lunes, 18 de enero de 2021

nuclear power plant - Core questions in focus

 When the spirits and reactor pressure vessels boil over, it is good to know what to bring up (or ignore) in order for your own following to reach a "critical mass".

Which physical process takes place in a nuclear power plant?

One kilogram of uranium contains two million times more energy than one kilogram of coal. In nuclear power plants, just like in coal-fired power plants, the energy is released as heat. The huge difference in energy density is due to the fact that the chemical energy of coal is used, but the nuclear (Greek nucleus = core) energy of uranium. This energy is released during nuclear fission (also nuclear fission, from the Latin fissio = fission): if the nucleus of a uranium atom is hit by a slow neutron, it absorbs the neutron and, shortly afterwards, decays into two parts (fission products) and two new ones, giving off strong energy Neutrons. These two neutrons can in turn split one uranium atom each, so that four neutrons are produced in the next generation. This is the notorious chain reaction that in atomic bombs releases the entire energy of the uranium in fractions of a second without any external intervention - if the critical mass, i.e. the minimum amount of uranium required for an unchecked chain reaction, is available (for uranium-235 it is 49 kg).

The neutrons responsible for the chain reaction are (as the name suggests) electrically neutral elementary particles. This is where the physicist's joke comes from: “A neutron walks into a bar, orders a beer and asks“ How much? ” and the bartender says, “For you, no charge [charge / costs]“ ». Like the proton, it is one of the hadrons (old Greek hadros = thick), weighs around 1800 times as much as an electron and, as radiation, is extremely dangerous for people (but easy to shield).

This fission process is regulated in two ways in nuclear power plants:

  • Absorber rods can be inserted into the reactor core and, depending on the depth of the penetration, absorb a desired proportion of all neutrons. They consist of special metals such as cadmium or hafnium.
  • Moderators are materials (e.g. water or graphite) that slow down neutrons and fill the reactor core. For nuclear fission to take place at all, the neutrons must be slow (thermal). If there is no moderator to slow down the chain reaction, the chain reaction stops because the neutrons are too fast. The slow neutrons are called thermal (Gr. Thermos = warm) because they are in thermal equilibrium with their surroundings and are slowed down to 2200 m / s.

What are the fuel assemblies?

A core catcher made of reinforced concrete is installed under the reactor. In newer nuclear power plants that have such a core catcher, the meltdown falls into the container (if all other safeguards fail). This prevents radiation from being emitted into the environment. The nuclear power plant in Fukushima does not have a core catcher. Since the Chernobyl accident (1986), all reactors have had at least one containment that encloses the actual reactor pressure vessel. However, the security container alone is not sufficient as a protective measure, as has been confirmed by developments in Japan (in Fukushima the container was partially destroyed).

The nuclear power plant does not allow a meltdown because it has no fuel rods. This is the case with high temperature reactors (HTRs). They are the safest of all reactors. They will be discussed here later.

Why does uranium need to be enriched?

Theoretically, almost all heavy atomic nuclei can be split by bombarding them with neutrons. However, this is only possible with uranium-233, uranium-235 and plutonium-239. Other atoms are out of the question because a chain reaction cannot be sustained with them. The following number (mass number) indicates the sum of protons and neutrons in a nucleus. Types of atoms that differ only in the number of their neutrons are called isotopes.

Uranium-233 and uranium-235 are two isotopes of uranium (92 protons). The natural occurrences of uranium contain 0.7% uranium-235 (easily fissile) and 99.3% uranium-238 (difficult to fissile). However, a proportion of 3-5% uranium-235 is required in the fuel rods. For this reason, gas centrifuges in enrichment plants increase the proportion of uranium-235 before the uranium can be used. The currently known uranium deposits will last for about 50 to 100 years. However, by so-called incubation, (useless) uranium-238, which is available in large quantities, can be used to produce (fissile) plutonium-239. That being said, so far, in the absence of a valid reason, no one has been interested in prospecting for new uranium deposits.

What types of nuclear power plants are there?

The mere revelation that there is more than one way to generate electricity using nuclear energy is surprising. Much more important than that, however, is that nuclear power plants differ dramatically in terms of safety. The four most important types, in order of increasing security, should be easy to understand with the help of my colorful pictures:

The fast breeder is not currently a type of power plant that should be built: it is extremely unsafe. Its purpose is to breed new fuel on the side. Plutonium-239 is hatched from the actually useless uranium-238 during operation. This in turn can be used very well as fissile material. Unfortunately also in bombs.

In order for uranium-238 to transform into plutonium, it has to be hit very centrally by a neutron. This also happens in other types of nuclear power plants. However, there the "breeding rates" (= new plutonium atoms per normal fission) are very low, because every neutron that creates a new atom is missing from the chain reaction. However, if plutonium is split with fast, unrestrained (not moderated) neutrons, not only two but three new neutrons are created. In this way, the fast breeder can breed more new fissile material than it consumes (breeding rate> 1).

That sounds tempting at first, because in this way the usable uranium reserves multiply to such an extent that fissile material is sufficiently available for crazy periods of time. But the problems are fatal. The plutonium has to be split with fast neutrons. And because the probability of a hit decreases, it must also be highly enriched (approx. 30%). As a result, there is an extremely high energy density in the reactor, which means that the absorber rods and the cooling must work highly effectively and reliably. Furthermore, since the neutrons must not be slowed down, no water can be used for cooling.

If the temperature in the reactor rises, the chain reaction continues undisturbed. You can only cool with liquid sodium, which does not absorb neutrons. However, it is an extremely aggressive representative of the chemical guild of alkali metals: in contact with water it explodes and in air it ignites.

In a heat exchanger, the hot sodium transfers its heat to water, which is under high pressure. In a steam generator, the hot water in turn gives off heat to a second water circuit. The water there evaporates and drives a steam turbine, which uses a generator to generate electricity.

Japan was still planning to build a fast breeder in 2009, as it promised an almost inexhaustible source of energy for the country, which is poor in natural resources. Unlike other reactors, due to the high concentration of plutonium, a fast breeder can in principle explode with a force similar to that of a detonating atomic bomb.

The boiling water reactor is a form of the light water reactor (just like the pressurized water reactor). Light, i.e. completely normal water (H2O) absorbs slightly more neutrons than heavy water (D2O, D: deuterium, an isotope of hydrogen), but is of course available en masse. The boiling water reactor is equipped with uranium fuel elements. The energy generated during nuclear fission heats the water that fills the reactor pressure vessel. It still evaporates in the reactor (at approx. 350 ° C). The hot steam is discharged and drives steam turbines, the mechanical rotation of which is converted into electrical power by a generator. The water cooled down in the process is returned to the reactor pressure vessel.

The cooling with water has a monumental advantage: if the cooling fails, the water evaporates and the chain reaction dies. Problems only arise when the cooling is not active for too long (as in Fukushima - a boiling water reactor).

The fission products generate heat by radioactively decaying (e.g. by shooting away an electron). This corresponds to about 7% of the power plant's nominal output. If there is no cooling, the zirconium of the fuel rods melts after a while and the core meltdown threatens. The inside of the fuel rod also melts: the uranium. The melt (called corium) can penetrate reactor walls.

To prevent the radioactive substances in the melt from leaving the reactor, a core catcher made from a concrete-ceramic mixture is the optimal solution. This is where the meltdown is caught and cools down. After a few years, the power plant can then be dismantled. The damage is then purely economic, as no radioactivity gets into the environment. To date, no German nuclear power plant has had a core catcher.

There are five boiling water reactors in Germany (Brunsbüttel, Philippsburg, Isar, Krümmel, Gundremmingen), all others are pressurized water reactors.

The second type of light water reactor is the pressurized water reactor. It increases safety by ensuring that no water from the reactor comes into direct contact with the environment. There is also an internal water circuit that runs through the reactor pressure vessel under high pressure and an external water circuit with lower pressure. A steam generator decouples the two circuits and transfers the heat. The steam in the secondary circuit then operates steam turbines with a generator.

The pressurized water reactor can be designed for slightly higher outputs than the boiling water reactor, but it also has the disadvantage that the high pressure of the internal water circuit places increased demands on the mechanical strength of the lines.

A particularly important and new type of pressurized water reactor is the EPR (European Pressurized Water Reactor). It is equipped with a variety of security technologies, including

A core catcher. In an emergency, it can be flooded with pre-stored water.

Double-walled containment (outer shell) 2.6 m thick.

Quadruple redundant, independent emergency cooling systems that are strictly separated.

Germany does not have or plan an EPR. It is now the preferred type of reactor for new construction in Europe.

The output of the two light water reactors is approx. 1.5 gigawatts (for comparison: typical coal-fired power plant - 1 gigawatt). According to the Federal Statistical Office, Germany consumed around 70 gigawatt years of electrical energy in 2010, i.e. the energy that around 47 continuously running nuclear power plants would generate per year.

The misunderstood genius, no, the unknown genius among nuclear power plants is the pebble bed reactor, also known as the high temperature reactor (HTR) because of its functionality. The great thing about it is that it is safe - even if all of the emergency and safety systems fail, a catastrophe cannot occur. How can this work?

The reactor makes use of a property of uranium: if the temperature of the uranium rises to over 1000 ° C, the uranium begins to absorb neutrons and withdraws them from the chain reaction. The chain reaction dies all by itself. On one condition - the reactor has to cope with the high temperatures.

The fissure material, hence the name, is packed in balls the size of a tennis ball made of graphite, i.e. pure carbon (the moderator). Instead of a lump, however, it is distributed in the spheres in the form of small "coated particles", each of which is again coated with graphite and a high-strength silicon carbide layer. This prevents the escape of fission products and is very heat-resistant.

If all cooling systems fail in such a reactor, the reactor must be able to cope with it. That is why it is designed in such a way that passive heat dissipation is sufficient. Physically, heat can only be transferred in three ways, through:

The bullets are piled up in the reactor, which is encased in prestressed concrete, and bombarded with neutrons as normal. The cooling is not done with water but with helium. This is because the water would also slow down the neutrons. In addition, it would have to be under enormous pressure at operating temperatures of 700 ° C. In addition, under extreme conditions it can lead to hydrogen explosions (as in Fukushima). Helium is a noble gas that hardly reacts chemically (this is called inert = inert) and is non-toxic.

Convection, i.e. through mass transport. This happens in the HTR with the regular helium cooling.

Radiation, i.e. through radiation. Warm bodies radiate in a temperature-dependent manner, humans for example in the infrared range. The HTR conducts the absorbed thermal radiation to the outside through its walls - regardless of whether the helium cooling is intact.

Conduction, i.e. through direct contact. The size of the HTR reactor is chosen so that as many spheres as possible touch the cooling wall. Therefore, HTRs can only achieve outputs in the range of approx. 250 megawatts per reactor.

Education comes from the screen

 And not from the book, otherwise it would mean booking. At least that's what the almost eighty-year-old German cabaret artist Dieter Hildebrandt says. We are constantly surrounded by screens of all kinds. One can certainly argue about the social and societal implications of such ubiquitous computing (“omnipresent computing”), but the technical side is undoubtedly fascinating. The machines communicate with us via extravagantly illuminated screens, of which there are many different types. However, experts unanimously agree that the future belongs to only one type of screen: the LED screen. Since its functionality is extremely simple and since the marketing apparatus of the electronics companies have constructed a completely impenetrable forest of terms (e.g. capacitive Super AMOLED touchscreen), here is a little overview of the subject of displays and screens.

We used to have screens with a weight of several centimeters using cathode ray tubes (CRT). Nobody likes these medieval “3D screens” anymore. With good reason - not only is the hernia preprogrammed at the next LAN party, the screens also flicker, have a high power consumption (that is to say: eat a lot of electricity) and shine like a medium-sized nuclear waste repository. The rough functional principle was simple (see picture, source: Wikipedia): In a vacuum, electrons are emitted from a glowing cathode (positive electrode) (hence cathode ray tube). These electrons are accelerated by an electric field and deflected by a magnetic field so that they hit a certain point on the screen. On this luminescent screen (the back of the screen you are looking at) a material (e.g. phosphor) is applied that generates visible light when the electron impacts, which reaches our eyes (the more intense the electron beam, the brighter). Three of these electron guns are required for the colored representation and accordingly three phosphors to produce the primary colors red, green and blue. The triple electron beam is focused on exactly one point on the screen at all times and races line by line from left to right and then jumps to the next line.

The era of flat screens began with the buzzword TFT. TFT stands for Thin Film Transistor (thin film transistor). The TFT screen is a form of the LCD screen that is controlled by these thin film transistors. LCD, in turn, stands for Liquid Crystal Display. Liquid crystals are special substances that are both liquids and solids (crystals). And they have an interesting property: if a voltage is applied to them, they preferentially only let light through that have a certain direction of oscillation (called polarization). And that is the secret of LCD screens. The LCD screen is a grid of pixels. These in turn each consist of three sub-pixels (red, green, blue). Each subpixel is controlled by the interplay of thin film transistor (regulates the voltage), liquid crystal (only allows a certain amount of light to pass through depending on the voltage) and color filter (only allows the respective subpixel color to pass). The light generated by backlighting first passes through the liquid crystal from behind. This was aligned by the applied voltage so that it lets the desired amount of light through. The transmitted light passes through the color filter and lets the subpixel glow in its color. Since the subpixels are very close together, the subpixels do not appear as individual basic colors of a certain brightness, but add up to one color - the color of the pixel they represent. This is called additive color mixing. Every single pixel can be controlled in this way.

In order to display an image, the control electronics of the screen only have to generate a signal for each subpixel that determines its brightness, e.g. the signal “red subpixel: light, green subpixel: light, blue subpixel: dark” for a yellow pixel. Since an electron beam no longer wanders over a fluorescent screen, but a constant backlight emits light, there is no longer any flickering. At the moment, LCD screens still dominate the market because they are inexpensive to manufacture, achieve high image quality and can also be manufactured in sizes that allow use in mobile devices.

Plasma screens are very similar to LCD screens. In physics, a gas with a very high temperature in which the electrons have separated from their atomic nuclei is called a plasma (ancient Greek structure). This plasma is generated in plasma screens according to the same principle as with fluorescent tubes: there is a gas (e.g. neon) between two electrodes. If a sufficiently high voltage is applied between the two electrodes, a voltage flashover occurs, i.e. a current flows briefly between the two electrodes - across the gas, which becomes plasma-shaped along the current path. This localized gas discharge produces ultraviolet light. The UV light itself is still invisible, but hits a fluorescent substance, which in turn emits light of a basic color (the fluorescent substance Zn2SiO4: Mn2 + e.g. green). Each subpixel is a small fluorescent tube that can be controlled individually. Now, however, the brightness of the subpixels must also be controlled. A trick is used here: instead of creating a continuous glow in one color, the subpixel is “ignited” at short intervals. The shorter the distance between two ignitions, the lighter the color of the subpixel is perceived.

Although plasma screens are very robust against interference fields, high temperatures and mechanical vibrations, they have not yet caught on. This is due to the fact that they are only practicable from screen sizes of approx. 37 inches, weigh a lot and are prone to burning in images that have been displayed for a long time (the fluorescent material loses its luminosity in the permanently exposed areas).

The most promising candidate among the screen types is the LED screen. An LED (Light Emitting Diode) is a semiconductor component very similar to a transistor. So-called OLEDs (organic LEDs) made of organic (carbon-based) semiconductors seem to be particularly promising, as they are extremely thin and can in principle be vapor-deposited onto almost all materials (e.g. also onto walls or paper). Since an OLED can emit light of a certain color, it is used for the sub-pixels of the screen. An LED is a sandwich: there are two semiconductors (a certain type of crystal) between two electrodes. One semiconductor has an excess of electrons (called n-layer) and the other lacks electrons (p-layer). These electron defects (holes) can migrate through the p-semiconductor. If a voltage is applied to the two surrounding electrodes, electrons and holes migrate towards each other. If they meet in the middle, energy is released, which is emitted in the form of photons (i.e. light). The color of the light can be determined by molecular modifications of the semiconductors.

In a sense, OLED screens take the most direct route; without background lighting and color filters, the light is generated directly at the position where it should appear on the screen. The resulting advantages are a very strong contrast (with LCDs a lot of contrast / brightness is lost due to the filter cascade), a low energy requirement (hardly any heat generation) and very small screen thicknesses (starting at 0.1 mm). So far, however, OLEDs have been very sensitive to contact with water and oxygen and must therefore be well shielded. Most cell phone manufacturers are already successfully offering models with OLED displays (e.g. the Nexus S built by Samsung) and Apple will probably also be offering the IPhone 5 in the future (patent applications).

The illustration shows the OLED display of the Nexus One mobile phone (source: Wikipedia). As you can see very nicely, OLEDs are not point-like but flat light sources, since the organic p- and n-semiconductors are flat as well. The arrangement of the subpixels may seem paradoxical at first, after all, there are twice as many green as blue and red subpixels. However, this is a clever move that makes use of empirical knowledge about the physiology of the human eye: [subjectively perceived overall brightness] = 0.6 * [green value] + 0.3 * [red value] + 0.1 * [blue value]. For this reason, the sensors of digital cameras are also arranged in a similar pattern (so-called Bayer matrix).

The seven deadly sins

 As an enlightened citizen of the world, one should know them, the famous seven deadly sins. Not to avoid them, but to swing them as a rhetorical club when necessary. It is particularly impressive, of course, when one can name the Latin technical terms from theology at the same time. Using the example of the seven deadly sins, I will demonstrate how any facts can be ordered and reliably memorized.

  1. The technique: route method
  2. Time to memorize: a few minutes
  3. Applicability of the technology: list-like information of all kinds

This oil painting (unchanged image) by Antonello da Messina, an Italian Renaissance painter (15th century), is a randomly chosen location for the route. This route consists of seven route points. Each of these route points will serve as a kind of universal drawer for information to be memorized. Let's learn the route first:

Imagine how you walk through the world of the painting along the marked route points. Now go through the route points again, this time without looking at the picture. If that works, we can start: now we will link the route points with information.

The actual memorizing of the facts takes place through arbitrary and strange connections between route point <-> fact. For this purpose, each deadly sin and its technical term are illustrated and mentally linked to a route point:

  1. Luxuria (lust) :  Leaning against the column stands a light lady (symbol of lust) with her leg bent. She wears a luxurious fur coat.Imagine this picture intensely (decorating it deepens the association immensely) and go to the next route point.
  2. Ira (anger): Like a madman, you tear out the peacock's feathers in a wild rage. Imagine the poor animal squirming on the ground. Sex & Crime adhere particularly well (in memory and good reputation).
  3. Gula  (Gluttony) :An overweight man eats goulash from a bowl like an animal.The gluttony becomes clear here all by itself. Don't be afraid of your own imagination!
  4. Avaritia (avaritia) : You see how the blue avatar searches the stairs, crack by crack. The stingy guy has lost a dime there and can't stop looking for it. Indeed, not every word has a perfect combination. Even the apparently most absurd association is a valuable safety net: we usually only miss the first syllable of a word. All the rest then comes to mind with the help of the first syllable (i.e. your picture - avatar).
  5. Invidia (envy) : A brand new Nvidia graphics card lies between the books. You look at the strange gem full of envy and instead take a frustrated book.Emotions play an important role in memory. This is because emotions are processed in the limbic system, a part of the brain in which the "control center" of conscious memory is located (the hippocampus). Therefore, evoking strong emotions helps learning / memorizing. However, physical pain wears off quickly. It is therefore not advisable to keep turning your nipple yourself while learning vocabulary.
  6. Superbia  (arrogance) : Annoyed by the haughty expression on the monk's face, they pour him super beer (particularly strong beer) on his robe. However, he only lifts an eyebrow in pity and haughtily shakes his head. Inventing things that don't really exist (super beer) can serve as a very strong link, provided the invention is visual and appealing.
  7. Acedia (laziness) : A wrecked soccer player lies on the bench. He is wearing an AC Milan jersey and is holding an old slide from his active days: the AC slide. But now he's just old and unspeakably lazy.
It is best to scroll up again to the route picture and go over all route points with the learned route points. If a link is lost, decorate the respective picture or build another, your own, instead. In fact, they often hold up much more strongly than strange images.
And now you're trying to go through all seven deadly sins in your head. The newly acquired knowledge should last about a week (even if it is not repeated). If you repeat it once in your head (e.g. when you are bored waiting for the subway) this period of time doubles. This example can give a first impression of what is possible with the route method. Incidentally, the 7-point route can easily be reused for new material (after about a day). And if the associations still seem to you to be unnecessary ballast of knowledge - ask someone to remember the deadly sins and foreign words. This is very likely only possible through multiple, mindless repetition.
Incidentally, Cicero already used this technique: before his virtuoso speeches he wandered around the venue. At prominent places (his route points) he put important key points of his speech as pictures. Methods of this kind (of which the route method is the most important) are called mnemotechnics (after Mnemosyne, the Greek goddess of memory).

Conclusion: with the route method you can remember information: 
1. sorted
2. reliable
3. pictorial.
How to create routes (the best ones lead through real places) and how to use them to memorize real learning material, we will show in future articles.

Historical note: The classic seven deadly sins go back to Pope Gregory II, who mopped them in the 6th century from an early Christian writer and varied them slightly. Whether he has to return his papal title because of unscientific citation methods is currently being examined by a special commission of the Catholic Church :-). In their modern interpretation one understands the seven deadly sins (also "main vices") today less as sins, but rather as the cause for them. Master Yoda already knew this: “Fear leads to anger, anger leads to hate, hatred leads to unspeakable suffering”.