The Music of Particles

    

    For a very long time, mankind wanted to know the complete set of laws that govern nature. In order to achieve this, you need to take a look at how macroscopic objects (which you can see with your bare eyes) behave and then go even further at the very small. We know (or at least we think we know) that everything is made out of molecules. Molecules are made from atoms. Atoms are made from protons, electrons, neutrons and a bunch of other particles that I don't want to mess you up with. 

    The logical question that arises is: what are subatomic particles made of? Good question indeed. Some say that there is no more sub level. The subatomic particles are what they are and period. Others, on the other hand, claim that there has to be something more deeper. What scientists (at least a part of them) realized that some phenomena indicated that the subatomic particles are made out of STRINGS of energy. 

    In other words, think of a guitar; mainly it's strings. When you strum a strings it produces a sound through it's vibration. In particle physics, when a string vibrates in some way it forms a proton. If the strings vibrates in another way it forms an electron and so on. Just like in guitar playing, when a string vibrates in one way it makes the E note, when it vibrates in another way it makes a F note and so on. 

    

    Furthermore, when we think of guitar chords, they can only vibrate bidimensionally: you can play only in up and down strokes. On the other hand, energy strings can vibrate throughout the whole space. Of course there are a significant number of string types. An interesting aspect of this theory is that it needs other dimensions included in the actual mathematical theory in order for it to work. After saying this, scientist have come to the conclusion that our entire "space" is not made out of 3 dimensions (as classical physicists would say), is not made out of 4 dimensions (as relativistic physicists would say) but it is made out of 11 dimensions(so they say :P). Not so easy to comprehend but I haven't solved any equations yet, related to this so I can't confirm or not this statement.

    In conclusion, it is not a question of are they or not right. I think that the most important aspect is that we are getting closer and closer to resolving some of our older questions about our Universe. You can't get it right from the first time, but I'm pretty sure that someone will figure it out and one day we will be able to fully understand nature. Until then, Cheers!   

 

LHC

    

    The LHC (Large Hadron Collider) is the world's largest particle accelerator (as the name suggests :P). It's role is to answer some of the fundamental questions in physics and to widen our understanding of the laws of nature. 

    The LHC lies in a tunnel 27 km in circumference beneath the Franco-Swiss border near Geneva, Switzerland. It is designed to collide particle beams at very high energies. The Large Hadron Collider was built by the European Organization for Nuclear Research (CERN) with the intention of testing various predictions of high-energy physics, including the existence of the hypothesized Higgs boson and of the large family of new particles predicted by supersymmetry. In physics, theory must be confirmed by experiments and this is just the purpose of this collider. 

    This scientific "toy" cost about 9 billion US dollars and is the most expensive piece of scientific equipment ever build. I hope that the money was well spent and I am pretty optimistic about the whole thing. 

     A wide-spread rumor was that these experiments at very high energies will create micro black holes and lead to apocalyptic phenomena. Of course, safety precautions have been taken in order to avoid such things and it was proven that such results are impossible. 

     The LHC was definitely an engineering challenge. It took all the efforts of the world's brightest minds to build and operate it. An operational challenge was to handle all the massive energies stored in huge magnets and in the beams. While operating, the total energy stored in the magnets is 10 GJ (equivalent to 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ (173 kilograms of TNT). 

    The Large Hadron Collider reached the attention not only from the scientific community but also the attention of science fiction writers, TV producers, film directors and so on. Popularity is a great thing but in some cases some may distribute the wrong message creating concern among the general public. I remember that movie, "Angels & Demons" in which the LHC was held responsible for creating so called "dark matter" which in the wrong hands can lead to the extinction of our world. I think that such messages belong only to the imagination and have no reasonable scientific explanation.

   

Radiation Shielding

    Of course, we can not talk about nuclear reactors without saying a thing or two about radiation shielding. In a reactor, the core especially, is covered lead. Lead is thought to be the most efficient shield against radiation (all types: neutron, alpha, beta, gamma and so on). The thickness of the lead shield is determined by the power of the reactor and also other factors.

    It is important to know that shielding is absolutely necessary mainly because some radiation types can penetrate deep into the human body and some can even alter the DNA. Some radiations can cause grave illnesses or even severe physical damages.

    The amount of damage radiation can cause is expressed in Siverds or Bequerrels (another unit of measurement is the Curie, but it is not Internationally recognized). The human body is naturally exposed to radiation absorbing about 250 mS (mili Sivert) per year (don't panic this is a normal thing). This amount of radiation comes from cosmic radiation, the Earth's radiation, the construction materials that your house is built from and so on. This is perfectly OK. The thing to worry about is when your body exceeds this amount. At that point (depending on the type and intensity of the radiation) illnesses and other disabilities will start to appear. Keep in mind that we do not react the same to the same quantity and intensity. It may take up to several years for effects to appear but maybe for others it may take up to a couple of months or even weeks.

Nuclear Reactor Vessel

 

    You might wonder where all the energy is produced in a nuclear reactor and how massive amounts of energy are controlled and kept away from bursting open the entire facility.

    The reactor vessel is a pressure vessel (a closed container designed to hold gases and liquids at a higher pressure than the ambient pressure) in which the coolant and the nuclear core are held. It is a container designed in such a way that it allows full control over the most important reactions that take place in a nuclear power plant. In order to produce optimum quantities of energy, the core, coolant and other components must be kept at a certain temperature and pressure. The vessel is build in order to make things easier and safer.

    The types of nuclear power plants are not classified by type of vessel but by type of nuclear coolant. For more information visit http://aboutnuclearphysics.blogspot.com/2010/07/types-of-nuclear-reactors.html

    Of the main classes of reactor with a pressure vessel, the PWR is unique in that the pressure vessel suffers significant neutron irradiation (called fluence) during operation, and may become brittle over time as a result. In particular, the larger pressure vessel of the BWR is better shielded from the neutron flux, so although more expensive to manufacture in the first place because of this extra size, it has an advantage in not needing annealing to extend its life.

    Note: fluence is the flux integrated over time. For particles, it is defined as the total number of particles that intersect a unit area in a given amount of time. It is considered one of the fundamental units in dosimetry.

Nuclear Reactor Coolant

   The nuclear reactor coolant is a substance used to remove heat from the nuclear reactor core. As nuclear reactions take place, the core heats up to enormous temperatures and to avoid damage it needs to be cooled down. Most frequently 2 coolant loops are used. This is so because the primary coolant loop takes up a bit of short-term radiation from the core.

    Almost all nuclear reactors are water cooled reactors. They use simple, plain water as coolant under high pressure. This is so mainly because it is cost effective (cheap :P). Other coolants are: heavy water (instead of one hydrogen molecule it contains a deuterium molecule), mercury, sodium, FLiBe, Lead. 

    Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase change to steam inside the reactor. About 2/3 are pressurized water reactors at even higher pressure. Current reactors stay under the critical point at around 374 °C and 218 bar where the distinction between liquid and gas disappears, which limits thermal efficiency, but the proposed supercritical water reactor would operate above this point.

    Fast reactors have a high power density and do not need neutron moderation. Most have been liquid metal cooled reactors using molten sodium. Lead and other metals have also been proposed and occasionally used.

    Gases have also been used as coolant. Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity, necessitating rapid circulation. Carbon dioxide has also been used. Gases of course need to be under pressure for sufficient density at high temperature.

Neutron Moderator

    

 A piece sample of graphite.

     In nuclear engineering, a neutron moderator is a medium that is used in order to reduce neutrons' speed, turning them into thermal neutrons capable of sustaining a nuclear chain reaction. The speed (as well as the energy of the neutrons) needs to be carefully monitored in order to make the processes as safe as possible and to obtain maximum efficiency. 

    

    Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors).

    

    Neutrons are slowed down due to the interactions with the nuclei within the moderator. These are elastic interactions. The neutron's speed can be calculated and predicted by the number of interactions that take place in the moderator. This way, if the initial velocity is known, then you can calculate how many interactions are needed in order to reduce the neutron's speed to the desired one. 

    

    In a thermal nuclear reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits ("fissions") into two smaller atoms ("fission products"). The fission process for 235U nuclei yields two fission products: two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. The free neutrons are emitted with a kinetic energy of ~2 MeV each. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become self sustaining — a chain reaction — under controlled conditions, thus liberating a tremendous amount of energy. 

    

    The probability of further fission events is determined by the fission cross section, which is dependent upon the speed (energy) of the incident neutrons. For thermal reactors, high-energy neutrons in the MeV-range are much less likely to cause further fission. (Note: It is not impossible for fast neutrons to cause fission, just much less likely.) The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down or "moderated", typically to speeds of a few kilometers per second, if they are to be likely to cause further fission in neighbouring 235U nuclei and hence continue the chain reaction. This speed happens to be equivalent to temperatures in the few hundred Celsius range. 

    

    In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; for example, in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a pressurized water reactor (PWR) a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled supercritical water reactor (SCWR), the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor.

    

    Good moderators are also free of neutron-absorbing impurities such as boron. In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The German World War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work. At that time, most graphites were deposited on boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. In the U.S., Leo Szilard, a former chemical engineer, discovered the problem.

Nuclear Fuel

    Nuclear fuel is a material that can be consumed in order to produce nuclear energy, whereas fossil fuel is burned for energy. Nuclear fuel in a nuclear fuel cycle can refer to the fuel itself or to physical objects such as fuel rods which contain the nuclear material and other materials designed for neutron moderating or neutron reflecting. The nuclear fuel cycle, also known as the nuclear fuel chain, is a series of steps describing the "life" of the nuclear fuel. It is basically composed of the front end, during which the nuclear material is processed; steps during the service period in which the nuclear fuel powers reactors in order to produce energy; and steps during the back end in which necessary operations are required in order to manage, contain and either reprocess or dispose of spent nuclear fuel. There has been much debate weather which of the steps of the back end is more economic and most of all, environmentally friendly. An open fuel cycle is when the spent nuclear fuel is not reprocessed and a closed fuel cycle is when it is reprocessed. Below is a diagram that shows the cycle more briefly. 

    PWR Fuel

    Pressurized water reactor (PWR) fuel consists of cylindrical rods put into bundles. A uranium oxide ceramic is formed into pellets and inserted into Zircaloy tubes that are bundled together. The Zircaloy tubes are about 1 cm in diameter, and the fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. Generally, the fuel bundles consist of fuel rods bundled 14x14 to 17x17. PWR fuel bundles are about 4 meters in length. In PWR fuel bundles, control rods are inserted through the top directly into the fuel bundle. The fuel bundles usually are enriched several percent in 235U. The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement. The Zircaloy tubes are pressurized with helium to try to minimize pellet-cladding interaction which can lead to fuel rod failure over long periods.

    BWR Fuel

    In boiling water reactors (BWR), the fuel is similar to PWR fuel except that the bundles are "canned"; that is, there is a thin tube surrounding each bundle. This is primarily done to prevent local density variations from affecting neutronics and thermal hydraulics of the reactor core. In modern BWR fuel bundles, there are either 91, 92, or 96 fuel rods per assembly depending on the manufacturer. A range between 368 assemblies for the smallest and 800 assemblies for the largest U.S. BWR forms the reactor core. Each BWR fuel rod is back filled with helium to a pressure of about three atmospheres (300 kPa).

    CANDU Fuel

    CANDU fuel bundles are about a half meter in length and 10 cm in diameter. They consist of sintered (UO2) pellets in zirconium alloy tubes, welded to zirconium alloy end plates. Each bundle is roughly 20 kg, and a typical core loading is on the order of 4500-6500 bundles, depending on the design. Modern types typically have 37 identical fuel pins radially arranged about the long axis of the bundle, but in the past several different configurations and numbers of pins have been used. The CANFLEX bundle has 43 fuel elements, with two element sizes. It is also about 10 cm (4 inches) in diameter, 0.5 m (20 in) long and weighs about 20 kg (44 lb) and replaces the 37-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters. Current CANDU designs do not need enriched uranium to achieve criticality (due to their more efficient heavy water moderator), however, some newer concepts call for low enrichment to help reduce the size of the reactors.

Types of Nuclear Reactors

   

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    There are several types of nuclear reactors, below I will describe some of them that are used today.

    Pressurized Water Reactors (PWR) these types of reactors use a pressurized vessel to contain the nuclear fuel, control rods, moderator, and coolant. The coolant and moderator  used is high pressurized water. The hot radioactive water that leaves the pressure vessel is looped through a steam generator, which in turn heats a secondary (non-radioactive) loop of water to steam that can run turbines. The majority of nuclear reactor are of this type, and are considered the safest and most reliable technology in current deployment. 

    Boiling Water Reactors (BWR) this type of reactor is similar to the PWR but without the steam generator. A boiling water reactor is cooled and moderated by water like a PWR, but at a lower pressure, which allows the water to boil inside the pressure vessel producing the steam that runs the turbines. In this case there is no primary and secondary loop. These reactors can have a higher thermal efficiency and can be more safe and stable than the PWR. 

    Pressurized Heavy Water Reactor (PHWR) these reactors ar of Canadian design (also know as CANDU) and are cooled by heavy water and moderated by pressurized water. The nuclear fuel is contained in hundreds of pressure tubes unlike the PWR where the fuel is contained in one large pressure vessel. The fossil fuel used is natural uranium (no need for enriched uranium) and the nuclear reactions are thermal-neutron acceptable. CANDUs have been built in Canada, Argentina, India, Pakistan, China, Romania and South Korea. 

    Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) (RBMK) this type of reactor is build to produce plutonium as well as power. They are water cooled and use graphite as a moderator. These reactors are in some way similar to the CANU by the fact that both can be refueled during power operation and use a pressure tube design rather than a pressurized vessel. However, unlike CANDU they are very unstable and large, making containment buildings for them expensive. Despite modern safety improvements, these types of reactors are considered the most dangerous reactors. The RBMKs have only been built in the former Soviet Union. 

    Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGCR) these reactors use graphite as a moderator and are cooled with carbon dioxide gas (CO2). They can have a high thermal efficiency compared with PWRs due to higher operating temperatures. There are a number of operating reactors of this design, mostly in the United Kingdom, where the concept was developed. 

    Liquid Metal Fast Breeder Reactor (LMFBR) this type of reactor is liquid metal cooled and the great thing about it is that it produces more fuel than it consumes. It is said that they "breed" fuel, mainly because of neutron capture. Their function is somehow similar to the PWR, but these reactors do not require high pressure containment because the liquid metal does not need to be kept at high pressure even at high temperatures. These reactors are fast neutron, not thermal neutron designs. There are 2 types of LMFBR:

          - Lead cooled: using liquid lead is a very good choice because of the great radiation shielding and operations can take place at very high temperatures. Also, lead is mostly transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive. Lead is mostly inert, so there is less risk of explosions or accidents but such large quantities of lead can present a bit of a health hazard and can be problematic when disposal operations are due. 

          - Sodium cooled: Most LMFBRs are of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually prevent corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken. 

    Pebble Bed Reactors (PBR) These use fuel molded into ceramic balls, and then circulate gas through the balls. The result is an efficient, low-maintenance, very safe reactor with inexpensive, standardized fuel. 

    Molten Salt Reactors. These dissolve the fuels in fluoride salts, or use fluoride salts for coolant. These have many safety features, high efficiency and a high power density suitable for vehicles. Notably, they have no high pressures or flammable components in the core. 

Nuclear Reactor

    

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    A nuclear reactor is a facility in which nuclear chained reactions are initiated and controlled. There are various types of nuclear reactors, but the most common uses are for producing electric power and for ship propulsion. The concept is fairly simple; the heat from the nuclear reaction is used to heat up water, then the steam spins some turbines in order to generate electricity.

    The idea is that when a large fissile atomic nucleus, such as uranium 235 or plutonium 239 absorbs a neutron, the nucleus splits into two lighter nuclei, along with the emission of other neutrons and gamma radiation. These new neutrons are later absorbed by other large nuclei and the process is happening all over again. The problem is that some neutrons are too fast, and might burst out of the reactor or cause damage. These neutrons are dealt with filters called moderators which reduces the velocity of the neutrons that pass through. In today's nuclear reactors, as moderators, it is used: water (75% of the world's nuclear reactors), solid graphite (20% of the world's reactors) and heavy water (5% of the world's reactors). Of course, other materials have been proposed but these are only in experimental stages. 

    The core of the reactor (where the nuclear reactions take place) emits an enormous quantity of thermal energy (heat) and therefore needs to be cooled. What do they use in order to cool the reactor ? The answer might surprise you - WATER. It's as simple as that. Sometimes, in certain conditions and in certain types of reactors, they might use a gas or a liquid metal or molten salt. Whatever the coolant may be, it is circulated past the core in order to absorb the heat that is produced. The heat is carried away from the reactor and is then used to generate steam. 

    The key components common to most nuclear reactors are:

• Nuclear fuel
• Nuclear reactor core
• Neutron moderator
• Coolant
• Control rods
• Reactor vessel
• Boiler feedwater pump
• Steam generators
• Steam turbine
• Electrical generator
• Spent fuel pool
• Nuclear safety systems
• Containment building
• Control room 

Fission bomb

    

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    A fission bomb, known also as the atomic bomb or the atom bomb, is a fission reactor designed to release as much energy as possible in a very short time before the reactor is destroyed by the huge amount of energy. If the reactor is destroyed too soon then the chain reaction is interrupted and the nuclear explosion will not take place. Development of nuclear weapons was the motivation behind early research into nuclear fission: the Manhattan Project of the U.S. military during World War II carried out most of the early scientific work on fission chain reactions, culminating in the Trinity test bomb and the Little Boy and Fat Man bombs that were exploded over the cities Hiroshima, and Nagasaki, Japan in August 1945. 

    Even the first fission bombs were thousands of times more explosive than a comparable mass of chemical explosive. For example, Little Boy weighted about 4 tons out of which 60 kg was the nuclear fuel, and was 3.4 m long and had an explosion equivalent to 15 kilotons of TNT. As technology evolved, nuclear bombs (or warheads) became more powerful and smaller. For example a modern nuclear warhead weighting less than 1/8th of Little Boy can have the energy of 475 kilotons of TNT. 

    The physics behind fission theory is the same in nuclear reactors and in nuclear bombs. The difference is in how they are constructed. It is impossible for a nuclear reactor to be converted into causing a nuclear explosion as a nuclear bomb would. 

    The strategic importance of nuclear weapons is a major reason why the technology of nuclear fission is politically sensitive. Although, the engineering process is not very complicated, the problem occurs when trying to obtain nuclear fissionable fuel. This process is not available to all countries, only to the most advanced and capable (politically and financially) of producing such types of fuel.  

    Enriched uranium represents the nuclear fuel. The problem is that in natural uranium contains only 0.7 % U235 of it's entire weight. During the Manhattan Project enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the location of the plants where the uranium was enriched. The term oralloy is still occasionally used to refer to enriched uranium. There are about 2,000 tonnes of highly enriched uranium in the world, produced mostly for nuclear weapons, naval propulsion, and smaller quantities for research reactors. All enrichment methods are expensive, elaborate and time consuming and in order to conduct such an operation higher understanding of the phenomena taking place is required. 

Fission energetics

    

    

    Typical fission events release about two hundred million eV (200 MeV) of energy for each fission event. By contrast, most chemical oxidation reactions (such as burning coal or TNT) release at most a few eV per event, so nuclear fuel contains at least ten million times more usable energy per unit mass than does chemical fuel. The energy of nuclear fission is released as kinetic energy of the fission products and fragments, and as electromagnetic radiation in the form of gamma rays; in a nuclear reactor, the energy is converted to heat as the particles and gamma rays collide with the atoms that make up the reactor and its working fluid, usually water or occasionally heavy water. Therefore we assume that it would be wiser to utilize nuclear fuel instead of conventional fuel in order to produce energy, mainly because our reserves of conventional fuel are running out quite rapidly. 

    When a uranium nucleus fissions into two daughter nuclei fragments, an energy of ~200 MeV is released. For uranium-235 (total mean fission energy 202.5 MeV), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion. Also, an average of 2.5 neutrons are emitted with a kinetic energy of ~2 MeV each (total of 4.8 MeV). The fission reaction also releases ~7 MeV in prompt gamma ray photons. The latter figure means that a nuclear explosion or criticality accident emits about 3.5% of its energy as gamma rays, less than 2.5% of its energy as fast neutrons, and the rest as kinetic energy of fission fragments ("heat"). In an atomic bomb, this heat may serve to raise the temperature of the bomb core to 100 million kelvin and cause secondary emission of soft X-rays, which convert some of this energy to ionizing radiation. However, in nuclear reactors, the fission fragment kinetic energy remains as low-temperature heat which causes little or no ionization.

    The total prompt fission energy amounts to about 181 MeV, or ~89% of the total energy. The remaining ~11% is released in beta decays which have various half-lives, but begin as a process in the fission products immediately; and in delayed gamma emissions associated with these beta decays. For example, in uranium-235 this delayed energy is divided into about 6.5 MeV in betas, 8.8 MeV in antineutrinos (released at the same time as the betas), and finally, an additional 6.3 MeV in delayed gamma emission from the excited beta-decay products (for a mean total of ~10 gamma ray emissions per fission, in all).

    As concerning the environment; if all nuclear waste is handled carefully, there should be nu threat to the ecosystem. During the process of converting the massive amount of nuclear energy into electric energy, only steam comes out the power plant's chimneys.  

Fission mechanics

    Nuclear fission can occur without neutron bombardment, as a type of radioactive decay. This type of fission (called spontaneous fission) is rare except in a few heavy isotopes. In engineered nuclear devices, essentially all nuclear fission occurs as a "nuclear reaction" — a bombardment-driven process that results from the collision of two subatomic particles. In nuclear reactions, a subatomic particle collides with an atomic nucleus and causes changes to it. Nuclear reactions are thus driven by the mechanics of bombardment, not by the relatively constant exponential decay and half-life characteristic of spontaneous radioactive processes.

    A great amount of nuclear reactions are known. Nuclear fission differs importantly from other types of nuclear reactions in that it can be amplified and sometimes controlled via a nuclear chain reaction. In such a reaction, free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions.

    The chemical element isotopes that can sustain a fission chain reaction are called nuclear fuels, and are said to be fissile. The most common nuclear fuels are 235U (the isotope of uranium with an atomic mass of 235 and of use in nuclear reactors) and 239Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 u (fission products). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha/beta decay chain over periods of millennia to eons. In a nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with another particle, a neutron, which is itself produced by prior fission events.

Nuclear Fission

    In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and protons (in the form of gamma rays), as well. Fission of heavy elements is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments. For fission to produce energy, the total binding energy of the resulting elements has to be higher than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. 

    Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are made possible because certain substances called nuclear fuels undergo fission when struck by free neutrons and in turn generate neutrons when they break apart. This makes possible a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.

    The nuclear chain reactions can be classified into 2 categories:

• Controlled chain reaction: this type of reaction is present in nuclear power facilities. By all sorts of methods, the number of neutrons that bombard the target nucleus is controlled and kept under high observation. 
• Uncontrolled chain reaction: this type of reaction is present in nuclear bombs, where the number of neutrons is not controlled, therefore resulting an enormous quantity of energy released in a very small space in a very short time.

    The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very tempting source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem.  

K-capture

    The K-capture is a form of decay in which the closest electron in the electron cloud is captured by the nucleus. It will occur when there are too many protons in the nucleus of an atom and insufficient energy to emit a positron; however, it continues to be a viable decay mode for radioactive isotopes that can decay by positron emission. It is sometimes called inverse beta decay, though this term can also refer to the capture of a neutrino through a similar process. If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden and electron capture is the sole decay mode. In this case, one of the orbital electrons, usually from the K or L electron shell (K-electron capture, also K-capture, or L-electron capture, L-capture), is captured by a proton in the nucleus, forming a neutron and a neutrino. 

p     +     e−     →     n     +     νe

    Note that a free proton cannot normally be changed to a free neutron by this process. The proton and neutron must be part of a larger nucleus. Since the proton is changed to a neutron, the number of neutrons increases by 1, the number of protons decreases by 1, and the atomic mass number remains unchanged. By changing the number of protons, electron capture transforms the nuclide into a new element. The atom moves into an excited state with the inner shell missing an electron. 

    The theory of electron capture was first discussed by Gian-Carlo Wick in a 1934 paper, and then developed by Hideki Yukawa and others. K-electron capture was first observed by Luis Alvarez, in vanadium-48. He reported it in a 1937 paper in the Physical Review. Alvarez went on to study electron capture in gallium-67 and other nuclides. 

Nuclear force

    Two or more nucleons are held together by what is called nuclear forces. These strong forces are the forces that maintain stable nuclei stable. 

    The nuclear force is now understood as a residual effect of an even more powerful strong force, or strong interaction, which is the attractive force that binds particles called quarks together, to form the nucleons themselves. This more powerful force is mediated by particles called gluons. Gluons hold quarks together with a force like that of electric charge, but of far greater power. 

    The concept of a nuclear force was first quantitatively constructed in 1934, shortly after the discovery of the neutron revealed that atomic nuclei were made of protons and neutrons, held together by an attractive force. The nuclear force at that time was conceived to be transmitted by particles called mesons, which were predicted in theory before being discovered in 1947. In the 1970’s, further understanding revealed these mesons to be combinations of quarks and gluons, transmitted between nucleons that themselves were made of quarks and gluons. This new model allowed the strong forces that held nucleons together, to be felt in neighboring nucleons, as residual strong forces. 

    The nuclear forces arising between nucleons are now seen to be analogous to the forces in chemistry between neutral atoms called van der Waals forces. Such forces between atoms are much weaker than the electrical forces that hold the atoms themselves together, and their range is shorter, because they arise from spontaneous separation of charges inside the atom. Similarly, even though nucleons are made of quarks and gluons that are in combinations which cancel most gluon forces, some combinations of quarks and gluons nevertheless leak away from nucleons, in the form of short-range nuclear force fields that extend from one nucleon to another close by. These nuclear forces are very weak compared to direct gluon forces inside nucleons, and they extend only over a few nuclear diameters, falling exponentially with distance. Nevertheless, they are strong enough to bind neutrons and protons over short distances, and overcome the electrical repulsion between protons in the nucleus.

Liquid drop model of the nucleus

    In nuclear physics, the nucleus is sometimes compared to a liquid drop of incompressible nuclear fluid. This was first proposed by George Gamow and developed by Niels Bohr and John Archibald Wheeler. The fluid is made of nucleons (protons and neutrons), which are held together by the strong nuclear force. This is a crude model that does not explain all the properties of the nucleus, but does explain the spherical shape of most nuclei. It also helps to predict the binding energy of the nucleus. 

    Mathematical analysis of the theory delivers an equation which attempts to predict the binding energy of a nucleus in terms of the numbers of protons and neutrons it contains. This equation has five terms on its right hand side. These correspond to the cohesive binding of all the nucleons by the strong nuclear force, the electrostatic mutual repulsion of the protons, a surface energy term, an asymmetry term and a pairing term. 

    The volume energy. When an assembly of nucleons of the same size is packed together into the smallest volume, each interior nucleon has a certain number of other nucleons in contact with it. So, this nuclear energy is proportional to the volume. 

    The surface energy. A nucleon at the surface of a nucleus interacts with fewer other nucleons than one in the interior of the nucleus and hence its binding energy is less. This surface energy term takes that into account and is therefore negative and is proportional to the surface area. 

    The Coulomb energy. The electric repulsion between each pair of protons in a nucleus contributes toward decreasing its binding energy. 

    The asymmetry energy (Pauli energy). If it wasn't for the Coulomb energy, the most stable form of nuclear matter would have N=Z, since unequal values of N and Z imply filling higher energy levels for one type of particle, while leaving lower energy levels vacant for the other type. 

    The pairing energy. An energy which is a correction term that arises from the tendency of proton pairs and neutron pairs to occur. An even number of particles is more stable than an odd number. 

    If we define A as the number of nucleons, Z the number of protons and N the number of neutrons, the mass of an atomic nucleus is given by:

    where mp and mn are the rest mass of a proton and a neutron, respectively, and EB is the binding energy of the nucleus, and c is the speed of light. 

    This formula states that the binding energy will take the following form:

    Every energy term can be calculated and introduced into the formula along with the numbers A and Z in order to calculate the binding energy. 

Gamma Rays

 

   

   

    Gamma radiation is an electromagnetic radiation of a very high frequency and a very short wavelength. It is symbolized by γ. These types of rays are produced by sub-atomic interactions. These radiations have a wavelength of about 10 picometers, sometimes smaller than an atom.

    These rays are a form of ionizing radiation and therefore are a health hazard because they destroy living tissue. 

    The measure of gamma rays' ionizing ability is called the exposure:

•     The coulomb per kilogram (C/kg) is the SI unit of ionizing radiation exposure, and is the amount of radiation required to create 1 coulomb of charge of each polarity in 1 kilogram of matter.
•     The gray (Gy), which has units of (J/kg), is the SI unit of absorbed dose, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.
•     The sievert (Sv) is the SI unit of equivalent dose, which for gamma rays is numerically equal to the gray (Gy).
•     The rem is the traditional unit of equivalent dose. For gamma rays it is equal to the rad or 0.01 J of energy deposited per kg. 1 Sv = 100 rem.

    Shielding from gamma rays requires large amounts of mass. They are better absorbed by materials with high atomic numbers and high density. The higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half. 

    All ionizing radiation causes similar damage at a cellular level, but because rays of alpha particles and beta particles are relatively non-penetrating, external exposure to them causes only localized damage, e.g. radiation burns to the skin. Gamma rays and neutrons are more penetrating, causing diffuse damage throughout the body (e.g. radiation sickness, increased incidence of cancer) rather than burns. External radiation exposure should also be distinguished from internal exposure, due to ingested or inhaled radioactive substances, which, depending on the substance's chemical nature, can produce both diffuse and localized internal damage.

    After gamma-irradiation, and the breaking of DNA double-strands, a cell can repair the damaged genetic material to the limit of its capability. However, a study of Rothkamm and Lobrich has shown that the repairing process works well after high-dose exposure but is much slower in the case of a low-dose exposure.  

Beta Decay

   

    In nuclear physics, beta decay represents the emission of a beta particle (an electron or a positron) from the nucleus. In the case of electron emission, it is referred to as beta minus (β−), while in the case of a positron emission as beta plus (β+). The emitted beta particles have a continuous kinetic energy spectrum, with energies starting from 0 to a maximum energy (Q) limited by the parent and daughter nuclear states. The most energetic beta particles are ultrarelativistic, with speeds close to the speed of light. 

•     β− decay

    In β− decay, the weak interaction converts a neutron (n) into a proton (p) while emitting an electron (e−) and an electron antineutrino (νe):

n     →     p     +     e−     +     νe

    This type of beta decay usually occurs in neutron rich nuclei. 

•     β+ decay

    In β+ decay, energy is used to convert a proton into a neutron, a positron (e+) and a neutrino (νe):

energy     +     p     →     n     +     e+     +     νe

    So, unlike β−, β+ decay cannot occur in isolation, because it requires energy, the mass of the neutron being greater than the mass of the proton. β+ decay can only happen inside nuclei when the value of the binding energy of the mother nucleus is less than that of the daughter nucleus. The difference between these energies goes into the reaction of converting a proton into a neutron, a positron and a neutrino and into the kinetic energy of these particles. 

•     Electron capture (K-capture)

    Another type of decay is the K-capture. It consists of an electron being captured from the electron cloud by the nucleus and the emission of a neutrino (anytime the β+ decay can occur energetically, it is accompanied by a K-capture). The decay can be described as:

energy     +     p     +     e−     →     n     +     ν

    This decay is called the K-capture because the closest electron is located in the K shell (the electron cloud is divided into shells, each one more further than the previous one from the nucleus; they are named K, L, M, N, P and so on) and it has the grater probability of being captured by the nucleus. 

Alpha Decay

   

    The alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (an alpha particle is actually an helium nucleus - see Natural Radioactive Decay). The initial nucleus transforms into a nucleus with a mass number with 4 less particles and an atomic number with 2 less particles. An alpha decay can be described by the following nuclear equation (this is an example):

238U     → 234Th     +     α    

     Alpha decay, like other cluster decays, is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the interplay between the nuclear force and the electromagnetic force. 

    Alpha decay typically occurs in the heaviest nuclides. In theory it can occur only in nuclei somewhat heavier than nickel (element 28), where overall binding energy per nucleon is no longer a minimum, and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitter being the lightest isotopes of tellurium (element 52).  

    Alpha particles have a typical kinetic energy of 5 MeV  and a speed of 15,000 km/s. This speed corresponds to about 0.05 of c ("c" being the speed of light, about 300.000 km/s).

    Most of the helium produced on Earth results from alpha decays of underground deposits of uranium and thorium. Helium is brought to the surface as a byproduct of natural gas.   

Natural Radioactive Decay

   

    Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting ionizing particles or radiation. The emission is spontaneous in that the nucleus decays without collision with another particle. This decay, or loss of energy, results in an atom of one type, called the parent nuclide, transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). However given a large number of similar atoms the decay rate, on average, is predictable.

    The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. A Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 10.9 decays per second) or TBq (terabecquerel, 1 x 10.12 decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 10.10 Bq. The use of Ci is presently discouraged by the SI.

    Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by X-rays might be connected with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent salts on it. All results were negative until he used uranium salts. The result with these compounds was a deep blackening of the plate. These radiations were called Becquerel Rays.

    At first it seemed that the new radiation was similar to the then recently discovered X-rays. Further research by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur, but Rutherford was the first to realize that they all occur with the same mathematical approximately exponential formula.

    Types of decay:

•     Alpha decay: it's seen only in heavier elements, atomic number 52, tellurium, and greater. The alpha rays carry a positive charge. From the magnitude of deflection, it was clear that alpha particles are massive. Passing alpha particles through a very thin glass window and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately prove that alpha particles are helium nuclei.
•     Beta decay: it represents the emission of one or more electrons from the nucleus. The beta rays carry a negative charge. Beta particles are much more lighter and smaller than alpha particles (these being helium nuclei). Due to their small size, the beta particles carry much more kinetic energy than the alpha particles, therefore these particles can penetrate deeper into the target material than alpha particles.
•     Gamma decay: this type of decay is neutral from an electric point of view. It usually assists beta and alpha decays. Gamma rays and X-rays are both high energy electromagnetic radiation. Gamma decay (as well as beta decay) has been observed in all types of elements, not only the heavier ones.  

Atomic Nucleus

    The atomic nucleus was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. 

    The diameter of the nucleus is in the range of 1.6 fm (1.6×10−15 m) for hydrogen to about 15 fm for the heaviest atoms, such as uranium. These dimensions are much smaller than the diameter of the atom itself. 

    The branch of physics concerned with studying and understanding the atomic nucleus, including its composition and the forces which bind it together, is called nuclear physics.

    The nuclear force is highly attractive at very small distances, and this overwhelms the repulsion between protons which is due to the electromagnetic force, thus allowing nuclei to exist. However, because the residual strong force has a limited range because it decays quickly with distance, only nuclei smaller than a certain size can be completely stable. The largest known completely stable nucleus is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons). 

    The nuclear radius (R) is considered to be one of the basic things that any model must explain. For stable nuclei the nuclear radius is roughly proportional to the cube root of the mass number (A) of the nucleus, and particularly in nuclei containing many nucleons, as they arrange in more spherical configurations. 

    In other words, packing protons and neutrons in the nucleus gives approximately the same total size result as packing hard spheres of a constant size (like marbles) into a tight spherical or semi-spherical bag. 

Short Introduction in Nuclear Physics

PHYSICS is a natural science that studies matter and the motion of matter in space-time. Since the beginning of mankind, we have tried to explain everything we see, hear, feel, making supernatural efforts to understand phenomena "unrolling under our noses". Mankind searched for answers to questions like: why are we here? or how it all began? or is there extraterrestrial life? and many more. These questions can be seen from a philosophical or a scientific point of view.

We now know that our Universe is made of tiny particles (when I say "tiny" I mean really really really small) held together by all sort of forces to form all the objects we recognize with our 5 senses. These particles were named ATOMS.

If objects are made out of atoms, what are atoms made of?

An atom is made of the NUCLEUS and the ELECTRONIC CLOUD.

If the atom is made of the nucleus and the electronic cloud, then what are these made of?

The electrons (as far as we know) have no inner structure. The nucleus on the other hand is made of PROTONS and NEUTRONS. Protons are pozitive-charged particles (whereas electrons are negative-charged particles); and neutrons (like the name suggests) are neutral from an electrical charge point of view. Almost the entire mass of an atom is concentrated in the nucleus, which is a lot more heavier that the electronic cloud. The atom is electrically stable, being composed of a pozitive nucleus and a negative electron cloud.

If we use the letter "Z" to express the number of electrons and "A" the number of particles in the nucleus, then "N" being equal to A-Z will be the number of neutrons. Keep in mind that the number of protons is equal to the number of electrons.

Imagine an atom similar to our solar system. The Sun being in the middle (it represents the pozitive nucleus) and the planets orbiting around it (they represent the negative electrons). This is the so-called Planetary Model of the Atom (I will cover this subject later).