Tritium Transfer Research Articles

Calculation of Recovery Rates of Tritium from Flibe Blanket

Rates of continuous tritium recovery from a Flibe stream are calculated for a permeation window and a counter-current gas-bubble column. The area of the permeation window is estimated to be comparatively small unless very low tritium partial pressure is demanded for the system. It is probable that the tritium transfer rate is controlled not by permeation through the window material (Nb or ferrite steel) but diffusion in Flibe. Since tritium diffusion in the molten salt is also determination of the recovery rate in the gas-bubble column from Flibe to He, the evaluation of the tritium diffusion resistance in the molten salt is crucial for its design.

Characterization of Two Polyketide Methyltransferases Involved in the Biosynthesis of the Antitumor Drug Mithramycin byStreptomyces argillaceus

A DNA chromosomal region of Streptomyces argillaceus ATCC 12596, the producer organism of the antitumor polyketide drug mithramycin, was cloned. Sequence analysis of this DNA region, located between four mithramycin glycosyltransferase genes, showed the presence of two genes (mtmMI and mtmMII) whose deduced products resembled S-adenosylmethionine-dependent methyltransferases. By independent insertional inactivation of both genes nonproducing mutants were generated that accumulated different mithramycin biosynthetic intermediates. The M3DeltaMI mutant (mtmMI-minus mutant) accumulated 4-demethylpremithramycinone (4-DPMC) which lacks the methyl groups at carbons 4 and 9. The M3DeltaM2 (mtmMII-minus mutant) accumulated 9-demethylpremithramycin A3 (9-DPMA3), premithramycin A1 (PMA1), and 7-demethylmithramycin, all of them containing the O-methyl group at C-4 and C-1', respectively, but lacking the methyl group at the aromatic position. Both genes were expressed in Streptomyces lividans TK21 under the control of the erythromycin resistance promoter (ermEp) of Saccharopolyspora erythraea. Cell-free extracts of these clones were precipitated with ammonium sulfate (90% saturation) and assayed for methylation activity using different mithramycin intermediates as substrates. Extracts of strains MJM1 (expressing the mtmMI gene) and MJM2 (expressing the mtmMII gene) catalyzed efficient transfer of tritium from [(3)H]S-adenosylmethionine into 4-DPMC and 9-DPMA3, respectively, being unable to methylate other intermediates at a detectable level. These results demonstrate that the mtmMI and mtmMII genes code for two S-adenosylmethionine-dependent methyltransferases responsible for the 4-O-methylation and 9-C-methylation steps of the biosynthetic precursors 4-DPMC and 9-DPMA3, respectively, of the antitumor drug mithramycin. A pathway is proposed for the last steps in the biosynthesis of mithramycin involving these methylation events.

Proton transfer in the mechanism of triosephosphate isomerase.

Triosephosphate isomerase (TIM) catalyzes the reversible interconversion of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), with Glu-165 removing the pro-R proton from C1 of DHAP and neutral His-95 polarizing the carbonyl group of the substrate. During the TIM reaction, approximately 2% of the pro-R tritium from C1 of DHAP is conserved and appears at C2 of GAP [Nickbarg, E. B., and Knowles, J. R. (1988) Biochemistry 27, 5939]. In the "classical" mechanism, 98% of the pro-R tritium exchanges with solvent from Glu-165 at the intermediate state and the remaining 2% is transferred by Glu-165 to C2 of the same substrate molecule. This intramolecular transfer of tritium is therefore predicted to be independent of DHAP concentration. On the basis of NMR detection of a strong hydrogen bond between Glu-165 and the 1-OH of an analogue of the enediol intermediate [Harris, T. K., Abeygunawardana, C., and Mildvan, A. S. (1997) Biochemistry 36, 14661], we have suggested a "criss-cross" mechanism for TIM in which Glu-165 transfers a proton from C1 of DHAP to O2 of the enediol, and subsequently from O1 of the enediol to C2 of the product GAP. Since the pro-R proton is transferred to O2 instead of C2 in the criss-cross mechanism, no intramolecular transfer of label from substrate to product would be expected to occur. However, intermolecular transfer of label could occur if the label exchanges from O2 into a group on the protein and is transferred to GAP in subsequent turnovers. The extent of intermolecular tritium transfer in the criss-cross mechanism would be predicted to be dependent on DHAP concentration. The extent of tritium transfer was studied as a function of initial DHAP concentration using DHAP highly tritiated at the pro-R position. At 50% conversion to GAP, triphasic tritium transfer behavior was found. For phase 1, between 0.03 and 0.3 mM DHAP, a constant extent of tritium transfer of 1.19 +/- 0.03% occurred. For phase 2, between 0.3 and 1.0 mM DHAP, the extent of transfer progressively increased as a function of DHAP concentration to 2.17 +/- 0.15%. For phase 3, between 1.0 and 7.0 mM DHAP, the extent of transfer slightly decreased to 1.68 +/- 0.17%. In a direct test for intermolecular isotope transfer, doubly labeled [1(R)-D, 13C3]DHAP and 13C-depleted [1(R)-H,12C3]DHAP were synthesized, mixed in equal amounts, and incubated at 1 mM total DHAP with TIM, GAP dehydrogenase, NAD+, and arsenate until 50% conversion to 3-phosphoglycerate occurred. Electrospray ionization mass spectral analysis of the stable 3-phosphoglycerate product detected an extent of 1.4 +/- 0.4% of intramolecular D transfer from [13C3]DHAP to the 13C3 product, but no intermolecular transfer (</=0.02%) of D from [13C3]DHAP to the 12C3 product. Hence, the entire transfer of hydrogen from substrate to product is intramolecular, providing no direct support for the criss-cross mechanism in wild-type TIM. The increase in the extent of intramolecular isotopic transfer with increasing initial DHAP concentration indicates site-site interaction in this dimeric enzyme which either (i) slows proton exchange with solvent from Glu-165 at the intermediate state in the classical mechanism or (ii) alters the partitioning of the abstracted proton between transfer to C2 by the classical mechanism or to O2 by the criss-cross mechanism in which no intermolecular transfer of label occurs.

Isotope Effect in Cryosorption of Tritium to Molecular Sieves and Activated Carbon at 77 K

The tritium bred in a deuterium-tritium fusion reactor is removed from its blanket by using helium sweep gas mixed with some amount of hydrogen to promote the release rate. From the viewpoint of uptake capacity at the partial pressure of tritium and ease of tritium transfer to the main fuel cycle, a cryosorption bed, which uses molecular sieves or activated carbon at the liquid nitrogen temperature, is attractive for recovery of bred tritium in the blanket sweep gas. The cryosorption bed is also applicable as a transfer pump of tritium in the fuel-handling process. Tritium cryosorbed from a certain subunit of the fuel-handling system is transferred to other subunits by an increase in temperature and the operation of valves. It is necessary to know the adsorption isotherm and the mass transfer coefficient of each hydrogen isotope for estimation of breakthrough performances of tritium in a cryosorption bed because the mixture of tritium with other hydrogen isotopes must be treated. It is observed that the isotope effects in adsorption capacity and surface diffusivity have a close connection with the quantum effect that is represented by the reduced molecular weight. The correlative equation between Langmuir constants in the adsorption isotherm with reduced molecular weight of hydrogen isotopes is proposed. The correlative equation between surface diffusivity and reduced molecular weight is also presented.

Deposition of D2O from air to plant and soil during an experiment of D2O vapor release into a vinyl house

Abstract Deuterium, a stable isotope of tritium, was released into a vinyl house in autumn 1995 and summer 1996 to study the transfer of tritium from air to plant and soil. Temporal variation of D2O concentrations in plant and soil water, and plant physiological parameters such as transpiration rate and leaf temperature, were measured during these experiments. D2O concentrations of plants were fitted to a first order kinetic model: Cp = Cmax (1 – e–kt), where Cp is the D2O concentrations in plants at time t, Cmax is the steady-state concentration in plants and k is the rate constant. The rate constant was also calculated using measured plant physiological parameters for comparison. The D2O uptake by paddy rice was most rapid and the value of k was 3.63 ± 0.31 h–1 followed by radish, cherry tomato, Komatsuna and orange. The day/night concentration ratio for cherry tomato and orange was higher than that for radish and Komatsuna.

Isotope exchange reaction in Li 2ZrO 3 packed bed

To understand the release behavior of bred tritium in a solid breeder blanker, the tritium transfer rate and tritium inventory for various mass transfer processes should be investigated. The contribution of the surface reactions (adsorption, desorption and two kinds of isotope exchange reactions) to the release process cannot be ignored. It is believed that two kinds of isotope exchange reactions (gaseous hydrogen-tritiated water and water vapor-tritiated water) occur on the surface of the solid breeder materials when hydrogen is added to the sweep gas to enhance the tritium release rate. The isotope exchange reaction study in H–D systems was carried out using a Li 2ZrO 3 packed bed. The exchange reaction between gaseous hydrogen and water was the rate controlling step among the two kinds of exchange reactions. The reaction rate constants were quantified, and experimental equations were proposed. The equilibrium constant of the isotope exchange reaction in the H–D system was obtained from experimental data and was found to be 1.17.

Transplacental transfer of radionuclides. A review.

Transplacental transfer of tritium (H), cesium (Cs), iodine (I), strontium (Sr) and plutonium (Pu) has been observed in all examined species with an active transfer mechanism for iodine, strontium and plutonium. Cesium and tritiated water freely cross the placenta and are distributed approximately uniformly in maternal as well as in fetal tissue. In pregnant mice, 137Cs concentrations in maternal organs were 4-6 times higher than in the foetus. With increasing weight, Cs amounts rise in the foetus. 131I distribution in the foetus before and after the beginning of thyroid function is totally different. In an 8-week-old human foetus, there was almost no accumulation of 131I in the thyroid gland, while on week 14 of gestation more than 40% of the fetal iodine was accumulated in the thyroid gland. Radioiodine given only once results in distinctly higher fetal-maternal ratios than after long-term exposure. Experiments performed to evaluate the extent of placental barrier of Sr in comparison to Ca showed a maximum of Sr/Ca discrimination early in pregnancy while at the end of pregnancy Sr and Ca are transported to almost equal proportions across the placenta. A well-known phenomenon is the accumulation of Pu in the yolk-sack of rats such that 80% of the total activity is contained in the feto-placental unit. For the extrapolation of results from animals to man, experiments should not be confined to laboratory animals but include primates. Furthermore, computer softwares like MIRDOSE could be used to simulate transplacental distribution.

Tritium release behavior from liquid tritium breeding materials for fusion reactor blanket under neutron irradiation

In-situ tritium release experiments on Li17-Pb83 molten alloy and LiF-BeF 2 molten salt were carried out using the fast neutron source reactor “YAYOI” of the University of Tokyo. Regarding Li17-Pb83, chemical form of released tritium was mainly HT or T 2. The diffusion coefficient in molten Li17-Pb83, the mass-transfer coefficient from Li17-Pb83 to purge gas, and the overall mass-transfer coefficient of tritium permeating through structural materials such as iron and SUS 304 were obtained. Stable oxide layer including Cr 2O 3 and FeCr 2O 4 formed on the surface reduced the mass-transfer coefficient. In particular, a kind of ceramic coating was quite effective as a tritium permeation barrier. Regarding LiF-BeF 2, on the other hand, chemical forms of released tritium from Flibe are HT and TF, depending on H 2 and F activity in the system. In case of low H 2 activity and high F activity, TF was dominant. Three tritium transfer processes were considered: direct release of T + to purge gas as TF (process (1)), conversion from T + to HT (process (2)), and release of HT to purge gas (process (3)). Among them, process (3) is much faster than process (1). The reaction rate of process (2) is so much influenced by H 2 and F activities in the system that each process could be the rate-determining process depending on H 2 and F activities. When process (3) was the rate-determining process, the overall mass transfer coefficient was estimated.

Elucidation of Mechanism of Coal Liquefaction Using Tritium and 35S Tracer Methods

The reactivity of hydrogen in tetralin and in Taiheiyo coal in the presence of iron catalysts (pyrrhotite and iron pentacarbonyl (Fe(CO)5)) and sulfur were investigated using tritium and 35 S tracer methods under liquefaction conditions. Coal liquefaction was performed for 30 min at an initial pressure of 5.9 MPa and a temperature of 400 °C in the presence of tetralin solvent and tritiated hydrogen, with and without a catalyst and 35 S-labeled sulfur. The rate of coal conversion and the tritium distribution in coal were higher when Fe(CO)5 and sulfur were used than were obtained with conventional pyrrhotite. Further, the coal products obtained with the use of Fe(CO)5 and sulfur were lighter than those produced in the presence of added pyrrhotite. The results suggest that the catalyst derived from Fe(CO)5 and sulfur was successfully dispersed in the coal and directly acted on the coal to increase both the rate of coal conversion and the tritium transfer from the gas phase to the coal. The iron catalysts greatly promoted the hydrogen exchange between the gas phase and the tetralin solvent in the absence of coal. Part of the added sulfur participated in the sulfur exchange reaction with the pyrrhotite catalyst. The addition of sulfur increased the amount of hydrogen incorporated into the coal. This suggests that sulfur promoted the formation of tetralyl radicals during the hydrogen transfer from the solvent to the coal.

Detritiation of water using microporous hollow-fiber membranes

A novel concept of tritium (HTO) removal with microporous hollow fiber membranes was evaluated in this study. Small-scale laboratory modules were constructed and tested to determine the mass transfer characteristics of the hollow fibers under varying system parameters. Tritiated water is pumped through the fiber lumen and air, saturated with water vapor, is pumped over the exterior of the fibers in a countercurrent mode. The high HTO concentration gradient encourages the HTO to diffuse across the porous membrane wall, and to transfer directly into the saturated air stream. A dimensionless mathematical correlation that predicts the tritium transfer coefficient across the membranes is presented for parallel flow modules. The measured overall mass transfer coefficients in the membrane module are two to three orders of magnitude greater than those of conventional bubble stripping. In addition, factors that influence the mass transfer performance of the membrane modules in practical applications are evaluated. The results indicate that very low concentrations of HTO can be separated from water using microporous hollow fiber membranes.

TRIF — An intermediate approach to environmental tritium modelling

The movement of tritium through the environment, from an initial atmospheric release to selected end points in the food chain, involves a series of closely coupled and complex processes which are, consequently, difficult to model. TRIF (tritium transfer into food) provides a semi-empirical approach to this transport problem, which can be adjusted to bridge the gap between simple steady state approximations and a fully coupled model of tritium dispersion and migration (Higgins et al., 1996). TRIF provides a time-dependent description of the behaviour of tritium in the form of tritium gas (HT) and tritiated water (HTO) as it enters and moves through the food chain into pasture, crops and animals. This includes a representation of the production and movement of organically bound tritium (OBT).

Tritium transfer from diet to human

The direct transfer of organically-bound3H (OBT) of diet to that in the human body is reasonable, since the human body needs essential amino acids and fatty acids. Metabolic models of3H which were recently developed for the human body took this direct incorporation into account. Data for fallout3H in diet and human tissues were used to validate model parameters, although such data are relatively scarce. However, a simple equilibrium between dietary3H and human body3H was assumed for such validations. The simple equilibrium assumption means completely neglecting past3H concentration in the human body. When the model consists of compartments which have long mean residence times, estimated3H concentrations in the human body from the simple equilibrium assumption can differ significantly from results of adequate calculation considering contribution of past human3H concentrations. The examples of such calculations are described in this paper. Results of the calculations show that suitable environmental3H transfer models are necessary to use fallout3H data for the validation of3H metabolism models.

Operational Problems with Uranium Pots Due to Helium Blanketing

Extensive experience with the refilling of the Amersham Mk IV 1.85 PBq uranium pots has shown that helium-3 resident in the uranium matrix causes `blanketing` of the metal surface and subsequent operational problems. This paper reviews the operational procedures and results observed at Ontario Hydro`s dispensing facilities. Experimental work performed to investigate the effect of this helium-3 on the reuse of the Amersham Mk IV pots demonstrated the necessity of an operational ability to evacuate the uranium pot at stages during the tritium transfer, without affecting the quantitative transfer measurement. 2 refs., 3 figs.

COWTRI - A Dynamic Compartment Model for the Transfer of Tritium to Milk in a Lactating Dairy Cow

A general dynamic model consisting of 14 compartments simulating the distribution of tritium into milk and its constituents in lactating dairy cows was developed. The model is calibrated to fit the results obtained in two experiments, where cows were given tritiated water in one experiment (A) and tritiated hay in the second (H) one. The model estimates the activity concentration in the whole milk and its constituents as a function of time. In the next step the COWTRI model will be fitted to a normalized cow for which the weight the daily intake of organic bound hydrogen (OBH), the milk faeces- and urine production are defined. In this version of the model the normalization procedure is only performed concerning intake of OBH. The purpose with this detailed model was to estimate the contribution of the ingested tritium to OBT in milk besides the whole milk itself. From this information it will be possible to simplify the model to one organic part and one non organic part of the milk. The model needs further testing against independent data before it can be simplified for a close assessment model attributed to accidental and continuous operational releases of tritium to an environment where the exposure pathway via milk consumption has to be considered.

Tritium isotope effects in adenosylcobalamin-dependent glutamate mutase: implications for the mechanism.

The transfer of tritium between adenosylcobalamin and substrate in the reaction catalyzed by glutamate mutase was examined to investigate the possibility of a protein-based radical intermediate. There was no evidence that tritium was transferred to the protein during the reaction, as tritium neither became stably bound to the protein nor exchanged with water. The kinetics of tritium transfer from adenosylcobalamin to 3-methylaspartate was investigated. Both the transfer of tritium to product and the exchange of enzyme-bound and free coenzyme contribute to the kinetics of tritium loss from adenosylcobalamin. By varying the experimental conditions, the rates of both coenzyme exchange and tritium transfer could be measured. Exchange of adenosylcobalamin with enzyme is very slow, k off = 0.01 s-1, which may reflect a conformational change in the coenzyme and/or protein involved in forming active holo enzyme. The rate constants for the loss of tritium from adenosylcobalamin and the appearance of tritium in 3-methylaspartate are much faster and very similar, k = 0.67 +/- 0.05 s-1 and k = 0.50 +/- 0.05 s-1, respectively, consistent with the transfer of tritium occurring directly between coenzyme and substrate. The isotope effect, calculated from the rate constants for tritium transfer, and kcat, determined for the overall reaction under the same conditions, are between 13.5 and 18. These values are typical of primary isotope effects seen for enzymes in which hydrogen transfer is substantially rate limiting. A protein radical, therefore, appears unlikely to feature in the mechanism of this enzyme.

Analysis of tritium behaviour and recovery from a water-cooled Pb17Li blanket

Abstract The question of the tritium recovery in water-cooled Pb 17Li blankets has been under investigation for several years at JRC Ispra. The method which has been more extensively analysed is that of slowly circulating the breeder out from the blanket units and of extracting the tritium from it outside the plasma vacuum vessel by helium gas purging or vacuum degassing in a suited process apparatus. A computerized model of the tritium behaviour in the blanket units and in the extraction system was developed. It includes four submodels: (1) tritium permeation process from the breeder to the cooling water as a function of the local operative conditions (tritium concentration in Pb 17Li, breeder temperature and flow rate); (2) tritium mass balance in each breeding unit; (3) tritium desorption from the breeder material to the gas phase of the extraction system; (4) tritium extraction efficiency as a function of the design parameters of the recovery apparatus. In the present paper, on the basis of this model, a parametric study of the tritium permeation rate in the cooling water and of the tritium inventory in the blanket is carried out. Results are reported and discussed in terms of dimensionless groups which describe the relative effects of the overall resistance on tritium transfer to the cooling water (with and without permeation barriers), circulating Pbl7Li flow rate and extraction efficiency of the tritium recovery unit. The parametric study is extended to the recovery unit in the case of tritium extraction by helium purge or vacuum degassing in a droplet spray unit.

Computer Aided Design of Operational Units for Tritium Recovery from Li17Pb83 Blanket of a Demo Fusion Reactor

The problem of tritium recovery from Li{sub 17}Pb{sub 83} blanket of a DEMO fusion reactor is analyzed with the objective of limiting tritium permeation into the cooling water to acceptable levels. To this aim, a mathematical model describing the tritium behavior in blanket/recovery unit circuit has been formulated. By solving the model equations, tritium permeation rate into the cooling water and tritium inventory in the blanket are evaluated as a function of dimensionless parameters describing the combined effects of overall resistance for tritium transfer from Li{sub 17}Pb{sub 83} alloy to cooling water, circulating rate of the molten alloy in blanket/recovery unit circuit and extraction efficiency of tritium recovery unit. The extraction efficiency is, in turn, evaluated as a function of the operating conditions of recovery unit. The design of tritium recovery unit is then optimized on the basis of the above parametric analysis and the results are herein reported and discussed for a tritium permeation limit of 10 g/day into the cooling water. 14 refs., 9 figs., 2 tabs.

Electron Paramagnetic Resonance Studies of the Methylmalonyl‐CoA Mutase Reaction

The substrate-dependent homolysis of the cobalt-carbon bond and generation of organic radicals in the coenzyme-B12-methylmalonyl-CoA-mutase complex have been demonstrated by EPR measurements. Both the natural substrate methylmalonyl-CoA, its 13C-substituted analogue and the non-hydrolysable synthetic substrates succinyl-dethia(carba)-CoA, succinyl-dethia(dicarba)-CoA and 4-carboxy-2-oxo-butyl-CoA induced similar but not identical EPR signals. 3-Carboxypropyl-CoA, a novel competitive inhibitor, has been synthesised. Its Ki value of 89 +/- 6 microM was in the same range as the Km of succinyl-CoA. Using [5'-3H]adenosylcobalamin, an enzyme-dependent tritium transfer to the inhibitor has been shown. The enzyme-coenzyme-inhibitor complex also exhibited EPR signals that were less structured and less intensive than the corresponding signals with active substrates. These results prove that the inhibitor also induces cobalt-carbon bond homolysis and undergoes reversible hydrogen transfer but not rearrangement.

Transfer of Tritium to Prenatal and Neonatal Rats from Their Mothers Exposed to Tritiated Compounds

The transfer of tritium through the placenta or milk was investigated to estimate the radiation dose to the fetus and newborn. Female rats at gestational stages or after delivery were exposed to tritium in the form of water, thymidine and lysine by a single oral administration and radioactivity in tissues including conceptuses (placenta, fetal membrane and fetus) and in the newborn was determined at various time after administration. In all cases of the investigated tritiated compounds, there was no significant difference between the tritium concentration in the fetus and that in the maternal tissues, suggesting that placenta has no effect in preventing or to accelerating the placental transfer of tritium

Transfer of Tritium to Prenatal and Neonatal Rats from Their Mothers Exposed to Tritiated Compounds

The transfer of tritium through the placenta or milk was investigated to estimate the radiation dose to the fetus and newborn. Female rats at gestational stages or after delivery were exposed to tritium in the form of water, thymidine and lysine by a single oral administration and radioactivity in tissues including conceptuses (placenta, fetal membrane and fetus) and in the newborn was determined at various time after administration. In all cases of the investigated tritiated compounds, there was no significant difference between the tritium concentration in the fetus and that in the maternal tissues, suggesting that placenta has no effect in preventing or to accelerating the placental transfer of tritium. The time course of tritium concentration and tritium content in the fetus and newborn were, however, dependent on the chemical form of tritium and on the prenatal or neonatal stages at the time of ingestion. In general, the tritium concentration and tritium content after the ingestion of 3H-lysine were higher than that after the ingestion of tritiated water of 3H-thymidine. The result of dose estimation showed that 3H-lysine gave higher prenatal and neonatal doses than tritiated water or 3H-thymidine by a factor of 1.5 to 6.0.