Because "ordinary" uranium is mostly 238U, which won't fission and create a chain like its lighter cousin 235U will. When critical mass is achieved with the 235U isotope of uranium, fission will occur spontaneously. Or with a significantly enriched uranium fuel (one where the natural concentration of 235U has been increased a bunch so the fuel has a much higher percentage of this isotope), fission and a chain reaction is also possible. But with just natural uranium, a big pile of it will just sit there. It won't fission and create a chain reaction. Note that 238U is radioactive and decays over time because it is unstable, but it has a long half-life. Also, the fact that it's unstable (radioactive) doesn't mean it's fissile. It isn't.
Plutonium can be used in nuclear reactors cooled with natural water - BWR, PWR.
Zerlina.
The moderator used in nuclear reactors with natural uranium is generally the heavy water (D2O).
The main types of nuclear power plant in use in the world, and exclusively in the US are PWR (Pressurised Water Reactor) and BWR (Boiling Water Reactor) types. These use normal water as moderator and enriched uranium fuel. In Canada a heavy water moderated type of reactor has been successfully used and exported to several countries. In the UK gas cooled reactors have mainly been used, firstly the magnox type and latterly the AGR. Both use carbon dioxide cooling and graphite moderator. In Russia the RBMK type was developed which combined graphite moderating with water cooling. This type has been largely discontinued after Chernobyl. High temperature helium cooled reactors and fast reactors liquid metal cooled have been tried as prototypes but not commercially.
The only example of nuclear fission in a naturally occurring material is of Uranium 235, which comprises 0.7 percent of natural uranium, the rest being Uranium 238 which is not fissile. To use U235 in a nuclear reactor it is usually enriched to about 4 percent first, though reactors have been designed to use natural uranium. These have to use graphite or heavy water as moderator, as normal water absorbs too many neutrons. During reactor operation some of the U238 absorbs a neutron and becomes Plutonium 239 which is also fissile, so this contributes to a proportion of the reactor power which increases as the fuel is used and the U235 diminishes.
This is the Candu type, which was uniquely developed in Canada to use heavy water moderator and natural uranium fuel
Because the neutron economics are against it, a critical reactor and a chain reaction would not be possible. Ordinary water absorbs too many neutrons. For natural uranium you must use graphite or heavy water for the moderator.
A nuclear reactor is an assembly of fuel elements (uranium usually), a moderator which can be ordinary water, heavy water, or graphite, and control rods. The reactor is made to reach criticality when uranium fuel will produce a steady power output as a result of nuclear fissions which release heat. The heat is used to produce steam which feeds a conventional steam turbine/generating unit.
Uranium ore (usually black oxide) is mined.Uranium ore is shipped to Mill.Mill separates Uranium from ore and converts black oxide to yellow oxide (yellowcake).Yellowcake is shipped to Enrichment plant.Preprocessing plant converts yellowcake to Uranium Hexafluoride (the only Uranium compound that is a gas near room temperature, but also corrosive to almost every metal except nickel and explosive in contact with either water or oil!!!)Enrichment plant enriches the Uranium in the Uranium Hexafluoride gas from 0.72% Uranium-235 (natural level) to 3% Uranium-235 (power reactor fuel level)Postprocessing plant converts enriched Uranium Hexafluoride to enriched yellowcake.Yellowcake is shipped to fuel pellet Canning plant.Canning plant cans yellowcake in Zirconium alloy fuel pellet cans.Fuel pellets are shipped to fuel rod Assembly plant.Assembly plant inserts fuel pellets into steel fuel rod tubes.Fuel rods are shipped to Reactor.Fuel rods are inserted in Reactor as needed.This is the basic outline from mine to a water moderated & cooled reactor. There are many additional minor steps along the way. For other reactor designs (e.g. metal cooled fast breeder reactor, gas cooled high temperature graphite moderated pebble bed reactor, liquid metal fueled reactor) several of the basic steps themselves will have to change.
In light water reactors it is uranium dioxide with the uranium enriched to 4-5 percent
Ordinary light water
I guess because it is the one with the fewest operational problems and the longest operational life. BWR's introduce the problems of a contaminated turbine. Gas cooled graphite reactors are efficient but there are life limitations on the graphite. Heavy water reactors have the benefit of using natural uranium but the heavy water is very expensive to produce. So the choice is between enriching uranium as for the PWR, or producing heavy water as for the CANDU. Most countries are now opting the PWR way as enriching uranium by centrifuge has become much less expensive than the old gaseous diffusion method.
Zerlina.
The moderator used in nuclear reactors with natural uranium is generally the heavy water (D2O).
Different types of nuclear plants: Pressurised Water Reactor (PWR); Boiling Water Reactor (BWR); Heavy Water Moderated Reactor (CANDU); Advanced Gascooled Reactor (AGR); Fast Breeder Reactor (FBR); Pebble Bed Gascooled Reactor; Water Cooled Graphite Reactor (RBMK). There are other ideas that only exist on paper.
The KAMINI (Kalpakkam Mini) reactor is a Uranium-233 fueled, demineralized light water moderated and cooled, beryllium oxide reflected, low power nuclear research reactor. It is located in the post irradiation examination facility of Radio Metallurgy Laboratory, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. A link is provided to the Wikipedia article.
There is some sort of confusion here. There are two types of water moderated/cooled reactors: boiling water and pressurized water.The boiling water reactor is at normal atmospheric pressure and the water in the reactor boils, producing steam directly.The pressurized water reactor is at elevated pressure to prevent the water from boiling. A heat exchanger/steam generator is used to produce steam indirectly.Other types of reactor (e.g. liquid metal, gas cooled, organic, aqueous homogeneous) also do not operate at pressures below atmospheric.
There is some sort of confusion here. There are two types of water moderated/cooled reactors: boiling water and pressurized water.The boiling water reactor is at normal atmospheric pressure and the water in the reactor boils, producing steam directly.The pressurized water reactor is at elevated pressure to prevent the water from boiling. A heat exchanger/steam generator is used to produce steam indirectly.Other types of reactor (e.g. liquid metal, gas cooled, organic, aqueous homogeneous) also do not operate at pressures below atmospheric.