Depth profiles radionuclides

Solution of equation (10) which involves sedimentation in the presence of mixing and that of equation (11) which contains the sedimentation term only, are exponential in nature. The major conclusion which arises from this is that the logarithmic nature of the activity-depth profiles by itself is not a guarantee for undisturbed particle by particle sediment accumulation, as has often been assumed. The effects of mixing and sedimentation on the radionuclide distribution in the sediment column have to be resolved to obtain pertinent information on the sediment accumulation rates. (It is pertinent to mention here that recently Guinasso and Schink [65] have developed a detailed mathematical model to calculate the depth profiles of a non-radioactive transient tracer pulse deposited on the sediment surface. Their model is yet to be applied in detail for radionuclides. )... 

Figure 9. Gamma-emitting radionuclide depth profiles at Sedan crater lip, 16A site...

The ejecta layer was a shallow surface stratum at distances between 1000 and 3000 feet from the crater lip. The radionuclide depth profiles at 1500 feet were studied in detail to determine whether or not any radionuclide had been leached from the surface stratum of radioactivity. At... 

Olsen, C.R., Simpson, H.J., Peng, T.H., Bopp, F., and Trier, R.M. (1981) Sediment mixing and accumulation rate effects on radionuclide depth profiles in Hudson Estuary sediments. J. Geophys. Res. 86, 11020-11028. 

Comparative studies of neodymium measurements in UOj fuel for the future application of local burn up calculations have been carried out by the SIMS analysis of a Nd implanted UOj single crystal. Samples for a round robin test were produced by the implantation of " Nd (5 x 10 ° at cm , 400 keV) in a UOj single crystal. Different depth profiles of the Nd "/ U+ ratio and especially the NdO /Nd, UO+/U+ and U02 /U+ ratios obtained by SIMS are not constant over time and indicate evidence of some fluctuations during the analysis. SIMS plays a dominant role for particle analysis, but also LA-ICP-MS with lower spatial resolution can be applied to identify anomalies in the isotopic composition of radionuclides. 

Fig. 27. Schematic representation of depth profiles of excess radionuclides (1,2,3) in a mixed deposit. Decay constants are assumed to decrease in the order k, > k, and...

If the interest in radionuclide distribution extends beneath the soil surface, coring is the preferred sampling technique. Cores are taken to a known depth at the sampling site. Depth profiles are separated at selected vertical intervals to provide information on area deposition and downward movement of a radionuclide. Numerous cores may have to be taken because radionuclide retention can vary with changes in soil constituents in a relatively small area. Samples at greater depths can be obtained from well-drilling cores. 

Besides the chemical and radiochemical composition, other properties of the collected materials are also often of interest, such as the natine of the chemical compounds present in these substances. For example, the structure of oxide compounds after isolation from the base material or from the coolant is analyzed by X-ray diffractometry or by Mdssbauer spectrometry. Other microanalytical techniques can be directly applied to oxide layers deposited on surfaces, e. g. of steam generator tube sections. Examples in this field are Auger electron spectroscopy for the determination of element concentrations in micrometer areas and X-ray induced photoelectron spectroscopy for the determination of the chemical states of the individual elements. In order to obtain depth profiles over the thickness of the oxide layer, these techniques often are combined with an argon sputtering process (e. g. Schuster et al., 1988), which removes nanometer fractions from the swface prior to the next analysis step. By y spectrometry of the specimen after each sputtering step, the profile of the radionuclides in the oxide layer can also be determined. 

Measure depth profile in rock after exposure to radionuclide solution... 

In addition to the identification of the important sorbing mineral phases, defocused ion-beam techniques can be used to provide depth profiles for strongly sorbing radionuclides that have short penetration depths. These profiles can be used to derive diffusion data and sorption coefficients, subject to assumptions regarding access to porespace within the sample. 

Equation (9) is quite generalized and allows for variations in the mixing rate, K, and in situ density, p, with depth in determining the activity-time relationship of radionuclide profiles. However, in all commonly used models, equation (9) is further simplified using assumptions such as K, p and S to be... 

Due to preferential scavenging and lateral transport of a daughter radionuclide, the activity of daughter Ap can be greater than that of the parent Ap in sediments. The inputs of daughter radionuclides that are not directly from the in situ decay of the parent (supported) are termed unsupported or excess activity. The unsupported Ap is equal to the supported A ) minus the Ap, as shown in the theoretical radionuclide profiles in figure 7.3. Moreover, the curve for the unsupported Ap decreases with depth more than the supported Ap because it is not being produced in situ from the parent. Consequently, the excess activity of a radionuclide can be used to calculate the time elapsed since the particles with unsupported Ap were last at the surface, relative to a particular depth (A). However, to calculate this it must be assumed that the sedimentation rate and supply of unsupported Ap has remained constant over time. 

Figure 7.3 Theoretical radionuclide profiles of unsupported or excess, Ad, which is equal to the supported A ) minus the Ap. The curve for the unsupported Ap decreases with depth more than the supported Ap because it is not being produced in situ from the parent. (Modified from Libes, 1992.)...

On 7 April 1989, a fire broke out in the stem section of the Komsomolets nuclear submarine. The submarine sank to a depth of 1685 m at 73°43T6"N, 13°15 52"E, near the south-west of Bear Island. The site is about 300 nautical miles from the Norwegian coast. The wreck contains one nuclear reactor and two nuclear warheads, one of which was fractured. The radionuclide inventory includes 1.5 PBq Sr, 2 PBq Cs, about 16 TBq " Pu in the two warheads and 5 TBq of actinides in the reactor s core. During June/July 1994, an international expedition to the Komsomolets site at the request of the Russian Federation was organised. The objectives of the scientific cmise on board the R/V Mstislav Keldysh were to close nine door holes, including torpedo tubes, by capping them with titanium metal cover caps, and to sample and monitor for ambient radioactivity. A series of 280-600 1 sea-water samples collected in profile, a suite of surface sediments and cores and various biota samples were returned to lAEA-MEL for analysis. The results showed that a very limited leakage of caesium and tritium had occurred from the submarine. 

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