Critical masses plutonium

A series of criticality eaqieriinents was begun in mid 1961 with plutonium solidions jn the new thmford Plutonium Critical Mass Laboratory. This laboratory was built for the purpose of obtaining needed criticality data on plutonium solutions and precipitates of plutonium in support of chemical processing. 

W. A. Reardon etal., Hazards Summary-Report for the Hanford Plutonium Critical Mass Laboratory, HW-66266 (Aug 1,1960). 

RICHEY, C. R., et al.. Hazard Summary Report for the Plutonium Critical Mass Laboratory Supplement No. 1, The Remote Split-Table Machine, HW-66266 Sup. 1 Rev. (October 1963). 

F. A. KLOVERSTROM, "Spherical and Cylindrical Plutonium Critical Masses, UCRL-4957, University of California Radiation Laboratory, Livermore (Sept. 1957). 

Since beginning operations in 1961, hundreds of criticality experiments have been performed at the Plutonium Critical Mass Laboratory of the Pacific Northwest Laboratory. The current research emphasis at this Laboratory is on the procurement of criticality data for plutonium-uranium mixtures that will serve as the basis for handling criticality problems subsequently encountered in the development of technology for the LHFBR community—FFTF, demonstration plants, and commercial LMFBRs. 

C. R. RICHEY et al., Hazards Stmmary Rq rt for the Hanford Plutonium Critical Mass Laboratoj, Supplement No. 1 The Remote Split-Tiable ktohiiie, HW-66266, Suppl. 1, rev.. General Electric Comply (1963). 

Because of the high rate of emission of alpha particles and the element being specifically absorbed on bone the surface and collected in the liver, plutonium, as well as all of the other transuranium elements except neptunium, are radiological poisons and must be handled with very special equipment and precautions. Plutonium is a very dangerous radiological hazard. Precautions must also be taken to prevent the unintentional formulation of a critical mass. Plutonium in liquid solution is more likely to become critical than solid plutonium. The shape of the mass must also be considered where criticality is concerned. 

The determination of critical si2e or mass of nuclear fuel is important for safety reasons. In the design of the atom bombs at Los Alamos, it was cmcial to know the critical mass, ie, that amount of highly enriched uranium or plutonium that would permit a chain reaction. A variety of assembhes were constmcted. Eor example, a bare metal sphere was found to have a critical mass of approximately 50 kg, whereas a natural uranium reflected 235u sphere had a critical mass of only 16 kg. 

Sloat, R.J., Critical Mass Control Specification Transporting of Plutonium-Bearing Materials by Motor Truck, HW-74716, General Electric Company, Richland, WA, 1963. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

A single kilogram of radioactive metallic plutonium-238 produces as much as 22 million kilowatt-hours of heat energy. Larger amounts of Pu-238 produce more heat. However, Pu-238 is not fissionable, and thus it cannot sustain a chain reaction. However, plutonium-239 is fissionable, and a 10-pound ball can reach a critical mass sufficient to sustain a fission chain reaction, resulting in an explosion, releasing the equivalent of over 20,000 tons of TNT. This 10-pound ball of Pu-239 is only about one-third the size of fissionable uranium-235 required to reach a critical mass. This makes plutonium-239 the preferred fissionable material for nuclear weapons and some nuclear reactors that produce electricity. 

The most common use of plutonium is as a fuel in nuclear reactors to produce electricity or as a source for the critical mass required to sustain a fission chain reaction to produce nuclear weapons. Plutonium also is used to convert nonfissionable uranium-238 into the isotope capable of sustaining a controlled nuclear chain reaction in nuclear power plants. It takes only 10 pounds of plutonium-239 to reach a critical mass and cause a nuclear explosion, as compared with about 33 pounds of fissionable, but scarce, uranium-235. 

Plutonium is the most important transuranium element. Its two isotopes Pu-238 and Pu-239 have the widest applications among all plutonium isotopes. Plutonium-239 is the fuel for nuclear weapons. The detonation power of 1 kg of plutonium-239 is about 20,000 tons of chemical explosive. The critical mass for its fission is only a few pounds for a solid block depending on the shape of the mass and its proximity to neutron absorbing or reflecting substances. This critical mass is much lower for plutonium in aqueous solution. Also, it is used in nuclear power reactors to generate electricity. The energy output of 1 kg of plutonium is about 22 million kilowatt hours. Plutonium-238 has been used to generate power to run seismic and other lunar surface equipment. It also is used in radionuclide batteries for pacemakers and in various thermoelectric devices. 

Fig 2 Critical Masses of Uranium and Plutonium (Data from Ref 24 as cited in Ref 29. The rapid increase in its critical mass makes iso-topically dilute U unusable as an expl. This is not true for Pu, making it a greater proliferation hazard)... 

Uranium-235 releases an average of 2.5 neutrons per fission, whereas plutonium-239 releases an average of 2.7 neutrons per fission. Which of these elements might you therefore expect to have the smaller critical mass ... 

An atomic bomb therefore has a core containing uranium or plutonium at a center. For a nuclear explosion the amount of the dore has to be greater than the critical mass that may explode itself. Therefore, 4he explosive core is divided into different portions and placed into the bomb. When the atomic bomb is going to be ignited, these portions have to come together and form a spherical shape. In order to form the spherical shape, trinitrotoluene (TNT) is used. First, TNT is exploded and then the nuclear explosives come together and the main explosion takes place. 

Plutonium presents particular problems in its study. One reason is that, since Pu is a strong o -emitter (ti = 24,100 years) and also tends to accumulate in bone and liver, it is a severe radiological poison and must be handled with extreme care. A further problem is that the accidental formation of a critical mass must be avoided. 

The metallurgical properties of metallic plutonium are even more unfavourable than those of uranium. The melting point of Pu is 639 °C and six solid phases are known. Furthermore, the critical mass of a reactor operating with pure Pu as fuel is below 10 kg, and it would be very difficult to take away the heat from such a small amount of material. A great number of plutonium alloys have been investigated with respect to their possible use as nuclear fuel, but they have not found practical application. 

Pu and can be used as nuclear explosives, because they have sufficiently high cross sections for fission by fast neutons. By use of the equations in section 11.1, it can be assessed that, in the absence of a reflector, a sphere of about 50 kg uranium metal containing 94% or a sphere of about 16 kg plutonium metal ( Pu) is needed to reach criticality. If a reflector is provided, the critical masses are about 20 kg for and about 6 kg for Pu. The critical masses for are similar to those for Pu. 

With uranium-235 separation started at Oak Ridge and plutonium-239 production under way at Hanford, a third laboratory was set up at Los Alamos, New Mexico, to work on bomb design. In order to create an explosion, many nuclei would have to fission almost simultaneously. The key concept was to bring together several pieces of fissionable material into a so-called critical mass, hi one design, two pieces of uranium-235 were shot toward each other from opposite ends of a cylindrical tube. A second design used a spherical shell of plutonium-239, to be detonated by an implosion toward the center of the sphere. 

Critical mass—The minimum amount of fissionable uranium or plutonium that is necessary to maintain a chain reaction. 

The critical mass of, for example, a sphere of pure plutonium-239 metal in its densest form (alpha-phase, density 19.8 g/cm) is about 10 kg. The radius of the sphere is about 5 cm, about the size of a small grapefruit. If the plutonium sphere were surrounded by a natural uranium neutron reflector, about 4.4 kg, the radius of the sphere would be about 3.6 cm, about the size of an orange. A 32 cm thick beryllium reflector reduces the critical mass to about 2.5 kg, a sphere with a radius of 3.1 cm, about the size of a tennis ball. 

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