Page 176 - Dynamics and Control of Nuclear Reactors

13.4 Reactivity feedbacks in BWRs 173

section with a constant diameter) resulting in an increase in the fluid pressure due to a
change in the fluid velocity in this section. A long diffuser is connected at the end of
the mixing section, causing an increase in the fluid pressure that drives the coolant
into the lower plenum and then up through the reactor core.

13.3.3 Other features of BWRs
The following are typical core parameters for a BWR-6:

6
• Total coolant flow rate: 105 10 lbm/h
• Plant efficiency: 34%
• Core diameter: 193in.
• Number of control rods: 177
• Coolant pressure: 1040 psia
• Core-exit (steam) temperature: 551°F
• Feed water temperature: 420°F
• Average coolant exit quality: 15%
• Vessel diameter: 19ft. Wall thickness: 5.7in./6.46in.
• Vessel height: 71ft.
• Core power density: 54kW/l.

The steam produced in the core and separated from liquid water passes through a
control valve into the turbine. The steam pressure is maintained at a constant value
by throttling the steam valve. Exhaust steam passes into a condenser and the conden-
sate passes through a series of feedwater heaters before returning to the reactor
vessel.
Note that a BWR system shares general features with U-tube steam generators
used in most PWRs (a heated riser, steam separators and driers, and a downcomer).
A BWR containment consists of a concrete “drywell” that encloses the reactor. If
steam escapes from the reactor vessel or related piping it flows into the drywell. The
drywell has piping that connects it to a large pool of water called the suppression
pool. The suppression pool water condenses the steam and reduces pressure in the
drywell. Three different types of suppression pool are used, Mark I, II and III [3].
See Fig. 13.5.

13.4 Reactivity feedbacks in BWRs

The BWR fuel temperature coefficient of reactivity is due to the Doppler effect (fuel
temperature feedback reactivity) and is always negative. Typically, the Doppler
coefficient in BWRs is around 2 10 5 Δρ/°C. The magnitude of the negative
Doppler coefficient increases as fuel temperature increases.
BWRs are under-moderated. Increases in moderator/coolant temperature cause
increased boiling and reduced in-core liquid water density. Thus, an increase in