Civil Engineering Reference
In-Depth Information
Pressurizer
Safety
valves and
Blowdown
valves
Fig. 5.11
PWR with the key components for steam production [
4
]
In this accident, the turbine and the generator are separated first from each other.
The main coolant pumps of the primary cooling circuits, the main feedwater pumps,
and the main cooling water pumps loose speed and coast down. When 93 % of the
design speed of the main coolant pumps is underrun, the absorber-shutdown rods
drop into the core (reactor scram) and the turbine emergency stop valve closes. Also
the valve for the main steam bypass closes, as cooling the turbine condenser fails
when the cooling water pumps coast down. Closure of these valves (turbine,
condenser) and steam production in the steam generators, which is continued for
the time being, cause the main steam pressure to rise. However, this rise of steam
pressure and coupled steam temperature on the secondary side of the steam
generators can be limited by opening the main steam blowdown valves downstream
of the steam generators when 7.0 MPa are exceeded. The escaping steam acts as a
temporary heat sink. Initially, heat conduction in the steam generator pipes slightly
raises the coolant temperatures and coolant pressure on the primary side. The
pressurizer valve opens briefly, limiting the primary pressure. As a result of
scram, the cooling water temperature in the reactor core decreases. Also the
primary coolant pressure drops at the same time. On a medium term, the primary
coolant temperature and the primary coolant pressure rise again slightly because the
main coolant pumps coast down to lower speeds. Over the longer term, however,
the primary coolant temperatures and the coolant pressure drop because the
afterheat heat production decreases and afterheat cooling systems start working
(Fig.
5.7
).
