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

VGB PowerTech 5 l 2021
Safety-related residual heat removal chains for pressure water reactors
Cooling System*. In this way, heat removal
from the fuel pool is possible in principle
via each of the four RHR lines.
NPPs with Pressurized Heavy
Water Reactors (PHWR)
The function of the Moderator System in
power operation of the plant requires identical
pressure and temperature design values
as for the Reactor Coolant System itself.
However, this also opens up the possibility
– by switching over valves inside the
Moderator System and with an appropriate
design of the RHR Intermediate Cooling
System as the middle link of the RHRC – to
take over the cooling of the reactor immediately
after shut down, even without additional
Steam Generator feed. This option
has not yet been implemented for the
MZFR as the first PHWR plant. Only CNA 1
and CNA 2 are equipped with a high pressure/high
temperature designed RHRC
and are therefore independent of the function
of the main heat sink (steam turbine
condenser) for cooling down the plant after
all shut-down occasions to be assumed.
Multi-purpose research reactor Karlsruhe
(MZFR), 50 MW el
The shutdown concept of the MZFR basically
corresponds to that of PLWR plants,
with priority on the Steam Generators [11]
(F i g u r e 7 ). Only when this – below a certain
coolant temperature – is no longer
thermodynamically possible, switch over
to RHRC operation has to be performed for
further cooling of the plant. Moderator
temperature and heat to be removed at this
time are already so low that the Moderator
Cooler on its secondary side can be operated
with inlet temperatures, which are accepted
by the other cooling points without
boiling at its outlet; even at the slight overpressure
with which the Component cooling
System is operated.
A special feature of the MZFR-RHRC is that
the operating pressure in the Secured Service
Cooling Water is higher than in the
Component Cooling System. In the event
of a heat tube leak in the Component Cooling
Heat Exchanger, transition of possibly
radioactive contaminated water to the environment
is thereby avoided, but pollution
of the deionized water in the component
cooling circuit may happen instead. In
subsequent plants, the pressure gradation
was implemented consistently from the
heat source (high) to the heat sink (low).
RCP
SG
Reactor
NPP Atucha 1 (CNA 1), 319 MW el
In contrast to MZFR, with CNA 1 one can
already speak of an important step towards
a multiple-line design of the RHRC (F i g -
u r e 8 ). The Moderator System consists of
two completely separate loops, each of
which adjacent to a circuit of the RHR Intermediate
Cooling System [12, 13, 14].
Deviating from MZFR, the task of this system
is to be able to take over the reactor
cooling already shortly after shut down of
the reactor. The associated temperature
and pressure values in the system preclude
the use of the Component Cooling System
for heat removal; this is designed to only
supply all other safety-related and the operational
cooling points as a single circuit.
It is fitted out with two Component Cooling
Heat Exchangers and Component Cooling
Pumps of full capacity each. The RHR Intermediate
Cooling System is equipped
with a third RHR Intermediate Cooling
Pump. In the event of failure of one of the
two regular pumps this additional pump
takes over the circulation in the affected
SG
2 1 1 2
Feed Water
System
3 3 3
4 4
5
RCP
6
Main Steam System
Reactor Coolant System
Moderator System
RHR Intermediate
Cooling System
Component Cooling
System
Secured Service
Cooling Water System
8
Fig. 8. CNA 1, Reactor Coolant System and RHR Chain.
7
9
SG Steam Generator
RCP Reactor Coolant Pump
1 Moderator Pumps
2 Moderator Coolers
3 RHR Intermediate
Cooling Pumps
4 RHR Intermediate
Cooling Heat Exchanger
5 Secured Service
Cooling Water Pumps
6 Component
Cooling Pumps
7 Component Cooling
Heat Exchanger
8 Component Cooling
Water Consumers
9 Fuel Pool Coolers
circuit. The principle of line separation for
the RHR Intermediate Cooling System is
not impaired by this. The return lines of the
RHR Intermediate Cooling Circuits cannot
be shut off to the area around the Moderator
Cooler flowed through by the feed water
during power operation, so that the
feed water pressure is impressed on them
in their standby state. After the feed water
lines at the outlet of the Moderator Cooler
have been shut off and transition to the
RHRC cycle operation is completed, the
water balance in the RHR Intermediate
Cooling Circuits (absorption of expansion
water when heating up, recovery of contraction
water when cooling down) can be
SG Reactor SG
RCP
Main Steam System
Reactor Coolant
System
RCP Reactor Coolant Pump
SG SteamGenerator
1 Moderator Pumps
2 Moderator Coolers
3
4
5
6
Moderator System Safety Component Cooling System Secured Service
Cooling Water
System
RHR Intermediate
Cooling System
Operation Component
Cooling System
RHR Intermediate Cooling Pumps
RHR Intermediate Cooling Heat Exchangers
Secured Service Cooling Water Pumps
Component Cooling Pumps
Fig. 9. CNA 2, Reactor Coolant System and RHR Chain.
RCP
7 Component Cooling Heat Exchangers
8 Component Cooling Water Consumers
9 Fuel Pool Coolers
10 Secured Intermediate Coolers
53