Civil Engineering Reference
In-Depth Information
5.6.6.3 Computer Codes for Accident Calculations for PWR and BWR
Computer codes, e.g. RELAP [
31
] or TRAC [
32
-
35
] etc. are available to compute
the course of accidents in PWRs and BWRs. The theoretical models underlying
these computer codes were verified repeatedly in out-of-pile test rigs.
5.6.7 Transients with Failure of Scram (Safety Level 3)
Safety level 3 of the safety concept also requires accidents including failure of the
scram system to be considered. This is why, again by way of example, the
emergency power case with failure of the off-site (auxiliary) power supply for the
main pumps etc. and subsequent failure of the scram system will be described
below. Also in this case, accident behavior is similar for BWRs.
In the previous case with an example for the KWU-PWR (Sect.
5.6.6.2
) the
reactor scram was supposed to function. As the scram system now is supposed to
fail and the nominal power will remain constant in the beginning of the accident, the
pressure and coolant temperature in the primary cooling system will rise. When the
pressurizer relief valves would not open (lower pressure limit), the primary coolant
pressure is limited to 17.6 MPa by opening of the pressurizer safety valves. As the
primary coolant temperature (Fig.
5.13a
) rises strongly together with the primary
pressure (Fig.
5.13b
), the negative coolant temperature coefficient takes effect,
initially automatically reducing the reactor power to roughly 25 % of nominal
power (Fig.
5.13c
). The high temperature in the primary cooling system also causes
temperatures and pressure on the secondary side in the steam generator to rise. The
main steam blowdown valves do limit the pressure to 7 MPa (escaping steam is the
temporary heat sink), but the emergency feed pumps are unable to supply enough
water as water for heat transfer of about 25 % of nominal power is still needed.
Consequently, the secondary steam temperature continues to rise and the steam
generators gradually run dry. This causes the primary coolant temperature and the
primary pressure to rise again. As a consequence, the reactor is shut down to the
afterheat level via the negative coefficient of coolant temperature after about 450 s.
Afterwards the reactor must be kept shut down by the boric acid (secondary)
shutdown system. In addition the pressure must be further decreased until the low
pressure emergency and afterheat cooling systems provide for further cooling after
about 800 s.
5.6.8 Loss-of-Coolant Accidents (LOCAs)
Loss-of-coolant accidents can arise from breaks or cracks of pipes or from faulty
sticking of valves in the open position. For large pipes, the leak size is assumed in
