13.32. The internal structure of a typical BWR core and reactor vessel is depicted in Fig. 13.9. The core consists of almost 800 fuel assemblies in square 8×8 arrays; 62 fuel rods are contained in each assembly with two hollow central (“water”) rods designed so that water flows through them to provide additional moderation (§10.52). Each fuel assembly is contained in a four-sided channel separator or “can” to prevent crossflow of coolant between assemblies. A control rod (or element) with a cruciform cross section is located within a set of four assemblies (see Fig. 5.16).

13.33. The fuel assemblies rest in support pieces mounted on top of the control-rod guide tubes. The guide tubes, in turn, rest on the control-rod penetration nozzles in the bottom of the reactor vessel. The core plate (Fig. 13.9) merely provides lateral support. Orifices in the fuel-assembly support permit adjustment of the coolant flow distribution among the as­semblies in the core. These orifices can be changed, if necessary, but only by disassembly of the core structure.

13.34. An important BWR consideration is the power stability based on the coupled hydrodynamic-neutronic feedback response resulting from the formation of steam in the core (§5.109). Power oscillations, which would cause fluctuations of the steam voids in the moderator, and hence in the reactivity, should be damped by a proper combination of feedback parameters (§5.132 et seq.); a detailed discussion is beyond the scope of this treatment. Stability tends to be improved by a decrease in power density, increase in coolant flow rate, and increase in rod diameter. Stability considerations govern the thermal power-coolant flow combination during startup and operation as shown in a “power map” (Fig. 14.1) which defines the acceptable operating region. Generally, a BWR of present design can be operated on natural circulation up to about 25 percent of rated power.

13.35. As with the PWR (§13.10) the H/U ratio in a BWR affects the fuel utilization. However, as a result of the low hydrogen density in the vapor fraction of the coolant (—0.4 by volume), the H/U ratio tends to be slightly lower than that for a PWR although the H20/U02 volume ratio (—2.4) is higher.* The void (vapor) fraction, and the H/U ratio which

Water in the channels between fuel assemblies is included in the calculation of the ratio.

depends on it, varies both axially and radially in the core, as well as with burnup.

13.36. A stainless steel cylindrical shroud within the reactor vessel sur­rounds the core and separates the upward flow of coolant through the core from the downward flow in the annulus between the shroud and the vessel. The jet pumps (see §13.39) within the annulus are supported by the shroud.

13.37. Liquid water entrained in the steam-water mixture leaving the fuel assemblies is separated in the upper part of the reactor vessel. The mixture first enters vertical standpipes in the shroud head and then passes through axial flow centrifugal separators. The swirling motion drives the water droplets to the outer wall from which they flow back to the core via the downcomer annulus outside the shroud (Fig. 13.9). The steam, still containing some moisture, passes on through a dryer assembly of vanes and troughs; here most of the remaining moisture is separated and returned to the downcomer.

13.38. The two-stage system represents a design challenge since the exit steam must contain no more than 0.1 percent water by weight. Steam leaving the first stage contains less than 10 percent water by weight. Since the system must process the entire two-phase mixture from the core, the drying capacity of the system has been one of the constraints on core power.