3.5 Reactor Design The sodium (lithium) cooled reactor, illustrated in Fig. 5, contains 780 UO2 8.5 mm in diameter (780 UN rods 9.0 mm in diameter) fuel rods. The active core of H/D = 270/290 mm (H/D = 290/310 mm) (leading to a content of 70 kg (110 kg) of 93% enriched uranium) is reflected axially by 7 and 14 cm thick BeO on the upper and lower parts. Both reactors are controlled with rotating beryllium drums located inside an 8 cm thick lateral reflector and equipped with neutron absorbing sectors, and are provided with seven safety rods, each replacing a hexagonal assembly of 19 fuel rods. Both reactors, of conventional design, are operated at a specific power on the order of 1 W/gU, well below the limit of their thermal hydraulic performance. The primary coolant flow rate and pressure drop, and the number of primary loops, are chosen so as to realize the optimum mass trade-off between the primary circuit (the electromagnetic pumps and their supplies, and the heat exchanger) and that of the conversion system, which depends on the power recycled in the E.M. pumps. The design of the gas cooled particle bed epithermal reactor was dominated by the search for a compact critical core assembly with a coolant routing scheme designed so as to keep the relative pressure drop across the core below 3%, which is necessary to prevent an excessive degradation of the conversion cycle efficiency. 'Hiese concerns resulted in a core consisting of a 38 cm high orthocylindrical vessel filled in bulk with UO2 BISO fuel particles (0.8 mm UO2 kernels with 0.1 mm coatings) cooled by 270 perforated drains inserted in the bed, and axially and radially reflected by 10 cm of BeO and 8 cm of Be respectively (see Fig. 6) [9]. The large porosity of this reactor concept leads to a uranium inventory of 140 kg, and makes 19 safety rods necessary to meet the desired subcriticality margin in case of immersion. As a consequence of its innovativeness, an important effort is required to qualify this reactor concept from the viewpoint of thermal hydraulics, loose particle bed behaviour, and porous barriers or cooling drain plugging risk. This also applies to the thermal spectrum gas cooled reactor concept proposed by Brookhaven National Laboratory [14], which uses ZrH moderator and annular fuel elements made of particle beds maintained between cylindrical porous barriers (see Fig. 7). Conceptual design studies performed on this concept demonstrated the advantage of thermal over fast reactors in terms of uranium inventory (12 kg for the ZrH moderated concept), but also revealed the great complexity of the internal architecture of the reactor (due to the need to specifically cool the ZrH significantly below 550°C in order to prevent its decomposition) and the requirement for permeation barriers to confine the hydrogen within the moderator.
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