Nuclear Reactor Physics, Physics tutorial

Introduction:

Nuclear reactor physics is a branch of science which manages the study and application of chain reaction to encourage the controlled rate of fission in the nuclear reactor for production of energy. Most nuclear reactors utilize chain reaction to induce the controlled rate of nuclear fission in fissile material, releasing both energy and free neutrons. The reactor comprises of the assembly of nuclear fuel, generally surrounded by the neutron moderator like heavy water, graphite, regular water, or zirconium hydride, and fitted with mechanisms like control rods which control rate of reaction.

One of the most varied applications of nuclear physics is in medicine. Work of nuclear physicists is behind the number of medical imaging methods that are utilized to give non-invasive looks in body. Radioactive isotopes used in treatment of some medical conditions like cancer are also product of research in nuclear physics, with physicists learning about such isotopes and how they can be applied carefully and efficiently to define medical problems.

Certain features of engineering need knowledge of nuclear physics, most particularly in nuclear engineering, a field that involves development of nuclear power plants that can do anything from producing electricity to powering submarines. Radiocarbon dating, a method utilized in geology and archeology, is also the product of nuclear physics. People in this line of work may also work with astronomers, employing their knowledge to assist date universe, describe physical phenomena, and design experiments.

Nuclear fission:

Nuclear fission is splitting of the atom's nucleus, therefore creating two products of roughly half the mass of the original. During the procedure few neutrons are also released. This procedure releases substantial amount of energy. Nuclear fission is physical process responsible for all kinds of power generation, comprising which utilized in both nuclear weapons and nuclear power plants.

There are several elements which can be utilized in nuclear fission, but the most common is uranium. This material is popular for various different reasons, but two of the most significant are that it is plentiful, and there are isotopes of uranium that are easy to split. The most commonly used isotope of uranium for nuclear energy production is known as U-235. In addition to U-235, plutonium is another substance at times utilized for nuclear power.

Nuclear fusion:

Nuclear fusion is procedure by which multiple atoms having same charge join together to form the heavier nucleus. In some cases, depending on mass, energy can be released or absorbed during procedure. It is a very significant energy source.

Nuclear fusion as the source of manmade energy is still mainly in developmental stage, though some fusion power plants are online. Most of the energy generated this way which benefits humans and other life forms comes from sun. Fusion is the procedure by which all stars produce energy.

Reactor Physics Methods and Applications:

Nuclear reactor physics R&D concentrates on reactor analysis applications and techniques development in areas like: lattice physics, reactor core physics, nuclear fuel cycle assessments, cross-section processing, source terms, radionuclide inventories, and decay heat. A main feature of our work is combination of techniques/software development and applications expertise. In methods development area, activities focus on development, improvement, and maintenance of many key codes and capabilities in scale code system, mainly those related with reactor physics, problem-dependent cross-section processing, isotopic depletion and decay, and with SCALE V&V and computational architecture development.

Applications focus areas comprise analysis of light water reactors in DOE Nuclear Energy Modeling and Simulation Hub called as Consortium for Advanced Simulation of LWRs; advanced reactor concepts like small modular reactors, molten-salt reactors and fluoride-salt-cooled high temperature reactors; and high performance research reactors like ORNL's High Flux Isotope Reactor.

Reactor physics or neutronics analysis plays the vital role in reactor design to find out fuel dimensions and core configuration, fissile and reactivity control needs, optimum fuel management system, heat generation and deposition, fuel composition evolution with burnup, and shielding of ex-core components. Kinetics parameters and reactivity changes for core geometry, temperature, and material density variations are also found to promise favorable safety features. Theory and governing equations for reactor physics analysis are well identified; Boltzmann equation for neutron and gamma transports and Bateman equation for fuel composition evolution. Boltzmann equation is the linear integro-differential equation with seven independent variables (three in space, two in angle, one in energy and time), and Bateman equation is the system of ordinary differential equations. Coefficients of such equations are found by nuclear data, geometry, and composition. Challenge in neutronics analysis is to find out solution efficiently by taking into consideration geometric complexity and complicated energy dependence of nuclear data.

Nuclear reactor types:

Several different reactor systems have been suggested and few of these have been developed to prototype and commercial scale. Six kinds of reactor (Magnox, BWR, AGR, PWR, CANDU and RBMK) have appeared as designs utilized to make commercial electricity around the world. Further reactor type, is fast reactor, has been developed to full-scale demonstration stage. These different reactor kinds will now be explained, together with current developments and few prototype designs.

PWR: Pressurized Water Reactor, were active international.  Water is both the coolant and moderator and is kept at the high pressure 70 to 150 atm.

BWR: Boiling Water Reactor, such as PWR, water is both coolant and moderator.  Though water is kept at the lower pressure 70 atm and therefore generates steam.  Steam directly powers turbine, therefore simplifying design.  The only downside is that over time turbine accumulates radioactivity.

Breeder Reactor: employs both fissile and fertile elements to generate reaction.  Due to this, breeder reactors can utilize materials which are more extensively available. It works by employing fast neutrons to convert fertile elements in fissile elements.

Uncontrolled Chain Reactions:

The other spectrum of chain reactions that generate enough energy to cause explosion.  The destructive chain reactions by accident are given below:

Chernobyl: Level 7 on Nuclear Event Scale, Chernobyl represents the chain reaction gone awry.  During the experiment which through several errors, coolant was turned off finally burning carbon control rods.  The complete meltdown, radioactive smoke spread across Europe causing widespread damage.

Three Mile Island: In Middleton Pennsylvania, Three Mile Island reactor suffered a level 5 Nuclear Event.  Chain reaction stopped due to too some slow neutrons.  Though chain reaction stopped, fission process continued sending fuel rods to severe temperatures.  Small fissure developed in one of the reactors, therefore sending radioactive steam in atmosphere.

Fukushima BWR reactors: Because of severe earthquake and tsunami in Japan numerous BWR nuclear reactors at Fukushima power plant lost electrical power for cooling, experienced explosions, and suffered reactor core damage as workers finally pumped seawater in reactors to cool them down and restrict any additional damage. 

Nuclear reactions and neutron cross sections:

Atomic nuclei can go through interactions with other nuclei, elemental particles (protons, neutrons, electrons) and electromagnetic radiation (photons). For reactor physics we can confine ourselves to interactions with neutrons that because of their electrical neutrality don't experience coulomb repulsion and can therefore become involved in interactions with nuclei already at very low energy. In description of neutron-nucleus interactions compound-nucleus model suggested by Bohr in 1936 can be utilized. According to the model two consecutive stages can be distinguished in nuclear reaction:

1) Incident particle is absorbed by nucleus and forms the compound nucleus with it.

2) After a short time compound nucleus breaks up by emission of particle that need not be equivalent to incident particle.

Nuclear Chain Reactions:

A chain reaction refers to the procedure in which neutrons released in fission produce the additional fission in at least one further nucleus. This nucleus in turn generates neutrons, and procedure repeats. The procedure may be controlled (nuclear power) or uncontrolled (nuclear weapons).

If each neutron releases two more neutrons then number of fissions doubles every generation. In that situation, in 10 generations there are 1,024 fissions and in 80 generations approx 6 x 10 23 (a mole) fissions.

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