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The acronym refers to its deuterium oxide heavy water moderator and its use of originally, reaktor 6 sale free uranium fuel. There have been two major types of CANDU reactors, the original design of around MW e that was intended to be used in multi-reactor installations in large plants, and reaktor 6 sale free rationalized CANDU 6 in the Reaktor 6 sale free e class that is designed to be used in single stand-alone units or in small multi-unit plants.
By the early /16731.txt, sales prospects for the original CANDU designs were dwindling due to the introduction of newer designs from other companies. ACR failed to find any buyers; its last potential sale was for an expansion at Darlington, but this was cancelled in Candu Energy offers support services for existing sites and is completing formerly stalled installations in Romania and Argentina through a partnership with China National Nuclear Corporation.
Sales effort for reaktor 6 sale free ACR reactor has ended. Ina consultation with industry reaktor 6 sale free Natural Resources Canada to establish a "SMR По этой ссылке [1] targeting the development of small modular reactors. Fission reactions in the reactor core heat pressurized water in a primary cooling loop.
A heat exchangeralso known as a steam generatortransfers the heat to a secondary cooling loopwhich reaktor 6 sale free a steam turbine with an electric generator attached to it for a typical Rankine thermodynamic cycle. The exhaust steam from the turbines is then cooled, condensed and returned as feedwater to the steam generator. The final cooling often uses cooling water from a nearby source, such as a lake, river, or ocean. Newer CANDU plants, such as the Darlington Nuclear Generating Station near TorontoOntario, use a diffuser to spread the warm reaktor 6 sale free water over a larger volume and limit the effects on the environment.
Where the CANDU design differs from most other designs is in the details of the fissile core and the primary cooling loop. Natural uranium consists of a mix of mostly uranium with small amounts of uranium and trace amounts reaktor 6 sale free other isotopes.
Fission in these elements releases high-energy neutronswhich can cause other U atoms in the fuel to undergo fission as well. This process is much more effective when the neutron energies are much lower than what the reactions release naturally. Most reactors use some form of reaktor 6 sale free moderator to lower the energy of the neutrons, or " thermalize " them, which makes the reaction more efficient.
The energy lost by the neutrons during this moderation process heats the moderator, and this heat is extracted for power. Most commercial reactor designs use normal water as the moderator. Water absorbs some of the neutrons, enough that it is not possible to keep the reaction going in natural uranium.
CANDU replaces this reaktor 6 sale free water with heavy reaktor 6 sale free. Heavy water's extra neutron decreases its ability to absorb excess neutrons, resulting in a better neutron economy.
This allows CANDU to run on unenriched natural uraniumor uranium mixed with a wide variety of other materials such as plutonium and thorium. This was a major goal of the CANDU design; by operating on natural uranium the cost of enrichment is removed. This also presents перейти на источник advantage in nuclear proliferation terms, as there is no need for enrichment reaktor 6 sale free, which might also be used for weapons. In conventional light-water reactor LWR designs, the entire fissile core is placed in a large pressure vessel.
The amount of heat that can be removed by больше информации unit of a coolant is a function of the temperature; by pressurizing the core, the water can be heated to much greater temperatures before boilingthereby removing more heat and allowing the core to be smaller and more efficient.
Building a pressure vessel of the required size is a significant challenge, and at the time of the CANDU's design, Canada's heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size.
This problem is amplified by natural uranium fuel's lower fissile density, which requires a larger reactor core. This issue was so major that even the relatively small pressure vessel originally intended for use in the NPD prior to its mid-construction redesign could not be fabricated domestically and had to be manufactured in Scotland instead.
Domestic development of the technology required to produce pressure vessels of the size required for commercial-scale heavy water moderated power reactors was thought to be very unlikely. The bundles are contained in pressure tubes within a larger vessel containing additional heavy water acting purely as a moderator. This larger vessel, known as a calandria, is not pressurized and remains at much lower temperatures, making it much easier to fabricate.
In order to prevent the heat from the pressure tubes from leaking into the surrounding moderator, each pressure tube is enclosed in a calandria tube.
Carbon dioxide gas in the gap between the two tubes acts as an insulator. The moderator tank also acts as a large heat sink that provides an additional safety feature.
In a conventional pressurized water reactorrefuelling the system requires to shut down the core and to open the pressure vessel. This allows the CANDU system to be continually refuelled without shutting down, another major design reaktor 6 sale free. In modern systems, two robotic machines attach to the reactor faces and open the end caps of a pressure tube.
One machine pushes in the new fuel, whereby the depleted fuel is pushed out and collected at the other end. A significant operational advantage of online refuelling is that a failed or leaking fuel bundle can be removed from the core once it has been located, thus reducing the radiation levels in the primary cooling loop. Each fuel bundle is a cylinder assembled from thin tubes filled with ceramic pellets of uranium oxide fuel fuel elements.
In older designs, the bundle had 28 or 37 half-meter-long fuel elements with 12—13 such assemblies lying end-to-end in a pressure tube. The newer CANFLEX bundle has 43 fuel elements, with two element sizes so the power rating can be increased without melting the hottest fuel elements.
It is about 10 centimetres 3. Natural uranium is a mix of isotopesmainly uraniumwith 0. A reactor aims for a steady rate of fission over time, where the neutrons released by fission cause an equal number of fissions in other fissile atoms. This balance is referred to as criticality. The neutrons released in these reactions are fairly energetic and don't readily react with get "captured" by the surrounding fissile material.
In order to improve this rate, they must have their energy moderatedideally to the same energy as the fuel atoms themselves. As these neutrons are in thermal equilibrium with the fuel, they are referred to as thermal neutrons. Since most of the fuel is usually U, most reactor designs are based on thin fuel rods separated by moderator, allowing the neutrons to travel in the moderator before entering the fuel again.
More neutrons are released than are needed to maintain the chain reaction; when uranium absorbs just the reaktor 6 sale free, plutonium is created, which helps to make up for the depletion of uranium Bugdom pc download free the build-up of fission products that are even more neutron-absorbing than U slows the reaction and calls for refuelling.
Light water makes an reaktor 6 sale free moderator: the light hydrogen atoms are very close in mass to a helper pc and can absorb a lot of energy in a single collision like a collision reaktor 6 sale free two reaktor 6 sale free balls.
Light hydrogen is also fairly effective at absorbing neutrons, and there will be too few left over to react with the small amount of U in natural uranium, preventing criticality. In order to allow criticality, the fuel must be enrichedincreasing the amount of U to a usable level. Enrichment facilities are expensive to build and operate. This can be remedied if the fuel is supplied and reprocessed by an internationally approved supplier.
The main advantage of heavy-water moderator over light water is the reduced absorption of the neutrons that sustain the chain reaction, allowing a lower concentration of reaktor 6 sale free atoms to the point of using unenriched natural uranium fuel. Deuterium "heavy hydrogen" already has the extra neutron that light hydrogen would absorb, reducing the tendency to capture neutrons.
Deuterium has twice the mass of a single neutron vs light hydrogen, which has about the same mass ; the mismatch means that more collisions are needed to moderate the neutrons, requiring a larger thickness of moderator between the fuel rods. This increases the size of the reactor core and the leakage of neutrons. It reaktor 6 sale free also the practical reason for the calandria design, otherwise, a very large pressure vessel would be needed.
In CANDU most of the moderator is at lower temperatures than in other designs, reducing the spread of speeds and the overall speed of reaktor 6 sale free moderator particles. This means that most of the neutrons will end up at reaktor 6 sale free lower energy and be more likely to cause fission, so CANDU not only "burns" natural uranium, but it does so more effectively as well.
This is a major advantage of the heavy-water design; it not only requires less fuel, but as the fuel does not have to be enriched, it is much less expensive as well. A further unique feature of heavy-water moderation is the greater stability of the chain reaction. This is due to the relatively low binding energy of the deuterium nucleus 2. Both gammas produced directly by fission and by the decay of fission fragments have enough energy, and the half-lives of the fission fragments range from seconds to hours or even years.
The slow response of these gamma-generated neutrons delays the response of the reaktor 6 sale free and читать the operators extra time in case of an emergency. Since gamma rays travel for meters through water, an increased rate of chain больше информации in one part of the reactor will produce a response from the rest of the reactor, allowing various negative feedbacks to stabilize the reaction. On the other hand, the fission neutrons are thoroughly slowed down before they reach another fuel rod, meaning that it takes neutrons reaktor 6 sale free longer reaktor 6 sale free to get from one part of the reactor to the other.
Thus if the chain reaction accelerates in one section of the reactor, the change will propagate itself only slowly to the rest of the core, giving time to respond in an emergency. The independence of the neutrons' energies from the nuclear fuel used is what allows such fuel flexibility in a CANDU reactor, since every fuel bundle will experience the same environment and affect its neighbors in the same way, whether the fissile material is uranium, uranium reaktor 6 sale free plutonium.
Canada developed the heavy-water-moderated reaktor 6 sale free in the post— World War II era to explore nuclear energy while lacking access to enrichment facilities. War-era enrichment systems were extremely expensive to build and operate, whereas the heavy water solution allowed the use of natural uranium in the experimental ZEEP reactor.
A much less expensive enrichment system was developed, but the United States classified work on the cheaper gas centrifuge process. Some of these are a side effect of the physical layout of the system. CANDU designs have a positive void coefficientas well as a small power coefficient, normally considered bad in reactor design. This implies that steam generated in the coolant will increase the reaction rate, which in turn would generate more steam.
This is one of the many reasons for the cooler mass of moderator in the calandria, as even a serious steam incident in the core would not have a major impact on the overall moderation cycle. Only if the moderator itself starts to boil, would there be any significant effect, and the large thermal mass ensures that this will occur slowly.
The deliberately "sluggish" response of the fission process reaktor 6 sale free CANDU reaktor 6 sale free controllers more time to diagnose and deal with problems. The fuel channels can only maintain criticality if they are mechanically sound. Reaktor 6 sale free the temperature of the fuel bundles increases to the point where they are mechanically unstable, their horizontal layout means that they will bend under gravity, shifting the layout of the bundles and reducing the efficiency of the reactions.
Because the original fuel arrangement is optimal for a chain reaction, and the natural uranium fuel has little excess reactivity, any significant deformation will stop the inter-fuel pellet fission reaction. This will not stop heat production from fission product decay, which reaktor 6 sale free continue to supply a considerable heat output. If this process further weakens the fuel bundles, reaktor 6 sale free pressure tube they are in will eventually bend far enough to touch the calandria tube, allowing heat to be reaktor 6 sale free transferred into the moderator tank.
The moderator vessel has a considerable thermal capability on its own and is normally kept relatively cool. The CANDU designs have several emergency cooling systems, as well as having limited self-pumping capability through thermal means the steam generator is well above the reactor.
Even in the event of a catastrophic accident and core meltdownthe fuel is not critical in light water. Normally the rate of fission is controlled by light-water compartments called liquid zone controllers, which absorb excess neutrons, and by adjuster rods, which can be raised or lowered in the core to control the neutron flux. These are used for normal operation, allowing the controllers to adjust reactivity across the fuel mass, as different увидеть больше would normally burn at different rates depending on reaktor 6 sale free position.
The adjuster rods can also be used to slow or stop criticality. Because these rods are inserted into the low-pressure calandria, not the high-pressure fuel tubes, they would ссылка на страницу be "ejected" by steam, a design issue for many pressurized-water reactors. There are reaktor 6 sale free independent, fast-acting safety shutdown systems as well. Shutoff rods are held above the reactor by electromagnets and drop under gravity into the core to quickly end criticality.
This system works even in the event of a complete power failure, as the electromagnets only hold the rods out of the reactor when power is available.
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