Decay of radionuclides

Alpha (a) particles An a particle is composed of two protons and two neutrons, with a charge of +2 essentially, it is a helium nucleus without orbital electrons. Alpha particles usually originate from the nuclear decay of radionuclides of atomic number >82, and are detected in samples containing U, Th, or Ra. Alpha particles react strongly with matter and consequently produce large numbers of ions per unit... 

Although comets are not expected to have experienced the thermal processing that asteroids have, some of the larger KBOs and Oort cloud objects may have been heated by decay of radionuclides. The relative proportions of rock and ice may determine the amount of heating, as radionuclides occur in the rock fraction. Comets and asteroids may have had similar impact histories, and many of these bodies may now be collisional fragments. 

The decay of radionuclide 232Th (t1/2 1.41 x 1010a) via the thorium decay line results in the emission of six alpha and four beta particles leading to the formation of the stable and most abundant lead isotope 208Pb ... 

Similar equations describe the decays of radionuclide to radiogenic pb and of Th to 2° Pb. The decay of radionuclide (t, 2 0.72 x 10° a) gives rise to the actinium series, which ends in the stable 2° Pb after the emission of seven a ( He) decays and four 3 decays ... 

Details of the decay of radionuclides are recorded in the form of decay schemes, in which the energy levels are plotted and the half-lives, the nuclear spins, the parity and the transitions are indicated. Nuclei with higher atomic numbers are put to the right, and energies are given in MeV. As an example, the decay scheme of is plotted in Fig. 5.1. 

Radioactive materials have been present in the environment since the accretion of the Earth. The decay of radionuclides provides an important source of heat that drives many large-scale planetary processes. The most abundant naturally occurring radionuclides are Th, and and The bulk of the natural global inventory of actinide radioactivity in the upper 100 m of the... 

The four 30,000-gal stainless steel tanks are buried in a horizontal position. They are 12 ft in diameter and have a 38-ft-long straight side. The shell and head of these tanks are 11/16 and 9/16 in. thick, respectively. They also are equipped with cooling coils and are connected to a shell-and-tube condenser to remove heat produced by the decay of radionuclides. These tanks rest on a concrete pad but are not surrounded by a vault. A curb surrounds the concrete pad, and a sump equipped with a level alarm collects any leakage from the tanks. A 24-in.-diameter pipe extends from the surface of the ground to the sump so that a portable unit can be used to empty the sump if necessary. 

Radioactive decay is an exception to this statement. The technique of neutron activation analysis is based on the measurement of the spontaneous decay of radionuclides created by irradiation of a sample in a nuclear reactor. 

The decay of radionuclides is represented by a decay scheme, an example of which is given in Fig. 1.3. 

Neutrino (v). A particle of no charge and mass emitted with variable energy during / + and electron capture decays of radionuclides. 

Using Eq. (7.15), one can introduce a more expressive quantity than the decay constant for the description of the decayability of radionuclides. The half-life Tyz is the time in which half of the atoms are expected to decay ... 

The external dose results primarily from y radiation arising from the decay of radionuclides at locations outside the specified body. Secondarily, the exposures from neutron radiation and to the skin from P radiation may be considered as an external pathway. The a, proton, and ion radiations are to be considered as an external impact only in very special cases, for example, for a geometry extremely close to the eyes. 

Extraterrestrial materials In some extraterrestrial material such as meteorites, elements may show isotopic compositions that are distinct from all terrestrial material investigated. This is related to decay of radionuclides that may already be extinct, due to half-lives which are very short compared with the age of the solar system of 4.6 x 10 years. Such variations are rare for terrestrial materials, in large part due to preferential sampling of the crust, whereas some extraterrestrial material, such as iron meteorites, resemble the Earth s core, in which parent to daughter element ratios may be much higher than in the cmst. 

Immediate Dismantling - Dismantling commences soon after shutdown of the plant (typically within 5 years) with radioactive material above a specific level being removed. This strategy does not allow for any significant decay of radionuclides. 

Deferred Dismantling or Safe Enclosure (Safestore) - Those parts of the facility containing radioactivity are processed or brought into a condition such that they can be stored and maintained in a safe manner (e.g., liquids are drained from the system and irradiated fuel and operational waste materials removed). The facility is placed in long-term storage (e.g., 50 years) prior to later dismantling. This option allows for decay of radionuclides. 

Most of the wastes dumped was low- and intermediate-level, but spent fuel and reactors fi-om nuclear submarines introduce high-level wastes. Since the time of the dumping total inventory of radioactivity has reduced due to the natural decay of radionuclides. Some of calculations suggest that less than 130,000 curies remains in the reactors and spent fuel deposited in the Arctic [OTA, p. 49]. 

---