Page 177 - Dynamics and Control of Nuclear Reactors

174 CHAPTER 13 Boiling water reactors

moderator/coolant temperature decreases neutron moderation (a negative reactivity
mechanism) and decreases neutron absorptions (a positive reactivity mechanism).
The moderation effect is larger, causing a net negative moderator/coolant tempera-
ture coefficient of reactivity. Typically, the moderator/coolant temperature coeffi-
cient is around 3 10 4 Δρ/°C.
The void coefficient depends strongly on reactor conditions, but it also is always
negative. Typically, the void coefficient is around 1.4 10 3 Δρ/% voids.
Pressure also affects reactivity because pressure affects boiling. For example, a
pressure increase causes a reduction in core voids. Because the void coefficient is
negative, a pressure increase causes a reactivity increase. That is, the pressure coef-
ficient of reactivity is positive. The control strategy in BWRs is to maintain constant
pressure by modulating the steam valve.

DRYWELL HEAD

DRYWELL FLANGE

DRYWELL SHEAR LUG
SUPPORT REACTOR
PRESSURE
VESSEL
DRYWELL
SHIELD WALL
CORE
RADIAL BEAM

MANWAY
RADIAL BEAM
VACUUM BREAKER
JET DEFLECTOR
VENT
VENT HEADER

DOWNCOMERS
WATER LEVEL
(A) SUPPRESSION CHAMBER (TORUS)
FIG. 13.5
(A) BWR Mark I containment with toroidal pressure suppression chamber.
(continued)