Fissile materials

The rapid fission of a mass of or another heavy nucleus is the principle of the atomic bomb, the energy liberated being the destructive power. For useful energy the reaction has to be moderated this is done in a reactor where moderators such as water, heavy water, graphite, beryllium, etc., reduce the number of neutrons and slow those present to the most useful energies. The heat produced in a reactor is removed by normal heat-exchange methods. The neutrons in a reactor may be used for the formation of new isotopes, e.g. the transuranic elements, further fissile materials ( °Pu from or of the... 

A D—T fusion reactor is expected to have a tritium inventory of a few kilograms. Tritium is a relatively short-Hved (12.36 year half-life) and benign (beta emitter) radioactive material, and represents a radiological ha2ard many orders of magnitude less than does the fuel inventory in a fission reactor. Clearly, however, fusion reactors must be designed to preclude the accidental release of tritium or any other volatile radioactive material. There is no need to have fissile materials present in a fusion reactor, and relatively simple inspection techniques should suffice to prevent any clandestine breeding of fissile materials, eg, for potential weapons diversion. 

A variation of the classical fuel cycle is the breeder cycle. Special breeder reactors are used to convert fertile isotopes iato fissile isotopes, which creates more fuel than is burned (see Nuclear reactors, reactor types). There are two viable breeder cycles U/ Pu, and Th/ U. The thorium fuels were, however, not ia use as of 1995. A breeder economy implies the existence of both breeder reactors that generate and nonbreeder reactors that consume the fissile material. The breeder reactor fuel cycle has been partially implemented ia France and the U.K. 

The recycle weapons fuel cycle rehes on the reservoir of SWUs and yellow cake equivalents represented by the fissile materials in decommissioned nuclear weapons. This variation impacts the prereactor portion of the fuel cycle. The post-reactor portion can be either classical or throwaway. Because the avadabihty of weapons-grade fissile material for use as an energy source is a relatively recent phenomenon, it has not been fully implemented. As of early 1995 the United States had purchased highly enriched uranium from Russia, and France had initiated a modification and expansion of the breeder program to use plutonium as the primary fuel (3). AH U.S. reactor manufacturers were working on designs to use weapons-grade plutonium as fuel. 

The role of the reactor may be either as a converter, which produces some plutonium by neutron absorption in uranium-238 but depends on uranium-235 for most of the fission, or as a breeder, which contains a large amount of plutonium and produces more fissile material than it consumes. Breeding is also possible using uranium-233 produced by neutron absorption in thorium-232. 

Fig. 2. Fuel for high temperature gas-cooled reactor. Fissile material is coated with carbon and siHcon carbide, fertile material with carbon. Particles mixed...

H. C. Paxton, Criticality Control in Operations with Fissile Material, LA-3366 (rev.), Los Alamos Scientific Laboratory, Los Alamos, N.M., 1972. 

The detection and determination of traces of cobalt is of concern in such diverse areas as soflds, plants, fertilizers (qv), stainless and other steels for nuclear energy equipment (see Steel), high purity fissile materials (U, Th), refractory metals (Ta, Nb, Mo, and W), and semiconductors (qv). Useful techniques are spectrophotometry, polarography, emission spectrography, flame photometry, x-ray fluorescence, activation analysis, tracers, and mass spectrography, chromatography, and ion exchange (19) (see Analytical TffiTHODS Spectroscopy, optical Trace and residue analysis). 

Besides fission products, the various forms of known but newly formed elements in the spent nuclear fuel, there is a small but significant amount of fissionable, or fissile, material in the SNF. This is quite important. There is some unused, unfissioned U-235 that has become too dilute to use. Like natural uranium ores in which chain reactions do not... 

Since the amount of fissile material in the fuel assemblies is only about 3 percent of the uranium present, it is obvious that there cannot be a large amount of radioactive material in the SNF after fission. The neutron flux produces some newly radioactive material in the form of uranium and plutonium isotopes. The amount of this other newly radioactive material is small compared to the volume of the fuel assembly. These facts prompt some to argue that SNF should be chemically processed and the various components separated into nonradioac-tive material, material that will be radioactive for a long time, and material that could be refabricated into new reactor fuel. Reprocessing the fuel to isolate the plutonium is seen as a reason not to proceed with this technology in the United States. 

Field desorption mass spectrometry radiopharmacological agents, 6,976 Filter dyes photography, 6, 104 Fissile material criticality... 

In a nuclear weapon, the fissile material is initially subcritical. The challenge is to produce a supercritical mass so rapidly that the chain reaction takes place uniformly throughout the metal. Supercriticality can be achieved by shooting two subcritical blocks toward each other (as was done in the bomb that fell on Hiroshima) or by implosion of a single subcritical mass (the technique used in the bomb that destroyed Nagasaki). A strong neutron emitter, typically polonium, helps to initiate the chain reaction. 

Nuclear energy can be extracted by arranging for a nuclear chain reaction to take place in a critical mass of fissionable material. with neutrons as the chain carriers. A moderator is used to reduce the speeds of the neutrons in a reactor that uses fissile material. 

Nuclear fuel reprocessing was first undertaken with the sole purpose of recovering plutonium, for weapons use, from uranium irradiated in nuclear reactors. These reactors, called the production reactors, were dedicated to transmuting as much of the uranium as possible to plutonium. From its original scope of recovering exclusively plutonium, with no attempts to either recover or recycle uranium, nuclear fuel reprocessing has since grown into a much more sophisticated and complex operation with expanded scope. It is now called upon to separate uranium and plutonium from the fission products, and to purify these elements to levels at which these fissile materials can be conveniently recycled for reuse. The present scope also extends to fission products separation and concentration. 

Another Issue which Some Classify under the Non-Proliferation Title Is the Diversion of Fissile Materials or Nuclear Weapons... 

The very problem of using uranium in the chemical industry is the very strict regulations in several countries that limit the fissile materials trade and, unfortunately, also that of depleted... 

Bunn, Matthew with Anthony Weir and Josh Freidman, The Demand for Black Market Fissile Material, NTI, June 16, 2005, available at www.nti.org/e Research/cnwm/threat/demand. asp. 

In fast (neutron) reactors, the fission chain reaction is sustained by fast neutrons, unlike in thermal reactors. Thus, fast reactors require fuel that is relatively rich in fissile material highly enriched uranium (> 20%) or plutonium. As fast neutrons are desired, there is also the need to eliminate neutron moderators hence, certain liquid metals, such as sodium, are used for cooling instead of water. Fast reactors more deliberately use the 238U as well as the fissile 235U isotope used in most reactors. If designed to produce more plutonium than they consume, they are called fast-breeder reactors if they are net consumers of plutonium, they are called burners . 

November 23, 1995 Jaeckle was sentenced to 2.5 years in prison for illegal possession of fissile material... 

Fissile materials are defined as materials that are fissionable by nentrons with zero kinetic energy. In nuclear engineering, a fissile material is one that is capable of snstaining a chain reaction of nuclear fission Nuclear power reactors are mainly fueled with manium, the heaviest element that occurs in natnre in more than trace qnantities. The principal nuclear energy soiuces are maninm-235, plutonium-239, uranium-233 and thorium. 

Accdg to Hammersley Handscomb (Addnl Ref N, p 8), S. Ulam, J. von Neumann and E. Fermi independently rediscovered Monte Carlo methods ca 1944 and started its systematic development. They also ensured that their scientific colleagues should become aware of the possibilities, potentialities and physical applications. The real use of Monte Carlo methods as research tools is attributed to von Neumann Ulam who applied them to random neutron diffusion in fissile material... 

The other main type of HLW are the liquid and sludge (a mixture of liquid, suspended colloids, and solids) that resulted from reprocessing to reclaim fissile material, either for weapons pro-... 

Albright, D. Barbour, L. 1999. Separated neptunium and americium. In Albright, D. O Neil K. (eds) The Challenges of Fissile Material Control. ISIS, Washington, DC, 85-96. 

Homogeneous distribution of radionuclides (especially if fissile materials are present) ... 

Maximum difficulty to recover radioactive constituents from waste form (especially fissile materials, such as Pu). 

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