Nuclear reactor

Nuclear power station in Kewaunee, Wisconsin.

A nuclear reactor is a device in which a nuclear chain reaction occurs in a controlled manner. It can be used to obtain energy in nuclear power plants, which may have one or more reactors; for the production of fissionable materials, such as plutonium or other isotopes used in medicine and other industries; to be used in nuclear weapons; in the nuclear propulsion of ships, submarines or artificial satellites or for research. Currently only nuclear fission reactors produce power commercially, although experimental nuclear fusion reactors exist.

The power of a nuclear fission reactor varies according to its size and designated function from the order of the thermal kW to the order of the thermal gigawatt. Depending on its application, this generated thermal power can be dissipated or used for electrical power generation. For their correct operation, they need a constant flow of cold water that allows their adequate cooling. For this reason, similar to conventional thermal power plants, they must be installed in areas close to water, generally rivers or the sea.

Nuclear reactors do not emit greenhouse gases or other air pollutants: cooling towers only release water vapor produced by cooling the reactor. However, they do produce radioactive waste, including isotopes with half-lives of the order of tens of thousands of years, a period in which they must be stored under safe conditions until their radioactivity levels are low enough or their final storage is decided on. deep geological storage.

The first prototype nuclear reactor was built by Enrico Fermi. The Oklo Natural Nuclear Reactor, located in Gabon, is an example of a natural nuclear reactor. It is a uranium deposit in which sustained chain fission nuclear reactions occurred over periods of thousands of years.

Main components

A nuclear fission reactor consists of the following essential parts:

• Fuel: Fissible (divisible) or fertile (convertible in physsionable by neutral activation): Uranium-235, Uranium-238, plutonium-239, Torium-232, or mixtures of these (MOX, Uranium oxides mix and plutonium). The usual fuel in light water cooled plants is enriched uranium dioxide, in which about 3% of uranium nuclei are U-235 and the rest of U-238. The proportion of U-235 in natural uranium is only 0.72%, so it is necessary to submit it to a process of enrichment in this nucleide.
• Moderator: Water, heavy water, graphite, metallic sodium: They fulfill the function of slowing down the neutrons produced by the fission, so that they have the opportunity to interact with other fissionable atoms and maintain the reaction. As a general rule, at lower neutron speed, greater probability of fission with other fuel cores in reactors using uranium 235 as fuel.
• Refrigerator: Water, heavy water, carbon dioxide, helium, metallic sodium: It leads the heat generated to a heat exchanger, or directly to the power generator turbine or propulsion.
• Reflector: Water, heavy water, graphite, uranium: reduces neutron exhaust and increases reactor efficiency.
• Armor: Hormigon, lead, steel, water: Avoid the radiation leak gamma and fast neutrons.
• Control material: Cadmio or boro: makes the chain reaction stop. They're very good neutron absorbers. They are usually used in the form of bars or dissolved in the coolant.
• Security Elements: All nuclear fission plants have, since 2007, multiple systems, assets (respons to electrical signals), or passive (act naturally, by gravity, for example). The containment of concrete that surrounds the reactors is the main one of them. They prevent accidents, or that, if it occurs, there is no release of radioactivity outside the reactor.

Types of nuclear fission reactors

There are several basic types in 2012:

• LWR - Light Water Reactors (Lightwater reactions): use as a coolant and moderator of water. Like enriched uranium fuel. The most used are the PWR (Pressure Water Reactor or pressure water reactors) and BWR (Boiling Water Reactor or boiling water reactors: 264 PWR and 94 BWR operating in 2007.

• CANDU - Canada Deuterium Uranium (Canada uranium deuterium): They use as a moderator and refrigerant heavy water (composed by two deuterium atoms and one of oxygen). Fuel uses natural uranium: 43 in operation in 2007.

• FBR - Fast Breeder Reactors (refeed fast trackers): they use fast neutrons instead of thermals to achieve the fission. Fuel uses plutonium and as a liquid sodium coolant. This reactor does not need moderator: 4 operations in 2007. Just one in operation.

• AGR - Advanced Gas-cooled Reactor (high gas cooled rig): uses uranium as fuel. How coolant uses CO₂ and as a graphite moderator: 18 in operation in 2007.

• RBMK - Reactor Bolshoy Moshchnosty Kanalny (High-Power Channels: Uses graphite as a moderator and water as a refrigerant. Uranium enriched as fuel. It can be recharged. It has a positive reactivity coefficient. The Chernobyl reactor was this guy. There were 12 in operation in 2007.

• FBNR Fijo Width Reactor, a fixed-bed reactor is a 4-generation modular reactor, in which the fuel chamber is separated from the reaction chamber

• ADS - Accelerator Driven System (accelerator-assisted system): uses a subcritical mass of torium, in which the fission is produced only by the introduction, through neutron particle accelerators, in the reactor. They are undergoing experimentation, and one of their core functions is expected to be the elimination of nuclear waste produced in other fission reactors.

Advantages and disadvantages of nuclear power plants

Nuclear power plants present a series of advantages and disadvantages as a source of electricity generation.

Advantages

One of the advantages of today's nuclear reactors is that they emit almost no pollutants into the air (although small amounts of radioactive gases are released periodically)^{[citation needed]}, and the waste produced is much smaller in volume and more controlled than the waste generated by plants powered by fossil fuels. In these conventional thermal power plants that use fossil fuels (coal, oil or gas), greenhouse gases are emitted (CO₂ mainly), gases that produce acid rain (SO₂ > mainly), soot, heavy metals, thousands of tons of ash annually, and even concentrated natural radioactive material (NORM). In a nuclear power plant, the solid waste generated is on the order of a million times smaller in volume than the pollutants from thermal power plants.

The enriched uranium used in nuclear power plants is not suitable for building a nuclear weapon or for using uranium from them. For this, the reactors are designed in high enrichment cycles or designs such as RBMK type reactors used for the generation of plutonium are used.

Lately, assisted fission power plants have been investigated, where part of the most dangerous waste would be destroyed by bombardment with particles from an accelerator (probably protons) that by spallation would produce neutrons that in turn would cause the transmutation of those most dangerous isotopes. This would be a kind of self-maintained radioactive waste neutralization plant. The performance of these plants would, in principle, be lower, since part of the energy generated would be used for the transmutation of waste. It is estimated that the construction of the first transmutation reactor (Myrrha) will begin in the year 2040.

Disadvantages

The danger to the population comes from several factors: 1) accident at an atomic power plant, 2) terrorist attack, 3) dangerousness of the waste and its high power to pollute the environment, 4) nuclear dumps, 5) possible diversion of waste for the production of weapons of mass destruction.

Nuclear reactors generate radioactive waste. Some of them have a high half-life, such as americium, neptunium or curium and are highly toxic. The detractors of nuclear energy emphasize the danger of this residue that lasts hundreds and even thousands of years.

Some nuclear reactors were used to generate plutonium 239 used in nuclear weapons. Civilian reactors generate plutonium but plutonium 239 (required in nuclear weapons) appears mixed with high proportions of plutonium 240, 238 and 242, making it unfeasible for military use.

The most serious nuclear accidents have been: Mayak (Russia) in 1957, Windscale (Great Britain) in 1957, Three Mile Island (United States) in 1979, Chernobyl (Ukraine) in 1986, Tokaimura (Japan) in 1999 and Fukushima (Japan) 2011.

The danger of nuclear waste is a highly controversial topic. These are usually associated with the generation of nuclear fission energy, however there are countless radioactive sources used in various uses that are also buried in nuclear cemeteries. Most countries have national companies in charge of managing this waste. In Spain, nuclear power plants pay a fee that goes to a fund for the management of this waste and the dismantling of nuclear power plants when they finish their life cycle. Currently, there is a definitive warehouse for the burial of spent fuel (Onkalo Spent Nuclear Fuel Deposit), before, they are usually kept in pools at the same reactor sites or in centralized warehouses. For many, this is the most reasonable option since 95% of the uranium is preserved in the spent fuel, which will allow it to be reused in the future. In fact, some countries already do so, but the technique is expensive, work is being done to make it cheaper, as it claims the BN-800 Nuclear Reactor, which has a 1200 MW brother (BN-1200 Nuclear Reactor) under construction, whose objective is to make profitable and make a commercial model of a reactor that feeds on nuclear waste, reduces its radioactivity and at the end of the cycle In this type of reactor, to be ready to be used again in a conventional nuclear reactor, this cycle would be repeated until the uranium and plutonium of the nuclear fuel were exhausted, which would turn it into waste again, with the exception that its radioactivity would be substantially less.

Applications

• Nuclear generation:
  • Heat production for power generation.
  • Heat production for domestic and industrial use.^{[chuckles]required]}
  • Hydrogen production by high temperature electrolysis.
  • Disalation.^{[chuckles]required]}
• Nuclear propulsion:
  • Maritime.
  • Nuclear thermal propulsion (proposal).
  • Pulsed nuclear propulsion (proposed).
• Transmutation of elements:
  • Production of plutonium, used for the manufacture of fuel from other reactors or nuclear weapon.
  • Creation of various radioactive isotopes, such as the americity used in smoke detectors, or cobalt-60 and others used in medical treatments.
• Research applications, including:
  • Its use as sources of neutrons and positrons (e.g. for its use of analysis by neutron activation or for the dating method of potassium-argon).
  • Development of nuclear technology.

Nuclear fusion reactor

Facility for the production of energy through nuclear fusion. After more than 60 years of research in this field, it has been possible to maintain a controlled reaction, although it is not yet energetically profitable.

The greatest difficulty lies in withstanding the enormous pressure and temperature required for nuclear fusion (which can only be found naturally in the core of a star). In addition, this process requires a huge initial injection of energy (although later it could be self-sustaining since the energy released is much greater).

There are currently two lines of research, inertial confinement and magnetic confinement.

Inertial confinement consists of containing fusion by pushing particles or laser beams projected against a fuel particle, causing it to ignite instantly.

The two most important projects worldwide are the NIF (National Ignition Facility) in the United States and the LMJ (Laser Mega Joule) in France.

Magnetic confinement consists of containing the material to be fused in a magnetic field while making it reach the necessary temperature and pressure. Hydrogen at these temperatures reaches the plasma state.

The first American magnetic models, known as Stellarators, generated the field directly in a toroidal reactor, with the problem that the plasma leaked between the field lines.

Russian engineers improved this model giving way to the Tokamak in which a primary coil winding induced the field on the plasma, taking advantage of the fact that it is conductive, and actually using it as a secondary winding. In addition, the electrical resistance of the plasma heated it.

The largest such reactor, the JET (Joint European Bull) has achieved nuclear fusion conditions with a Q factor >0.7. This means that the ratio between the energy generated by fusion and that required to sustain the reaction is 0.7. For the reaction to sustain itself, parameters higher than Q>1 must be reached and even more so for its economic viability. The first objective must be achieved with the ITER project and the second with DEMO.

The creation of an even larger reactor, ITER, has been committed, joining the international effort to achieve fusion. Even in the case of achieving it, it would continue to be an experimental reactor and another prototype would have to be built to test power generation, the so-called DEMO project.

Possible fuels for nuclear fusion reactors

The optimal reaction to produce energy by fusion is that of deuterium and tritium due to its high effective section. It is also, therefore, the most used in experimental tests. The reaction is the following:

D + T → ⁴He + n

Obtaining deuterium is not difficult as it is a stable and abundant element that formed in large quantities in the primordial soup of particles (see Big Bang). In water, one part per 6,500 contains deuterium instead of hydrogen, so it is considered that there is an inexhaustible reserve of deuterium. In a self-sustaining reactor, the deuterium-tritium reaction would generate energy and neutrons. Neutrons are the negative part of the reaction and must be controlled since neutron uptake reactions on the reactor walls or on any atom of the reagent can induce radioactivity. In fact, neutrons, given enough time, can weaken the structure of the container itself, with the consequent risk of dangerous cracks. For this, there are neutron moderators and shields such as heavy water, beryllium, sodium or carbon as moderators widely used in fission plants, or boron and cadmium, used as products that completely stop neutrons by absorbing them. If you want to make a really clean reactor, you will have to look for other formulas. A double solution to the problem of neutrons and to the abundance of tritium has been proposed. Tritium is not found in nature as it is unstable so it must be manufactured. To obtain it, you can resort to fission plants, where it can be generated by the activation of the hydrogen contained in the water, or by bombarding lithium, an abundant material in the earth's crust, with neutrons.

⁶Li + n → ⁴He + T

⁷Li + n → ⁴He + T +n

There are two stable isotopes of lithium, lithium-6 and lithium-7, the latter being much more abundant. Unfortunately, the reaction that absorbs neutrons is the one with lithium-6, the least abundant. All this does not prevent many neutrons from ending up hitting the walls of the reactor itself with the subsequent manufacture of radioactive atoms. Despite this, one of the proposals for ITER is to cover the walls with lithium-6 which would stop a good part of the neutrons to produce more tritium. Due to all these problems, other cleaner high cross section reactions are being investigated. One of the most promising is deuterium plus helium-3.

D + 3He → ⁴He + p

The problem in this reaction resides in the smaller effective section with respect to that of deuterium-tritium and in the actual obtaining of helium-3, which is the rarest isotope of said element. Protons do not pose as much danger as neutrons since they will not be easily captured by atoms due to the Coulomb barrier that they must cross, something that does not happen with neutrally charged particles such as neutrons. In addition, a proton can be manipulated by electromagnetic fields. A solution to obtain helium-3 artificially would be to incorporate, in the reactor itself, the deuterium-deuterium reaction.

D + D → 3He + n

The problem is that, again, we get a residual neutron, which brings us back to the neutron problem. Perhaps the key was obtaining natural helium-3, but this is extremely rare on Earth. It must be taken into account that the little natural helium-3 that is produced by radioactivity tends to escape from our dense atmosphere. The curious thing is that this isotope is abundant on the Moon. It is scattered on its surface and comes from the solar wind that for billions of years has bathed the bare lunar surface with its ionized particles. This lunar helium could be, in the future, the key to fusion reactors.

In the meantime, research is being carried out on materials that, even if activated, only give rise to isotopes with a short half-life, with which, leaving these materials to rest for a short period, they could be considered as conventional (non-radioactive) waste. The main problem, in any case, would continue to be the difficulty of keeping the core frame in good condition without it deteriorating and having to be changed every so often.