Investigation and assessment of natural radioactivity in commercial animal feeds in Texas.

Commercial animal feed in Texas was characterized by determining natural gamma emitters including 40K,137Cs, and Uranium (235U and 238U) and Thorium (232Th) series to obtain basic radioactivity values. The measured activity concentration of natural radionuclides in animal feed was low enough for safe consumption by animal and largely depended on the type of animal feed.40K was the predominant radionuclide showing the highest activity concentration in animal feed. The radioactivity concentration of 214 Bi and 214Pb in 238U decay series was 1.39 and 1.33 Bq/kg in corn, respectively, lower than in other animal feed types. On the other hand, the vitamin/mineral mix samples showed higher concentrations of 214 Bi (9.04 Bq/kg) and 214Pb (10.19 Bq/kg). Beef cattle feed, poultry feed, and vitamin/mineral mix exhibited higher activity concentration of 228Ac and 212Pb in 232Th decay series. Gamma radionuclides appeared to be highly and significantly correlated within each decay series. 235U was present at low levels in all feed samples while the anthropogenic radionuclide of 137Cs was not detected irrespective of the type of animal feed. This study highlights an importance of establishing a current baseline of radioactivity concentration in animal feed in Texas in which the largest animal feed consumption in the US exists.

Radiation safety analysis of retrieval and conditioning of naturally occurring radioactive material waste / 中国辐射卫生

Objective The radioactivity level in historical waste from a plant is high, and decommissioning operators may be exposed to high radiation doses. The objective of the study is to carry out a radiation safety analysis of the retrieval and conditioning of naturally occurring radioactive material (NORM) waste, put forward appropriate radiation protection measures, and minimize the exposure doses of operators. Methods The source terms of NORM waste in the temporary storehouse were analyzed; MicroShield, a point kernel integration program, was used for modeling and calculation; and radiation protection measures under decommissioning conditions were put forward based on on-site monitoring data. Results The maximum gamma dose rate calculated near the waste pile in the temporary storehouse was 313.9 μGy/h, equivalent to the monitoring level; distance decay and appropriate shielding measures significantly reduced gamma dose rates for operators. Conclusion Decommissioning sites of temporary storehouses are of great harm to operators. Measures such as shielding, isolation, remote operations, and personal protection can effectively reduce the exposure doses of operators.

Thorotrast and in vivo thorium dioxide: numerical simulation of 30 years of α radiation absorption by the tissues near a large compact source.

BACKGROUND: The epidemiology of the slightly radioactive contrast agent named Thorotrast presents a very long latency period between the injection and the development of the related pathologies. It is an example of the more general problem posed by a radioactive internal contaminant whose effects are not noteworthy in the short term but become dramatic in the long period. A point that is still to be explored is fluctuations (in space and time) in the localized absorption of radiation by the tissues. METHODS: A Monte Carlo simulation code has been developed to study over a 30-year period the daily absorption of α radiation by µm-sized portions of tissue placed at a distance of 0-100 µm from a model source, that approximates a compact thorium dioxide source in liver or spleen whose size is ≳20 µm. The biological depletion of the daughter nuclei of the thorium series is taken into account. The initial condition assumes chemically purified natural thorium. RESULTS: Most of the absorbed dose is concentrated in a 25-µm thick layer of tissue, adjacent to the source boundary. Fluctuations where a target region with a volume of 1 µm(3) is hit by 3-5 α particles in a day or in a shorter period of time are relevant in a 1-10 µm thick layer of tissue adjacent to the source boundary, where their frequency is larger than the Poisson-law prediction.

Thorotrast: analysis of the time evolution of its α activity concentration, in the 70 years following the chemical purification of Thorium.

We simulate the α-activity of the Thorium series elements present in the contrast medium named Thorotrast, used until 1960 and cause of certified deaths until today. Assuming, as active components at t=0, (232)Th and (228)Th in the same relative concentration they have in nature, α-activity oscillates for some decades before reaching a stationary value that in absence of biological depletion would be AST =24000Bq/g. Our Montecarlo code generates the nuclear decays of the Thorium series with and without in-vivo biological depletion, arriving to three kinds of results for the activity: 1) Theoretical activity concentration (no biological depletion). Our result is fitted by: A(t)=A(ST).{[1-exp(-t/10)]+[exp(-t/tB)(1-0.8exp(-t/tA))]}, with t in years, tA=1.07.10(-2) years, and tB=2.38 years. 2) Weak biological depletion (228Ra/232 Th equilibrium activity ratio 0.6, 224Ra/228Ra e.a.r 0.9, 10% excretion for 220Rn). The ratio of the activity concentration to the theoretical activity concentration is fitted by: A weak (t)/A(t)=0.61+0.29 exp[-(t/15)2] (t in years). 3) Strong biological depletion (228Ra/232Th e.a.r 0.4, 224Ra/228Ra e.a.r. 0.8, 10% excretion for 220Rn). The ratio of the activity concentration to the theoretical activity concentration is fitted by A(strong)(t)/A(t)=0.44+0.4 exp[-(t/13)2](t in years). We also report fluctuation calculation for two cases where standard statistical behavior is not expected.