in the design, some substances with a light
nucleus could absorb a large amount of energy from a neutron in one collision.
On the other hand, fast reactors use just the opposite method. They attempt to
keep the neutrons fast with high energies as long as possible (Nuclear Electric,
2005).

Fast reactors do not require a moderator to operate
because a neutron may have high kinetic energy or be a fast neutron, so we use
moderator in thermal reactors to thermalize this fast neutron. Fast neutron has
high kinetic energy. Fast reactors use 238U as fuel which has a high cross section for fast neutron
but very low cross section for thermal neutron. Therefore, they do not need any
moderator and immediately go into fission reaction with 238U (Nuclear
Electric, 2005).

Nowadays,
thermal reactors play a major role commercially as they are reactor systems
that use slow or thermal neutrons to initiate the fission reaction in the 235U fuel. Even
with the different enrichment levels used in the fuel in these reactors, yet,
large numbers of atoms present are 238U, which cannot be split. Therefore, when an additional neutron is absorbed by these
atoms, their nuclei do not divide but are transformed into another substance,
Plutonium. Plutonium is fissile and some of it is consumed, and some of this
plutonium is left with 235U that was not utilised (Nuclear Electric,
2005). After that, these fissile elements can be taken
from the fission reaction wastes. Then, they can be recycled. Therefore, that
would diminish the usage of uranium in thermal reactors by 40 percent, even
though thermal reactors still need a huge stock of natural uranium. However,
it is possible to design a reactor which sufficiently produces more fissile substances
than it consumes. This is the fast reactor in which a moderator is not
needed. Fast reactors, however, are now still at the preparation stage (Nuclear
Electric, 2005).

Fast reactors are extremely expensive to build
than other types of nuclear reactors and will therefore become economically
beneficial only if uranium prices noticeably go up.

To sum up, the great improvement of fast reactors has dramatic
effect in principle. This is because they have the potential to increment the
energy available from a specific amount of uranium by a factor of 15 or even
more, and can utilise the existing quantities of used uranium, which would have
no benefit if it is not used (Santics, Monti
and Ripani, 2016). Since fast reactors, the
unmoderated reactors, produce more energy than thermal reactors, fast reactors
are preferred to be built even it is expensive if we look at this situation in
the long-term.

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