13.22. The Combustion Engineering design evolved from that of the three-unit Palo Verde Generation Station in Arizona, which began com­mercial operation in 1986. A summary of design specifications for the proposed system is given in Table 13.2. The format follows that used in Table 13.1.

13.23. The fuel assemblies are in a 16 x 16 array with rod dimensions similar to those used in the Westinghouse core. However, as seen in Fig. 13.6, there are five large thimbles in each assembly. Four of these are for control rods and one for instrumentation. There are clusters of both four and 12 elements in the core. The 12-element cluster spans five assemblies. Each rod cluster is connected by a “spider” to the drive mechanism. Groups of these clusters are moved as units for power regulation. Burnable ab­sorber rods are substituted for fuel rods in the lattice, as needed, rather than inserted in the control rod thimbles.

13.24. The reactor cooling system arrangement is shown in Fig. 13.7. Four circulating pumps serve two large steam generators. The steam gen­erator secondary-side water inventory has been increased by 25 percent to improve the response to upset conditions and to extend the time available for countermeasures should the feedwater supply be interrupted. Similarly, the pressurizer volume has been increased by 33 percent to reduce the pressure changes during transients such as reactor trip and load rejection.

13.25. The reactor operating margins relative to previous designs were increased by the foregoing improvements as well as by a reduction in the hot-leg temperature and improved monitoring methods. Also, the new design provides for carrying out operating power level maneuvers using control rods only. Thus, the need for short-term boron-level adjustments is reduced.

13.26. Numerous other evolutionary design features contribute to sim­plification and improved safety margins. A probabilistic risk analysis in­dicated a reduction in the risk of severe accidents by two orders of mag­nitude compared with that for present systems.

General Thermal-Hydraulics Power Thermal 3800 MW Electrical (net) 1280 MW Specific power 37 kW(th)/kg U Power density 95.4 MW(th)/m3 Coolant Pressure 15.5 MPa(a) (2250 psia) Inlet temp. 292°C (558°F) Outlet/temp. 324°C (615°F) Flow rate 21.4 Mg/s (1.61 x 108 lb/ hr) Mass velocity 3.6 Mg/s • m2 (2.64 x 106 lb/hr) Rod surface heat flux Ave. 0.599 MW/m2 (1.90 x 105 Btu/hr-ft2) Max. 1.41 MW/m2 (4.46 x 105 Btu/hr-ft2) Steam pressure 6.90 MPa(a) (1000 psia) Core Length 3.81 m (12.5 ft) Diameter 3.66 m (12 ft) Fuel Rod, OD 9.7 mm (0.382 in.) Clad thickness 0.64 mm (0.025 in.) Pellet diameter 8.3 mm (0.325 in.) Rod lattice pitch 12.8 mm (0.506 in.) Rods per assembly 236 (16 x 16 array) Assembly overall 202 mm (7.97 in.) width Assemblies 241 Fuel loading, U02 116 x 103 kg (2.57 x 105 lb) Enrichment levels 3.3, 2.8, 1.9 percent by weight Control Control assemblies 68 full length, 25 part strength

13.27. A 61-m (200-ft)-diameter spherical containment for this system provides both safety and cost advantages. As shown in Fig. 13.8, a concrete shield building encloses an inner steel sphere which has a relatively large internal free volume of 9.5 x 104 m3 (3.4 x 106 ft3). The steel shell acts as a heat sink and the large volume provides energy-absorbing capability in the event of an accident. An in-containment refueling water storage tank (IRWST) combines the functions of a refueling water tank and a post — LOCA containment sump. Economic advantages also result from efficient space utilization and ease of construction.[27]

13.28. To control costs, this and other new reactor proposals feature extensive standardization, not only for the Nuclear Steam Supply System, but the entire plant. This is in accordance with NRC’s standardization rules given in 10 CFR 52, which also provides for design certification so that the plant can be pre-licensed prior to purchase. A 48-month construction period is estimated, which should help meet a construction cost goal for new plants established by EPRI of $1500 per kW(el).