rapid decrease in reactor power following the automatic shutdown
((3) in Figure 8.2.1).
(5) The minimum DNBR
decreases with the decrease in reactor core coolant flow, reaching
approximately 2.21 (no less than 1.42 as safety limit) about 3.2s
after the event occurs (@
in Figure 8.2.1). But after that the minimum DNBR rapidly recovers
with the rapid decrease of reactor power due to the automatic
shutdown. Throughout the
transient, the minimum DNBR, reactor pressure, etc. are maintained
within their respective sufficiently safe ranges.
In this postulated transient control rods are continuously withdrawn
during reactor startup operation due to a malfunction or an
operational error of the control rod driving system which results in
increase of the reactor power. Since in this event the inserted
reactivity exceeds 1 dollar ($1), the event is belongs to the
“reactivity insertion event"
category.
When inserted reactivity exceeds 1 dollar, neutron flux rapidly
increases, but the increase is suppressed by negative feedback of
reactivity due to the Doppler effect. If nothing is done against
this abnormal withdrawal of control rods, the rod withdrawal will
continue with the continuous reactor power increase, and will lead
to fuel failure.
However, actually a number of measures, as listed below, are
provided so that the abnormal event is terminated:
Control rod cluster withdrawal is by a magnetic jack mechanism, and
the maximum withdrawal speed is limited by the speed of the coil
excitation sequence.
Withdrawal of control rod clusters is automatically blocked by the
following signals. •High intermediate range neutron flux
High power range neutron flux
The reactor is shutdown automatically by the following signals from
the reactor protection system.
•High source range neutron flux
High intermediate range neutron flux
•High power range neutron flux (low setpoint) •High power range
neutron flux (high setpoint)
•High rate of power range neutron flux
The analytical results given below are for the case of an automatic
reactor trip by the signal for high power range neutron flux (Low
setpoint), and neglecting the signals for high intermediate range
neutron flux, high power range neutron flux, and high source range
neutron flux in order to make the results conservatives.
Figure 8.2.2 shows the analytical results of neutron flux, fuel
enthalpy and reactor pressure. The changes of the parameters with
time are as follows,
© Due to the withdrawal of control rod cluster banks, neutron flux
rapidly increases. Approximately 9.5s after occurrence of the event
the signal “high power range neutron flux'*
(low setpoint) is generated, and approximately 10s after
occurrence of the event the control rod clusters start to drop and
the reactor is shutdown automatically (© in Figure 8.2.2). Before
the control rods start to drop, the reactor power increase is
suppressed by feedback due to the Doppler effect ((2) in Figure
8.2.2), and then the reactor is brought to the subcritical condition
as the control rods are fully inserted.
(D Fuel enthalpy at the
hottest spot in the core increases along with withdrawal of control
rod cluster banks and reaches the maximum value of approximately
343kJ/kg ■ UO2.
Then it decreases along
with reactor automatic shutdown.
(3) Since the event is assumed to occur at the time of reactor
startup with no load in the secondary cooling system, heat removal
from the primary system becomes insufficient due to the increase of
neutron flux and the temperature and pressure of the reactor system
increase. But with automatic shutdown of the reactor, the
temperature and pressure of the reactor system gradually decrease,
and the transient change comes to an end. Throughout the course of
the event as described above, fuel enthalpy and reactor pressure are
maintained within their respective sufficiently safe ranges.
This transient is postulated to occur during the reactor power
operation. Heat removal from the
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Uncontrolled Control Rod Withdrawal at Reactor Startup
Loss of Normal Feedwater Flow
Chapter
8 Safety Evaluation of PWR Plants
primary system is reduced as a consequence of loss of normal
feedwater to the steam
generators due to a malfunction of the main feedwater pumps or
condensate pumps, or the feedwater control system or other causes.
If nothing is done against the loss of normal feedwater flow, the
cooling water in the secondary side of the steam generators will
decrease and removal of heat from the reactor system will become
insufficient which will result in increased temperature of the
reactor system. As a further result, the volume of the coolant will
increase and so will the pressure. If such a situation spreads, the
pressurizer relief valve and then the pressurizer safety valve will
open because of the over-pressure of the reactor coolant system, and
coolant will flow out
However, certain measures are provided to control the event They
are as follows.
(D Generating an alarm
signal at low water level in the steam generators.
Automatic tripping of the reactor via the following signals from
the reactor protection systems.
•Low feedwater to steam generators
Low water level in steam generator
High reactor pressure
In response to the signal “low water level in steam generator"
or other signals, two
motor-driven auxiliary feedwater pumps start automatically, resuming
supply of water to the steam generators. These pumps are powered by
the emergency diesel generators in case of loss of external power.
(D Furthermore, in response
to simultaneous signals of “low water level in steam generator”
for two or more steam generators, the turbine- driven auxiliary
feedwater pump starts automatically to supply water to the steam
generators.
Figure 8.2.3 (1) shows the analytical results of reactor pressure,
reactor coolant average temperature and steam generator water level.
Event development with time is as follows.
© The reactor is
automatically tripped by start of control rod cluster to drop in
response to the signal of “high reactor pressure ” about 30s
after the event start.
(D The
water level of the steam generators drops rapidly due to loss of
normal feed water and reactor trip (see (D
in Fig. 8.2.3 (1)). With
the drop in water level, reactor coolant temperature increases due
to decreased heat removal from the steam generators, causing first
the coolant volume expansion, and then the pressurizer water level
rise.
(3) Reactor pressure reaches a maximum of approximately 17.4MPa
(gage) immediately
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Figure 8.2.2 Uncontrolled control rod withdrawal at reactor startup (•Indicates the initial value)
after the reactor trip. The rise of pressure is limited by the
actuation of the pressurizer safety valve (in the analysis the
depressurization effect by actuations of the pressurizer spray valve
and the pressurizer relief valve is ignored) ((2)
in Figure 8.2.3 (1)).
(D At 60s after reactor
trip a motor-driven auxiliary feedwater pump is actuated, resuming
water supply to the steam generators, and the water levels are
gradually recovered. Heat removal from the secondary side is
gradually restored and reactor system temperature and pressure
gradually decrease, and the transient change is brought
to a stable state ((3) in
Figure 8.2.3(1)).
The results of the water
inventory in the pressurizer in the case of pressurizer water level
analysis are shown in Figure 8.2.3(2).
The event development with
time is as follows.
(§) By the signal of
“low-low steam generator water level” causes the control rod
clusters to start dropping about 52s after the event starts, and the
reactor is automatically tripped.
® The maximum pressurizer
water volume reaches about 78% of the pressurizer volume and the
pressurizer is not filled.
Throughout the course of the transient, the reactor pressure is
maintained within sufficiently safe range.
Figure
8.2.3(1)
Loss of normal feedwater flow (‘indicates
the initial value)
s
40
>g
20 -
CL>
I
fe
o 1 1 * 1 I
I
0 1000 2000 3000 4000 5000 6000
(L Time
(s)
Figure
8.2.3(2)
Loss of normal feedwater flow
(‘indicates
the initial value)
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