Nuclear reactors

Nuclear Reactors
 Fusion Power Reactors
 Light Water Reactor
 Heavy Water Reactor

Fusion Power Reactors
 Fusion power is the generation of energy by nuclear
fusion. Fusion reactions are high energy reactions in
which two lighter atomic nuclei fuse to form a heavier
nucleus. When they combine, some of the mass is
converted into energy in accordance with the
formula . This major area of plasma physics research is
concerned with harnessing this reaction as a source of
large scale sustainable energy.

 In large scale commercial proposals, heat from the
fusion reaction is used to operate a steam turbine that
drives electrical generators, as in existing fossil
fuel and nuclear fission power stations. Many different
fusion concepts have come in and out of vogue over the
years. The current leading designs are
the tokamak and inertial confinement fusion (laser)
approaches. As of November 2015, these technologies
are not yet commercially viable. Currently, it takes
more energy to initiate and contain a fusion reaction,
than the energy it produces.[2]

The Sun is a natural fusion reactor

Mechanism
 Fusion reactions occur when two (or more) atomic
nuclei come close enough for the strong nuclear
force pulling them together to exceed the electrostatic
force pushing them apart, fusing them into heavier
nuclei. For nuclei lighter than iron-56, the reaction
is exothermic, releasing energy. For nuclei heavier
than iron-56, it is endothermic, requiring an external
source of energy. Hence, nuclei smaller than iron-56
are more likely to fuse while those heavier than iron-56
are more likely to break apart.

 To fuse, nuclei must be brought close enough together
for the strong force to act, which occurs only at very
short distances. The electrostatic force keeping them
apart acts over long distances, so a significant amount
of kinetic energy is needed to overcome this "Coulomb
barrier" before the reaction can take place. There are
several ways of doing this, including speeding up
atoms in a particle accelerator, or more commonly,
heating them to very high temperatures.

 Once an atom is heated above its ionization energy,
its electrons are stripped away, leaving just the bare
nucleus (the ion). The result is a hot cloud of ions and
the electrons formerly attached to them. This cloud is
known as a plasma. Because the charges are separated,
plasmas are electrically conductive and magnetically
controllable. Many fusion devices take advantage of
this to control the particles as they are being heated.

 Theoretically, any atoms can be fused if enough
pressure and temperature are applied, and studies
have been made of the conditions required to create
fusion conditions for a variety of atoms. Power plants,
however, are currently limited to only the lightest
elements. Hydrogen is ideal for this purpose because
of its small charge, making it the easiest atom to fuse,
and producing helium.

Power production
 Steam turbines It has been proposed that steam
turbines be used to convert the heat from the fusion
chamber into electricity. The heat is transferred into
a working fluid that turns into steam, driving electric
generators.
 Neutron blankets Deuterium and tritium fusion
generates neutrons. This varies by technique (NIF has
a record of 3E14 neutrons per second while a
typical fusor produces 1E5 - 1E9 neutrons per second).
It has been proposed to use these neutrons as a way to
regenerate spent fission fuel or as a way to breed
tritium from a liquid lithium blanket.

 Direct conversion This is a method where the kinetic
energy of a particle is converted into voltage. It was first
suggested by Richard F. Post in conjunction with magnetic
mirrors, in the late sixties. It has also been suggested
for Field-Reversed Configurations. The process takes the
plasma, expands it, and converts a large fraction of the
random energy of the fusion products into directed
motion. The particles are then collected on electrodes at
various large electrical potentials. This method has
demonstrated an experimental efficiency of 48 percent.


Lawson criterion
 This equation shows that energy varies with the
temperature, density, speed of collision, and fuel used. This
equation was central to John Lawsons' analysis of fusion
power plants working with a hot plasma. Lawson assumed
an energy balance, shown below.
 Net Power = Efficiency * (Fusion - Radiation Loss -
Conduction Loss)
 Net Power is the net power for any fusion power plant.
 Efficiency how much energy is needed to drive the device
and how well it collects power.
 Fusion is rate of energy generated by the fusion reactions.
 Radiation is the energy lost as light, leaving the plasma.
 Conduction is the energy lost, as mass leaves the plasma.

 Plasma clouds lose energy
through conduction and radiation. Conduction is
when ions, electrons or neutrals hit a surface and
transfer a portion of their kinetic energy to the atoms
of the surface. Radiation is when energy leaves the
cloud as light. This can be in the visible, UV, IR or X-
Ray light. Radiation increases as the temperature rises.
To get net power from fusion, you must overcome
these losses.

Light water Reactor
 The light water reactor (LWR) is a type of thermal-
neutron reactor that uses normal water, as opposed
to heavy water, as both its coolant and neutron
moderator – furthermore a solid form of fissile
elements is used as fuel. Thermal-neutron reactors are
the most common type of nuclear reactor, and light
water reactors are the most common type of thermal-
neutron reactor.
 There are three varieties of light water reactors:
the pressurized water reactor (PWR), the boiling water
reactor (BWR), and (most designs of) the supercritical
water reactor (SCWR).

Reactor Design
 The light water reactor produces heat by controlled nuclear
fission. The nuclear reactor core is the portion of a nuclear
reactor where the nuclear reactions take place. It mainly
consists of nuclear fuel and control elements. The pencil-
thin nuclear fuel rods, each about 12 feet (3.7 m) long, are
grouped by the hundreds in bundles called fuel assemblies.
Inside each fuel rod, pellets of uranium, or more
commonly uranium oxide, are stacked end to end. The
control elements, called control rods, are filled with pellets
of substances like hafnium or cadmium that readily capture
neutrons. When the control rods are lowered into the core,
they absorb neutrons, which thus cannot take part in
the chain reaction. On the converse, when the control rods
are lifted out of the way, more neutrons strike the
fissile uranium-235 or plutonium-239 nuclei in nearby fuel
rods, and the chain reaction intensifies. All of this is
enclosed in a water-filled steel pressure vessel, called
the reactor vessel.

Control:
 Control rods are usually combined into control rod
assemblies — typically 20 rods for a commercial
pressurized water reactor assembly — and inserted
into guide tubes within a fuel element. A control rod is
removed from or inserted into the central core of a
nuclear reactor in order to control the number of
neutrons which will split further uranium atoms. This
in turn affects the thermal power of the reactor, the
amount of steam generated, and hence the electricity
produced. The control rods are partially removed from
the core to allow a chain reaction to occur. The number
of control rods inserted and the distance by which
they are inserted can be varied to control the reactivity
of the reactor.

 Usually there are also other means of controlling
reactivity. In the PWR design a soluble neutron
absorber, usually boric acid, is added to the reactor
coolant allowing the complete extraction of the control
rods during stationary power operation ensuring an
even power and flux distribution over the entire core.
Operators of the BWR design use the coolant flow
through the core to control reactivity by varying the
speed of the reactor recirculation pumps. An increase
in the coolant flow through the core improves the
removal of steam bubbles, thus increasing the density
of the coolant/moderator with the result of increasing
power.

Coolant:
 The light water reactor also uses ordinary water to
keep the reactor cooled. The cooling source, light
water, is circulated past the reactor core to absorb the
heat that it generates. The heat is carried away from
the reactor and is then used to generate steam. Most
reactor systems employ a cooling system that is
physically separate from the water that will be boiled
to produce pressurized steam for the turbines, like the
pressurized-water reactor. But in some reactors the
water for the steam turbines is boiled directly by the
reactor core, for example the boiling-water reactor.

Moderator:
 A neutron moderator is a medium which reduces the
velocity of fast neutrons, thereby turning them
into thermal neutrons capable of sustaining a nuclear
chain reaction involving uranium-235. A good neutron
moderator is a material full of atoms with light nuclei
which do not easily absorb neutrons. The neutrons
strike the nuclei and bounce off. After sufficient
impacts, the velocity of the neutron will be
comparable to the thermal velocities of the nuclei; this
neutron is then called a thermal neutron.

 The light water reactor uses ordinary water, also called light
water, as its neutron moderator. The light water absorbs
too many neutrons to be used with unenriched natural
uranium, and therefore uranium enrichment or nuclear
reprocessing becomes necessary to operate such reactors,
increasing overall costs. This differentiates it from a heavy
water reactor, which uses heavy water as a neutron
moderator. While ordinary water has some heavy water
molecules in it, it is not enough to be important in most
applications. In pressurized water reactors the coolant
water is used as a moderator by letting the neutrons
undergo multiple collisions with light hydrogen atoms in
the water, losing speed in the process. This moderating of
neutrons will happen more often when the water is denser,
because more collisions will occur.

 The use of water as a moderator is an important safety
feature of PWRs, as any increase in temperature causes
the water to expand and become less dense; thereby
reducing the extent to which neutrons are slowed
down and hence reducing the reactivity in the reactor.
Therefore, if reactivity increases beyond normal, the
reduced moderation of neutrons will cause the chain
reaction to slow down, producing less heat. This
property, known as the negative temperature
coefficient of reactivity, makes PWR reactors very
stable.

Heavy water reactor:
 A pressurized heavy-water reactor (PHWR) is
a nuclear power reactor, commonly using
unenriched natural uranium as its fuel, that
uses heavy water (deuterium oxide D2O) as
its coolant and moderator. The heavy water coolant is
kept under pressure, allowing it to be heated to higher
temperatures without boiling, much as in
a pressurized water reactor. While heavy water is
significantly more expensive than ordinary light water,
it creates greatly enhanced neutron economy, allowing
the reactor to operate without fuel-enrichment
facilities (offsetting the additional expense of the
heavy water) and enhancing the ability of the reactor
to make use of alternate fuel cycles.

Purpose of using heavy water:
 Heavy water is used as a moderator.
 It is used to slow the neutrons being directed at the
fissionable material, by means of the molecules of the
moderator physically impacting the incoming
neutrons and absorbing some of the kinetic energy
they posses, thus slowing them down.
 The reason that the neutrons have to be slowed is that
most fissionable materials are more likely to absorb
thermal neutrons (2.2km/s) than fast neutrons
(14,000km/s).
 This means that when heavy water is used as a
moderator, enough neutrons get through that even
with very low levels of U-235 (even the very low levels
found in natural uranium), criticality can be
maintained, and power is produced.

Advantages and Disadvantages
 The use of heavy water as the moderator is the key to
the PHWR (pressurized heavy water reactor) system,
enabling the use of natural uranium as the fuel (in the
form of ceramic UO2), which means that it can be
operated without expensive uranium enrichment
facilities. The mechanical arrangement of the PHWR,
which places most of the moderator at lower
temperatures, is particularly efficient because the
resulting thermal neutrons are "more thermal" than in
traditional designs, where the moderator normally is
much hotter. These features mean that a PHWR can
use natural uranium and other fuels, and does so more
efficiently than light water reactors (LWRs).

 Pressurised heavy-water reactors do have some
drawbacks. Heavy water generally costs hundreds of
dollars per kilogram, though this is a trade-off against
reduced fuel costs. The reduced energy content of
natural uranium as compared to enriched uranium
necessitates more frequent replacement of fuel; this is
normally accomplished by use of an on-power
refuelling system. The increased rate of fuel movement
through the reactor also results in higher volumes
of spent fuel than in LWRs employing enriched
uranium. However, since unenriched uranium fuel
accumulates a lower density of fission products than
enriched uranium fuel, it generates less heat, allowing
more compact storage.

Nuclear Proliferation
 Opponents of heavy-water reactors suggest that such
reactors pose a much greater risk of nuclear
proliferation than comparable light water reactors.
These concerns stem from the fact that during normal
reactor operation, uranium-238 in the natural uranium
fuel of a heavy-water reactor is converted
into plutonium-239, a fissile material suitable for use
in nuclear weapons, via neutron capture followed by
two β− decays. As a result, if the fuel of a heavy-water
reactor is changed frequently, significant amounts
of weapons-grade plutonium can be chemically
extracted from the irradiated natural uranium fuel
by nuclear reprocessing. In this way, the materials
necessary to construct a nuclear weapon can be
obtained without any uranium enrichment.

 In addition, the use of heavy water as a moderator
results in the production of small amounts
of tritium when the deuterium nuclei in the heavy
water absorb neutrons, a very inefficient reaction.
Tritium is essential for the production of boosted
fission weapons, which in turn enable the easier
production of thermonuclear weapons,
including neutron bombs. It is unclear whether it is
possible to use this method to produce tritium on a
practical scale.

Nuclear reactors

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Nuclear reactors (20)

Recently uploaded (20)

Nuclear reactors