13.29. The boiling-water reactor (BWR), as the name implies, generates steam directly in the core. Since this steam then goes directly to the turbine — generator, the system is inherently simpler than a PWR. However, a com­plication is the need to maintain the fraction of steam in the core less than about 14 percent by weight for stability purposes. Thus, the exiting water, which has not been vaporized, must be separated from the steam at the top of the core and recycled.

13.30. Since the core steam conditions are comparable to those on the secondary side of a PWR steam generator, the BWR core pressure is about one-half that of a PWR. Therefore, a larger core and lower power density are practical as well as larger-diameter fuel rods. In Table 13.3 are listed typical design specifications for a large BWR of the type marketed in the 1970s. The thermal and electrical powers are essentially the same as for

General	Thermal-Hydraulic
Power Thermal Electrical Specific power Power density
3830 MW 1330 MW 25.9 kW(th)/kg U 56 MW(th)/m3
Core
3.76 m (12.3 ft) —4.8 m (15.8 ft)
Length Diameter
Fuel
12.52 mm (0.493 in.) 0.864 mm (0.034 in.) 10.57 mm (0.416 in.) 16.3 mm (0.64 in.) 62 (8 x 8 array) 4.47 mm (14.7 ft.) 134 mm (5.28 in.) 0.01 m2 (15.5 in.2) 168 x 103 kg (3.69 x 105 lb)
Rod, OD Clad thickness Pellet diameter Rod lattice pitch Rods per assembly Assembly overall height Assembly width Coolant flow area/ assembly Fuel loading, 1Ю2
Control
Movable cruciform elements Overall length Poison section
193 4.42 m (14.5 ft) 3.66 m (12.0 ft)
Ave. feed enrichment —2.6% Ave. core enrichment —1.9% Burnup 2.38 TJ/kg (27,500 MW • d/t)

the PWR described in Table 13.1. Hence, the parameters quoted for the two types of light-water reactors may be compared.

13.31. A variety of BWR fuel designs are offered by various vendors. Worthy of note is a 9 x 9 lattice design containing 272 fueled rods and a central water channel (§10.57). An average linear heat rating of 15.9 kW/m is specified for rods having an outside diameter of 11 mm.