Safety and Risk of Light Water Reactors - The Risks of Nuclear Energy Technology - page 117

- Faulty opening of the main steam line valves.
- Complete failure of feedwater supply.
- Maximum reactivity increase as a consequence of withdrawal of control ele-
ments or groups of control elements at full power.
- Depressurization as a consequence of inadvertent opening of the pressure vessel
safety valve.
- Maximum reduction of core inlet temperature due to disturbances on the steam
generator secondary side.
In these accidents, the permissible stresses and temperatures of the reactor
pressure vessel and the cooling system must not be exceeded. The boration system
(secondary shutdown system) and the heat removal systems must be designed so
that the reactor core can be shut down safely in these accidents and remains
subcritical (Fig. 5.7 ).
5.6.6.1 Operating Transients of LWRs with the Reactor Shutdown
System Functioning (Safety Level 3)
Disturbances of steady-state reactor operation arise from imbalances between heat
generation and heat removal. This raises the coolant temperatures and coolant
pressure. In all cases, the reactor protection system will shut down the reactor in
the shortest possible time when limits of power or coolant temperatures have been
exceeded or the pump speed limit of the reactor protection system has been
underrun. For instance, the shut down rods of the reactor scram system, operating
by the failsafe principle, drop into the reactor core from the top under gravity in
about 2 s (PWR) or are pushed into the reactor core under pressure from below
(BWR) in a similar time period. There must be another, diverse shutdown system
(boric acid system) in case the first shutdown system was to fail (Sects. 3.1 and 3.2 ).
One example is described below for a PWR. The accident sequence for a BWR is
similar.
5.6.6.2 Loss of Off-Site (Auxiliary) Power Supply (Emergency Power
Case) with Scram Functioning
Failure of off-site power supply causes the emergency power diesel systems to start
up and supply the most important components of the PWR with electricity. The
instruments and some smaller electrical components are supplied from batteries.
However, the power of the emergency diesel systems is generally not sufficient to
supply the large main coolant pumps of the primary system, the main feedwater
pumps for the steam generators, and the main cooling water pumps of the turbine
condenser. Figure 5.11 shows the key components of the steam circuit of as, e.g. the
KWU-PWR.