Thorium specific activity

Metzger R, McKIveen JW, Jenkins R, et al. 1980. Specific activity of uranium and thorium in marketable rock phosphate as a function of particle size. Health Phys 39 69-76. 

According to strict interpretation of transuranic, these wastes could be so classified only if they contained sufficient amounts of elements having an atomic number greater than 92. Despite this, wastes that contained sufficient amounts of 233U and other alpha-emitting non-transuranium radionuclides often were classified and managed as transuranic waste because their specific activities (activities per unit mass) are more similar to those of the transuranium elements than to natural uranium or thorium. 

The decay constant of 220Rn is 6000 times greater than that of 222 Rn, so from (1.4) L is about 80 times less, that is only about 1 cm. The fraction of 220Rn atoms which escape from the rock crystals to the interstitial air is apparently about the same as for 222Rn, and since the specific activities of the thorium and radium chains are similar, equation (1.6) implies that the emanation of 222Rn should be 80 times greater... 

Hutchings (170) plotted (Figure 31) the activity against the surface area for a number of promoted catalysts and deduced that most of the catalysts conform to a linear correlation. The only enhancement of the specific activity was observed for the cerium-promoted catalyst. This result shows that care must be taken in the interpretation of the catalyst performance data, particularly when catalysts prepared by different methods are compared. In a separate study, Hutchings and Higgins (171) found that chromium, niobium, palladium, antimony, ruthenium, thorium, zinc, and zirconium each had very little effect on the specific activity of (VO)2P207. A significant increase in surface area was observed with zirconium, zinc, and chromium, which could be of use as structural promoters. Iron-, cesium-, and silver-doped catalysts decreased the specific activity, and cobalt and molybdenum were the only promoters found to increase the specific activity. 

Thorium is a radioactive element and a natural a-emitter (specific activity 4250 Bq/g and half-life 1.4 x lO a). Health and environmental considerations (evaporation and inhalation of thorium during welding disposal of grinding dust) have therefore in the past led to a partial replacement of thoriated tungsten for welding operations and in the lighting industry by thoria-free ODS tungsten alloys [6.24]. 

As the detection technique for radioactivity has been refined, a number of long-lived radionuclides have been discovered in nature. The lightest have been motioned in 5.1. The heavier ones, not belonging to the natural radioactive decay series of uranium and thorium, are listed in Table 5.2. is the nuclide of lowest elemental specific activity ( 0.(XX)1 Bq/g) while the highest are Rb and Re (each —900 Bq/g). As our ability to make reliable measurements of low activities increases, the number of elem ts between potassium and lead with radioactive isotopes in nature can be expected to increase. 

The term unirradiated thorium in the definition of low specific activity material is intended to exclude any thorium which has been irradiated in a nuclear reactor so as to transform some of the Th-232 into U-233, a fissile material. The definition could have prohibited the presence of any U-233, bnt naturally occurring thorium may contain trace amounts of U-233. The limit of 1(U g of U-233 per gram of Th-232 is intended to clearly prohibit any irradiated thorium while recognizing the presence of trace amounts of U-233 in natural thorium. 

Many individual components in the series are referred to by names derived from their positions in the series and other details that are no longer of much interest except to reveal the history of discovery of structure of each series. The most common names of this type for the Th series are given in O Fig. 13.1. Since Ra and Ac are sandwiched between two isotopes of thorium, they were called mesothorium 1 and 2 meso intermediate or in the middle), with symbols MsThi and MsTh2- Th has higher specific activity (activity per unit mass) than Th and was called radiothorium (RdTh). Tn ( Rn) is short for thoron, the gaseous component similar to radon from radium. Relatively simple chemical procedures could show that there were three species following Tn, and these were named... 

The most significant chemical property of most members of the chains is that each element is carrier-free — there are no inactive atoms to compete with the radioactive ones for chemical sites, such as ion-exchange sites on the inner surface of a container. In extreme cases, such as polonium in the Th chain, there will be few if any atoms at a given time. The specific activity of Th is 4,070 Bq/g. If 1 g of thorium is in equilibrium with its chain, the activities of 0.15 s Po and 0.3 ps Po will be 4,070 Bq and 2,609 Bq (64.1%), respectively. On average there will be 881 atoms of Po present at any given time. Only 0.1% of the time is even one atom of Po present. Francium is even more rare. It occurs only in the chain (and the extinct 4n + 1), and then only in a 1.4% branch. There are only a few grams of Fr in the entire crust of the Earth. 

In chemical investigations, thorium is often used as an analog for tetravalent actinides. The reasons are that it has only one, stable valence state and also that it is more easy to handle than, e.g., Pu(rV). Th is, however, not always suitable because of its low specific activity. In, e.g., solvent extraction experiments Th Ty = 24.1 days) is more convenient because the chemical concentration can be kept at a tracer level. Tb is formed in the decay of and, thus it can be isolated from uranium. Here one procedure will be described in more detail. 

Similarly, natural thorium is a mononuclidic element, i.e., it has only one radioactive isotope thorium-232, parent radionuclide of the natural decay chain (4n), therefore, for a naturally occurring radioactive material containing a mass fraction of natural thorium, the specific activity of the parent radionuclide is given by ... 

Therefore, for natural thorium, the specific activity of the radionuclide Th is 4.046 MBq.kg. However, because the radionuclide is also in secular equilibrium with all its decaying daughters, the total specific activity of natural thorium in secular equilibrium is given by the previous specific activity times the number of decaying radionuclides in the 4n decay chain as follows ... 

It is concluded from this review that measurements of Bi-212, Tl-208 and Pb-212 all give consistent estimates of Th-232 by y-spectrometry and Ra-224 does not. Experience is that Pb-212 is detected with the highest reliability. The mean value Th-232 at Dounreay, assumed to be in equilibrium with Pb-212, is 0.027 Bq/g, which exceeds the threshold of radioactive waste unless the material is natural. The value is within the range reported above for natural soils. Although there are no features of the decay chain that would allow manufactured thorium materials to be separated from natural materials since secular equilibrium is re-established so rapidly, because pure thorium has a specific activity of 4100 Bq/g, activity concentrations alone will distinguish such material. 

McCoy. H. N. and Ross, W. H. The specific radioactivity of thorium and the variation of the activity with chemical treatment and with time. J. Am. Chem. Soc. 29, 1709-1718 (1907). 

Some of the methods commonly used for the determination of thorium in biological materials are given in Table 6-1. The colorimetric methods are not capable of isotope-specific determination of thorium isotopes. Alpha spectrometric and neutron activation analysis are useful in the quantification of isotope-specific thorium and thorium-232, respectively, and have better sensitivities than colorimetric methods. Alpha spectrometry is the commonly used isotope-specific analysis for the determination of thorium-232 and the thorium-230 derived from the decay of uranium-238 (Wrenn et al. 1981). Standard reference materials (SRMs) containing thorium in human liver (SRM-4352) and human lung (SRM-4351) necessary for the determination of absolute recovery in a given sample are available from the National Institute of Standards and Technology (Inn 1987). 

In the second (or development) phase, effort in the process types being pursued consists largely of laboratory experimental work and engineering scale-up aimed at providing process information. This activity includes experiments to define the behavior of specific important elements uranium, thorium,... 

Naturally occurring oxoanions like SO/ and H2P04 at concentrations representative of those encountered in natural waters can inhibit dissolution and weathering reactions. A very low concentration of inhibitors can often be effective, because it may suffice to block the functional groups of solution-active sites (such as the kink sites). The effect of specifically adsorbable cations on the reduction of dissolution (weathering) rates of minerals is important. A case was documented by Grandstaff (32), who showed that thorium, Pb(II),... 

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