Nuclear Reactor

What Is Nuclear Reactor?

A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, producing radionuclides (for industry and medicine), conducting research, and military purposes. All of the various designs of power-producing reactors accomplish the same simple task: spinning a generator. Many commercial reactors pass water over heat-producing fuel rods to generate steam and run a turbine. Some designs call for the passage of helium over a pile of heat-producing fuel pebbles. Yet another design uses liquid sodium as a coolant.

Nuclear Reactor Technology.

A nuclear reactor is a device to initiate, and control, a sustained nuclear chain reaction. The most common use of nuclear reactors is for the generation of electrical power and for the power in some ships.  This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam turbines..

Components of Nuclear Reactors                          

There are main component of nuclear reactor.

The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (<5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.

The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, helium, or something else. In the US fleet of power reactors, water is the standard.

The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.

The containment is the structure that separates the reactor from the environment. These are

usually dome-shaped, made of high-density, steel-reinforced concrete. Chernobyl did not have a containment to speak of.

Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapor. 

This image shows a nuclear reactor heating up water and spinning a generator to produce electricity. It captures the essence of the system well. The water coming into the condenser and then going right back out would be water from a river, lake, or ocean. It goes out the cooling towers. As you can see, this water does not go near the radioactivity, which is in the reactor vessel.

Nuclear Reactor Core

A nuclear reactor core is the portion of a nuclear rector containing the nuclear fuel components where the nuclear reactions take place. The nuclear reactor is the region within a nuclear reactor where the nuclear fuel assemblies are located and the nuclear reaction consequently takes place.

Fuel Pins

The smallest unit of the reactor is the fuel pin. These are typically uranium-oxide (UO₂). They are surrounded by a zirconium clad to keep fission products from escaping into the coolant.

Fuel Assembly

Fuel assemblies are bundles of fuel pins. Fuel is put in and taken out of the reactor in assemblies.

Fuel Core

This is a full core, made up of several hundred assemblies. Some assemblies are control assemblies. Various fuel assemblies around the core have different fuel in them. They vary in enrichment and age, among other parameters. The assemblies may also vary with height, with different enrichments at the top of the core from those at the bottom.

To understand more about nuclear reactor you all can see this video

References 

http://en.wikipedia.org/wiki/Nuclear_reactor_technology
http://en.wikipedia.org/wiki/Nuclear_reactor_core

The Chernobyl Reactor Incident

On April 25, 1986, Russian engineers and scientists begin preliminary tests on Chernobyl power plant's 4th reactor. What they didn't realize was that they were about to cause a meltdown that would kill them instantly and would have severe consequences that would extend even to the present day.

The test was conducted in order to create a sufficient supply of energy to prevent overheating in the event of a shutdown. In order to do this properly, several alterations in the generator's magnetic fields had to be made, requiring the engineers to lower the power output to unstable levels. In order to control the experiment, the automatic control system was shut down. After some work, stability was reached at very low power outputs. Unfortunately, manual control of the water pressure wasn't maintained. The reactor began to create excess heat. Without the automatic control, the control rods couldn't be reinserted in time; a deadly chain reaction had begun.

Within a matter of 3-4 seconds, the reactor went from 5% output to 100 times its normal level. The water in the reactor flash-boiled, creating an explosion that leveled thousands of tons of concrete and steel, including the housing for the reactor. The steam carried almost 70% of the nuclear material out of the reactor into the surrounding environment.

Several thousand volunteers died on the scene, and it is estimated that 7,000 to 10,000 volunteers died in total, considering short and long-term effects. Thousands of miles from the scene, the birth defect rate became double the world average.

It is also estimated that 150,000 were put at risk for thyroid cancer, and over 800,000 children were put at risk of contracting leukemia. 2 million acres of land (1/5 of the usable farmland in the Ukraine) was, and still is, completely unusable.

It remains difficult to determine the scope of the disaster; radiation resulting from the event was detected all over the globe. It is estimated that it may cost up to $400 billion and will take up to 200 years to correct the damage done to the area, and to compensate those affected by the meltdown. 

The Chernobyl site and plant

The damaged Chernobyl unit 4 reactor building

References:

http://en.wikipedia.org/wiki/Chernobyl_disaster 

http://seriousaccidents.com/legal-articles/what-happened-at-chernobyl/ 

http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/cherno.html 

Reactivity Coeffivients

Definition Of Reactivity Coefficient

The temperature coefficient of reactivity is the change in reactivity per degree change in temperature.

Important Coefficients of Reactivity

1. Moderator Temperature Coefficient.

The change in reactivity per degree change in moderator temperature. As the moderator (water) increases in temperature, it becomes less dense and slows down fewer neutrons, which results in a negative change of reactivity. This negative temperature coefficient acts to stabilize an atomic power reactor operation.

 A  reactor  is  under  moderated  when  a  decrease  in  the  moderator-to-fuel  ratio decreases  keff due  to  the  increased  resonance absorption. A reactor is over moderated when an increase in the moderator-to-fuel ratio decreases keff due to the decrease in the thermal utilization factor. Reactors are usually designed to operate in an under moderated condition so that the moderator temperature coefficient of reactivity is negative.

2. Fuel Temperature Coefficient

Fuel temperature coefficient of reactivity is the change in reactivity of the nuclear fuel per degree change in the fuel temperature. 

The coefficient quantifies the amount of neutrons that the nuclear fuel (uranium-238) absorbs from the fission process as the fuel temperature increases. It is a measure of the stability of the reactor operations. This coefficient is also known as the Doppler coefficient.

A negative temperature coefficient of reactivity is desirable because it makes the reactor more self-regulating.   An increase in power, resulting in an increase in temperature, results   in negative reactivity addition due to the temperature coefficient.  The negative reactivity addition due to the temperature increase will slow or stop the power increase.

The fuel temperature coefficient is more effective than the moderator temperature coefficient in terminating a rapid power rise because the fuel temperature immediately increases   following a power increase,   while   the   moderator temperature does not increase for several seconds.

The Doppler broadening of resonance peaks occurs because the nuclei may be moving  either  toward  or  away  from  the  neutron  at  the  time  of  interaction. Therefore, the neutron may actually have either slightly more or slightly less than the  resonant  energy,  but  still  appear  to  be  at  resonant  energy  relative  to  the nucleus.

3. Pressure Coefficient

The reactivity in a reactor core can be affected by the system pressure.  The pressure coefficient of reactivity is defined as the change in reactivity per unit change in pressure.

 The pressure coefficient of reactivity for the reactor is the result of the effect of pressure on the density of the moderator. For this reason, it is sometimes referred to as the moderator density reactivity coefficient. As pressure increases, density correspondingly increases, which increases the moderator-to-fuel ratio in the core. In the typical under moderated core the increase in the moderator-to-fuel ratio will result in a positive reactivity addition. In reactors that use water as a moderator, the absolute value of the pressure reactivity coefficient is seldom a major factor because it is very small compared to the moderator temperature coefficient of reactivity.

4. Void Coefficient

The void coefficient of reactivity is defined as the change in reactivity per percent change in void volume.

 As the reactor power is raised to the point where the steam voids start to form, voids displace moderator from the coolant channels within the core. This displacement reduces the moderator-to-fuel ration, and in an under moderator core, results in negative reactivity addition, thereby limiting reactor power rise. The void coefficient is significant in water-moderated reactors that operate at or near saturated conditions.

Effective neutron multiplication factor

The effective neutron multiplication factor, k, is the average number of neutrons from one fission that cause another fission. The remaining neutrons either are absorbed in non-fission reactions or leave the system without being absorbed. The value of k determines how a nuclear chain reaction proceeds:

~ k < 1 (subcriticality): The system cannot sustain a chain reaction, and any beginning of a chain reaction dies out over time. For every fission that is induced in the system, an averagetotal of 1/(1 − k) fissions occur.

~ k = 1 (criticality): Every fission causes an average of one more fission, leading to a fission (and

power) level that is constant. Nuclear power plants operate with k = 1  unless the power level is being increased or decreased.

~ k > 1 (supercriticality): For every fission in the material, it is likely that there will be "k" fissions

after the next mean generation time. The result is that the   number of fission reactions increases exponentially, according to the equation e^{(k − 1)t / Λ}, where t is the elapsed time. Nuclear weapons are designed to operate under this state. There are two subdivisions of supercriticality: prompt and delayed.

When describing kinetics and dynamics of nuclear reactors and also in the practice of reactor operation is used the concept of Reactivity (nuclear), which characterizes the deflection of reactor from the critical state. ρ=(k-1)/k.

In a nuclear reactor, k will actually oscillate from slightly less than 1 to slightly more than 1, due primarily to thermal effects (as more power is produced, the fuel rods warm and thus expand, lowering their capture ratio, and thus driving k lower). This leaves the average value of k at exactly 1. Delayed neutrons play an important role in the timing of these oscillations.

In an infinite medium, the multiplication factor may be described by the four factor formula, in a non-infinite medium, the multiplication factor may be described by the six factor formula.

 

The effective multiplication factor may be expressed mathematically as shown below:

Reactivity

Definition of Reactivity.

~Chemistry :- The relative capacity of an atom, molecule, or radical to combine chemically with

another atom, molecule, or radical. 

~Physics :- it means a measure of the deviation from the condition at which reactor is critical.

Reactivity is a measure of the departure of a reactor from criticality. The reactivity is related to the value of keff. Reactivity also is a useful to predict how the neutron population will change over time.

When keff remains constant from generation to generation, it is possible to determine the number of neutrons beginning any particular generation by knowing only the value of keff  and the number of neutrons staring the first generation, No.

Number of neutron :- Nn = No ( keff )

Reactivity is the fractional change in neutron population per generation

-          A little description about the reactivity of Uranium.

Uranium is a heavy silvery-white metallic element, radioactive and toxic, easily oxidized. The uranium isotope with mass number 235 and half-life 7.04 × 10⁸ years, fissionable with slow neutrons and capable in a critical mass of sustaining a chain reaction that can proceed explosively with appropriate mechanical arrangements.

Nuclear fission chain reaction

A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g. ²³⁵U) or the fusion of light isotopes (e.g. ²H and ³H). The nuclear chain reaction is unique since it releases several million times more energy per reaction than any chemical reaction.

Units of Reactivity

Reactivity is a dimensionless number. It does not have dimensions of time, length, mass, or any combination of these dimensions. It is simply a ratio of two quantities that are dimensionless. As shown in the calculation in the previous example, the value of reactivity is often a small decimal value. In order to make this value easier to express, artificial units are defined.

By definition, the value for reactivity is in units of change of k/k.   Alternative units for reactivity are % change of k/k and pcm (percent millirho). The conversions between these units of reactivity are shown below.

 

Another unit of reactivity is used at some reactors is equivalent to 10^-4 change of k per k. This unit of reactivity does not have a unique name.