Very long-lived nuclides

Very long-lived nuclides lighter than lead 97... 

The reason that nuclear wastes must be isolated from the environment for a very long time is that they contain relatively long-lived radioactive nuclides, such as technetium-99 with a half-life of over 2.1 x 10 years. One proposed solution is to bombard the waste with neutrons so as to convert the long lived nuclides into nuclides that decay more quickly. When technetium-99 absorbs a neutron, it forms technetium-100, which has a half-life of 16 seconds and forms stable ruthenium-100 by emitting a beta particle. Write the nuclear equations for these two changes. 

Albarede (1998) argued that the model mantle cannot be in steady state due to the progressive decay of the parent elements in the upper mantle, and so is selfcontradictory. However, the notion of steady state upper mantle conditions clearly is an approximation. While a strict steady state does not hold, the deviation from this due to decay of the very long-lived parent nuclides over the residence time of upper mantle noble gases is minor. 

Finally, the existence of SHEs in nature remains an open question. Experimental results and new calculations do not exclude the possibility that the very heavy elements can be found in nature (O Fig. 19.26). In contrast to numerous earlier searches that concentrated on long-lived nuclides expected to be seen around the magic proton number Z = 114, new calculations suggest a search for hassium or a lighter chemical element may be more productive. For these elements, the most stable species are expected with fission half-lives of the order of 10 to 10 years (Smolanczuk 1997) comparable to the half-life of New investigations have already begun at Dubna (Oganessian 2007) with renewed interest. 

Accelerator Mass Spectrometry is a special tool for quantitative measurements of very long-lived radionuclides. This technique consists of counting the nuclides themselves instead of waiting for their decay. As for as the tandem accelerator is concerned, no molecular background exists because of the stripping process. Any isobaric background is also drastically suppressed due to the nuclear techniques involved in the measurements. There occurs an extremely wide area of applications in different fields because of the capability of the AMS technique to determine the radioisotopes at the faintest level (the isotopic concentrations down to the range of 10 in some cases). 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

Figure 1. Number of stable isotopes relative to atomic number (Z) for the elements. Mono-isotopic elements shown in gray diamonds. Elements discussed in this volume are shown as large gray circles. Other elements that have been the major focus of prior isotopic studies are shown in small white circles, and include H, C, O, and S. Nuclides that are radioactive but have very long half-lives are also shown in the diagram.

Deep geological disposal is the most favored solution for the permanent disposal of nuclear wastes with long half-lives. Although the locations of the burial places are selected with outmost care to avoid migration of the wastes in nature over a very long period of time, no barrier can be safe forever, so, numerous studies are in progress to determine the main factors that could cause leaks of radioactive nuclides. Soluble compounds in ground water are likely to play a major role in the release of actinides. 

A selective elution technique from the lon-exchange column Is often successful for a very rapid Isolation of a short-lived daughter nuclide from relatively long-lived parent activities which are strongly held to the Ion-exchange column. 

A number of short-lived radionuchdes also existed at the time that the Sun and the rocky bits of the solar system were forming (Table 1). These nuclides are sufficiently long-lived that they could exist in appreciable quantities in the earhest solar system rocks, but their mean fives are short enough that they are now completely decayed from their primordial abundances. In this sense they are referred to as extinct nuchdes. Although less familiar than the still-extant radionuclides, these short-lived isotopes potentially play similar roles their relative abundances can, in principle, form the basis of various chronometers that constrain the timing of early chemical fractionations, and the more abundant radioisotopes can possibly provide sufficient heat to drive differentiation (i.e., melting) of early accreted planetesimals. The very rapid rate of decay of the short-lived isotopes, however, means that inferred isotopic differences translate... 

Naturally occurring nuclides of very long half-life which have persisted since the formation of the Earth, and their shorter lived daughter nuclides. 

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