Very high temperature reactor fuel design

This appendix provides a brief description of the Fast Flux Test Facility (FFTF) fuel handling system and its operation as described by Cabell (1980) and in the Fast Flux Test Facility System Design Description (FFTF, 1983). The description is limited to those system features that are potentially relevant to the refueling of a liquid-salt very high-temperature reactor (LS-VHTR). Because the FFTF was designed as a reactor to test fuel, it has additional capabilities and equipment compared with a sodium-cooled fast reactor designed only to produce electricity. 

Of the twenty-six concepts and designs addressed, 13 (50%) are water cooled SMRs, 6 (23%) are gas cooled SMRs-high temperature gas cooled reactors (HTGRs), 6 are sodium or lead-bismuth cooled fast reactors, and 1 is a non-conventional very high temperature reactor concept, a liquid salt cooled reactor with HTGR type prismatic fuel. 

Most fission products (including cesium and iodine) and all actinides escaping the solid AHTR fuel are soluble in the molten salt and will remain in the molten salt at very high temperatures. Fluoride salts were chosen for the liquid-fueled molten-salt reactor because actinides and fission products dissolve in the molten salt at very high temperatures. This same characteristic applies to the AHTR and provides the reactor with a second, independent beyond-design-basis-accident mitigation system to prevent radionuclide release to the environment. 

Among the reactor accidents which have occurred up to the present, Windscale-1 and Chernobyl-4 (see Section 7.4.3.) represent by far the most serious ones with respect to the thyroid burdens to the public as well as to the field contamination of the surrounding areas. In the days and weeks after the Windscale-1 accident, it was possible to measure the released radionuclides in low concentrations even in Norway and in Germany. As was pointed out before, accidents of this type cannot happen in light-water reactors on the other hand, such release values are not to be expected from gas-cooled reactors of modern design, since the fission products in their fuels are confined in graphite-coated fuel elements which are stable even under very high temperatures. 

Figure 11.10 Design of a spherical fuel element ( pebble ) of a very high-temperature (VHTR) gas-cooled pebble bed reactor (PBR).

The effect of reactor inlet temperature is shown in Figure 5.3. For an inlet temperature of 446 K, the reaction rate is small. Therefore there is only a small increase in temperature and little consumption of the reactants (low conversion). However, a quite small increase in inlet temperature to 448 K results in very rapid increases in temperature and conversion. With an inlet temperature of 450 K, the reactants are essentially completely consumed. The adiabatic temperature rise is about 330 K This example illustrates one of the difficult problems associated with tubular reactors. They can be very sensitive to reactor inlet temperature. The problem is analogous to that seen in earlier chapters in CSTRs that are designed for low conversions. The reactor inlet stream contains high concentrations of both reactants, so there is plenty of fuel to generate a runaway reaction. If the maximum temperature limitation in the system is 550 K, this runaway could do real damage to the catalyst or result in a vessel meltdown. 

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