VGB POWERTECH 5 (2021) - International Journal for Generation and Storage of Electricity and Heat

Safety-related residual heat removal chains for pressure water reactors VGB PowerTech 5 l 2021
carried out via expansion tanks as well as
discharges to the feed water tank on the one
hand, and feed from the feed water tank or
the deionized water tank by means of system-associated
pumps on the other hand.
A line assignment has not yet been made for
the outer RHRC link, the Secured Service
Cooling Water System. Three parallel Secured
Service Cooling Water Pumps can feed
a manifold, from which all intercoolers as
well as the Fuel Pool Coolers are supplied.
NPP Atucha 2 (CNA 2), 692 MW el
A clear line separation concept has been
implemented at CNA 2. Although the plant
only has two reactor cooling circuits, the
Moderator System and the entire RHRC are
constructed with four lines, each of them
having a capacity of 50 % of the total power
to be removed in the design case. Thus the
“repair + single-failure” criterion for accidents
is fulfilled. Not only the RHR Intermediate
Cooling System, but also the Safety
Component Cooling System here consists
of four circuits, which supply the
respective associated consumers – i.e.
pumps and their motors – with cooling water.
The circuits of the two outer redundancies
in F i g u r e 9 can also be optionally
switched on to cooling points of the fuel
assembly transport devices (not shown in
F i g u r e 9 ). One circuit of the two inner
redundancies serves not only its safety-relevant
consumers, but also the Operation
Component Cooling System, the other one
stands by for that. The design of the RHR
Intermediate Cooling System enables – if
necessary – a takeover of heat transfer
from the reactor cooling system after shutdown
of the plant without the aid of steam
generator feed. To achieve the maximum
possible heat removal capacity, the bypasses
inside the RHR Intermediate Cooling
Circuit around Moderator Cooler and RHR
Intermediate Cooling Heat Exchanger
must be closed. If it is necessary for the
RHRC to keep the reactor cooling system in
a desired temperature state or to cool it
down according to a specified shutdown
gradient, this is done by opening/closing
the bypass around the Moderator Cooler
(without intermediate positions) and by
controlling the flow rate through the primary
side of the RHR Intermediate Cooling
Heat Exchanger on the one hand and the
bypass around the cooler on the other
(Shutdown control).
An important further development compared
to CNA 1 is the handling of the water
balance in the RHR Intermediate Cooling
Circuits. Facilities for absorbing expansion
water and re-feeding it when the circuit
cools down as well as replacing operational
medium losses in the first period after an
accident occurs (in the event of failure of
operational demineralized water supply)
are set up for each circuit self-sufficient
and spatially separated from each other in
the Reactor Building Annulus.
Each of the four subsystems of the Secured
Service Cooling Water System with one Secured
Service Cooling Water Pump each,
supplies all of the assigned heat exchangers
in parallel, that are:
––
One RHR Intermediate Cooling Heat Exchanger,
––
One Component Cooling Heat Exchanger,
––
One Secured Intermediate Cooler,
––
This heat exchanger removes the heat
loss from the line-assigned Emergency
Diesel Engine and the Secured Chilled
Water System, which is absorbed in the
so-called Secured Closed Cooling Water
System.
––
One Fuel Pool Cooler (Really only a total
of two coolers, each of which con-
SG
SG
RCP
RHR Intermediate
Cooling System
Main Steam System
Reactor Coolant System
RCP
Main Steam System
Reactor Coolant System
SG
RCP
Residual Heat Removal System
Safety Component Cooling System
Moderator System
RHR Intermediate
Cooling System
Reactor
nected to two Secured Service Cooling-
Water Pumps for alternating supply)
Comparison DWR 1,300 MW –
Atucha 2
By comparing the RHRC configurations of
the latest PLWR- and PHWR plants in F i g -
u r e 10 it is intended to show at a glance
their differences in the type and scope of
process engineering equipment for the removal
of residual heat from the reactor
cooling system. Furthermore, it is made
clear which or how many subsystems/lines
must be active during power operation of
the plant.
SG
RCP
Safety Component Cooling System
Operation Component
Cooling System
DWR 1,300 MW
RCP
Operation Component Cooling System
Secured Service Cooling Water System
SG
SG
CNA 2
Secured Service
Cooling Water
System
Fig. 10. DWR 1,300 MW – CNA 2, Comparison of RHR Chains regarding their necessary use
during power operation of the plant; Explanation of Numbers: see Figures 6 and 9.
RCP
54