Hydrogen Isotope Retention

The hydrogen-isotope retention and recycling properties of first-wall materials are also the determinant of the isotopic ratio in a multi-component plasma. Maximization of the reactivity of D-T plasmas (16) and the efficiency of minority-specie RF heating schemes require precise control of the isotopic mixtures, a task which is considerably easier in low recycling situations. Titanium sublimation over 20-40% of the vacuum vessel of the PLT tokamak was sufficient for density and isotopic mixture control for both the high power neutral beam (55) and ion cyclotron... 

Hydrogen isotope retention in stainless steel is extremely important from both the plasma physics and environmental points of view. Recent measurements60) on Tokamaks have demonstrated that trapped hydrogen isotopes are desorbed from the first wall during a discharge and dominate the plasma refueling. 

The hydrogen isotope retention properties of beryllium surfaces are fairly well understood. Two excellent review articles [36,37] have been written that deal extensively with this issue. This section will briefly summarize the understanding presented in those articles. Readers desiring more details are advised to find it within those papers. 

The nature and position of substituents relative to the ring oxygen atom have an important effect on the acid-catalyzed isomerization of oxepins. It has been observed that, in addition to the hydrogen isotopes ( H, 2H, 3H), chloro, bromo, methyl (72E1129) and alkoxycarbonyl (79JA2470) substituents also show the migration-retention sequence found in the NIH shift. 

Experimental results indicate that hydrogen isotopes reside in the near surface region as well as in the bulk stainless steel after low energy bombardment. The total retention of hydrogen isotopes is relatively low, indicating that tritium retention in... 

Of course, oxygen is not the only impurity that will react with beryllium. Another material that is important in forming mixed-material layers with beryllium is carbon. The saturated value of retention that has been found in beryllium surfaces exposed to a large deuterium ion fluence could easily be overshadowed if a carbon rich layer forms on the beryllium surface due to impurity carbon ions in the incident plasma flux. The hydrogen retention properties of plasma deposited carbon films has been shown to dominate the total retention in beryllium samples exposed to the plasma at lower temperature. Once the sample temperature during exposure approaches 500°C there is little difference between the retention in Be/C mixed-material layers compared to clean beryllium samples [48]. The temperature dependence of the retention of carbon containing mixed material layers, as well as that of clean beryllium surfaces is shown in Fig. 14.10. There are two possible explanations for the reduced retention in the mixed-material layers formed at elevated temperature. The first is that beryllium carbide forms more readily at elevated temperature and less retention is expected in beryllium carbide [11]. The second is that carbon films deposited at elevated temperature also tend to retain less hydrogen isotopes [49]. 

The conversion of lysine into piperidine alkaloids involves retention of hydrogen isotope at C-2/° The sequence is suggested to be that shown in Scheme 1, and catalysis of the reaction may be attributed to L-lysine decarboxylase. This enzyme, from the micro-organism Bacillus cadaveris, has been found to carry out the conversion of L-lysine into cadaverine with retention of configuration. Decarboxylation of L-[2- H]lysine by this enzyme then affords [15- H]-cadaverine. When this material is converted into alkaloids, e.g. iV-methyl-pelletierine (4 R = Me), the tritium attached to what becomes C-2 is lost cf. refs. 5 and 6. On the other hand, conversion of lysine into sedamine (27) in Sedum acre results in retention of the tritium originally present at C-2. The simplest explanation is that protonation of (26) in the micro-organism and plant proceeds with opposite stereochemistry. This is at variance, however, with current ideas on the stereochemistry of reactions that are catalysed by pyridoxal phosphate. ... 

The method of analysis developed is for acetic acid and involves the use of two enzymes. Chiral acetic acid [as 1.26)] is irreversibly condensed as its CoA-derivative with glyoxylic acid (/.27), using the enzyme malate synthase, to give malic acid 1.28). The condensation occurs with loss of hydrogen isotope by a primary kinetic isotope effect (A h kj). This means that loss of H is favoured over loss of D which is in turn favoured over loss of T. The result is a high retention of tritium. 

Rosenfeld and his co-workers (1954, 1956, 1964) have studied the mechanism of reduction of cholesterol to coprostanol using cholesterol labeled at the 3 a position with deuterium or tritium and in the 4 position with carbon-14. Their data indicate that the coprostanol formed exhibits up to 81 % retention of the isotope at the 3a position thus, there is no oxidation at that position. However, the variable incorporation of hydrogen isotope into the B ring leaves the complete course of the conversion in doubt. The possibility exists that the products are the results of the action of more than one organism. 

Figure 4.12. The NIH shift—some examples (Retention of the hydrogen isotope is shown in brackets) ...

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