Half-life magnitude

A D—T fusion reactor is expected to have a tritium inventory of a few kilograms. Tritium is a relatively short-Hved (12.36 year half-life) and benign (beta emitter) radioactive material, and represents a radiological ha2ard many orders of magnitude less than does the fuel inventory in a fission reactor. Clearly, however, fusion reactors must be designed to preclude the accidental release of tritium or any other volatile radioactive material. There is no need to have fissile materials present in a fusion reactor, and relatively simple inspection techniques should suffice to prevent any clandestine breeding of fissile materials, eg, for potential weapons diversion. 

From the alpha-activity remaining in the supernatant liquid after the final precipitation as a fluoride, it can be calculated, using 30,000 years as the half-life of 94239, that this salt of 94 has a solubility of the order of magnitude of 10 mg of the element per liter of 6 N HF solution. This value is necessarily somewhat tentative. 

A simple order of magnitude estimate of the rate constants for reaction with ethylene can be made for the high intensity ions in the 5-torr spectrum. Since the average reaction time, limited by neutralization or removal from the ion source is a few milliseconds (see section dealing with sampling conditions and section on ethylene in xenon) we can take 1 msec, as the half-life of these ions in 5-torr ethylene. This leads to k = 10-14 to 10-15 cc. molecule-1 sec.-1 as a rate constant for further reaction with ethylene. The value for 5a found by the kinetic treatment above was 8 X 10 -14. 

Slope expressed as the contribution from each R radical. If the AE(t) values of Table I are used, the overall magnitude of the slope would be 9.76 for reaction 1 at one hour half life, for example. 

Pyrethroids can also persist in sediments. In one study, alpha-cypermethrin was applied to a pond as an emulsifiable concentrate (Environmental Health Criteria 142). After 16 days of application, 5% of the applied dose was still present in sediment, falling to 3% after a further 17 days. This suggests a half-life of the order of 20-25 days—comparable in magnitude to half-lives measured in temperate soils. 

Radioactivity serves as a useful clock only for times that are the same order of magnitude as the decay half-life. At times much longer than t j2, the amount of radioactive nuclide is too small to measure accurately. At times much... 

It is important to note however that Equation (10) assumes steady state in the Th distribution so that production truly is balanced by decay and export. It is easy to imagine a scenario after a phytoplankton bloom, when the export of POC (and " Th) has decreased or even ceased, such that the water column " Th profile would still show a deficit with respect to caused by prior high flux events. This relief deficit will disappear as " Th grows into equilibrium with on a time scale set by the " Th half-life. The magnitude by which the Th flux is over- or under-estimated depends on whether deficits are increasing or decreasing and at what rate. 

The rate of hydrolysis of various organic chemicals, under environmental conditions can range over 14 orders of magnitude, with associated half-lifes (time for one-half of the material to disappear) as low as a few seconds to as high as 10 years and is pH dependent. It should be emphasized that if laboratory rate constant data are used in soil models and not corrected for environmental conditions — as is often the only choice — then model results should be evaluated with skepticism. 

It is not difficult to show by conductance measurements that when the polymerisation of dioxolan becomes of first order, roughly at the first half-life, all the available acid has reacted with monomer or oligomer to produce active centres [10]. Thus on the Mainz theory one would expect to find a number of OH groups of the same order of magnitude as the number of perchloric acid molecules introduced this is evidently not so, as long as the water concentration in the system is significantly less than that of the acid [10]. For 1,3-dioxepan the protonation is very much faster than for DXL and is complete long before the first half-life of the polymerisation. 

First, the computer calculated uncertainties shown for the calculated values of kj, k and kg are an indication that the model has considerable validity for describing the kinetics of the system, at least over one half-life in the disappearance of chlorpyrifos. Second, the values of k and kj are all similar and their magnitude indicates that in this case the assumption of rapid sorption/desorption kinetics compared to hydrolysis is valid. 

The control via activation or inhibition of the rate(s) of an enzyme-catalyzed reaction(s). This control includes the increase or decrease in the stability or half-life of the enzyme(s). There are many different means by which control can be achieved. These include 1. Substrate availability and reaction conditions (e.g., pH, temperature, ionic strength, lipid interface activation) 2. Magnitude of Vraax sud valucs) 3. Activation (particularly, feedforward activation) 4. Isozyme formation 5. Com-partmentalization and channeling 6. Oligomerization/ polymerization 7. Feedback inhibition and cooperativity (particularly, allosterism and/or hysteresis) 8. Covalent modification and 9. Gene regulation (induction repression)... 

Verschueren 1983). The magnitude of the estimated Henry s law constant (4.4-7.7x10 atm- m /mole) indicates that 2-hexanone will volatilize from water, with a half-life in river water of about 10-15 days (Mabey et al. 1982). Volatilization will be slower from lakes or ponds (Mabey et al. 1982). There is no information on whether 2-hexanone in water is expected to partition to soils and sediments. 

---