# Evaluation of Heat Removal from RBMK-1500 Core Using Control Rods Cooling Circuit

Authors: A. Kaliatka; E. Uspuras; M. Vaisnoras
Journal: Science and Technology of Nuclear Installations
 (2008)

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## Abstract

The Ignalina nuclear power plant is a twin unit with two RBMK-1500, graphite moderated, boiling water, multichannel reactors. After the decision was made to decommission the Ignalina NPP, Unit 1 was shut down on December 31, 2004, and Unit 2 is to be operated until the end of 2009. Despite of this fact, severe accident management guidelines for RBMK-1500 reactor at Ignalina NPP are prepared. In case of beyond design basis accidents, it can occur that no water sources are available at the moment for heat removal from fuel channels. Specificity of RBMK reactor is such that the channels with control rods are cooled with water supplied by the system totally independent from the reactor cooling system. Therefore, the heat removal from RBMK-1500 reactor core using circuit for cooling of rods in control and protection system can be used as nonregular mean for reactor cooldown in case of BDBA. The heat from fuel channels, where heat is generated, through graphite bricks is transferred in radial direction to cooled CPS channels. This article presents the analysis of possibility to remove heat from reactor core in case of large LOCA by employing CPS channels cooling circuit. The analysis was performed for Ignalina NPP with RBMK-1500 reactor using RELAP5-3D and RELAP5 codes. Results of the analysis have shown that, in spite of high thermal inertia of graphite, this heat removal from CPS channels allows to slow down effectively the core heat-up process.


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## Body

## 1. Introduction

RBMK-1500 reactor, which is located at the Ignalina
NPP site, is multichannel boiling water and graphite moderated nuclear reactor.
Reactor core consists of 2488 graphite columns with the vertical bore openings
[1]. These openings are used for positioning of the fuel channels,
which in
turn are used for placing fuel assemblies, reactivity regulating control rods,
and several types of instruments into the core. Approximately, 95% of energy is
generated in fuel assemblies, while 5% in graphite columns. Specificity of RBMK reactor is such that the control rods are placed in
the individual channels that are independent from the channels with fuel
bundles. The channels with control rods are cooled with water supplied by the
system totally independent from the reactor cooling system. The pressure in
channels with control rods is always close to atmospheric. The geometry of the graphite columns and
availability of FC and the channels of control and protection system located
near by results in nonuniformity of heat fluxes and temperature distributions
around the graphite columns periphery. Thus, in order to model the thermal
hydraulic behavior in the RBMK core, it is necessary to represent heat
conduction between the graphite columns. On the other hand, the heat removal
from reactor core using CPS cooling system can be used as nonregular mean for
reactor cooldown in case of beyond design-basis accidents. In BDBA, it
can occur that no water sources are available at the moment for heat removal
from FC [2], but the removal of heat from reactor core by CPS cooling circuit
is possible, since 211 CPS channels are distributed evenly in the reactor core
(see Figure 1). These channels are filled with water supplied by the low-pressure
system totally independent from the reactor cooling system (RCS). Thus, the
large break in reactor cooling system does not disturb usage of the CPS cooling
circuit. The proportion between channels with fuel assemblies and channels with
control rods is 1661/211=7.87. Thus, we can assume that one CPS channel is
environed by 8 FCs. The heat from fuel channels, where heat is generated,
through graphite bricks is transferred in radial direction to cooled CPS
channels (see Figure 2).Figure 1
Distribution of fuel channels, CPS channels and graphite reflector cooling channels in the reactor core.Figure 2
Heat transfer from FCs to cooled CPS channel.Thus,
this article presents the analysis of heat removal by employing CPS channels
cooling circuit. The analysis was performed for Ignalina NPP with RBMK-1500
reactor.


## 2. Assessment of Heat Transfer Transferred in Radial Direction between Adjacent Graphite Columns

### 2.1. Simulation of Heat Transfer Transferred in Radial Direction by Employing the Interstructure Heat Conduction Model

For
the simulation of heat transfer between fuel channels and CPS channels the
RELAP5-3D code [3] was used. RELAP5-3D code has the interstructure heat
conduction model. This heat conduction enclosure model gives RELAP5-3D a
general multidimensional heat conduction capability. As it was demonstrated by Paik [4], this model is applied to calculation of heat
transfer through the gas gap between the RBMK reactor core graphite blocks.The
nodalization scheme of the developed model is presented in Figure3. The one
single CPS channel (3) with surrounding graphite column is modeled by “pipe"
element with heat structure. The water in this channel is supplied from water
tank of additional hold-down system (1) through pipeline (2) and bottom
distribution header (4). The tank of AHS is elevated 30 m up to the bottom of
reactor core. The steam from the CPS channel is removed in to top distribution
header (5). The pressure in this header is atmospheric.Figure 3
Simplified model of Ignalina NPP for thermal-hydraulic analysis of processes in RCS (for RELAP5-3D code): (1) water tank of AHS, (2) pipeline, (3) CPS channel, (4)
bottom distribution header, (5) top distribution header, (6) model of DS and steam lines, (7) steam-water pipeline, (8) eight fuel channels, (9) bottom water pipeline, (10) individual
control valve, (11) GDH, (12) water supply from MCP pressure header.Eight FCs (8), placed in the
graphite columns, are surrounding
this single CPS channel. These channels are fed by water from volume (12),
which models water supply from MCP pressure header. Water from GDH, which is
modeled by “branch” element (11), is supplied into bottom water pipeline (9)
and later into fuel channels (8). The water flow through FC is regulated using
individual control valve (10). The steam water mixture is directed to the DS
and steam-line model (6), through steam water pipeline (7). The volumes (1, 5,
6, and 12) are modeled by “time-dependent” elements with specified boundary
conditions.


### 2.2. Gas Gap Conductance between Adjacent Graphite Blocks

The graphite columns
in RBMK-type reactors are separated by a small(∼1mm) gap. The gas gaps
between graphite columns are filled with a gas mixture (40% of He and 60% of N2 by volume fraction) [1]. For establishing of the gas gap conductance between
adjacent graphite blocks, the additional calculation was performed. The problem
consists of two graphite blocks separated by a gap (see Figure 4).Figure 4
Schematic of two adjacent graphite blocks.Heat
transfer coefficient of the gas gap (gap conductance) can be expressed [5]: (1)α=λeδ, where (2)λe=λgas⋅εk.In (2), λgas is the thermal conductivity coefficient of gas, and εk is the convection coefficient. Convection
coefficient is equal to (3)εk=c⋅(Grδ⋅Prδ)n, where
coefficient c and exponent n depend from Grashof number (Grδ) and Prandtl number (Prδ) for the gap product. If Grδ⋅Prδ<1⋅103, then c=1.0,n=0.0. If 1⋅103≤Grδ⋅Prδ≤1⋅106, then c=0⋅105,n=0⋅3. If 1⋅106≤Grδ⋅Prδ≤1⋅1010, then c=0.4,n=0.2.Since
for He and N2 gas Grδ⋅Prδ<1⋅103, then εk=1 and λe=λgas.According
to RBMK designers [5], thermal conductivity coefficient can be expressed as
follows: (4)λgas=(a+b⋅Tav)⋅10−3, where
coefficients a and b depend on the He concentration (for the gas mixture 40%
of He and 60% of N2,a=58 and b=0.09), Tav-average gas temperature in the gap (it is
assumed that the gas temperature is equal to the average temperature of the
graphite surface 750 K). Using (4), we obtain that λgas=0.1255W/m⋅K. Then, the heat
transfer coefficient of the gas gap (according to (1)) α=104.6W/m2⋅K.


### 2.3. Validation of the Gas Gap Conductance Model

The
values of the determined heat transfer coefficient of the gas gap between
adjacent graphite blocks were
used in the “heat conductance” model [3] of the RELAP5-3D input, set for the
simulation of the problem. Using the RELAP5-3D code, the steady-state condition
of RBMK-1500 reactor, including the reactor cooling system operating conditions
during the reactor operation on 4000 MW thermal power, was simulated. As could
be seen from Figure 5, the calculated values of the graphite blocks outer
surface temperature for the maximum and minimum power fuel channels are in
reasonable agreement with the measured plant data [6].Figure 5
The calculated values of the graphite blocks outer surface temperature for the maximum and minimum power fuel channels (reactor power 4000 MW).Such
achieved correspondence of the measured data and the calculation results
obtained using the RELAP5-3D code allows to conclude that the selected
methodology for the modeling of the heat transfer in RBMK reactor core is
suitable.


## 2.1. Simulation of Heat Transfer Transferred in Radial Direction by Employing the Interstructure Heat Conduction Model

For
the simulation of heat transfer between fuel channels and CPS channels the
RELAP5-3D code [3] was used. RELAP5-3D code has the interstructure heat
conduction model. This heat conduction enclosure model gives RELAP5-3D a
general multidimensional heat conduction capability. As it was demonstrated by Paik [4], this model is applied to calculation of heat
transfer through the gas gap between the RBMK reactor core graphite blocks.The
nodalization scheme of the developed model is presented in Figure3. The one
single CPS channel (3) with surrounding graphite column is modeled by “pipe"
element with heat structure. The water in this channel is supplied from water
tank of additional hold-down system (1) through pipeline (2) and bottom
distribution header (4). The tank of AHS is elevated 30 m up to the bottom of
reactor core. The steam from the CPS channel is removed in to top distribution
header (5). The pressure in this header is atmospheric.Figure 3
Simplified model of Ignalina NPP for thermal-hydraulic analysis of processes in RCS (for RELAP5-3D code): (1) water tank of AHS, (2) pipeline, (3) CPS channel, (4)
bottom distribution header, (5) top distribution header, (6) model of DS and steam lines, (7) steam-water pipeline, (8) eight fuel channels, (9) bottom water pipeline, (10) individual
control valve, (11) GDH, (12) water supply from MCP pressure header.Eight FCs (8), placed in the
graphite columns, are surrounding
this single CPS channel. These channels are fed by water from volume (12),
which models water supply from MCP pressure header. Water from GDH, which is
modeled by “branch” element (11), is supplied into bottom water pipeline (9)
and later into fuel channels (8). The water flow through FC is regulated using
individual control valve (10). The steam water mixture is directed to the DS
and steam-line model (6), through steam water pipeline (7). The volumes (1, 5,
6, and 12) are modeled by “time-dependent” elements with specified boundary
conditions.


## 2.2. Gas Gap Conductance between Adjacent Graphite Blocks

The graphite columns
in RBMK-type reactors are separated by a small(∼1mm) gap. The gas gaps
between graphite columns are filled with a gas mixture (40% of He and 60% of N2 by volume fraction) [1]. For establishing of the gas gap conductance between
adjacent graphite blocks, the additional calculation was performed. The problem
consists of two graphite blocks separated by a gap (see Figure 4).Figure 4
Schematic of two adjacent graphite blocks.Heat
transfer coefficient of the gas gap (gap conductance) can be expressed [5]: (1)α=λeδ, where (2)λe=λgas⋅εk.In (2), λgas is the thermal conductivity coefficient of gas, and εk is the convection coefficient. Convection
coefficient is equal to (3)εk=c⋅(Grδ⋅Prδ)n, where
coefficient c and exponent n depend from Grashof number (Grδ) and Prandtl number (Prδ) for the gap product. If Grδ⋅Prδ<1⋅103, then c=1.0,n=0.0. If 1⋅103≤Grδ⋅Prδ≤1⋅106, then c=0⋅105,n=0⋅3. If 1⋅106≤Grδ⋅Prδ≤1⋅1010, then c=0.4,n=0.2.Since
for He and N2 gas Grδ⋅Prδ<1⋅103, then εk=1 and λe=λgas.According
to RBMK designers [5], thermal conductivity coefficient can be expressed as
follows: (4)λgas=(a+b⋅Tav)⋅10−3, where
coefficients a and b depend on the He concentration (for the gas mixture 40%
of He and 60% of N2,a=58 and b=0.09), Tav-average gas temperature in the gap (it is
assumed that the gas temperature is equal to the average temperature of the
graphite surface 750 K). Using (4), we obtain that λgas=0.1255W/m⋅K. Then, the heat
transfer coefficient of the gas gap (according to (1)) α=104.6W/m2⋅K.


## 2.3. Validation of the Gas Gap Conductance Model

The
values of the determined heat transfer coefficient of the gas gap between
adjacent graphite blocks were
used in the “heat conductance” model [3] of the RELAP5-3D input, set for the
simulation of the problem. Using the RELAP5-3D code, the steady-state condition
of RBMK-1500 reactor, including the reactor cooling system operating conditions
during the reactor operation on 4000 MW thermal power, was simulated. As could
be seen from Figure 5, the calculated values of the graphite blocks outer
surface temperature for the maximum and minimum power fuel channels are in
reasonable agreement with the measured plant data [6].Figure 5
The calculated values of the graphite blocks outer surface temperature for the maximum and minimum power fuel channels (reactor power 4000 MW).Such
achieved correspondence of the measured data and the calculation results
obtained using the RELAP5-3D code allows to conclude that the selected
methodology for the modeling of the heat transfer in RBMK reactor core is
suitable.


## 3. Evaluation of Amount of Heat, Which Can Be Removed by Employing Cooling of CPS Channels

The
evaluation of amount of heat, which can be removed by employing cooling of CPS
channels, was performed for two boundary cases:(i)
the possibility to remove the heat from graphite in case of station
blackout, when all pumps are tripped;(ii)
the possibility to remove the heat from graphite in case of large break
LOCA, when CPS channels cooling system remains in operation.


### 3.1.
Evaluation of Amount of Removed Heat from Graphite Stack by CPS Channels in
Station Blackout Case

In the station blackout case due to
of loss electrical power supply MCPs, the pumps of the service water system and
feedwater supply pumps are switched off. Station blackout case assumes the
failure on start-up of all diesel generators, which leads to the unavailability
of the ECCS pumps. The reactor scram signal is activated due to signal of MCP
trip. The pressure in RCS remains nominal after the accident. The water is
evaporating in the fuel channels due to decay heat; the generated steam is
removed through main safety valves that keep the nominal pressure in RCS (see
Figure6).Figure 6
Station blackout case. Pressure behaviour in RCS.The
analysis of processes in reactor core and RCS in case of station blackout (when
heat removal using CPS channels is not taken into account) is presented in
articles [7, 8]. It was shown that within first 0.5 hour after beginning of
accident the reactor is cooled by natural circulation. Later, after water
resource in DSs is evaporated and discharged through MSV, the heat-up of core
components begins. It appears only ∼1.5hours after the beginning of the
accident (see Figure 7). In case of station blackout, ∼3hours after the
beginning of the accident, the failure of fuel channels is expected because the
pressure in RCS is nominal, and the acceptance criterion for FC wall 650°C
[9] will be reached (Figure 7). In the present analysis, the opening of one
safety relief valve at time moment ∼3.8hours after the accident beginning by
an operator to discharge steam was assumed. Due to pressure decrease after
valve opening the rest of coolant in pipelines below reactor core starts to
boil and steam cools down the core for a short time span. However, at time
moment 4.3 hours the second (repeated) heat-up of core elements starts. After operator
starts depressurization, the accident scenario would continue at low pressure
(Figure 7).Figure 7
Station blackout case. Heat removal using CPS channels is not taken into account. Behaviour of core components temperatures.The analysis of possibility to
remove heat from reactor core in case of station blackout by employing cooling
of CPS channels was performed using RELAP5-3D model, shown in Figure3. In case
of station blackout, the water supply in the CPS rods cooling channels is
interrupted due to loss of power to pumps. The control rods will be cooled the
first 10 minutes by water flow from the top distribution tank of CPS because of
water resource in this tank. After emptying this tank, the temperature of CPS
channels walls is increasing (see Figure 7). However, there is a possibility to
fill the tank of AHS (see Position 1 in Figure 3) by artesian water. The tank
of AHS, which is elevated 30 m up to the bottom of reactor core, can be
connected to the bottom distribution header of CPS (see Position 4 in Figure 3)
for water supply to control rod channels (see Position 3 in Figure 3). It was
assumed during the modeling that the tank of AHS is connected to CPS channels 1 hour after the beginning of the accident. It takes 10 additional minutes for
water to enter the bottom part of CPS channels. The supplied water starts to
boil and vapor will escape through pipelines, connected to the top part of
channels (see Position 5 in Figure 3). At the time moment, when the water
supply in to CPS channels starts, the control rods and their channels walls are already hot. As it
is presented in Figures 7–10, the
CPS channel wall temperature reaches approximately 400°C. It is
because these channels have received heat from hot graphite stack. The
temperature of FCs walls is lower (∼300C°) because the
rest of water from RCS cools fuel channels at this moment. After the start of
water supply, the bottom part of CPS channels will be cooled. Temperature of
CPS channel wall is approximately 100°C that means boiling of water
in channels. The water in CPS channels removes the heat from graphite bricks,
thus temperature of graphite of CPS channel is below the temperature of
graphite of fuel channels (Figures
8 and 9). The part of heat from FCs is transferred in radial direction through
graphite columns and gaps between graphite columns into cooled CPS channels.
However, the flow rate of water from AHS tank is not sufficient for effective
cooling of CPS channels. As it is presented in Figures 9 and 10, the temperature of CPS channels
walls in the top part is much higher. 17 hours after the beginning of the
accident peak, CPS channel wall temperature reaches 1000°C. To
prevent generation of huge amount of hydrogen, the operator should stop the
water supply into CPS channels at this moment. As it is shown in Figure 10, the
maximum capacity of heat removal in such way is equal to 15 MW (the water is
supplied into 211 CPS channels). The decay heat, generated in the reactor,
decreases down to such power level only approximately 100 hours after the
beginning of the accident (see Figure 11). Thus, the water supply from AHS tank
into CPS channels in case of station blackout is not effective enough and
cannot remove all heat from reactor core, but such restoration of water supply
into CPS channels allows to slow down the core heat-up process and may be used
in complex with other measures.Figure 8
Station blackout case. Heat removal using CPS channels is taken into account. Behaviour of core components temperatures at the elevation 1.75 m from core bottom.Figure 9
Station blackout case. Heat removal using CPS channels is taken into account. Behaviour of core components temperatures at the elevation 5.25 m from core bottom.Figure 10
Station blackout case. Heat removal using CPS channels is taken into account. Behaviour of core components temperatures. Capacity of heat removal from the core by CPS cooling circuit.Figure 11
Comparison of reactor decay heat with capacity of heat removal from core by CPS cooling circuit in station blackout case.


### 3.2. Evaluation of Amount of Removed Heat from Graphite Stack by CPS Channels in
Large Break LOCA Case

For
illustration of possibility to remove heat from reactor core using CPS cooling
system, the case of break of RCS pipeline with the largest diameter (guillotine
break of MCP pressure header) with failure of ECCS is presented in this
article. In this BDBA case, only two means for reactor cooldown are assumed: injection
of∼180m3 of water from ECCS hydroaccumulators and availability of
CPS channels cooling circuit with capacity of heat removal 28.5 MW. The
structure of coolant flows in case of this event is presented in Figure 12. It
is assumed in the modeling that before the accident the reactor operates at
maximum allowed power level 4200 MW.Figure 12
MCP pressure header break with failure of ECCS (except ECCS hydro
accumulators): (1) pressure header break; (2) discharge of coolant from drum separators in
the affected loop; (3) discharge of coolant from lower part of reactor cooling circuit in the
affected loop; (4, 5) supply of water from ECCS hydroaccumulators; (6) supply of steam
from DSs in in the intact loop; (7) circuit for cooling of rods in reactor control and protection
system.The
behavior of pressure in the GDH and DS, coolant flow rate through channels in
the affected RCS loop, is
presented in Figures13 and 14. It was assumed during the modeling that supply of water into CPS
channels is keeping constant the temperature in the CPS channels. It was
assumed that these channels are always filled with boiling water.Figure 13
MCP pressure header break with failure of ECCS. Coolant flow rate through the single FC.Figure 14
MCP pressure header break with failure of ECCS. Pressure behaviour in DS and GDH.The
behavior of maximum temperatures of fuel, cladding, FC wall, graphite columns,
and CPS channel wall in case of MCP pressure header break with failure of ECCS
within first 220 minutes is
presented in Figure15. As it is shown in this figure, after temperature in
graphite columns with FC starts increasing, the temperature of graphite column
with CPS starts to increase too but with some delay. This indicates that heat
from overheated fuel channels is transferred in radial direction through the
graphite bricks and gas gaps between these graphite bricks into cooled CPS
channels, as shown schematically in Figure 2. The temperature of graphite with
CPS channels is always lower than temperature of graphite with FC. The code
RELAP5-3D is not designed to model severe accident, thus the calculation is
terminated after fuel channel wall temperature reaches 950C°.Figure 15
MCP pressure header break with failure of ECCS. The heat transfer from FC channels to CPS cooling channels is taken into account. Behaviour of maximum temperatures of fuel, cladding, FC wall, graphite columns and CPS channel wall.The amount of heat that is
transferred from fuel channels to CPS channels was calculated using the
presented RELAP5-3D model. The total amount of heat removed by 211 CPS channels
is presented in Figure16. As it is seen from this figure, during normal
operation, approximately 7–8 MW of heat is
removed from reactor core by CPS channel cooling system. In case of accident,
when graphite temperature increases, the amount of removed heat also increases
up to 40 MW. The amount of removed heat is proportional to the temperature
difference between graphite column with overheated FC and graphite column with
cooled CPS channels. As it is seen from Figure 15, during normal operation of
reactor, the temperatures of graphite columns with fuel channels and with CPS
channels are similar—only insignificant amount of heat is removed
from reactor core by system of CPS channels cooling. In case of accident, after
dryout and overheating of fuel channels (the cooling of CPS channels remains
available), the temperature difference between CPS and FC graphite columns
starts to increase (see Figure 15). This leads to increasing of amount of removed
heat from hot FCs (see Figure 16).Figure 16
MCP pressure header break with failure of ECCS. Heat removal from fuel channels through 211 CPS channels by CPS cooling system.Based
on the simulation performed using RELAP5-3D code, the dependency of capacity of
removed heat from temperature of FCs graphite column was established. As it is
presented in Figure17, if graphite temperature is increasing, the capacity of
removed heat is increasing as well. At graphite temperature 700°C,
the maximal constant value of heat removal from fuel channels (28.5 MW) is
reached. The modeling of processes in reactor fuel channels in case of
mentioned BDBA, when part of decay heat is removed by employing cooling of CPS
channels, was performed using RELAP/SCDAPSIM model, described in [8]. The
behavior of temperature of core components is presented in Figure 18. Due to
loss of coolant in reactor cooling system, the heat-up of core components
starts a few minutes after the beginning of the accident (Figure 18). The
graphite stack, which is a moderator in RBMK-1500 reactor, is an efficient heat
sink and slows down the heat-up of the core. As it is presented in Figure 18,
due to the removal of heat from CPS channels, the peak fuel temperatures are
below 1000°C. Assuming heat removal, using CPS channels cooling
circuit, approximately 16 hours after the beginning of the accident, a
slow decrease of core components temperature begins. This is because decay
heat, generated in the core, at this time moment decreases down to the level of
heat, which is removed from CPS channels—28.5 MW, when CPS channels cooling
circuit is available (Figure 19).Figure 17
Dependency of removed heat capacity on the temperature of FCs graphite column, when regular CPS cooling circuit is available.Figure 18
MCP pressure header break with failure of ECCS pumps. Behaviour of core components temperatures.Figure 19
Comparison of reactor decay heat with capacity of heat removal from core by CPS cooling circuit (when regular CPS cooling system is available).The cool-down
process is very slow because after the decrease of graphite temperature the
temperature gradient from fuel channels to control rods channels is decreasing
as well. Thus, the amount of removed heat is decreasing. To evaluate the
influence of the heat removal effect from the reactor core using a CPS cooling
system, the simulated temperatures are compared with the case where the heat
transfer to CPS channels is not taken into account. As it is presented in
Figure18, in the last case the temperatures of core components are increasing
continuously because all decay heat, generated in the reactor core, is used for
heating of core components. In this case after ∼10hours, the 1450°C
temperature is reached and the melting of stainless steel grids in fuel
assemblies starts. Later, ∼40hours after the beginning of the accident the
temperatures increase up to 2400–2600°C and ceramic formation in
fuel assemblies starts.


## 3.1.
Evaluation of Amount of Removed Heat from Graphite Stack by CPS Channels in
Station Blackout Case

In the station blackout case due to
of loss electrical power supply MCPs, the pumps of the service water system and
feedwater supply pumps are switched off. Station blackout case assumes the
failure on start-up of all diesel generators, which leads to the unavailability
of the ECCS pumps. The reactor scram signal is activated due to signal of MCP
trip. The pressure in RCS remains nominal after the accident. The water is
evaporating in the fuel channels due to decay heat; the generated steam is
removed through main safety valves that keep the nominal pressure in RCS (see
Figure6).Figure 6
Station blackout case. Pressure behaviour in RCS.The
analysis of processes in reactor core and RCS in case of station blackout (when
heat removal using CPS channels is not taken into account) is presented in
articles [7, 8]. It was shown that within first 0.5 hour after beginning of
accident the reactor is cooled by natural circulation. Later, after water
resource in DSs is evaporated and discharged through MSV, the heat-up of core
components begins. It appears only ∼1.5hours after the beginning of the
accident (see Figure 7). In case of station blackout, ∼3hours after the
beginning of the accident, the failure of fuel channels is expected because the
pressure in RCS is nominal, and the acceptance criterion for FC wall 650°C
[9] will be reached (Figure 7). In the present analysis, the opening of one
safety relief valve at time moment ∼3.8hours after the accident beginning by
an operator to discharge steam was assumed. Due to pressure decrease after
valve opening the rest of coolant in pipelines below reactor core starts to
boil and steam cools down the core for a short time span. However, at time
moment 4.3 hours the second (repeated) heat-up of core elements starts. After operator
starts depressurization, the accident scenario would continue at low pressure
(Figure 7).Figure 7
Station blackout case. Heat removal using CPS channels is not taken into account. Behaviour of core components temperatures.The analysis of possibility to
remove heat from reactor core in case of station blackout by employing cooling
of CPS channels was performed using RELAP5-3D model, shown in Figure3. In case
of station blackout, the water supply in the CPS rods cooling channels is
interrupted due to loss of power to pumps. The control rods will be cooled the
first 10 minutes by water flow from the top distribution tank of CPS because of
water resource in this tank. After emptying this tank, the temperature of CPS
channels walls is increasing (see Figure 7). However, there is a possibility to
fill the tank of AHS (see Position 1 in Figure 3) by artesian water. The tank
of AHS, which is elevated 30 m up to the bottom of reactor core, can be
connected to the bottom distribution header of CPS (see Position 4 in Figure 3)
for water supply to control rod channels (see Position 3 in Figure 3). It was
assumed during the modeling that the tank of AHS is connected to CPS channels 1 hour after the beginning of the accident. It takes 10 additional minutes for
water to enter the bottom part of CPS channels. The supplied water starts to
boil and vapor will escape through pipelines, connected to the top part of
channels (see Position 5 in Figure 3). At the time moment, when the water
supply in to CPS channels starts, the control rods and their channels walls are already hot. As it
is presented in Figures 7–10, the
CPS channel wall temperature reaches approximately 400°C. It is
because these channels have received heat from hot graphite stack. The
temperature of FCs walls is lower (∼300C°) because the
rest of water from RCS cools fuel channels at this moment. After the start of
water supply, the bottom part of CPS channels will be cooled. Temperature of
CPS channel wall is approximately 100°C that means boiling of water
in channels. The water in CPS channels removes the heat from graphite bricks,
thus temperature of graphite of CPS channel is below the temperature of
graphite of fuel channels (Figures
8 and 9). The part of heat from FCs is transferred in radial direction through
graphite columns and gaps between graphite columns into cooled CPS channels.
However, the flow rate of water from AHS tank is not sufficient for effective
cooling of CPS channels. As it is presented in Figures 9 and 10, the temperature of CPS channels
walls in the top part is much higher. 17 hours after the beginning of the
accident peak, CPS channel wall temperature reaches 1000°C. To
prevent generation of huge amount of hydrogen, the operator should stop the
water supply into CPS channels at this moment. As it is shown in Figure 10, the
maximum capacity of heat removal in such way is equal to 15 MW (the water is
supplied into 211 CPS channels). The decay heat, generated in the reactor,
decreases down to such power level only approximately 100 hours after the
beginning of the accident (see Figure 11). Thus, the water supply from AHS tank
into CPS channels in case of station blackout is not effective enough and
cannot remove all heat from reactor core, but such restoration of water supply
into CPS channels allows to slow down the core heat-up process and may be used
in complex with other measures.Figure 8
Station blackout case. Heat removal using CPS channels is taken into account. Behaviour of core components temperatures at the elevation 1.75 m from core bottom.Figure 9
Station blackout case. Heat removal using CPS channels is taken into account. Behaviour of core components temperatures at the elevation 5.25 m from core bottom.Figure 10
Station blackout case. Heat removal using CPS channels is taken into account. Behaviour of core components temperatures. Capacity of heat removal from the core by CPS cooling circuit.Figure 11
Comparison of reactor decay heat with capacity of heat removal from core by CPS cooling circuit in station blackout case.


## 3.2. Evaluation of Amount of Removed Heat from Graphite Stack by CPS Channels in
Large Break LOCA Case

For
illustration of possibility to remove heat from reactor core using CPS cooling
system, the case of break of RCS pipeline with the largest diameter (guillotine
break of MCP pressure header) with failure of ECCS is presented in this
article. In this BDBA case, only two means for reactor cooldown are assumed: injection
of∼180m3 of water from ECCS hydroaccumulators and availability of
CPS channels cooling circuit with capacity of heat removal 28.5 MW. The
structure of coolant flows in case of this event is presented in Figure 12. It
is assumed in the modeling that before the accident the reactor operates at
maximum allowed power level 4200 MW.Figure 12
MCP pressure header break with failure of ECCS (except ECCS hydro
accumulators): (1) pressure header break; (2) discharge of coolant from drum separators in
the affected loop; (3) discharge of coolant from lower part of reactor cooling circuit in the
affected loop; (4, 5) supply of water from ECCS hydroaccumulators; (6) supply of steam
from DSs in in the intact loop; (7) circuit for cooling of rods in reactor control and protection
system.The
behavior of pressure in the GDH and DS, coolant flow rate through channels in
the affected RCS loop, is
presented in Figures13 and 14. It was assumed during the modeling that supply of water into CPS
channels is keeping constant the temperature in the CPS channels. It was
assumed that these channels are always filled with boiling water.Figure 13
MCP pressure header break with failure of ECCS. Coolant flow rate through the single FC.Figure 14
MCP pressure header break with failure of ECCS. Pressure behaviour in DS and GDH.The
behavior of maximum temperatures of fuel, cladding, FC wall, graphite columns,
and CPS channel wall in case of MCP pressure header break with failure of ECCS
within first 220 minutes is
presented in Figure15. As it is shown in this figure, after temperature in
graphite columns with FC starts increasing, the temperature of graphite column
with CPS starts to increase too but with some delay. This indicates that heat
from overheated fuel channels is transferred in radial direction through the
graphite bricks and gas gaps between these graphite bricks into cooled CPS
channels, as shown schematically in Figure 2. The temperature of graphite with
CPS channels is always lower than temperature of graphite with FC. The code
RELAP5-3D is not designed to model severe accident, thus the calculation is
terminated after fuel channel wall temperature reaches 950C°.Figure 15
MCP pressure header break with failure of ECCS. The heat transfer from FC channels to CPS cooling channels is taken into account. Behaviour of maximum temperatures of fuel, cladding, FC wall, graphite columns and CPS channel wall.The amount of heat that is
transferred from fuel channels to CPS channels was calculated using the
presented RELAP5-3D model. The total amount of heat removed by 211 CPS channels
is presented in Figure16. As it is seen from this figure, during normal
operation, approximately 7–8 MW of heat is
removed from reactor core by CPS channel cooling system. In case of accident,
when graphite temperature increases, the amount of removed heat also increases
up to 40 MW. The amount of removed heat is proportional to the temperature
difference between graphite column with overheated FC and graphite column with
cooled CPS channels. As it is seen from Figure 15, during normal operation of
reactor, the temperatures of graphite columns with fuel channels and with CPS
channels are similar—only insignificant amount of heat is removed
from reactor core by system of CPS channels cooling. In case of accident, after
dryout and overheating of fuel channels (the cooling of CPS channels remains
available), the temperature difference between CPS and FC graphite columns
starts to increase (see Figure 15). This leads to increasing of amount of removed
heat from hot FCs (see Figure 16).Figure 16
MCP pressure header break with failure of ECCS. Heat removal from fuel channels through 211 CPS channels by CPS cooling system.Based
on the simulation performed using RELAP5-3D code, the dependency of capacity of
removed heat from temperature of FCs graphite column was established. As it is
presented in Figure17, if graphite temperature is increasing, the capacity of
removed heat is increasing as well. At graphite temperature 700°C,
the maximal constant value of heat removal from fuel channels (28.5 MW) is
reached. The modeling of processes in reactor fuel channels in case of
mentioned BDBA, when part of decay heat is removed by employing cooling of CPS
channels, was performed using RELAP/SCDAPSIM model, described in [8]. The
behavior of temperature of core components is presented in Figure 18. Due to
loss of coolant in reactor cooling system, the heat-up of core components
starts a few minutes after the beginning of the accident (Figure 18). The
graphite stack, which is a moderator in RBMK-1500 reactor, is an efficient heat
sink and slows down the heat-up of the core. As it is presented in Figure 18,
due to the removal of heat from CPS channels, the peak fuel temperatures are
below 1000°C. Assuming heat removal, using CPS channels cooling
circuit, approximately 16 hours after the beginning of the accident, a
slow decrease of core components temperature begins. This is because decay
heat, generated in the core, at this time moment decreases down to the level of
heat, which is removed from CPS channels—28.5 MW, when CPS channels cooling
circuit is available (Figure 19).Figure 17
Dependency of removed heat capacity on the temperature of FCs graphite column, when regular CPS cooling circuit is available.Figure 18
MCP pressure header break with failure of ECCS pumps. Behaviour of core components temperatures.Figure 19
Comparison of reactor decay heat with capacity of heat removal from core by CPS cooling circuit (when regular CPS cooling system is available).The cool-down
process is very slow because after the decrease of graphite temperature the
temperature gradient from fuel channels to control rods channels is decreasing
as well. Thus, the amount of removed heat is decreasing. To evaluate the
influence of the heat removal effect from the reactor core using a CPS cooling
system, the simulated temperatures are compared with the case where the heat
transfer to CPS channels is not taken into account. As it is presented in
Figure18, in the last case the temperatures of core components are increasing
continuously because all decay heat, generated in the reactor core, is used for
heating of core components. In this case after ∼10hours, the 1450°C
temperature is reached and the melting of stainless steel grids in fuel
assemblies starts. Later, ∼40hours after the beginning of the accident the
temperatures increase up to 2400–2600°C and ceramic formation in
fuel assemblies starts.


## 4. Conclusions

The
analysis of possibility to remove heat from RBMK-1500 reactor core in case of
large LOCA by employing cooling of CPS channels was performed using RELAP5-3D
and RELAP5 codes. The part of heat that can be removed by CPS in long-term
accident becomes significant, when level of decay heat is decreased. In spite
of high thermal inertia of graphite, this heat removal from CPS channels allows
to slow down effectively the core heat-up process.If
regular CPS cooling circuit with the pumps and heat exchangers are available,
this circuit can remove up to 28.5 MW from reactor core. Decay heat, generated
in the core, decreases down to this level of heat, which is removed from CPS
channels, 16 hours after the beginning of the accident. Starting from this time
moment, the slow decrease of core components temperature begins. RELAP5
analysis shows that due to removal of heat from CPS channels, the peak fuel
temperatures are below 1000°C. Thus, in this case the use of this system
allows to prevent zirconium oxidation and melting of stainless steel grids and
fuel.If
the regular CPS cooling circuit is not available, there is a possibility to
supply artesian water into CPS channels from AHS tank. The results of the
analysis, performed using RELAP5-3D code, show that in this case only 15 MW of
heat can be removed from the core. This amount of removed heat is not enough to
prevent serious consequences, but it allows to slow down the core heat-up
process and may be used in complex with other measures.



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*Source: 430768-2008-05-28.xml*