Page 178 - Dynamics and Control of Nuclear Reactors

13.4 Reactivity feedbacks in BWRs 175

The moderator temperature also affects the thermal neutron spectrum. Increases
in moderator temperature cause hardening of the thermal neutron spectrum. This
causes a negative component of the reactivity change due to changes in U-235
absorptions and a positive component due to changes in Pu-239 absorptions. See
Section 7.3.
As in all thermal spectrum power reactors, burnup and production of Xe-135
causes a reactivity feedback effect. See Section 6.2.
The power coefficient for BWRs is negative, thereby ensuring that reactor power
achieves a new steady state level following a change in external reactivity.

DRYWELL HEAD

DRYWELL

REACTOR VESSEL

SACRIFICIAL SHIELD WALL
STEEL LINER
REACTOR
PEDESTAL S/R VALVE TAILPIPE
(18)
EQUIPMENT HANDLING
DRYWELL PLATFORM
DECK
DOWNCOMER (VENT)
VACUUM
BREAKERS PRESSURE SUPPRESSION
(5) CHAMBER
SUPPORT
COLUMN WATER LEVEL
(12)
QUENCHER (18)
REINFORCED CONCRETE
(B)

FIG. 13.5, cont’d
(B) BWR Mark II containment chamber.
(continued)