13.39. An important feature of BWR operation is the coolant recir­culation system, shown schematically in Fig. 13.10. This system provides the forced convection flow through the core necessary to achieve the re­quired power density in a BWR. About 14 weight percent of the water passing through the core is vaporized and the remainder must be recir­culated. A portion of the coolant is withdrawn from the lower end of the shroud region and forced by a centrifugal recirculation pump into the jet pumps. Most BWRs have two recirculation pumps, each of which provides by way of a manifold the “driving flow” for 10 to 12 jet pumps. The flow of coolant from the pumps, up through the core and back by way of the annular region, is indicated in Fig. 13.10.

13.40. In the German KWU-BWR design, the recirculation pumps are within the reactor vessel. The electric drive motors, which are outside the vessel, are connected to the axial flow pumps by mechanical seals flanged to the bottom of the pressure vessel. By eliminating large external coolant circulation loops, the loss-of-coolant accident becomes less probable, and the coolant depressurization rate is reduced. This internal circulation pump arrangement has been adopted by the General Electric Co. for its evolu­tionary Advanced BWR design (§13.49).

Control System

13.41. The control rods in a BWR, driven by hydrostatic pressure, enter from the bottom of the core; this is both necessary and desirable. In the first place, the reactor vessel above the core is occupied by the steam — water separators and dryers; furthermore, movement of the controls in and out of the lower part of the core permits compensation for the reactivity decrease in the upper part arising from the steam voids. The absorber rods are used for reactor startup and shutdown and also to flatten the power distribution. Power adjustment during operation is commonly made by changing the coolant recirculation rate, as will be explained shortly. Boric acid solution is not used in BWRs because solid would be deposited on the fuel rods in the boiling region, thereby interfering with heat transfer. However, in newer designs for the advanced BWR, an electrical-hydraulic control rod drive system provides motion in fine steps suitable for power adjustment purposes.

13.42. Early BWR practice was to “scatter load” the core so that fuel assemblies having different burnups would be adjacent to partially inserted control rods. Since the influence of the neighboring absorber would cause
uneven burnup, particularly with fuel of higher enrichment, it was common practice to exchange or “swap” groups of control rods used for deep or shallow insertion during the operating cycle. In this way, the effects on adjacent assemblies would be reduced. However, in the newer control cell core (CCC) loading (§10.55), control rod groups intended for partial in­sertion service have only low enrichment assemblies in the adjacent four positions. Therefore, burnup effects are minimal and these rods may be used for reactivity and power distribution control throughout the operating cycle, with no need for rod pattern exchanges.

13.43. The excess reactivity of fresh fuel is partly compensated by the inclusion of gadolinium oxide (Gd203) as a burnable poison mixed with uranium dioxide in some of the fuel elements (§5.197). These rods are located where they will improve the uniformity of the power distribution.

13.44. The use of the coolant recirculation rate in the adjustment and automatic control of BWR power output is based on the following consid­erations. Let wf be the feedwater mass flow rate, wr the recirculation flow rate, and wc(= wr + wf) the core coolant mass flow rate. The reactor power P can then be represented by

P = wf(hr — hf) + (13

wc = wr + wf,

where hr, hf, and hvap are the enthalpies of the saturated recirculating water, the feed water, and of water vaporization, respectively; X is the quality (weight fraction of vapor) of coolant leaving the core. If the recirculation flow rate wr is increased without changing wf, the quality will be momen­tarily lowered, since there will not have been time for P to change. The reduction in steam voids will tend to increase the reactivity and hence the power; as a result, X will increase toward the original value and the reac­tivity will decrease. The reactor will thus become stabilized at a higher power level [5].

13.45. The foregoing behavior, which leads to a roughly linear de­pendence of the BWR power on the recirculation flow rate, is effective over a range of about 25 percent of the reactor design power without movement of the control rods. The coolant recirculation rate is determined by the injection rate into the jet pumps, and this is controlled by flow control valves at the recirculation pumps.

13.46. The critical heat flux (§9.99), at which DNB occurs, depends on the core coolant flow rate and the exit quality. Either a decrease in the flow rate or an increase in the quality (or both) tends to decrease the critical heat flux. Changes in the recirculation rate, wr, and consequently in the core flow rate wc (at constant uy), must therefore be such that the critical heat flux ratio (§9.179) is maintained above the design minimum of 1.90 at 120 percent of rated power. However, this classical figure of merit does not give a true picture of the thermal margin since the axial heat flux profile changes with power. Therefore, the critical power ratio (CPR), defined as the critical power divided by the operating bundle power at the condition of interest, is also used (§9.181).