NUCLEAR REACTORS

 

Nuclear energy, also called atomic energy, energy that is released in significant amounts in processes that affect atomic nuclei, the dense cores of atoms. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced by nuclear fission of uranium and plutonium. Nuclear decay processes are used in niche applications such as radioisotope thermoelectric generators. Generating electricity from fusion power remains at the focus of international research.

Components of Nuclear Reactor:

BACK THEN:

It would be worthwhile here to have a look at first in which the Noble Prize winner Enrico Fermi and his colleagues proved the possibility of sustained fission reaction. The very first time man created a chain reaction in a reactor named as Chicago Pile on 2^nd December 1942. 

The figure shows the picture of Chicago Pile. Fermi and his colleagues showed that the critical condition (self-sustaining) was reached when 400 tonnes of graphite (used as a moderator and control rods made up of Cadmium were used), 6 tonnes of Uranium metal and 37 tonnes of Uranium Oxide were piled up in carefully planned arrangement (this explains the origin of term atomic pile).In a picture above we see two operators in the left corner. One who is sitting in front of a display monitor is keeping a count of number of neutrons and one person has his hand on the lever which is used to raise or lower the control rods. These two persons obviously communicated verbally as electronics was not too much developed at that time.

As safety is one of the most important thing in nuclear reactions, Fermi too was aware of it that if reaction goes out of control or if any emergency arises then there should be some means to shut down the reactor. Line of defense consisted of a tank with Cadmium salt. And as soon as Fermi would signal emergency, the tank would be turned over and poured into the reactor core to stop the reaction. 

TYPES OF NUCLEAR REACTORS:

Pressurised Water Reactor (PWR):

A pressurized water reactor (PWR) is a type of light-water nuclear reactor (Light-water reactors (LWRs) are power reactors that are cooled and moderated with ordinary water. There are two basic types: the pressurized-water reactor (PWR) and the boiling-water reactor (BWR).). PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator.

Uranium (Uranium – 235) is used as a fuel. Chain reaction inside a core produces high level of heat. Heat tubes pass the heat to the primary cooling system water simply by contact. Primary cooling system is a closed circuit of pressurized water. Primary water enters the reactor vessel at 296 °C and exit at 327 °C. The water than passes into a steam generator where it transfers it heat to a secondary system. To ensure that water in the primary system remains in the liquid state a pressurizer will maintain the constant level of 155 bars. Hence the name “Pressurized Water Reactor”. Within the steam generator heat stored in the primary system water is transferred to a secondary system. The heated water enters the bottom of the steam generator transfers its heat to the secondary system water through the U tubes then returns to the reactor vessel for a new cycle. Water in the secondary system is heated to boiling temperature turns into steam and then travels to the turbine set. After passing through the turbine the steam is recondensed into liquid water and return to the steam generator for another cycle. Steam pressure from the secondary system drives the generators to produce electricity.

PWRs were originally designed to serve as nuclear marine propulsion for nuclear submarines and were used in the original design of the second commercial power plant at Shippingport Atomic Power Station.

 

Boiling Water Reactor (BWR):

A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power.

The core inside the reactor vessel creates heat. A steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat. The steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line.

The streamline directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted to the condenser, where it is condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated, and pumped back to the reactor vessel.

The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If offsite power is lost, emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators. Boiler water reactor contain between 370-800 fuel assemblies. The cooling water is maintained at about 75 atm (7.6 MPa, 1000–1100 psi) so that it boils in the core at about 285 °C (550 °F).

The BWR was developed by the Argonne National Laboratory and General Electric (GE) in the mid-1950s. The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor.

Advanced Boiling Water Reactor (ABWR):

The advanced boiling water reactor (ABWR) is a Generation III boiling water reactor. The ABWR is currently offered by GE Hitachi Nuclear Energy (GEH) and Toshiba. The ABWR generates electrical power by using steam to power a turbine connected to a generator; the steam is boiled from water using heat generated by fission reactions within nuclear fuel. Kashiwazaki-Kariwa unit 6 is considered the first Generation III reactor in the world. The standard ABWR plant design has a net electrical output of about 1.35 GW, generated from about 3926 MW of thermal power.

The ABWR represents an evolutionary route for the BWR family, with numerous changes and improvements to previous BWR designs.

Major areas of improvement include:

1. The addition of reactor internal pumps (RIP) mounted on the bottom of the reactor pressure vessel (RPV) - 10 in total - which achieve improved performance while eliminating large recirculation pumps in containment and associated large-diameter and complex piping interfaces with the RPV (e.g. the recirculation loop found in earlier BWR models).
2. The control rod adjustment capabilities have been supplemented with the addition of an electro-hydraulic Fine Motion Control Rod Drive (FMCRD), allowing for fine position adjustment using an electrical motor, while not losing the reliability or redundancy of traditional hydraulic systems which are designed to accomplish rapid shutdown in 2.80 s from receipt of an initiating signal, or ARI (alternate rod insertion) in a greater but still insignificant time period.
3. The FMCRD also improves defence-in-depth in the event of primary hydraulic and ARI contingencies.
4. A fully digital Reactor Protection System (RPS) (with redundant digital backups as well as redundant manual backups) ensures a high level of reliability and simplification for safety condition detection and response. This system initiates rapid hydraulic insertion of control rods for shutdown (known as SCRAM by nuclear engineers) when needed.
5. Two-out-of-four per parameter rapid shutdown logic ensures that nuisance rapid shutdowns are not triggered by single instrument failures. RPS can also trigger ARI, FMCRD rod run-in to shut down the nuclear chain reaction. The standby liquid control system (SLCS) actuation is provided as diverse logic in the unlikely event of an Anticipated Transient Without Scram.
6. Fully digital reactor controls (with redundant digital backup and redundant manual backups) allow the control room to easily and rapidly control plant operations and processes. Separate redundant safety and non-safety related digital multiplexing buses allow for reliability and diversity of instrumentation and control.
7. The Emergency Core Cooling System (ECCS) has been improved in many areas, providing a very high level of defence-in-depth against accidents, contingencies, and incidents.

If the fission reaction is controlled well and the nuclear waste released is exposed properly then it’s a great boon to the human society. Thus in this post we take a brief look at some of the common nuclear reactors.

"The release of atomic power has changed everything except our way of thinking... the solution to this problem lies in the heart of mankind. If only I had known, I should have become a watchmaker."

-Albert Einstein

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