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.

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