Tritium confinement

Pulsed plasmas containing hydrogen isotopes can produce bursts of alpha particles and neutrons as a consequence of nuclear reactions. The neutrons are useful for radiation-effects testing and for other materials research. A dense plasma focus filled with deuterium at low pressure has produced 10 neutrons in a single pulse (76) (see Deuterium AND TRITIUM). Intense neutron fluxes also are expected from thermonuclear fusion research devices employing either magnetic or inertial confinement. 

The confinement region in which nuclear fusion proceeds is surrounded by a blanket in which the neutrons produced by the fusion reaction are captured to produce tritium. Because of its favorable cross section for neutron capture, lithium is the favored blanket material. Various lithium blanket... 

Another approach to nuclear fusion is shown in Figure 19.6. Tiny glass pellets (about 0.1 nun in diameter) filled with frozen deuterium and tritium serve as a target. The pellets are illuminated by a powerful laser beam, which delivers 1012 kilowatts of power in one nanosecond (10 9 s). The reaction is the same as with magnetic confinement unfortunately, at this point energy breakeven seems many years away. 

The intracellular distribution of steroid hormone receptors has long been the object of controversy. The first theoretical formulation on the intracellular location of the ERs was elaborated by Jensen in 1968 and is known as the two-step theory. Its execution was based entirely on biochemical observations obtained by means of tritium-marked estradiol. The ERs, in cells not exposed to hormones, are found abundantly in the soluble cell fraction, or cytosol (Fig. 1.1). Treatment with hormones confines the receptors to the particulated or nuclear fraction and causes their disappearance from the cytosol. The two-step theory established that the receptor is found in the cytoplasm naturally and upon the arrival of a hormone it is transformed into a complex hormone-receptor (first step) capable of translocating itself to the nucleus and of modifying gene expression (second step). 

Inertial Confinement. In the inertial approach, the fuel is heated as it is compressed to a very high density, estimated at about I(KK) times that or the normal density of the solid fuel. An intense energy source is focused onto Ihe outer surface of a specially formed spherical pellet. This produces ablation on the outer surface somewhat similar lo the ablation of a rocket as it is exposed to extremely high temperatures. The energy also causes an implosion (an inward bursting) of the deuterium-tritium fuel mixture in the inneT portion of Ihe pellet. The compression process heats the fuel to ignition temperature and also contributes to the quantity of fuel that can be burned. Inasmuch as the compressed fuel is restrained by its own inertia, the fuel hums before it can fly apart. This is a time span of a billionth of a second or less. ... 

Groundwater in unconfined systems receives recharge over the entire surface and, hence, is expected to have a recent age (section 10.4). If a well included in a model of an unconfined system reveals an absence of tritium, the model has to be modified to include a confined aquifer beside the unconfined one (Fig. 3.14). Water in a confined aquifer is expected to have the same age or reveal a certain age increase downflow. However, if the age varies suddenly between adjacent wells, for example, gets younger in the downflow direction, the connectivity assumption is disproved (Fig. 3.13). 

Fig. 3.13 A transect through wells that display a gradually lower water table, yet the chloride concentration is lower in the well with the lower water table. In this case the explanation is that the wells tap different aquifers and are not hydraulically interconnected. Tritium values further clarify the picture wells I and II tap a confined aquifer, whereas well III taps a phreatic through-flow system.

Fig. 4.17 Tritium and 14C measurements during a pumping test conducted in a confined aquifer at the Aravaipa Valley, Arizona (Adar, 1984). Recent water of high tritium and 14C concentrations intruded from the overlying phreatic aquifer.

Four samples (no. 4, the July 1969 sample no. 6, no. 17, and no. 19 Table 11.6) have post-bomb tritium, yet they are from the confined part of the aquifer. Does this necessarily mean rapid flow into the confined aquifer, or is there another explanation These cases may represent mixed pumping of... 

These examples demonstrate the importance of tritium measurements in every 14C study. In the Paris basin a relatively large number of tritium measurements were done and, except for the four samples mentioned, no tritium was detected in the samples assigned to the confined aquifer. 

Fig. 11.12 13C and tritium as a function of 14C in the Albian aquifer of the Paris basin (Table 11.6). x, free water table dots, confined. Initial 14C value is 58-80 pmc (see text). 

This system provides another example of a phreatic-confined interrelationship. Data are given in Table 11.7. A clear phreatic-confined discontinuity is established by 14C, <513C, and tritium. 

This complex has been intensively studied (Andrews et al., 1984). Drastic discontinuities were reported for groundwater properties passing from the phreatic part of the aquifer to the confined part. The transition is observed between two wells, about 400 m apart. For example, 14C drops from 114pmc to 48pmc, tritium drops from 49 TU to 0.8 TU, dlsO drops from —8.6%o to —9.3%o, and bicarbonate rises from 100mg/l to 320mg/l. 

Answer 11.6 The Migennes well has already been discussed in the text. It is reported to be a confined well, but the tritium-14C combination indicates two water types are pumped in this well as a mixture. This nature is distinct from the comparison of the repeated measurements (10/67 and 7/69). In the last sample the percentage of recent water was greater than in the first sample. Corrosion and rupturing of the well casing may have deteriorated the well, so it pumps an increasing amount of shallow recent water. However, such a well may cause a short circuit between the two aquifers due to different hydraulic pressures, water from one aquifer may flow uncontrolled into the other aquifer. Repair of the well may be necessary. 

The factors that affect the selection of plasma facing materials for ITER come primarily from the requirements of plasma performance (e.g., need to minimize impurity contamination and the resulting radiation losses in the confined plasma), engineering integrity, component lifetime (e.g., need to withstand thermal stresses, acceptable erosion), and safety (e.g., need to minimize tritium and radioactive dust inventories and avoid explosion hazards). 

While still in its infancy the prospect of liquid plasma facing surfaces may offer a potential solution to many of these concerns. Of course, many unanswered questions remain regarding the use of liquids in a magnetic confinement environment. The capability to continuously renew a plasma facing surface is attractive from both an erosion and a tritium accumulation viewpoint. It is also this ability of the material to flow that removes any rigidity from the equation and restricts the ability of the material to withstand applied forces. The main challenge faced in the deployment of liquids as plasma facing surfaces is to control their motion in an environment where all the parameters encountered by the surface may not be well known and will most certainly vary in both space and time. 

Nuclei ( Li, Li) for Tritium Breeding. For either magnetically confined or inertially confined fusion, the near term nuclear species involved will be deuterium (0) and tritium (T). 

New fusion applications include the concept of production of intense negative ion beams ( ) (for neutral beam injection for heating and diagnostics in tokamaks or other magnetically confined plasmas (26, 28)) by using photodissociation to ion pairs (e.g. NaLi + hVyy Na + Li") in supersonic molecular beams. Another promising concept is the use of laser induced fluorescence to monitor very low tritium concentrations (as little as 10 Tj/cm ) under fusion reactor conditions (29). 

The fusion reaction least difficult to initiate is the deuterium-tritium (DT) reaction which releases a 14.1 MeV neutron and a 3.5 MeV alpha particle. However, because neutrons activate the reactor structure, other fusion reactions have been considered. These reactions are either neutron free, or they produce fewer and less energetic neutrons. The required quality of confinement for these more desirable fusion reactions is much higher than for DT, and it is not yet clear if it will be achieved. Hence, fusion reactor designers have concentrated on the DT reaction for at least the first generation of fusion plants. 

Compared with fission reactors, operation of fusion reactors is more complicated because of the high ignition temperatures, the necessity to confine the plasma, and problems with the construction materials. On the other hand, the radioactive inventory of fusion reactors is appreciably smaller. Fission products are not formed and actinides are absent. The radioactivity in fission reactors is given by the tritium and the activation products produced in the construction materials. This simplifies the waste problems considerably. Development of thermonuclear reactors based on the D-D reaction would reduce the radioactive inventory even further, because T would not be needed. The fact that the energy produced by fusion of the D atoms contained in 1 litre of water corresponds to the energy obtained by burning 120 kg coal is very attractive. 

Nuclear reactors and reprocessing plants are constructed and operated in such a way that the radioactive inventory is confined to shielded places. Only limited amounts of radionuclides are allowed to enter the environment. The amounts of T and produced in nuclear reactors vary with the reactor type, between about 10 and 10 Bq of T and about lO Bq of per GWg per year. Tritium is released as HTO and about one-third of the is in the form of " C02. Under normal operating conditions, very small amounts of fission products and radioelements are set free from nuclear reactors and reprocessing plants. In this context, the actinides and long-lived fission products, such as °Sr, Tc, I, and Cs, are of greatest importance. 

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