Neutron chain reactions

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

According to the notes, the object of the project was to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission. 

When neutrons are in uranium they are also caused to disappear by another process represented by the equation 28 + 29 + Y- The resulting element 29 undergoes two successive p transformations into elements 39 and 49. The occurrence of this process in 28 acts to consume neutrons and works against the possibility of a fast neutron chain reaction in material containing 28. 

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). 

Among the neutron sources related to peaceful uses, the most important are nuclear reactors, which are intended for industrial power production or power supply of special engines from the heat released during a nuclear reaction. Nuclear reactors operate using a controllable neutron chain reaction the system does not reach the explosion state, and, therefore, the neutron flux through the reactor protective shell is relatively weak. 

The shutdown of SMART can be achieved by a function of one of two independent systems. The primary shutdown system is 32 shutdown banks of CEA of which absorbing material is B4C. The control banks are dropped into the reactor core by the gravity force and immediately stops die neutron chain reactions. These control banks have sufficient shutdown margin to bring the reactor fi-om hot full power to hot shutdown, even with a most reactive bank stuck out of the core. For the case of failure of the primary shutdown system, the emergency boron injection system is provided as a backup system and consists of two tanks, 6m ... 

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. 

They laid their plans, often during hikes into the uninhabited wild surroundings of the mesa. They had to rely heavily on theoretical anticipations of the effects they wanted to study that was their basic constraint. Any experimental device that demonstrated a fast-neutron chain reaction to completion would use up at least one critical mass there could be no controlled, laboratory-scale bomb tests, no squash-court demonstrations. They decided they had to analyze the explosion theoretically and work out ways to calculate the stages of its development. They needed to understand how neutrons would diffuse through the core and the tamper. They needed a theory of the explosion s hydrodynamics—the complex dynamic motions of its fluids, which the core and tamper would almost instantly become as their metals heated from solid to liquid to gas. 

No one doubted that Little Boy would work if any design would. Otto Frisch s Dragon experiments had proven the efficacy of the fast-neutron chain reaction in uranium. The gun mechanism was wasteful and inefficient but U235 was forgiving. It remained to test implosion. While doing so... 

Note that the water loss of 0.012 r units is the greatest loss of those listed. However, by means of our inven-25 tion using materials of high purity and the proper geometry, this loss may be overcome and a neutronic chain reaction is achieved. 

Before proceeding to a detailed description of a control circuit that may be utilized in controlling the neutron chain reaction, it is desirable to point out the manner in which the control rods operate to regulate the neutron density. In any self-sustaining chain reacting structure... 

If the control rods 168 are used alone to control the neutron chain reaction then one method of control would be to withdraw the control rods from the active portion of the reactor to a point where there is an exponential and preferably slow rise in neutron density within the... 

The neutron chain reaction may be prevented merely by immersing into the heavy water one or more rods,... 

This invention is concerned with the establishment of a self-sustaining neutron chain reaction in a suspension of a fissionable material in a liquid moderator and is particularly concerned with the establishment of such a reaction in a suspension wherein deuterium oxide is used as the moderator. 

In accordance with the present invention, a novel process and apparatus for establishment of a self-sustaining neutron chain reaction of neutrons with a neutron fissionable isotope such as U, and 94 2 is provided. The invention is particularly advantageous since it may be applied to establishment of such a reaction in compositions such as natural uranium where the concentration of fissionable material is low. Thus, we have found that a self-sustaining reaction may be established by use of a suspension of natural uranium in a liquid moderator containing about 0.0025 to 0.04 atom of uranium per molecule of a moderator such as deuterium oxide or about 0.0013 to 0.02 atom of uranium per atom of deuterium. Where the liquid moderator is less efficient, and absorbs more neutrons than deuterium oxide, this range of uranium concentration is somewhat narrower. 

In order that a self-sustaining neutron chain reaction Diphenyl--------------------------------------- 0.00453... 

The variation in critical mass which is required to sustain a neutron chain reaction depends to a very substantial degree upon the nature and thickness of the neutron reflector. 

The amount of fissionable isotope which should be present in reaction tank 10 in order to establish a self-sustaining neutron chain reaction depends to a substantial degree upon the concentration of the fissionable isotope in the moderator, upon the shape of the tank, and also upon the neutron absorption characteristic of the moderator used. In general, it can be said that the amount of 94 present should be at least about 200 grams with optimum concentration and using pure 94 . The exact amount required will also depend upon the fissionable isotope which is used. 

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. 

It is known in the art that a controlled slow neutron chain reaction may be established by combining fission-able material and neutron moderator to form a reactive mass, the moderator being effective to reduce the average neutron energy to a value at which the fission cross section of said material is relatively great. 

A. M. Weinberg and E. P. Wigner, The physical theory of neutron chain reactions, Chicago, University of Chicago Press, 1958. 

We can say that the conditions (7)-(7a) define a discrete multiplicative process, and that conditions (8)-(8a) define a continuous multiplicative process. This terminology is borrowed from Everett and Ulam [6], who first realized the relevance of such processes to neutron chain reactions. (See [11, p. 169].)... 

Analytical difficulties. The results of [1] thus serve to isolate the central problem of criticahty in any specific model for a neutron chain reaction, as that of proving the uniform primitivity of the associated multi-phcative process. Though Drs. Habetler and Martino [8] proceed in a different way, using the non-constructive Jentzsch theorem of [9] as their main technical tool instead of the constructive version of [1] (which requires stronger hypotheses), I beheve that the concept of uniform primitivity provides the best systematic approach. 

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