Neutron sources, actinide elements

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

Decay by SF has been observed in the elements heavier than thorium (Z = 90). At californium (Z = 98), SFbegins to compete favorably with alpha-particle emission as a mode of decay, and becomes the primary decay mode for many of the higher atomic number actinides and the transactinides. As with all fission, SF releases neutrons. Unlike the induced fission process described in Section 2.3.2, SF takes place without addition of energy. Radioisotopes for which SF is an important decay mode may be used as neutron sources. 

Many schemes have been considered for disposal of both fission products and actinide elements. A succinct and informative discussion of these proposals has been given by Choppin and Rydberg [66]. Nuclear incineration is one possibility. For the actinide elements, which are the predominant source of radioactivity after about 600 years, prolonged neutron irradiation in an ordinary nuclear reactor, or... 

The principal application of the actinide elements is in the production of nuclear energy. Although this is by far the most important use for any of the actinide elements, a surprising number of other uses have been found. These include the use of short-lived actinide isotopes as portable power supplies for satellites in ionization smoke detectors in the therapy of cancer in neutron radiography in mineral prospecting and oil-well logging as neutron sources in nuclear reactor start-up and as neutron sources in a variety of analytical procedures, the most important of which are neutron activation analysis and heavy-ion desorption mass spectroscopy. 

The decay heat power comes mainly from five sources (1) unstable fission products, which decay via a, p-, p+, and y ray emission to stable isotopes (2) unstable actinides that are formed by successive neutron capture reactions in the uranium and plutonium isotopes present in the fuel (3) fissions induced by delayed neutrons (4) reactions induced by spontaneous fission neutrons (5) structural and cladding materials in the reactor that may have become radioactive. Heat production due to delayed neutron-induced fission or spontaneous fission is usually neglected. Activation of light elements in structural materials plays a role only in special cases. 

As opposed to the lanthanide elements, all of which are found in nature except for Pm, less than half of the actinides are naturally occurring. The first six actinides, Ac through Pu, have been found in nature, although Np and Pu exist in only minute amounts as a result of natural neutron reactions in some uranium ores. Practicably, only Th and U are extracted from natural sources (small amounts of Pa have also been obtained). The remaining nine elements. Am through Lr, are available only as man-made materials. For these latter elements, there are many different isotopes that can be prepared by various synthetic, nuclear reactions. A discussion of all of these isotopes is beyond the scope of this chapter and the reader is referred to other sources (Seaborg and Loveland 1990, Ahmad and Fields 1986). However, it is pertinent in a discussion of actinide oxides to mention certain aspects of sources and specific isotopes of the actinides as they pertain to the solid-state science of the oxides. 

The principal sources of radioactivity in the reactor are the fission products, supplemented by the transuranic elements, or actinides, formed as a result of successive captures of neutrons in the uranium fuel. The fresh fuel loaded into the core is only mildly radioactive (about 300 Ci for the initial core of a typical BWR), but the activity increases steadily over the core life to a value of the order of 1.7 x 10 Ci just prior to refueling. 

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