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sâmbătă, 25 septembrie 2010

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!   


miercuri, 8 septembrie 2010



    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.


marți, 17 august 2010

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.

luni, 9 august 2010

Nuclear Reactor Vessel

Photo of a nuclear power plant 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

    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.

luni, 26 iulie 2010

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.

duminică, 25 iulie 2010

Neutron Moderator

A piece sample of graphite used as a 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. 
Brief nuclear fuel cycle.

    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.