Page 190 - Dynamics and Control of Nuclear Reactors

13.13 BWR safety 187

13.12 BWR control strategy
We have seen that a BWR with no control action responds initially in the wrong
direction following an increase in steam flow. So, the basic idea in BWR control
is to increase reactor power before releasing more steam to the turbine following
an increase in demand. A reactor operated in this way is called a “turbine following
boiler”. That is, following a power demand maneuver, the reactor power is adjusted
first. The turbine waits until the reactor changes power level before experiencing a
change in steam flow.
The main control systems in a BWR are the reactor power controller, the feed-
water controller, and the pressure controller.
The reactor controller uses control rod motion and core flow adjustment to con-
trol reactivity. As shown above, the choice of control action (control rods or core
flow) depends on reactor condition. The power-flow map provides information on
allowable flows at all power levels.
The feedwater controller is a so-called three element controller. Measurements
provide the downcomer level, the feedwater flow rate and the steam flow rate. Feed-
water flow rate is adjusted to eliminate a deviation in level from its set point and to
eliminate a mismatch between feedwater flow rate and steam flow rate. This type of
control is necessary because shrink and swell occur in BWRs (just like in U-tube
steam generators).
The pressure controller adjusts the steam valve to maintain constant steam pres-
sure. Called the electro-hydraulic controller, it modulates the steam valve to achieve
constant pressure.
Consider the response to an increase in power demand. The first action is an
increase in core flow. This increases reactivity, power level and steam production.
The resulting pressure increase causes the pressure controller to open the steam valve
and consequently provide the steam flow needed to satisfy the increase in power
demand. The feedwater flow is regulated to maintain the downcomer level at a
set point. This scenario illustrates the “turbine following boiler” approach used
in BWRs.

13.13 BWR safety

Like PWRs, generation II plants require emergency power supplies to provide cool-
ant water pumping in the event of an accident. As shown in Section 11.5.3 emergency
power was lost due to a tsunami at the Fukushima Dai-ichi power plant in Japan. This
led to catastrophic failures, but it was a result of placing emergency power facilities
in a vulnerable location rather than a failure of BWR safety philosophy. Changes at
other generation II BWRs provides increased security of emergency power supplies.
New BWR designs eliminate the need for electrically driven emergency coolant
pumps by using gravity feed and flow from pressurized tanks.