Uranium slow-neutron chain reaction

The absorption cross section, as Fermi and Anderson subsequently calculated it, proved usefully small 3 X 10 cm. And could be made smaller still, they thought, with purer graphite. The measurement strongly supported Fermi s and Szilard s plan to attempt to induce a slow-neutron chain reaction in natural uranium. 

In high concentrations or substantially pure form, 10 plutonium can also be used, when properly combined with a neutron moderator, to sustain a slow neutron chain reaction in a neutronic reactor of exceptionally small size as compared to the size of reactors using natural uranium. The neutron leakage is high in small reactors. 15 In other words, such reactors can be used as efficient sources of large quantities of neutrons, and the neutrons thus produced can be used to produce another fissionable isotope. 

Created by a slow neutron chain reaction in natural uranium. 

The discovery of nuclear fission in 1938 proved the next driver in the development of coordination chemistry. Uranium-235 and plutonium-239 both undergo fission with slow neutrons, and can support neutron chain reactions, making them suitable for weaponization in the context of the Manhattan project. This rapidly drove the development of large-scale separation chemistry, as methods were developed to separate and purify these elements. While the first recovery processes employed precipitation methods (e.g., the bismuth phosphate cycle for plutonium isolation). 

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 23su while occuring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. 

In the light water reactor, the circulating water serves another purpose in addition to heat transfer. It acts to slow down, or moderate, the neutrons given off by fission. This is necessary if the chain reaction is to continue fast neutrons are not readily absorbed by U-235. Reactors in Canada use heavy water, D20, which has an important advantage over H20. Its moderating properties are such that naturally occurring uranium can be used as a fuel enrichment in U-235 is not necessary. 

But making a bomb was not so easy. Natural uranium comes in two forms, or isotopes (see page 119). These have the same number of protons (92) in their nuclei, but different numbers of neutrons. One isotope has 143 neutrons (uranium-235 or U), the other has 146 (uranium-238 or U). Only undergoes fission induced by the slow, low-energy neutrons that the fission products emit. So only this isotope can be used to create a runaway chain reaction. But natural uranium is mostly only 1 per cent is A bomb requires a critical mass of only a few pounds of - with... 

Unenriched uranium—which contains more than 99 percent of the nonfissionable isotope U-238—undergoes a chain reaction only if it is mixed with a moderator to slow down the neutrons. Uranium in ore is mixed with other substances that impede the reaction and has no moderator to slow down the neutrons, so no chain reaction occurs. 75- Nuclear fission is a poor prospect for powering automobiles primarily because of the massive shielding that would be required to protect the occupants and others from the radioactivity and the problem of radioactive waste disposal. 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

Nuclear power plants use fissionable materials such as uranium-235 as sources of energy. In the core of a nuclear power plant, rods of uranium dioxide (U02) are placed in a matrix containing moderators such as heavy water or graphite that slow neutrons so they can be captured. The neutrons impact uranium nuclei, splitting them to release lighter nuclei and converting a small amount of mass to energy. In order for a chain reaction to occur, a critical mass of fissionable material must be present. 

The source of energy in a nuclear reactor is a fission reaction in which neutrons collide with nuelei of uranium-235 or plutonium-239 (the fuel), causing them to split apart. The products of a fission reaction include not only energy but also new elements (known as fission products) and free neutrons. A constant and reliable flow of neutrons is insured in the reactor by a moderator, which slows down the speed of neutrons, and by control rods, which limit the number of neutrons available in the reactor and, hence, the rate at which fission can occur. In a nuclear weapon, the fission chain reaction, once triggered, proceeds at an exponentially increasing rate, resulting in an explosion in a nuclear reactor, it proceeds at a steady, controlled rate. Most commercial nuclear power plants are incapable of undergoing an explosive nuclear chain reaction, even should their safety systems fail this is not true of all research reactors (e.g., some breeder reactors). 

The physical properties of isotopes differ slightly because of differences in atomic mass. For example, water that contains deuterium is called heavy water because the neutrons in deuterium add mass to the water molecule. Some nuclear reactors use heavy water to help keep the chain reaction going. The heavy water slows down (or moderates) the neutrons produced during nuclear fission so that they can be absorbed by the uranium fuel. You will learn more about nuclear reactions in Chapter 25. 

Uranium-235 in fuel rods produces fast-moving neutrons and heat in a fission chain reaction. The neutrons are slowed down by a moderator such as water or graphite so that they are not moving too quickly to be absorbed by other uranium-235 nuclei. The rate of the reaction is maintained using control rods that absorb some of the neutrons. These rods can be raised or lowered in the reaction chamber to slow or speed the reaction, respectively. 

The so-called fast neutrons which this fission produces have energies of about 2MeV (190 x 10 kJmol ) and are not very effective in producing fission of further nuclei. Better in this respect are slow or thermal neutrons whose energies are of the order of 0.025 eV (2.4kJmol ), i.e. equivalent to the thermal energy available at ambient temperatures. In order to produce and sustain a chain reaction in uranium it is therefore necessary to counter the inefficiency of fast neutrons by either... 

When the control rods are moved between the fuel rods, the chain reaction slows because fewer neutrons are available to bombard uranium atoms when the control rods are removed, the chain reaction speeds up. 

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