U-238 decay chain

Mr. Petr discussed the source term foimd in mining wastes, as a function of components of the U-238 decay chain and the relative equilibrium between U-238, Ra-226, and the contribution from the Th-232 series. Contaminated items (e.g., scrap metals) can be characterised by using approximate mass to surface specific activity ratios. 

Adsley, I, Backhouse, J.S., Nichols, A.L. and Toole, J. (1998). U-238 Decay Chain Resolution of Observed Anomalies in the Measured Secular Equilibrium Between Th-234 and Daughter Pa-234m, Appl. Radiat. Isotopes, 49, 1337-1344. 

Both decay chains involve a gaseous isotope. Rn-220 in the Th-232 decay chain is short-lived (<1 min) and is unlikely to diffuse from solids Rn-222 in the U-238 decay chain is longer-lived (3.8 d) and diffusion before daughters grow in is more credible. Short-lived daughters after the radon gap will exist for analytical purposes if their radon parents are present. It is noted that Pb-210 at the end of the U-238 chain is long-lived on the time scale of sample analysis (22 y), and will remain after sample collection even if Rn-222 is lost subsequently. 

Results for U-238 Decay Chain. Considering the U-238 chain, Pb-214 was chosen as the reference nuclide for the data review, since it is has the smallest number of results in the dataset below detection (7/1865). The Pb-214 results from all three labs are similar, although those from Lab 3 are typically higher than those from the other two labs and the difference is statistically significant. 

Empirically, a better fit is achieved by increasing the estimated p-emission from the Th-232 and U-238 decay chains by around x3.4. This is unphysical, equating to -10+13 decays, rather than the actual 4+6. The discrepancy is left unexplained here and the factor is treated simply as an empirical calibration term. In summary, there is an effective gross-P activity of around 0.9 Bq/g associated with these nuclides in materials from Dounreay. This is split approximately 0.7 Bq/g from K-40 and 0.2 Bq/g from Th-232 and U-238 daughters. It is less than the typical measured gross-p activity of 1.3 Bq/g (Labs 2 and 3)... 

One-sixteenth will remain after 4 half-lives, so 4 X 30 = 120 years. 71. Eight alpha particles and six beta particles are emitted in the decay chain from U-238 to Pb-206. (The numbers are the same for the alternate routes.)... 

Some of the neutrons released in the controlled chain reaction strike the nuclei of non-fissionable U-238 atoms. In this case, the U-238 captures the neutron and becomes a heavier uranium isotope, U-239, which eventually decays to produce plutonium-239. 

Transmutation-decay chains for U-238 and Th-232. (From Knief, R.A., Nuclear Engineering, Taylor Francis Group, Washington, DC, 1992. With permission.)... 

Reactor isotopic changes occur for two reasons irradiation effects (primarily due to fission and transmutation) and radioactive decay. Figure 21.5 shows the transmutation decay chains for U-238 and Th-232. 

Effect of Fuel Processing on Actinide Decay Chains. Chemical separation of thorium results in material containing Th-232 and Th-228 only. Secular equilibrium of Pb-212 and Bi-212 from Th-228 takes 40 days governed by the half-life of Ra-224. Secular equilibrium of Ac-228 takes around 50 years, governed by the half-life of Ra-228. Chemical separation of uranium results in material containing U-234, U-235 and U-238 only. Subsequent enrichment for fuel increases the proportions of U-235 and inevitably of U-234. Th-234 and Pa-234m achieve secular equilibrium with U-238 within a year, but... 

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