What is nuclear reactor ?

[Saqib Riaz]

What is nuclear reactor
Nuclear reactor is An equipment in which the nuclear chain reaction is carried out in a controlled manner is called a nuclear reactor. The energy liberated in a controlled manner is used to produce steam, which can run turbines and produce electricity. In nuclear reactors, the nuclear fission is controlled by controlling the number of neutrons released during the fission.
The main part of the nuclear reactor is called the reactor core. It consists of the following parts:


a) Fuel rods

The fissionable material used in the reactor is called fuel. The fuel used is enriched uranium 235 (in the form of U2O3). The solid fuel is made into rods or pellets, which are shielded by placing in stainless steel tubes.

b) Control rods

To control the fission process, rods made of cadmium or boron are suspended between the fuel rods. These rods can be raised or lowered and control the fission process by absorbing neutrons. That is why they are called 'control rods'. Controlling of neutrons is based upon the fact that cadmium and boron can absorb neutrons to form the corresponding isotopes, which are not radioactive.

c) Moderator

The speeds of the neutrons produced in the fission have to be slowed down so that they are easily captured by the fuel and the fission process can take place most effectively. This is done by surrounding the fuel rods with heavy water (D2O). The material used to slow down the neutrons (without absorbing them) is called a moderator. Graphite is also used as a moderator sometimes.

d) Coolant

To carry away the heat produced during fission, a liquid is circulated in the reactor core. This liquid enters the base of the reactor core and leaves at the top. The heat carried by the outgoing liquid is used for producing steam. The liquid cools down and is recycled again. This liquid is called the coolant. Usually heavy water is used as coolant so that it may also act as moderator.

e) Shield

To prevent the losses of heat and to protect the persons operating the reactor from the radiation and heat, the entire reactor core is enclosed in a heavy steel or concrete some called the shield.

All nuclear reactors are devices designed to maintain a chain reaction producing a steady flow of neutrons generated by the fission of heavy nuclei. They are, however, differentiated either by their purpose or by their design features. In terms of purpose, they are either research reactors or power reactors.
Research reactors are operated at universities and research centres in many countries, including some where no nuclear power reactors are operated. These reactors generate neutrons for multiple purposes, including producing radiopharmaceuticals for medical diagnosis and therapy, testing materials and conducting basic research.
Power reactors are usually found in nuclear power plants. Dedicated to generating heat mainly for electricity production, they are operated in more than 30 countries (see Nuclear Power Reactors). Their lesser uses are drinking water or district water production. In the form of smaller units, they also power ships.
Differentiating nuclear reactors according to their design features is especially pertinent when referring to nuclear power reactors (see Types of Nuclear Power Reactors).

Nuclear Power Reactors

There are many different types of power reactors. What is common to them all is that they produce thermal energy that can be used for its own sake or converted into mechanical energy and ultimately, in the vast majority of cases, into electrical energy.
In these reactors, the fission of heavy atomic nuclei, the most common of which is uranium-235, produces heat that is transferred to a fluid which acts as a coolant. During the fission process, bond energy is released and this first becomes noticeable as the kinetic energy of the fission products generated and that of the neutrons being released. Since these particles undergo intense deceleration in the solid nuclear fuel, the kinetic energy turns into heat energy.

In the case of reactors designed to generate electricity, to which the explanations below will now be restricted, the heated fluid can be gas, water or a liquid metal. The heat stored by the fluid is then used either directly (in the case of gas) or indirectly (in the case of water and liquid metals) to generate steam. The heated gas or the steam is then fed into a turbine driving an alternator.

Since, according to the laws of nature, heat cannot fully be converted into another form of energy, some of the heat is residual and is released into the environment. Releasing is either direct – e.g. into a river – or indirect, into the atmosphere via cooling towers. This practice is common to all thermal plants and is by no means limited to nuclear reactors which are only one type of thermal plant.

Types of Nuclear Power Reactors

Nuclear power reactors can be classified according to the type of fuel they use to generate heat.

Uranium–fuelled Reactors

The only natural element currently used for nuclear fission in reactors is uranium. Natural uranium is a highly energetic substance: one kilogram of it can generate as much energy as 10 tonnes of oil. Naturally occurring uranium comprises, almost entirely, two isotopes: U238 (99.283%) and U235 (0.711%). The former is not fissionable while the latter can be fissioned by thermal (i.e. slow) neutrons. As the neutrons emitted in a fission reaction are fast, reactors using U235 as fuel must have a means of slowing down these neutrons before they escape from the fuel. This function is performed by what is called a moderator, which, in the case of certain reactors (see table of Reactor Types below) simultaneously acts as a coolant. It is common practice to classify power reactors according to the nature of the coolant and the moderator plus, as the need may arise, other design characteristics.

Reactor Type

Coolant
Moderator
Fuel
Comment

1)
Pressurised water reactors (PWR, VVER)
Light water
Light water
Enriched uranium
Steam gener-ated in secondary loop

2)
Boiling water reactors (BWR)
Light water
Light water
Enriched uranium
Steam from boiling water fed to turbine

3)
Pressurised heavy water reactor (PHWR)
Heavy water
Heavy water
Natural uranium
---

4)
Gas-cooled reactors (Magnox, AGR, UNGG)
CO2
Graphite
Natural or enriched uranium
------
5)
Light water graphite reactors (RBMK)
Press-urised boiling water
Graphite
Enriched uranium
Soviet design



PWRs and BWRs are the most commonly operated reactors in Organisation for Economic Cooperation and Development (OECD) countries. VVERs, designed in the former Soviet Union, are based on the same principles as PWRs. They use “light water”, i.e. regular water (H2O) as opposed to “heavy water” (deuterium oxide D2O). Moderation provided by light water is not sufficiently effective to permit the use of natural uranium. The fuel must be slightly enriched in U235 to make up for the losses of neutrons occurring during the chain reaction. On the other hand, heavy water is such an effective moderator that the chain reaction can be sustained without having to enrich the uranium. This combination of natural uranium and heavy water is used in PHWRs, which are found in a number of countries, including Canada, Korea, Romania and India.
Graphite-moderated, gas-cooled reactors, formerly operated in France and still operated in Great Britain, are not built any more in spite of some advantages.
RBMK-reactors (pressure-tube boiling-water reactors), which are cooled with light water and moderated with graphite, are now less commonly operated in some former Soviet Union bloc countries. Following the Chernobyl accident (26 April 1986) the construction of this reactor type ceased. The operating period of those units still in operation will be shortened.

Plutonium-fuelled Reactors

Plutonium (Pu) is an artificial element produced in uranium-fuelled reactors as a by-product of the chain reaction. It is one hundred times more energetic than natural uranium; one gram of Pu can generate as much energy as one tonne of oil. As it needs fast neutrons in order to fission, moderating materials must be avoided to sustain the chain reaction in the best conditions. The current Plutonium-fuelled reactors, also called “fast” reactors, use liquid sodium which displays excellent thermal properties without adversely affecting the chain reaction. These types of reactors are in operation in France, Japan and the Commonwealth of Independent States (CIS).

Light Water Reactors

The Light Water Reactors category comprises pressurised water reactors (PWR, VVER) and boiling water reactors (BWR). Both of these use light water and hence enriched uranium. The light water they use combines the functions of moderator and coolant. This water flows through the reactor core, a zone containing a large array of fuel rods where it picks up the heat generated by the fission of the U235 present in the fuel rods. After the coolant has transferred the heat it has collected to a steam turbine, it is sent back to the reactor core, thus flowing in a loop, also called a primary circuit.
In order to transfer high-quality thermal energy to the turbine, it is necessary to reach temperatures of about 300 °C. It is the pressure at which the coolant flows through the reactor core that makes the distinction between PWRs and BWRs.

In PWRs, the pressure imparted to the coolant is sufficiently high to prevent it from boiling. The heat drawn from the fuel is transferred to the water of a secondary circuit through heat exchangers. The water of the secondary circuit is transformed into steam, which is fed into a turbine.

In BWRs, the pressure imparted to the coolant is sufficiently lower than in a PWR to allow it to boil. It is the steam resulting from this process that is fed into the turbine.

This basic difference between pressurised and boiling water dictates many of the design characteristics of the two types of light water reactors, as will be explained below.
Despite their differing designs, it must be noted that the two reactor types provide an equivalent level of safety.

Pressurised Water Reactors

The fission zone (fuel elements) is contained in a reactor pressure vessel under a pressure of 150 to 160 bar (15 to 16 MPa). The primary circuit connects the reactor pressure vessel to heat exchangers. The secondary side of these heat exchangers is at a pressure of about 60 bar (6 MPa) - low enough to allow the secondary water to boil. The heat exchangers are, therefore, actually steam generators. Via the secondary circuit, the steam is routed to a turbine driving an alternator. The steam coming out of the turbine is converted back into water by a condenser after having delivered a large amount of its energy to the turbine. It then returns to the steam generator. As the water driving the turbine (secondary circuit) is physically separated from the water used as reactor coolant (primary circuit), the turbine-alternator set can be housed in a turbine hall outside the reactor building.


Nuclear power plant with pressurized water reactor

Boiling Water Reactors

The fission zone is contained in a reactor pressure vessel, at a pressure of about 70 bar (7 MPa). At the temperature reached (290 °C approximately), the water starts boiling and the resulting steam is produced directly in the reactor pressure vessel. After the separation of steam and water in the upper part of the reactor pressure vessel, the steam is routed directly to a turbine driving an alternator.

The steam coming out of the turbine is converted back into water by a condenser after having delivered a large amount of its energy to the turbine. It is then fed back into the primary cooling circuit where it absorbs new heat in the fission zone.
Since the steam produced in the fission zone is slightly radioactive, mainly due to short-lived activation products, the turbine is housed in the same reinforced building as the reactor.


Principle of a nuclear power plant with boiling water reactor



In nuclear power plants, the large amount of energy which is released in the form of heat during controlled fission process is converted into electrical energy. Twelve such nuclear power plants have been set up in our country.

In nuclear power plants, the large amount of energy which is released in the form of heat during controlled fission process is converted into electrical energy. Twelve such nuclear power plants have been set up in our country.

[Saqib Riaz]

What is radioactivity?

Source - About Uranium

Radioactivity is the spontaneous emission of energy from unstable atoms.
Atoms are found in all natural matter. There are stable atoms, which remain the same forever, and unstable atoms, which break down or 'decay' into new atoms. These unstable atoms are said to be 'radioactive', because they emit radioactivity from the nucleus as they decay.

Radioactive elements, such as uranium, thorium and potassium break down (decay) fairly readily to form lighter atoms. The energy that is released in the process is made up of small, fast-moving particles and high-energy waves. These particles and waves are, of course, invisible. (The level of radioactivity of an element varies according to how stable its atoms are). Other elements with naturally occurring radioactive forms, (isotopes) are carbon, bismuth, radon, and strontium.
Radioactivity is a random process that happens naturally as the isotopes in particular elements decay. The isotopes continue to break down over time. The length of time that is taken for half of the nuclei in an element to decay is called its 'half-life'. A half-life can be very short (milliseconds to hours) or very long (hundreds of thousands of years).
Radiation also arises from nuclear fission. Fission can be spontaneous but is usually initiated in a nuclear reactor. Fission is a radioactive process; it releases energy as the heavy nucleus is split into two.