Fission Nuclides

The fissionable nuclide must be concentrated enough to become critical. 

Subcritical portions of this fissionable nuclide must be combined into a critical mass. 

The critical mass of a fissionable material is the minimum mass of a particular fissionable nuclide in a set volume that is necessary to sustain a nuclear chain reaction. 

Just as earlier we were able to observe mass-yield distributions of the fission products from the fissionable nuclide used in the Chinese nuclear device, it is possible to see part of the mass-yield curve from the fission of 244Pu, which was synthesized originally in a supernova. Figure 6 shows the mass-yield distribution of the excess fissiogenic xenon observed in the meteorite Pasamonte (15). 

In early work gross counting of delayed neutrons was used to determine the abundance of a single fissionable nuclide known to be in the sample. Brownlee 101> has reported techniques by which two or more fissionable species may be determined at the submicrogram level in a single irradiated sample. Nuclides fissionable only with fast neutrons may also be determined by this technique. One of the more interesting applications of the method is in the non-destructive determination of uranium and thorium at trace levels in minerals, rocks, and stony meteorites 102,108). 

Fissionable Nuclides (includes fission) Moderating Materials ... 

Due to the production of neutrons, spontaneously fissioning nuclides are of practical interest as neutron sources. An example is Cf ( i/2 = 2.64 y), which is used for neutron activation. 

The prior presence of " Pu, the only transura-nic nuclide known to have been present in the early solar system, can be inferred from its spontaneous-fission decay branch, through production of fission tracks and, more diagnostically, by production of fission xenon and krypton. The identification of " Pu as the fissioning nuclide present in meteorites is unambiguous, since the meteoritic fission spectrum is distinct from that of but consistent with that of artificial " Pu (Alexander et al, 1971). The demonstration of the existence of " Pu in the solar system reinforced the requirement (from the presence of I) of a relatively short time between stellar nucleosynthesis and solar-system formation and made it incontrovertible, since while it might be possible to make in some models of early solar system development, the rapid capture of multiple neutrons (the r-process) needed to synthesize Pu could not plausibly be supposed to have happened in the solar system. 

In this concept, the transuranium elements are considered as waste, due to their quantity and value (table 1), since they contribute only 0.6 % to the total spent fuel and from that amount only 5 % (239Pu and 2i+1Pu) are fissionable nuclides. 

On-line Experiments with Spontaneously Fissioning Nuclides... 

Pu. The isotope Pu is produced by neutron capture in Pu. It is not fissionable by thermal neutrons, but, like all other plutonium isotopes, it fissions with fast neutrons. Pu is converted to a fissionable nuclide by neutron capture. Therefore, like Th and it is a fertile material. It undergoes alpha decay, with a half4ife of 6580 years, to form which then decays to Th, the parent of the 4n decay series discussed in Chaps. 6 and 8. Like the other even-mass plutonium isotopes, Pu produces neutrons by spontaneous fission. It is present in greater concentration in reactor plutonium than any of the other even-mass plutonium isotopes. 

Darmstadtium — (Darmstadt, city in Germany), Ds. In 1987 Oganessian et al., at Dubna, claimed discovery of this element. Their experiments indicated the spontaneous fissioning nuclide with a half-life of 10 ms. More recently a group led by... 

The SF process that results in two nearly equal mass fragments (a process called symmetric fission ) has been observed in Fm (1.5 s). More commonly, SF occurs as asymmetric fission, a split of the parent radionuclide into two unequal large FF. As in neutron-induced fission, many different asymmetric mass (and charge) divisions with varying yields can result, with mass numbers from about 70 to 170, each with many isotopes. Hundreds of different nuclides can be produced. Figure 2.1 displays the predominantly asymmetric mass yields as a function of mass number (dubbed mass-yield curves ) that have been measured for several SF and neutron-induced fission nuclides. 

Indirect methods such as half-life systematics, excitation functions for the production reactions, and cross bombardments have been used to reinforce this information. In order to positively identify the atomic number of a spontaneously fissioning nuclide from detection of the fragments, the atomic numbers of both primary fragments from the same SF event must be determined in coincidence and added together to determine the Z of the new, unknown fissioning nuclide. Detection of only SF decay has resulted in much controversy concerning discovery and identification of the transactinide elements. 

There also is the usual problem of positively determining the atomic number of the fissioning nuclide whose chemistry was being studied. 

Although other technologies are now coming into use for this purpose, gaseous diffusion has played an important role in the enrichment of uranium for use in nuclear reactors. Natural uranium is mostly 2, which cannot be fissioned to prodnce energy. It contains only about 0.7% of the fissionable nuclide IfU. For uranium to be useful as a nuclear fuel, the relative amount of IfU must be increased to abont 3%. In the gas diffusion enrichment process, the natnral nraninm (containing IfU and a small amount of 9iU) reacts with fluorine to form a mixtnre of and UFg. Because these... 

The so-called average -energy of fission nuclides arising as decay... 

In the r on of symmetric fission or in the neighborhood of shell closures, the four parameters behave differently and show some abrupt changes. The behavior of the four parameters mentioned, as a function of the fission fi- ment mass and also as a function of the mass, charge, and excitation energy of the fissioning nuclide, is the subject of the fission yield systematics mentioned (Wahl 1988, 1989 IAEA 2000). A version (YCALC) of the model calculations is attached to ref. IAEA 2000, and it is also available for downloading (YCALC 2003). 

A beam of Ne or was directed on a target of By the impact, the fission isomers formed were projected onto a rotating collector wheel and transported in front of two ionization chambers. Any fission fragments formed from a spontaneously fissioning nuclide on the wheel were to be detected by the ionization chambers. The beam of Ne or O ions was stopped in a Ta-collector connected to a current meter for monitoring the beam intensity. The apparatus was equipped to allow a calibration of the chambers using fission fragments from the reaction U(nth,f). 

The ratio of count rates in the two ionization chambers at a known rotational speed of the wheel (800-1,400 rpm) allowed to calculate the half-life of the spontaneously fissioning nuclides. The value obtained was 0.02 s - about lo times shorter than expected for a normal spontaneously fissioning nuclide of this fissility parameter (see O Fig. 4.4). (The shortest-lived normal spontaneously fissioning nuclide that could be considered is pm formed in the reaction ( 0,2n) Fm. Its fissility parameter would be Z /A = 39.7. The half-life extrapolated from the systematics (O Fig. 4.4) would be of the order of 10 -10 s. The species observed here was later identified to be a shape isomer of Am (fissility parameter of 37.3). The ground state of Am decays by P decay and electron capture with a half-life of 16 h.)... 

Mass number assignments were based on quantitative analysis of the chemically resolved fractions. Fission is known to produce primarily products with neutron/proton ratios similar to the fissioning nuclide (here two to three neutrons less which accompany the fragments). For A = 147, the most likely products are 4-s La and 56-s which join their short-lived isobars by quickly cumulating in 11-day Very little Pm is produced as a primary... 

O Table 56.7 lists independent fission product yields (England and Rider 1994) per 100 fissions for the most important fissionable nuclides. For thermal, fast neutron (fission spectrum), and high-energy neutron (14 MeV) induced, for fast neutron induced,... 

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