# Effect of Coolant Inventories and Parallel Loop Interconnections on the

Authors: Avinash J. Gaikwad; P. K. Vijayan; Sharad Bhartya; Kannan Iyer; Rajesh Kumar; A. D. Contractor; H. G. Lele; S. F. Vhora; A. K. Maurya; A. K. Ghosh; H. S. Kushwaha
Journal: Science and Technology of Nuclear Installations
 (2008)

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## Abstract

Provision of passive
means to reactor core decay heat removal
enhances the nuclear power plant (NPP) safety
and availability. In the earlier Indian
pressurised heavy water reactors (IPHWRs), like
the 220 MWe and the 540 MWe, crash
cooldown from the steam generators (SGs) is
resorted to mitigate consequences of station
blackout (SBO). In the 700 MWe PHWR
currently being designed an additional passive
decay heat removal (PDHR) system is also
incorporated to condense the steam generated in
the boilers during a SBO. The sustainability of
natural circulation in the various heat
transport systems (i.e., primary heat transport
(PHT), SGs, and PDHRs) under station blackout
depends on the corresponding system's
coolant inventories and the coolant circuit
configurations (i.e., parallel paths and
interconnections). On the primary side, the
interconnection between the two primary loops
plays an important role to sustain the natural
circulation heat removal. On the secondary side,
the steam lines interconnections and the initial
inventory in the SGs prior to cooldown, that is,
hooking up of the PDHRs are very
important. This paper attempts to open up
discussions on the concept and the core issues
associated with passive systems which can
provide continued heat sink during such accident
scenarios. The discussions would include the
criteria for design, and performance of such
concepts already implemented and proposes
schemes to be implemented in the proposed
700 MWe IPHWR. The designer feedbacks
generated, and critical examination of
performance analysis results for the added
passive system to the existing generation II
& III reactors will help ascertaining that
these safety systems/inventories in fact perform
in sustaining decay heat removal and augmenting
safety.


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## Body

## 1. Introduction to 700 MWe Pressurised Heavy Water Reactor (PHWR)

In the 700 MWe PHWR, the primary
coolant heavy water under pressure removes (with partial boiling at channel
exit) the fission heat generated in the reactor core and transfers it to the
secondary coolant (light water) in the steam generators (SGs). The primary heat
transport (PHT) system consists of 392 fuel channels. The PHT system is divided
into two identical loops. Each loop consists of two primary circulating pumps (PCPs)
and two SGs in a figure of eight loop configuration as shown in Figure1. There
are two passes through the core for each loop. As the primary coolant flows
over the fuel bundles placed inside the channels, it picks up the fission heat
in four passes through the reactor core. In each pass, 98 channels are
connected to a common header at each end of the reactor. After picking up heat
from the reactor core, the coolant flows through the reactor outlet header (ROH)
into the tube side of the SGs. After transferring heat in the SGs, the primary
coolant is pumped (by the primary circulating pumps (PCPs)) back to the reactor
core through reactor inlet header (RIH).Figure 1
700 MWe PHWR primary heat
transport system.The SG provides the thermal linkage
between the PHT system and the secondary coolant system. The SGs deployed in
the 700 MWe reactors are of the inverted U-tube type with integral drum. The secondary
fluid flows in the shell side, and the hot primary coolant from the reactor
core (ROH) flows inside the U tubes. The total boiler feed is given at the top
of the downcomer. The recirculation flow
from the steam drum, after mixing with the feed water, flows down the annulus
(downcomer). Then, it rises up through the main boiling zone, as it picks up
the heat. After extracting the heat, the secondary fluid (steam-water mixture)
rises through the riser and then passes through the steam separator and dryers.
Here, the two-phase mixture gets separated into saturated water and steam. The
former is led downwards, after mixing with the feed water, to the annular
downcomer of the SG, while the latter goes to the steam outlet and then to the
turbine.The average of the two ROH pressures
is controlled around a set point of 101.0 kg/cm2, to avoid excessive
boiling and over-pressurisation in the PHT system. In the 700 MWe PHWR, for
controlling the PHT system pressure, a pressuriser (surge tank) is also provided
along with the Feed/Bleed system for maintaining the coolant inventory. The feed/bleed
system is provided for controlling the water level in the pressuriser. Steam bleed
valves (SBVs) are provided on top of the pressuriser vapor volume to control
the increase in the PHT system pressure, by relieving the heavy water steam
into the bleed condenser (BC) through the PHT system pressure controller.
Electrical heaters are provided to take care of the low-pressure transient, by
switching on the heater banks to increase the pressuriser pressure. The hot
bleed from the RIH, and the relief from the south ROH flashes into a two-phase
mixture inside the BC. The BC pressure is controlled at 34 Kg/cm2.
In the event of an increase in the PHT system pressure, the SBVs also start
relieving heavy water steam into the BC. At 100% full power steady state, the
reactor core inlet temperature is 266 C, the core outlet temperature at the ROH
is 310 C, and the reactor core exit quality is around 3% only.


## 2. Experience during 220 MWe PHWR NAPS Fire Incident

An incident of fire in the generator at
one of the units of Narora Atomic Power Station had led to gradual loss of class-IV and class-III
power supplies for all the plant loads such as primary coolant pumps, pressurising
pumps, shutdown cooling pumps and main and auxiliary boiler feed pumps on
secondary side resulting in a station blackout like scenario. Fire water had to
be manually hooked up by going to boiler room. Thermosyphon in the primary and
the secondary systems did work, and there were no fuel failures, with no
activity release. Nuclear power plants have safety systems which are designed
to be highly reliable. In spite of the various built-in provisions for very
high reliability, requirement of analysis and suitable provisions for beyond design
basis event (BDBE) scenario of station blackout, LOCA without ECCS actuation, and
so forth, are necessary.A debate on the concepts to be
adopted for nuclear power plants to be built in the future has been underway
with several different approaches being in vogue. One approach being the
“evolutionary” approach which recommends that the plant design is similar to
well proven design with some enhancements in safety. Other approach is to go
for passive design. The “evolutionary” approach considers greater redundancy
and diversity whereas latter relies on features such as lower-core power
density, greater RB volumes, and greater reliance on thermosyphon. Broad
lessons from Chernobyl,
NAPS fire incident, and even recent Tsunami incident with respect to Kalpakkam
show that continued availability of heat sink is the major issue to be
addressed even if provision of this is done in somewhat “simple” manner.


## 3. Passive Decay Heat Removal System (PDHRS) for 700 MWe PHWR

Long term removal of decay heat is
essential to avoid fuel heat up even after reactor trip or shutdown. Different
heat sinks are available for various states of reactor shutdown such as a
normal shutdown with class IV available or accident conditions such as LOCA. During
normal shutdown, initially the decay heat is removed by steam generators with
steam being dumped to condenser and/or through atmospheric steam discharge
valves (ASDVs). Feed water make up to steam generators is by main or auxiliary
boiler feed pumps (MBFPs or ABFPS). Further primary cool down to room
temperature is by shutdown cooling system with the heat getting transferred to active
process water system (APW) and subsequently to service water loop and then to
atmosphere.In case of station blackout, the
envisaged heat sink is the passive decay heat removal system (PDHRS) for
recirculating the steam generator secondary side inventory through the U-tube
condenser inside PDHRS tanks.For removing the heat generated by
the PHT, PDHRS is provided for condensing the steam and recirculating the steam
generator inventory during station blackout scenario (see Figure2). This
system consists of a horizontal U-tube condenser inside a tank having inventory
of 125 m3 of water. The U-tube condenser is connected to a 150 mmNB line taken
off from the main steam line, this steam gets condensed inside the 50 mmNB tubes
of the condenser, and the condensate returns back to steam generator. During
this process, the decay heat from primary side is given up to the tank
inventory which would initially heat up, later starts boiling and the steam gets
vented to the atmosphere. Four sets of such PDHR tanks, gets, and piping
are provided one set connected to each of the four steam generators. The stored
inventory in the tank is adequate to provide decay heat removal for more than 8
hours during which inventory make up to the tank can be initiated. During the
normal operation of plant, periodic make up to this tank is envisaged by DM
water connection. During the station black out scenario, the make up to this
tank is envisaged from the firewater.Figure 2
Layout for the four SGs and
the 4 PDHRs.As a design practice, it is always
followed that the tested PHT system layouts (with multiple loops and parallel
paths) of the earlier generation and operating power reactors are augmented
with new concepts/systems such as the PDHRS and other passive systems. It is
conveniently assumed that addition of these systems will enhance the NPP safety
by continued removal of decay heat under adverse conditions. The performance
analysis and the present studies point to another aspect which is very
important, and it points out at the degradation/failure of heat removal in the
presence of more than adequate coolant inventories in the primary, secondary,
and the  PDHRS. With detailed parametric studies and analysis of all the
anticipated scenarios, this problem can be overcome and the effective use of
all the available safety systems and coolant inventories can be achieved for
the SBO case.It has been reported in recent literature that RELAP5/MOD3.2
is capable of simulating natural circulation phenomena [1–4]. The SG boil-off
and SBO response for a PWR are described in detail [5]. Reference [6] describes
the incorporation of PDHRS, its design, and modeling. Reference [7] describes the
application of RELAP5 for SBO analysis. The present study deals with boil-off
in PDHRS connected to the secondary side of SGs and the effects of inter-loop
connection leading to depletion of heat removal in the presence of large
coolant inventories in the SGs and the PDHRS. Such study aims at analysing all
the worst possible SBO scenarios and design verification to avoid severe
accident conditions [8]. The thermal hydraulics modeling methodology and
simulation philosophy of 700MWe PHWR for the present study are based on [9–16],
though RELAP5/MOD3.2 code has been used here. Sensitivity studies were carried
out to finalize the present nodalisation, which are not presented due to space
limitation.


## 4. Modeling & Nodalisation

Primary heat transport (PHT) system model has been developed
with two loops connected to the pressuriser and four passes through the core (see Figures3 and 4). In each pass, 98 channels are modeled using 10 axial volumes. 10 heat slabs are connected to the fuel in each pass of core.
The feed and bleed systems are
connected to the headers on one side of the reactor, and on the
other side the pressuriser is connected to both outlet headers through the
surge line. Surge line is modeled using two pipe volumes and a branch. All the
headers are modeled as branches. The pressuriser is modeled using 12 control
volumes with 1.5 MW electrical heaters and 10 heat slabs. The switching logic
for pressuriser heaters has been developed based on the error in the PHT
pressure, the steam bleed valves (SBVs) open following an increase in the PHT system pressure. The secondary system model (see Figure 5)
includes simulation of the steam generator with pressure controller, level
controller, and all the steam lines. The heated riser region is simulated with
10 control volumes; the unheated riser volume is also modeled. 20 heat slabs
are used for connecting the primary and secondary systems thermally. The steam drum model includes
10 control volumes. The downcomer model also includes 10 control volumes. The PDHR system (see Figures 6 and 7) model is
also integrated with all the 4 steam generators. All the steam lines up to governor,
CSDV, and ASDV are simulated using several pipe and single volumes. Steady
state conditions were achieved on both the PHT (ROH quality ~3.5%) and SG side
integrated together for a plant simulation model.Figure 3
700 MWe PHWR loop-1 nodalisation.Figure 4
700 MWe PHWR loop-2 nodalisation.Figure 5
Steam generator nodalisation.Figure 6
Schematic of a PDHR.Figure 7
PDHR nodalisation.


## 5. Results and Discussion

SBO scenario includes the loss of all
the operating pumps, that is, 4 primary circulating pumps (PCPs), primary pressurising
pumps (PPPs), and all the boiler feed pumps (BFPs). The reactor trip signal based on the loss of all the PCPs; is generated within one second. To study
the effect of the various inventories and the parallel multiple loop
interconnections, three case studies carried out, are is, (i) effect of
initial coolant inventory in the SGs prior to cooldown, (ii) effect of the
steam line interconnection, and (iii) effect of the PHT loop isolation.


### 5.1. Effect of Initial SG
Inventory

Two cases are presented in this category, that is, (a) SBO analysis
with PDHR valving in after 6 minutes. (b) SBO analysis with no delay in valving
of the PDHR. Because of the delay in valving in of the PDHRS (which is a closed
system), certain amount of SG inventory is lost through the ASDVs in the first
case. In the second case, the initial inventory of the SG coolant is higher
prior to the initiation of cooldown with the PDHRS.


#### 5.1.1. Station Blackout
Analysis with PDHR Valving in after 6 Minutes

Station blackout was initiated by
tripping all the PCPs, PPPs, and the boiler feed pumps (BFPs) att=0 second. The reactor trip on no
PCP available signal was delayed by one second. The actual reactor power
reduction was further delayed by one second considering the delays for rod
insertions on conservative side. Hot shutdown condition was maintained for
initial six minutes with the help of atmospheric steam discharge valves (ASDVs)
mounted on the SG steam lines, after this all the four PDHRs valves were opened
to condense the steam from SGs, that is, at t=361 seconds. Once the PCPs are tripped, the differential
pressure across the headers/channel decreases, and all header pressures start
falling together.Following the reactor trip and the
valving in of the PDHR, the PHT system pressure (see Figure8) starts falling
(80 bar at t=508 seconds), and the pressuriser level falls below 1.7 m, which
leads to isolation of the pressuriser. After this, the PHT system pressure
falls rapidly to 8 bar at t=7000 seconds, then it remains around this value
approximately up to 27000 seconds. Later on the PHT system pressure, Pressure shows an increasing
trend again following depletion of shell side inventory in the PDHRs. It comes
down to about 43 ton from 121 ton. At t=47000 seconds, it falls to 11 ton, and
thereafter it remains almost constant as the liquid level in the PDHRS falls
below the tube bundle.Figure 8
PHT system pressure case-6 minutes.The SG pressure (see Figure9) shows a peak of 49.3 bar at
t=45 seconds initially following the turbine trip initiated due to reactor trip,
it falls to 41.8 bar, at t=7000 seconds, then it comes down to 7.03 bar at
t=27000 seconds.Figure 9
SG Pressure variation case-6
minutes delay in valving in PDHRS.All the four SGs are connected through the steam lines. Any
PHDR through the steam lines can draw the steam from all the four SGs, but it
sends back condensate only to the SG to which it is connected. Though the
difference between the four SG pressures is very small, the SG with maximum SG
pressure sends more steam to the other PDHR in addition to the PDHRs to which
it is directly connected. The return flow to this SG is only equal to the steam
flow which was going to the corresponding PDHR (directly connected), the steam
flow to other PDHRs sends the condensate to the other SGs. This initiates an inventory transfer, beyondt=1800 seconds, due to low-driving forces
encountered during natural circulation at low pressure in the PHTs, SGs, and
PDHR, it is observed that the SG level (see Figure 10) in two SGs goes down and the
other two SGs,  it shows an increasing trend. Inventory transfer through steam lines is
observed but the total SGs inventory remains constant. Another initiation cause
(for difference in the four SG pressures) is the difference in the PHT flow
through all the four SGs during natural circulation.Figure 10
SG level variation case-6 minutes
delay in valving in PDHRS.The total primary core flow (see Figure11) remains around 7%
at t=7000 seconds after this it shows a slow-decreasing trend. The PHT system
core exit quality remains low up to 15000 seconds, then it shows lot of
oscillation (also observed in the core flow), it even reaches values up to 50%
and above, up to 30000 seconds, then it comes back to lower values (<2%). It
can be concluded that the PDHRs can remove the decay heat safely up to 10 hours, during this period additional water inventories can be lined
up.Figure 11
Core flow variation case-6
minutes delay in valving in PDHRS.


#### 5.1.2. SBO with No Delay in Valving in of the PDHRs

In the previous station blackout case, it was assumed that
the PDHRs valves are opened after 6 minutes delay. In the present case, it is
assumed that the PDHRs valves are opened without any delay immediately after
sensing the station blackout att=1 second. The results obtained are similar to
the earlier predictions (see Figures 12, 13, 14, 15, and 16) but the oscillation/fluctuation
in the PHT and the SG flows are relatively dampened, and more stable flow
conditions are observed due to higher SG inventories. In the 6 minutes PDHR
valving delay cases, the SG inventories go down from 32 ton to almost 27 ton, a
loss of 5 ton through atmospheric steam discharge valves (ASDVs) in the initial
6 minutes. For the no delay case, the ASDVs open only for a short period, and
the SG inventory remains around 32 ton with negligible loss. As the steam lines
were not isolated, diverging trend was observed for SG levels (see Figure 13).
It can be concluded that valving in of the PDHR should not be delayed if a SBO
is confirmed.Figure 12
PHT pressure variation case-no
delay in valving in PDHRS.Figure 13
SG level variation case-no
delay in valving in PDHRS.Figure 14
Core exit quality variation
case-no delay in valving in PDHRS.Figure 15
Clad temperature variation
case-no delay in valving in PDHRS.Figure 16
PDHR shell inventory variation
case-no delay in valving in PDHRS.


### 5.2. SBO with Steam Line Isolation

All the steam lines interconnections were isolated to avoid
any inter loop inventory transfer through the parallel paths available in the
steam lines. Each of the SGs is connected only to the corresponding PDHR, and all other connections are
snapped.Station Blackout
Analysis with Isolation of Steam Lines
In the previous station blackout transient analysis, it was
observed that the level in two SGs was increasing significantly after 30000 seconds, and in the other two
SGs, it was going down correspondingly, keeping the total inventory of all the four
SGs constant. This
was attributed to low-driving forces encountered during natural circulation at
low pressure in the PHTs, SGs, and PDHR, resulting in an inventory transfer
through steam lines. To support these explanations, another hypothetical station
blackout with complete isolation of the steam lines was
carried out. Here, it was observed that the SG levels (see Figure17) do not
diverge and remain almost at the same value for all the four SGs, though the
levels are not exactly same, but they follow a similar trend (variation around
9 m). For most of the other parameters, the trend is almost similar as compared
to the previous blackout analysis without isolation of the steam lines. It can
be concluded that the alternative parallel path provided by the steam lines
leads to inventory transfer. This leads to drying out of one SG in each loop, but
the decay heat removal is unaffected as the other SG in the loop with its own
inventory and the transferred coolant inventory can carry on the decay heat
removal effectively for the complete loop.Figure 17
SG level case-steam line isolation.


### 5.3. 3 PDHRs with/without Primay Loop Isolation

In these cases, one PDHR connected to
one of the SG in loop-1 was kept isolated during the transient. As there is no
heat removal from one of the bank, the primary coolant at higher temperature
enters in the core through the return pass. After some time as the two phases appear in the
return pass, there is a drastic reduction in core flow leading to a stagnation
phase att>4200 seconds (see Figure 18). Since the affected loop pressure was
slightly higher as compared to healthy loop (cooling unaffected), there is a
continuous transfer of primary inventory from loop-1 to loop-2 through the ROH
connection though the pressuriser is isolated, but the PHT loops are connected.Figure 18
Core flow case-3PDHRs with no loop isolation.To avoid this inter-loop PHT
inventory transfer, a case study with the isolation of these two primary loops
was also carried out. With the arrest of primary inventory removal from the
affected loop-1 (with one working and another nonworking PDHR), the core
flow (see Figure20) through the two
passes of both affected loop-1 and healthy loop-2 remains at a higher-positive
value for a considerable period of time (~21 000 seconds), and core cool
ability is maintained. The PDHR isolation valve failure leads to the failure of
the corresponding PDHR, probability of this event cannot be neglected. Based on
this study, it is highly recommended that the two primary loops should be
isolated when there is an unsymmetrical mode of PDHR operation.


#### 5.3.1. Station Blackout
(SBO) Analysis with Only 3 PDHRs Available and with PHT Loops Isolation

In this SBO transient analysis, it
is assumed that, one passive decay heat removal (PDHR) heat exchanger fails, as
a result only 3 PDHRs are available to cool the steam from all the four SGs. It
was also assumed that the pressuriser and the 2 PHT loops are isolated from
each other on pressuriser low level. The results obtained are similar to the
all PDHRs available case for the initial period, but the decay heat removal is
hampered within 7 hours, whereas for SBO with all PDHRs available case, the
decay heat removal is not affected for more than 17 hours.For the PHT loop isolation case, the nonfunctional PDHR
inventory remains at 121 ton throughout. For the other only working PDHR, in
the affected PHT loop-1, the PDHR inventory falls to 26.7 ton in 8.3 hours, whereas
in the healthy PHT loop-2, with both PDHRs working, the PDHR inventory falls to
16.5 ton in the corresponding period. After 7 hours, the affected loop clad
temperature (see Figure21) shoots up sharply following core flow (see Figure
20) reduction and exposure of the PDHR heat exchanger tubes in the only working
PDHR in that loop. For the SG without a working PDHR, the SG-inventory falls to
almost one ton within 1.1 hour, after this it remains around this value and
does not fall to zero. For the SG in the affected PHT loop-1 with only working
PDHR, the SG-inventory comes down below one ton after 5.8 hours, that is, both
the SGs in the affected PHT loop-1 dryout due to inventory transfer to the SGs
in healthy PHT loop-2.This unfavorable situation is caused by inventory transfer
from the SGs of the affected loops to the other loops where both PDHRs are
operational. Steam flow from all the four SGs to 4 PDHRs is guided based on the
differential pressure between these components. More steam flow goes from the
SG drum with the highest pressure (i.e., low-PHT flow) to the PDHRs, based on
the pressure, all the SGs send steam to the PDHRs. The SG receiving maximum steam/condensate
will accumulate more inventory. This phenomenon was observed for all the SBO
cases analysed (except for the SBO without steam lines). It leads to total SG inventory transfer from
loop with one PDHRs working to the SGs of the loop with 2 PDHRs working, as a
result both the SGs in the one PDHR available loop-1 dryout after 5.8 hours for
the PHT loop isolation case, and the clad temperature (see Figure20) increases
rapidly.


#### 5.3.2. SBO Analysis with Only 3 PDHRs Available
and without PHT Loops Isolation

Here, only 3 PDHRs are available following SBO, and it was
also assumed that only the pressuriser isolates from the 2 PHT loops, and the 2 PHT loops remain hydraulically connected through the surge lines as the pressuriser
level falls below 1.7 m. For this case, the primary flow reduces almost zero
in the one of the core passes in the affected loop, leading to an increase in
clad surface temperature (see Figure19, more than 1000 C) after ~1.5 hours.
The primary coolant flow (see Figure 18) in this core path is hampered by the
inter-loop inventory transfer, as the affected loop which is at higher pressure
and temperature tries to equalise pressure, forcing flow out from this core
path. As the flow reduces and stagnates (see Figure 18), the core exit quality
increases sharply (>1.0). Though a lot of coolant inventory is available in
the SGs and also in the PDHRs (89 ton), the clad temperature (see Figure 19)
shoots up due to core flow stagnation. For the SG without PDHR working, the SG
level comes down to almost to zero, in one hour. For the other SG in the healthy
loop-2 with working PDHRs, it shows an inventory corresponding to 12 m level at
about 1.5 hours. Here, also the phenomenon of SGs inventory transfer from the
affected loop to the healthy loop is observed, but the clad temperature shoots
up far ahead of dryout due to PHT flow stagnation following PHT inventory
transfer from affected loop to healthy loop.Figure 19
Clad temperature case-3PDHRs
with no loop isolation.Figure 20
Core flow case-3PDHRs with
loop isolation.Figure 21
Clad temperature case-3PDHRs with loop isolation.The results obtained with RELAP5
model show a similar behavior for natural circulation as reported in the
literature [1–4]. It can be concluded that the 2 PHT loops should be
isolated following an SBO to avoid inter-loop inventory transfer through the
surge lines, which leads to stagnation of core flow in the affected loop due to
unfavorable pressure distribution. This undesirable situation is further aggravated
by the inventory transfer from the SGs of the affected loops to another loop
with both PDHRs operational.


## 5.1. Effect of Initial SG
Inventory

Two cases are presented in this category, that is, (a) SBO analysis
with PDHR valving in after 6 minutes. (b) SBO analysis with no delay in valving
of the PDHR. Because of the delay in valving in of the PDHRS (which is a closed
system), certain amount of SG inventory is lost through the ASDVs in the first
case. In the second case, the initial inventory of the SG coolant is higher
prior to the initiation of cooldown with the PDHRS.


### 5.1.1. Station Blackout
Analysis with PDHR Valving in after 6 Minutes

Station blackout was initiated by
tripping all the PCPs, PPPs, and the boiler feed pumps (BFPs) att=0 second. The reactor trip on no
PCP available signal was delayed by one second. The actual reactor power
reduction was further delayed by one second considering the delays for rod
insertions on conservative side. Hot shutdown condition was maintained for
initial six minutes with the help of atmospheric steam discharge valves (ASDVs)
mounted on the SG steam lines, after this all the four PDHRs valves were opened
to condense the steam from SGs, that is, at t=361 seconds. Once the PCPs are tripped, the differential
pressure across the headers/channel decreases, and all header pressures start
falling together.Following the reactor trip and the
valving in of the PDHR, the PHT system pressure (see Figure8) starts falling
(80 bar at t=508 seconds), and the pressuriser level falls below 1.7 m, which
leads to isolation of the pressuriser. After this, the PHT system pressure
falls rapidly to 8 bar at t=7000 seconds, then it remains around this value
approximately up to 27000 seconds. Later on the PHT system pressure, Pressure shows an increasing
trend again following depletion of shell side inventory in the PDHRs. It comes
down to about 43 ton from 121 ton. At t=47000 seconds, it falls to 11 ton, and
thereafter it remains almost constant as the liquid level in the PDHRS falls
below the tube bundle.Figure 8
PHT system pressure case-6 minutes.The SG pressure (see Figure9) shows a peak of 49.3 bar at
t=45 seconds initially following the turbine trip initiated due to reactor trip,
it falls to 41.8 bar, at t=7000 seconds, then it comes down to 7.03 bar at
t=27000 seconds.Figure 9
SG Pressure variation case-6
minutes delay in valving in PDHRS.All the four SGs are connected through the steam lines. Any
PHDR through the steam lines can draw the steam from all the four SGs, but it
sends back condensate only to the SG to which it is connected. Though the
difference between the four SG pressures is very small, the SG with maximum SG
pressure sends more steam to the other PDHR in addition to the PDHRs to which
it is directly connected. The return flow to this SG is only equal to the steam
flow which was going to the corresponding PDHR (directly connected), the steam
flow to other PDHRs sends the condensate to the other SGs. This initiates an inventory transfer, beyondt=1800 seconds, due to low-driving forces
encountered during natural circulation at low pressure in the PHTs, SGs, and
PDHR, it is observed that the SG level (see Figure 10) in two SGs goes down and the
other two SGs,  it shows an increasing trend. Inventory transfer through steam lines is
observed but the total SGs inventory remains constant. Another initiation cause
(for difference in the four SG pressures) is the difference in the PHT flow
through all the four SGs during natural circulation.Figure 10
SG level variation case-6 minutes
delay in valving in PDHRS.The total primary core flow (see Figure11) remains around 7%
at t=7000 seconds after this it shows a slow-decreasing trend. The PHT system
core exit quality remains low up to 15000 seconds, then it shows lot of
oscillation (also observed in the core flow), it even reaches values up to 50%
and above, up to 30000 seconds, then it comes back to lower values (<2%). It
can be concluded that the PDHRs can remove the decay heat safely up to 10 hours, during this period additional water inventories can be lined
up.Figure 11
Core flow variation case-6
minutes delay in valving in PDHRS.


### 5.1.2. SBO with No Delay in Valving in of the PDHRs

In the previous station blackout case, it was assumed that
the PDHRs valves are opened after 6 minutes delay. In the present case, it is
assumed that the PDHRs valves are opened without any delay immediately after
sensing the station blackout att=1 second. The results obtained are similar to
the earlier predictions (see Figures 12, 13, 14, 15, and 16) but the oscillation/fluctuation
in the PHT and the SG flows are relatively dampened, and more stable flow
conditions are observed due to higher SG inventories. In the 6 minutes PDHR
valving delay cases, the SG inventories go down from 32 ton to almost 27 ton, a
loss of 5 ton through atmospheric steam discharge valves (ASDVs) in the initial
6 minutes. For the no delay case, the ASDVs open only for a short period, and
the SG inventory remains around 32 ton with negligible loss. As the steam lines
were not isolated, diverging trend was observed for SG levels (see Figure 13).
It can be concluded that valving in of the PDHR should not be delayed if a SBO
is confirmed.Figure 12
PHT pressure variation case-no
delay in valving in PDHRS.Figure 13
SG level variation case-no
delay in valving in PDHRS.Figure 14
Core exit quality variation
case-no delay in valving in PDHRS.Figure 15
Clad temperature variation
case-no delay in valving in PDHRS.Figure 16
PDHR shell inventory variation
case-no delay in valving in PDHRS.


## 5.1.1. Station Blackout
Analysis with PDHR Valving in after 6 Minutes

Station blackout was initiated by
tripping all the PCPs, PPPs, and the boiler feed pumps (BFPs) att=0 second. The reactor trip on no
PCP available signal was delayed by one second. The actual reactor power
reduction was further delayed by one second considering the delays for rod
insertions on conservative side. Hot shutdown condition was maintained for
initial six minutes with the help of atmospheric steam discharge valves (ASDVs)
mounted on the SG steam lines, after this all the four PDHRs valves were opened
to condense the steam from SGs, that is, at t=361 seconds. Once the PCPs are tripped, the differential
pressure across the headers/channel decreases, and all header pressures start
falling together.Following the reactor trip and the
valving in of the PDHR, the PHT system pressure (see Figure8) starts falling
(80 bar at t=508 seconds), and the pressuriser level falls below 1.7 m, which
leads to isolation of the pressuriser. After this, the PHT system pressure
falls rapidly to 8 bar at t=7000 seconds, then it remains around this value
approximately up to 27000 seconds. Later on the PHT system pressure, Pressure shows an increasing
trend again following depletion of shell side inventory in the PDHRs. It comes
down to about 43 ton from 121 ton. At t=47000 seconds, it falls to 11 ton, and
thereafter it remains almost constant as the liquid level in the PDHRS falls
below the tube bundle.Figure 8
PHT system pressure case-6 minutes.The SG pressure (see Figure9) shows a peak of 49.3 bar at
t=45 seconds initially following the turbine trip initiated due to reactor trip,
it falls to 41.8 bar, at t=7000 seconds, then it comes down to 7.03 bar at
t=27000 seconds.Figure 9
SG Pressure variation case-6
minutes delay in valving in PDHRS.All the four SGs are connected through the steam lines. Any
PHDR through the steam lines can draw the steam from all the four SGs, but it
sends back condensate only to the SG to which it is connected. Though the
difference between the four SG pressures is very small, the SG with maximum SG
pressure sends more steam to the other PDHR in addition to the PDHRs to which
it is directly connected. The return flow to this SG is only equal to the steam
flow which was going to the corresponding PDHR (directly connected), the steam
flow to other PDHRs sends the condensate to the other SGs. This initiates an inventory transfer, beyondt=1800 seconds, due to low-driving forces
encountered during natural circulation at low pressure in the PHTs, SGs, and
PDHR, it is observed that the SG level (see Figure 10) in two SGs goes down and the
other two SGs,  it shows an increasing trend. Inventory transfer through steam lines is
observed but the total SGs inventory remains constant. Another initiation cause
(for difference in the four SG pressures) is the difference in the PHT flow
through all the four SGs during natural circulation.Figure 10
SG level variation case-6 minutes
delay in valving in PDHRS.The total primary core flow (see Figure11) remains around 7%
at t=7000 seconds after this it shows a slow-decreasing trend. The PHT system
core exit quality remains low up to 15000 seconds, then it shows lot of
oscillation (also observed in the core flow), it even reaches values up to 50%
and above, up to 30000 seconds, then it comes back to lower values (<2%). It
can be concluded that the PDHRs can remove the decay heat safely up to 10 hours, during this period additional water inventories can be lined
up.Figure 11
Core flow variation case-6
minutes delay in valving in PDHRS.


## 5.1.2. SBO with No Delay in Valving in of the PDHRs

In the previous station blackout case, it was assumed that
the PDHRs valves are opened after 6 minutes delay. In the present case, it is
assumed that the PDHRs valves are opened without any delay immediately after
sensing the station blackout att=1 second. The results obtained are similar to
the earlier predictions (see Figures 12, 13, 14, 15, and 16) but the oscillation/fluctuation
in the PHT and the SG flows are relatively dampened, and more stable flow
conditions are observed due to higher SG inventories. In the 6 minutes PDHR
valving delay cases, the SG inventories go down from 32 ton to almost 27 ton, a
loss of 5 ton through atmospheric steam discharge valves (ASDVs) in the initial
6 minutes. For the no delay case, the ASDVs open only for a short period, and
the SG inventory remains around 32 ton with negligible loss. As the steam lines
were not isolated, diverging trend was observed for SG levels (see Figure 13).
It can be concluded that valving in of the PDHR should not be delayed if a SBO
is confirmed.Figure 12
PHT pressure variation case-no
delay in valving in PDHRS.Figure 13
SG level variation case-no
delay in valving in PDHRS.Figure 14
Core exit quality variation
case-no delay in valving in PDHRS.Figure 15
Clad temperature variation
case-no delay in valving in PDHRS.Figure 16
PDHR shell inventory variation
case-no delay in valving in PDHRS.


## 5.2. SBO with Steam Line Isolation

All the steam lines interconnections were isolated to avoid
any inter loop inventory transfer through the parallel paths available in the
steam lines. Each of the SGs is connected only to the corresponding PDHR, and all other connections are
snapped.Station Blackout
Analysis with Isolation of Steam Lines
In the previous station blackout transient analysis, it was
observed that the level in two SGs was increasing significantly after 30000 seconds, and in the other two
SGs, it was going down correspondingly, keeping the total inventory of all the four
SGs constant. This
was attributed to low-driving forces encountered during natural circulation at
low pressure in the PHTs, SGs, and PDHR, resulting in an inventory transfer
through steam lines. To support these explanations, another hypothetical station
blackout with complete isolation of the steam lines was
carried out. Here, it was observed that the SG levels (see Figure17) do not
diverge and remain almost at the same value for all the four SGs, though the
levels are not exactly same, but they follow a similar trend (variation around
9 m). For most of the other parameters, the trend is almost similar as compared
to the previous blackout analysis without isolation of the steam lines. It can
be concluded that the alternative parallel path provided by the steam lines
leads to inventory transfer. This leads to drying out of one SG in each loop, but
the decay heat removal is unaffected as the other SG in the loop with its own
inventory and the transferred coolant inventory can carry on the decay heat
removal effectively for the complete loop.Figure 17
SG level case-steam line isolation.


## 5.3. 3 PDHRs with/without Primay Loop Isolation

In these cases, one PDHR connected to
one of the SG in loop-1 was kept isolated during the transient. As there is no
heat removal from one of the bank, the primary coolant at higher temperature
enters in the core through the return pass. After some time as the two phases appear in the
return pass, there is a drastic reduction in core flow leading to a stagnation
phase att>4200 seconds (see Figure 18). Since the affected loop pressure was
slightly higher as compared to healthy loop (cooling unaffected), there is a
continuous transfer of primary inventory from loop-1 to loop-2 through the ROH
connection though the pressuriser is isolated, but the PHT loops are connected.Figure 18
Core flow case-3PDHRs with no loop isolation.To avoid this inter-loop PHT
inventory transfer, a case study with the isolation of these two primary loops
was also carried out. With the arrest of primary inventory removal from the
affected loop-1 (with one working and another nonworking PDHR), the core
flow (see Figure20) through the two
passes of both affected loop-1 and healthy loop-2 remains at a higher-positive
value for a considerable period of time (~21 000 seconds), and core cool
ability is maintained. The PDHR isolation valve failure leads to the failure of
the corresponding PDHR, probability of this event cannot be neglected. Based on
this study, it is highly recommended that the two primary loops should be
isolated when there is an unsymmetrical mode of PDHR operation.


### 5.3.1. Station Blackout
(SBO) Analysis with Only 3 PDHRs Available and with PHT Loops Isolation

In this SBO transient analysis, it
is assumed that, one passive decay heat removal (PDHR) heat exchanger fails, as
a result only 3 PDHRs are available to cool the steam from all the four SGs. It
was also assumed that the pressuriser and the 2 PHT loops are isolated from
each other on pressuriser low level. The results obtained are similar to the
all PDHRs available case for the initial period, but the decay heat removal is
hampered within 7 hours, whereas for SBO with all PDHRs available case, the
decay heat removal is not affected for more than 17 hours.For the PHT loop isolation case, the nonfunctional PDHR
inventory remains at 121 ton throughout. For the other only working PDHR, in
the affected PHT loop-1, the PDHR inventory falls to 26.7 ton in 8.3 hours, whereas
in the healthy PHT loop-2, with both PDHRs working, the PDHR inventory falls to
16.5 ton in the corresponding period. After 7 hours, the affected loop clad
temperature (see Figure21) shoots up sharply following core flow (see Figure
20) reduction and exposure of the PDHR heat exchanger tubes in the only working
PDHR in that loop. For the SG without a working PDHR, the SG-inventory falls to
almost one ton within 1.1 hour, after this it remains around this value and
does not fall to zero. For the SG in the affected PHT loop-1 with only working
PDHR, the SG-inventory comes down below one ton after 5.8 hours, that is, both
the SGs in the affected PHT loop-1 dryout due to inventory transfer to the SGs
in healthy PHT loop-2.This unfavorable situation is caused by inventory transfer
from the SGs of the affected loops to the other loops where both PDHRs are
operational. Steam flow from all the four SGs to 4 PDHRs is guided based on the
differential pressure between these components. More steam flow goes from the
SG drum with the highest pressure (i.e., low-PHT flow) to the PDHRs, based on
the pressure, all the SGs send steam to the PDHRs. The SG receiving maximum steam/condensate
will accumulate more inventory. This phenomenon was observed for all the SBO
cases analysed (except for the SBO without steam lines). It leads to total SG inventory transfer from
loop with one PDHRs working to the SGs of the loop with 2 PDHRs working, as a
result both the SGs in the one PDHR available loop-1 dryout after 5.8 hours for
the PHT loop isolation case, and the clad temperature (see Figure20) increases
rapidly.


### 5.3.2. SBO Analysis with Only 3 PDHRs Available
and without PHT Loops Isolation

Here, only 3 PDHRs are available following SBO, and it was
also assumed that only the pressuriser isolates from the 2 PHT loops, and the 2 PHT loops remain hydraulically connected through the surge lines as the pressuriser
level falls below 1.7 m. For this case, the primary flow reduces almost zero
in the one of the core passes in the affected loop, leading to an increase in
clad surface temperature (see Figure19, more than 1000 C) after ~1.5 hours.
The primary coolant flow (see Figure 18) in this core path is hampered by the
inter-loop inventory transfer, as the affected loop which is at higher pressure
and temperature tries to equalise pressure, forcing flow out from this core
path. As the flow reduces and stagnates (see Figure 18), the core exit quality
increases sharply (>1.0). Though a lot of coolant inventory is available in
the SGs and also in the PDHRs (89 ton), the clad temperature (see Figure 19)
shoots up due to core flow stagnation. For the SG without PDHR working, the SG
level comes down to almost to zero, in one hour. For the other SG in the healthy
loop-2 with working PDHRs, it shows an inventory corresponding to 12 m level at
about 1.5 hours. Here, also the phenomenon of SGs inventory transfer from the
affected loop to the healthy loop is observed, but the clad temperature shoots
up far ahead of dryout due to PHT flow stagnation following PHT inventory
transfer from affected loop to healthy loop.Figure 19
Clad temperature case-3PDHRs
with no loop isolation.Figure 20
Core flow case-3PDHRs with
loop isolation.Figure 21
Clad temperature case-3PDHRs with loop isolation.The results obtained with RELAP5
model show a similar behavior for natural circulation as reported in the
literature [1–4]. It can be concluded that the 2 PHT loops should be
isolated following an SBO to avoid inter-loop inventory transfer through the
surge lines, which leads to stagnation of core flow in the affected loop due to
unfavorable pressure distribution. This undesirable situation is further aggravated
by the inventory transfer from the SGs of the affected loops to another loop
with both PDHRs operational.


## 5.3.1. Station Blackout
(SBO) Analysis with Only 3 PDHRs Available and with PHT Loops Isolation

In this SBO transient analysis, it
is assumed that, one passive decay heat removal (PDHR) heat exchanger fails, as
a result only 3 PDHRs are available to cool the steam from all the four SGs. It
was also assumed that the pressuriser and the 2 PHT loops are isolated from
each other on pressuriser low level. The results obtained are similar to the
all PDHRs available case for the initial period, but the decay heat removal is
hampered within 7 hours, whereas for SBO with all PDHRs available case, the
decay heat removal is not affected for more than 17 hours.For the PHT loop isolation case, the nonfunctional PDHR
inventory remains at 121 ton throughout. For the other only working PDHR, in
the affected PHT loop-1, the PDHR inventory falls to 26.7 ton in 8.3 hours, whereas
in the healthy PHT loop-2, with both PDHRs working, the PDHR inventory falls to
16.5 ton in the corresponding period. After 7 hours, the affected loop clad
temperature (see Figure21) shoots up sharply following core flow (see Figure
20) reduction and exposure of the PDHR heat exchanger tubes in the only working
PDHR in that loop. For the SG without a working PDHR, the SG-inventory falls to
almost one ton within 1.1 hour, after this it remains around this value and
does not fall to zero. For the SG in the affected PHT loop-1 with only working
PDHR, the SG-inventory comes down below one ton after 5.8 hours, that is, both
the SGs in the affected PHT loop-1 dryout due to inventory transfer to the SGs
in healthy PHT loop-2.This unfavorable situation is caused by inventory transfer
from the SGs of the affected loops to the other loops where both PDHRs are
operational. Steam flow from all the four SGs to 4 PDHRs is guided based on the
differential pressure between these components. More steam flow goes from the
SG drum with the highest pressure (i.e., low-PHT flow) to the PDHRs, based on
the pressure, all the SGs send steam to the PDHRs. The SG receiving maximum steam/condensate
will accumulate more inventory. This phenomenon was observed for all the SBO
cases analysed (except for the SBO without steam lines). It leads to total SG inventory transfer from
loop with one PDHRs working to the SGs of the loop with 2 PDHRs working, as a
result both the SGs in the one PDHR available loop-1 dryout after 5.8 hours for
the PHT loop isolation case, and the clad temperature (see Figure20) increases
rapidly.


## 5.3.2. SBO Analysis with Only 3 PDHRs Available
and without PHT Loops Isolation

Here, only 3 PDHRs are available following SBO, and it was
also assumed that only the pressuriser isolates from the 2 PHT loops, and the 2 PHT loops remain hydraulically connected through the surge lines as the pressuriser
level falls below 1.7 m. For this case, the primary flow reduces almost zero
in the one of the core passes in the affected loop, leading to an increase in
clad surface temperature (see Figure19, more than 1000 C) after ~1.5 hours.
The primary coolant flow (see Figure 18) in this core path is hampered by the
inter-loop inventory transfer, as the affected loop which is at higher pressure
and temperature tries to equalise pressure, forcing flow out from this core
path. As the flow reduces and stagnates (see Figure 18), the core exit quality
increases sharply (>1.0). Though a lot of coolant inventory is available in
the SGs and also in the PDHRs (89 ton), the clad temperature (see Figure 19)
shoots up due to core flow stagnation. For the SG without PDHR working, the SG
level comes down to almost to zero, in one hour. For the other SG in the healthy
loop-2 with working PDHRs, it shows an inventory corresponding to 12 m level at
about 1.5 hours. Here, also the phenomenon of SGs inventory transfer from the
affected loop to the healthy loop is observed, but the clad temperature shoots
up far ahead of dryout due to PHT flow stagnation following PHT inventory
transfer from affected loop to healthy loop.Figure 19
Clad temperature case-3PDHRs
with no loop isolation.Figure 20
Core flow case-3PDHRs with
loop isolation.Figure 21
Clad temperature case-3PDHRs with loop isolation.The results obtained with RELAP5
model show a similar behavior for natural circulation as reported in the
literature [1–4]. It can be concluded that the 2 PHT loops should be
isolated following an SBO to avoid inter-loop inventory transfer through the
surge lines, which leads to stagnation of core flow in the affected loop due to
unfavorable pressure distribution. This undesirable situation is further aggravated
by the inventory transfer from the SGs of the affected loops to another loop
with both PDHRs operational.


## Concluding Remarks

(1)
It can be concluded that the 4 PDHRs, with an initial inventory of 121 ton each at 40 C, can remove the core
decay heat without any increase in the clad temperature for about 17 hours
without the help of make up system, if all the 4 PDHRs are available.(2)
No delay in valving in of the PDHRs
is recommended after confirmation of a station blackout situation, as a higher
inventory in the SG leads to more stable natural circulation in the secondary
and the primary heat transport system. The rate of change of primary and secondary coolants
structure temperature is also moderate.(3)
The secondary inventory transfer
from SGs in one loop to SGs in another occurs due to parallel paths
interconnected steam lines. Following the SBO and cooldown with natural
circulation at low pressure and low-driving forces in the PHT, SGs, and PDHRS, this
phenomenon cannot be avoided.(4)
For the SBO with three PDHRs
available case, the PHT system inventory transfer takes place from the affected
loop (cooling affected due to inventory transfer to other loop) to healthy loop
(cooling unhindered), due to pressure imbalance and parallel paths
inter-connected surge lines available. In this case, the isolation of the two
PHT system loops is helpful in mitigating the consequences of failure of one of
the 4 PDHRs, and without the loop interconnection the decay heat removal is not
hampered for 7 hours.(5)
The designer feedbacks generated
from the analysis, and critical examination of performance analysis results for
the added passive system to existing generation II and III reactors will help
ascertaining that the these safety systems/inventories in fact perform in sustaining
decay heat removal and augmenting safety.



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*Source: 458316-2008-04-30.xml*