Nuclear Fusion - One Noble Goal and a Variety of Scientific and Technological Challenges


4. Target protection system

DOI: http://dx.doi.org/10.5772/intechopen.81518

bottom.

Figure 16.

73

the process of target acceleration and injection.

demonstrate the practical possibilities of this direction.

Target delivery into IFE power plant requires target acceleration (accompanied

A promising direction for survivability of fuel layers is application of external target protective coatings, which reduce the risks of the fuel damage under the radiation exposure from the hot walls of the reaction chamber: cryogenic coatings (from the solid D2, H2, or Xe), metal coatings from Au, Pt, Pd, and their alloys, application of a double protection: "metal + cryogenic" (Figure 16a). Below, we

To obtain the results in Figure 16b, an additional procedure is added in the FST

formation cycle. First, the metal coating made from Pt/Pd is deposited on the CH-shell. Then, the shell is filled with the D2 gaseous fuel with 3% Ne as doping agents. The next step is D2 layer fabrication by the FST layering method.

The sabot is also a special element of the target protection system [12]. An important feature of its design is the shape of a target nest. Our study shows that a proper choice of the nest shape makes it possible to significantly increase the upper limit of the permissible overloads and to minimize the injector size. Being based on the discrete-continuous physical model of the shell stress, a simulation code SPHERA is developed that makes it possible to define the stress and deformation arising in the target during the acceleration. A shape analysis of the sabot bottom (in the target nest area) during the target acceleration is carried out for three sufficiently different cases: (1) flat bottom, (2) semispherical bottom with Rn > Rt (Rn and Rt are the nest and the target radii, respectively), and (3) conical bottom (Figure 7). Important conclusions followed from these calculations are listed below:

• Permissible target overloads for a flat bottom are 50 times smaller than those for semispherical supports with clearance less than 5 μm; at clearance higher than 20 μm, the stresses growing in the target are close to those of the flat

• Use of a conical bottom with the angle in cone basement equal to 87° provides a 20-times increase of the permissible overloads; technologically, the conical bottom has much better predictive estimate than the semispherical one.

Different protective coatings. In (a): double protection "Pd-coating of 150 Å thick and cryogenic O2-coating": 1 —1.2-mm CH-shell at 14.6 K before the experiment (liquid H2 inside as temperature indicator), 2—in the top part of the shell (from the outside), there is a solid deposit of oxygen (Ttp = 54.3 K), 3—after operation of the piezoelectric vibrator [22], the oxygen snow becomes redistributed onto the outer shell surface; in (b): single protection "Pt/Pd-coating of 200 Å thick": 1—1.5-mm CH-shell before the experiment, 2—cryogenic target at

5.0 K with a uniform D2 layer of 50 μm fabricated by the FST-layering method.

with mechanical and thermal loads) and repeatable injection into the reaction chamber (additional thermal loads). For this reason, the problem of using the cryogenic targets in the IFE experiments or in a future reactor includes not only an issue of fabricating the qualitative cryogenic fuel layer (nonuniformity <1%, roughness <1 μ), but also an issue of target delivery at the laser focus under conditions of the layer parameter survival. In our study, a number of protection techniques have been proposed and examined with the aim of risk minimization in

Mechanical Mockup of IFE Reactor Intended for the Development of Cryogenic Target Mass…

Table 3.

MgB2-driving body acceleration efficiency at TS = 20 K.

the pinning force density Fpin can be found for the superconducting coils proposed for the sabot acceleration. For MgB2-cables of 1.18 mm in diameter, the critical current vs. the magnetic field at temperatures of 4.2, 9.8, 15, 20, and 25 K was measured in [30]. Using these data, we have made the calculations under the actual operating conditions: (a) the target design is CHGT, and its mass is 5 mg (see Section 2.2); (b) the HTSC-sabot is "open parallelepiped" to exclude a bend of the Gd123 tapes (see Section 3.2); and (c) the mass of the assembly "HTSC-sabot- + CHGT" is 0.5 g. The calculation results are presented in Table 3.

Thus, using the MgB2-driving body allows not only to accelerate the reactorscaled targets to the required injection velocities, but also to provide the system performance without exceeding the acceleration limits at 500 g. As one can see from the Table 3, the MgB2 coils (with parameters 2πRSC = 24 mm, B0 = 0.25 T, JC = 5000 A) yield Vinj = 200 m/s at 400 g on the acceleration length of 5 m.

Note that several important aspects related to a practical engineering are as follows:


Especially, note that the injection velocities Vinj ≥ 200 m/s are not a problem for the proposed noncontact schedule of the target delivery. It can successfully be used in creation of a hybrid accelerator for future IFE power plants.

Mechanical Mockup of IFE Reactor Intended for the Development of Cryogenic Target Mass… DOI: http://dx.doi.org/10.5772/intechopen.81518
