# Simulation of MASPn Experiments in MISTRA Test Facility with COCOSYS Code

Authors: Mantas Povilaitis; Egidijus Urbonavičius
Journal: Science and Technology of Nuclear Installations
 (2008)

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## Abstract

An issue of the stratified atmospheres in the containments of nuclear power plants is still unresolved; different experiments are performed in the test facilities like TOSQAN and MISTRA. MASPn experiments belong to the spray benchmark, initiated in the containment atmosphere mixing work package of the SARNET network. The benchmark consisted of MASP0, MASP1 and MASP2 experiments. Only the measured depressurisation rates during MASPn were available for the comparison with calculations. When the analysis was performed, the boundary conditions were not clearly defined therefore most of the  attention was concentrated on MASP0 simulation in order to develop the nodalisation scheme and define the initial and boundary conditions. After  achieving acceptable agreement with measured depressurisation rate, simulations of MASP1 and MASP2 experiments were performed to check the influence of sprays. The paper presents developed nodalisation scheme of MISTRA for the COCOSYS code and the results of analyses. In the performed analyses, several parameters were considered: initial conditions, loss coefficient of the junctions, initial gradients of temperature and steam volume fraction, and characteristic length of structures. Parametric analysis shows that in the simulation the heat losses through the external walls behind the lower condenser installed in the MISTRA facility determine the long-term depressurisation rate.


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## Body

## 1. Introduction

Loss-of-coolant accidents
from ruptures in the primary loop of light water reactors (LWRs) are generally
fully controlled by the engineered safety systems. Nevertheless, there is a
small probability that due to failure of designed emergency core cooling
measures during accident, the reactor core could overheat and chemical reaction
of steam and strongly overheated zircaloy could produce significant amounts of
hydrogen. This hydrogen would then be released into the containment. Without
counter measures, the flammable mixtures could then form and cause combustion
loads that could threaten the integrity of the containment. The hydrogen
combustion had occurred in the containment of Three Mile Island NPP and caused
pressure spike of ~2 bar [1].Detailed
experimental and analytical hydrogen mixing research has been performed at
several laboratories and experiment facilities TOSQAN, MISTRA, and THAI in the
frame of international standard problem (ISP-47) [2]. Performed analyses identified the phenomena, which
should be further addressed in the code development and what further
experiments could be performed to enhance knowledge about hydrogen mixing in
the containments.The installed
water sprays could enhance gas mixing in the containments and prevent local
accumulation of hydrogen. In order to evaluate the ability of containment
modelling codes to simulate the spray behaviour and the interaction with gas
atmosphere, severe accident research network (SARNET)
initiated the spray benchmark in two test facilities: TOSQAN and MISTRA [1]. Lithuanian Energy Institute participated in the
benchmark by simulating MASPn tests performed in the MISTRA facility. The MISTRA
spray tests MASP1 and MASP2 concern the depressurisation of the containment
atmosphere by spray, while MASP0 is the reference case without spray.The simulation of
MASPn experiments was performed with the lumped-parameter code COCOSYS. Only
the measured depressurisation rates during MASPn were available for the
comparison with calculations at this stage of benchmark. In this work, most of
the attention was concentrated on MASP0 experiment simulation, since MASP0 is the
less complex experiment than MASP1 and MASP2 (sprays are not used during MASP0).
Therefore, it was decided to test developed nodalisation, defined initial and
boundary conditions and selected modelling parameters by simulating this
experiment. When the MASP0 test simulation was performed and acceptable
agreement with measured depressurisation rate was achieved then the analyses of
MASP1 and MASP2 experiments were performed to check the influence of sprays on
the depressurisation rate.This paper presents
a short description of MISTRA facility, specification of MASPn tests performed
in this facility, developed nodalisation for COCOSYS code and the obtained
results. The performed analysis showed that a clear and detailed specification
of initial and boundary conditions of the tests is essential in order to
perform the correct simulation of the experiments. The performed parametric
analysis revealed the importance of the conditions in the so-called “dead-end”
volume behind the condensers, installed in MISTRA test facility.


## 2. Experiments

There were three
experiments—MASP0, MASP1, and MASP2—performed in MISTRA test facility. Each MASP
experiment followed after an M5 experiment, during which the stratified
atmosphere of the containment was created by a centred steam release into the
facility and a high-thermal gradient of the temperatures of the condensers.
Therefore, initial conditions of the MASP tests were the conditions of the M5
tests steady-states. Temperatures of the condensers during the MASP tests were
the same as during the M5 tests.MASP1 and MASP2 experiments can be divided into two phases—phase of the depressurisation due to the
thermal losses (0 second–2100 seconds) and spray phase
(2100 seconds–3900 seconds). During the first phase,
the pressure of the containment is decreasing only due to thermal loses through
the external walls of the facility and steam condensation on the lower
condenser. During the second phase, water sprays are activated and the dominant
processes affecting depressurisation is steam condensation on the water
droplets and atmosphere mixing. MASP0 experiment is a reference case and only
has one phase—depressurisation due to the thermal losses
(0 second–3900 seconds). Detailed description of
experiments can be found in [3].


### 2.1. MISTRA Test Facility

MISTRA facility is
a vertical stainless steel cylindrical vessel with curved bottom (Figure1). The free volume of the cylinder is ~99.5 m3,
internal diameter −4.25 m, maximal internal height −7.38 m. Vessel is thermally insulated with 20 cm of rock wool. Three
cylinders with controlled surface temperature, called condensers, are inserted
inside the vessel. Inner diameter of every condenser is equal to 3.8 m. The condensate is
collected by gutters. Condensers are situated between 1.285 m and 7.28 m of cylinder height.
Condensation occurs only on surfaces of condensers facing the inside of MISTRA
facility. Total condensing surface is about 69 m2. Outside
surfaces of the condensers are insulated with 2 cm synthetic foam layer.
There is some space, the so-called “dead volume,” between condensers and
facility walls. Vertical spaces between condensers (~0.12 m) are partially
hidden by gutters. Further description of MISTRA facility can be found in [3, 4].Figure 1
Schematics of MISTRA test facility [1].


### 2.2. Experimental Results

During the MASP tests phase of the
depressurisation due to thermal losses containment pressure dropped from the
initial value of 2.4 bar to ~2.2 bar in the first ~500 seconds and then was
slowly decreasing (Figure2) [3]. At the end of MASP0 experiment (~4000 seconds) the
pressure had decreased down to ~2.03 bar. During MASP1 and MASP2 tests the
sprays were activated after 2100 seconds. Until this time, the pressure
behaviour in all three cases was very similar and small differences were caused
by the different conditions that had appeared after each M5 test. After spray
activation, a rapid depressurisation was observed. In the MASP1 experiment, the
pressure after 4000 seconds was ~1.6 bar and in the MASP2 experiment it was
~1.45 bar. The different final pressures of the MASP1 and MASP2 experiments
spray phase were due to the different temperatures of injected water (Table 1). Lower spray water temperature induced a larger
containment pressure decrease.Table 1
Benchmark specifications—Part A [1].
ParameterMASP0MASP1MASP2Pressure2.4 bar2.4 bar2.4 barMean
gas temperature124°C124°C124°CSteam
volume fraction0.450.450.45Air
mass~115 kg~115 kg~115 kgWater
temperature in the nozzle—40°C60°CDroplets mass flow-rate—0.87 kg/s0.87 kg/sTemperature
of lower condenser80°C80°C80°CTemperature
of medium and upper condensers140°C140°C140°CFigure 2
Experimental results [3].


## 2.1. MISTRA Test Facility

MISTRA facility is
a vertical stainless steel cylindrical vessel with curved bottom (Figure1). The free volume of the cylinder is ~99.5 m3,
internal diameter −4.25 m, maximal internal height −7.38 m. Vessel is thermally insulated with 20 cm of rock wool. Three
cylinders with controlled surface temperature, called condensers, are inserted
inside the vessel. Inner diameter of every condenser is equal to 3.8 m. The condensate is
collected by gutters. Condensers are situated between 1.285 m and 7.28 m of cylinder height.
Condensation occurs only on surfaces of condensers facing the inside of MISTRA
facility. Total condensing surface is about 69 m2. Outside
surfaces of the condensers are insulated with 2 cm synthetic foam layer.
There is some space, the so-called “dead volume,” between condensers and
facility walls. Vertical spaces between condensers (~0.12 m) are partially
hidden by gutters. Further description of MISTRA facility can be found in [3, 4].Figure 1
Schematics of MISTRA test facility [1].


## 2.2. Experimental Results

During the MASP tests phase of the
depressurisation due to thermal losses containment pressure dropped from the
initial value of 2.4 bar to ~2.2 bar in the first ~500 seconds and then was
slowly decreasing (Figure2) [3]. At the end of MASP0 experiment (~4000 seconds) the
pressure had decreased down to ~2.03 bar. During MASP1 and MASP2 tests the
sprays were activated after 2100 seconds. Until this time, the pressure
behaviour in all three cases was very similar and small differences were caused
by the different conditions that had appeared after each M5 test. After spray
activation, a rapid depressurisation was observed. In the MASP1 experiment, the
pressure after 4000 seconds was ~1.6 bar and in the MASP2 experiment it was
~1.45 bar. The different final pressures of the MASP1 and MASP2 experiments
spray phase were due to the different temperatures of injected water (Table 1). Lower spray water temperature induced a larger
containment pressure decrease.Table 1
Benchmark specifications—Part A [1].
ParameterMASP0MASP1MASP2Pressure2.4 bar2.4 bar2.4 barMean
gas temperature124°C124°C124°CSteam
volume fraction0.450.450.45Air
mass~115 kg~115 kg~115 kgWater
temperature in the nozzle—40°C60°CDroplets mass flow-rate—0.87 kg/s0.87 kg/sTemperature
of lower condenser80°C80°C80°CTemperature
of medium and upper condensers140°C140°C140°CFigure 2
Experimental results [3].


## 3. Nodalisation Scheme, Initial and Boundary Conditions

Experiments were
simulated with lumped-parameter code COCOSYS version V2.3v11 [5]. Developed nodalisation scheme of the MISTRA test
facility (Figure 3) consisted of 31 nodes, which were connected by 50
atmospheric junctions. Water film flow along condensers was modelled with 4
junctions. Condensers were simulated by 14 structures, walls of containment—11 structures. This nodalisation scheme was
developed on the basis of the MISTRA facility nodalisation, which was used in
the frame of ISP-47 [2]. In order to simulate the temperature and steam
fraction gradients in more detail the previous nodalisation was refined. In the
current nodalisation, there are four vertical subdivisions at the level of
lower condenser instead of two in the previous ISP-47 nodalisation because the
level of lower condenser corresponds to a region between 1 m and 4 m of the facility height and
the gradients of both temperature and steam volume fraction are created in this
region. Regions of middle and upper condensers were left subdivided into two
vertical parts.Figure 3
Nodalisation scheme for COCOSYS code.Nodalisation
scheme includes 3 radial nodes in the central part of the containment and one
in the “dead volume” behind the condensers. There are 8 vertical levels with
detailed radial subdivision. A single node defined below the lower condenser
simulates the sump. The heat transfer through convection, radiation, and
condensation is considered. Outer sides of the facility walls in the model were
kept at room temperature during whole simulations. COCOSYS code is capable of
calculating gradients of temperature inside the wall from this external
condition and conditions in the control node the wall is facing.For the simulation
of MASP1 and MASP2 experiments, several paths of the falling spray droplets
were identified according to nodalisation. Fraction of the total water inlet
mass flow of the spray system related to each spray path was calculated
according to the part of the base area of the cone, which corresponds to a
given path. The spray droplet size distribution was not given in the
specification; a value of around 1 mm was
specified [3]. The spray model, which is
implemented in COCOSYS code, simulates only monodisperse droplets, that is,
does not allow defining a droplet size distribution.


## 4. Calculations

Simulations of the
experiments in the MASPn benchmark were performed in two parts: (1) part A,
where only mean values of gas temperature and steam volume fraction were
specified (Table1), and (2) part B, where the steam content and
temperature stratification conditions were specified.At first, the initial
and boundary conditions for the base case calculations were set according to
the part A specification [1], where the gas stratification was not identified. Temperature
of the external sides of the facility walls was set equal to 20°C. When using
mean gas temperature and mean steam volume fraction as homogenous initial
conditions for vessel volume, the calculated pressure drop in MASP0 case was
too high compared to the experimental results (Figure 4). Calculations showed that after 4000 seconds the
pressure would drop to almost 1.8 bar, but in experiment it was still higher
than 2 bar. This result shows that it is not possible to model MASP tests using
homogenous initial conditions and experiments have to be simulated using gas
temperature and steam fraction gradients for initial data.Figure 4
Pressure evolution using homogenous initial conditions.Stratifications of
steam content and gas temperature specified in part B are presented in Figure5 [6]. One can see that after M5 test the temperature
gradient is created between elevations of 1.25 m and 4.5 m. Temperature difference
in this region is 40°C. The stratification of steam volume fraction
changes from 0.25 at elevation of 1.25 m to 0.6 at elevation of 3.5 m.Figure 5
Additional experimental data in part B specification [6].Therefore, MISTRA
nodalisation was modified to include gradients from the part B specification (Figure5) [6]. But it was not possible to perform calculations
using these conditions because they correspond to the relative humidity greater
than 100% at low elevations of the facility (Figure 6). Nevertheless, it should be noted that with such
temperature and steam volume fraction distribution, the calculated mass of air
in the facility was 115.7 kg and corresponded to the test specification (see Table 1).Figure 6
Relative humidity obtained with conditions from part B specification.Considering that
better definition of experimental boundary conditions was not available at the
moment, when analysis was performed it was decided to perform parametric
analyses with slightly modified conditions in order to start calculations and
study the effect of these conditions on the results. As well there were
performed several analyses to study the effect of modelling assumptions.At first, it was
chosen to reduce the steam volume fraction at lowest elevations of the facility
from 25% to 20%. This reduced saturation in the lower part of the facility
below 100% and allowed to start calculations but the obtained depressurisation
rate (Figure7) was not much different from the one received
assuming homogeneous mixture in the facility (Figure 4), that is, they are significantly different from the
experimental values.Figure 7
Pressure evolution with decreased steam volume fraction at low elevation.The next step was
to investigate the influence of modelling parameter—loss coefficient in junctions—on the depressurisation rate. In COCOSYS code,
the atmospheric junctions are described by junction area, hydraulic length, and
loss coefficient. Area of the junction is unambiguously defined by dividing
volume of the facility into the calculation nodes. Hydraulic length is selected
as distance between the centres of the nodes connected by the junction. The
loss coefficient was selected equal to the value of 1.0 in the base case for all
junctions. The loss coefficient is not very well defined in this model because
there are no obstacles for gas flow and mass transfer takes place between
virtually divided volumes. With the purpose of evaluating the influence of this
parameter additional calculations were made, where the value of the loss
coefficient was changed to 0.5 and 1.5 as different variants. The results
showed that variation of the loss coefficient in junctions does not influence
the results significantly (Figure8) and could not be the reason for large differences
between calculations and experiments.Figure 8
Pressure evolutions with different loss coefficients.Later it was
decided to perform variation of initial temperature and steam volume fraction
at lower and upper elevations since these measurements of these parameters
include some uncertainty. At first the temperature in the lower part of the
containment was defined to be85°C instead of 87°C, then the steam fraction at
low elevations was defined to be 18% instead of 20% and then the steam fraction
at high elevations was defined to be 55% instead of 60%. These values were
taken considering the measurement errors. Only results of the variant with 55%
of steam volume fraction at high elevations showed more different pressure
evolution (Figure 9). But the containment pressure was still too low
compared with experiment and, furthermore, such change of the gas content
changed the air mass of the facility and it became noncompliant with the test
specification.Figure 9
Pressure evolutions with different initial parameters.Structures of the
facility in the developed model are described by the area of the structure,
applicable models of the heat transfer (convection, condensation, and
radiation), composition of the structure, and the characteristic height and
length. The characteristic length of the structure is not well defined in the
modelling methodology. According to COCOSYS user’s manual [5], it is defined as 1=A/U, where A is an area of the
structure, and U is its perimeter, but for the vertical walls the
characteristic length should be equal to the height of the wall. Also,
according to this user’s manual, for the simulation of real power plants the
value of 0.01 can be used for all vertical walls. During simulations of aerosol
deposition in the KAEVER experimental facility, the value of 0.05 for all
vertical walls was determined as best for simulation of aerosol deposition [7]. Since the value of the characteristic length is not
very clear, it was varied to investigate its influence on the calculation
results. The performed analysis showed that this parameter had significant
influence on results, but the calculated pressure was still too low compared to
experiment (Figure 10). The variation of this parameter led to 0.1 bar
pressure difference at the end of calculations. As well it should be noted that
changing this parameter does not give initial faster pressure drop and slower
decrease after 500 seconds.Figure 10
Pressure evolutions with different characteristic length values.The last tested
assumption was simulation of the external walls of the test facility. The
calculations were performed assuming that there are no external walls, that is, the
depressurisation in the MASP0 case is determined only by the heat transfer to
the lower condenser. The obtained results showed that the calculated
depressurisation rate is similar to the measured (Figure11). After further examinations, it was determined that
only structures behind the lower condenser, which is colder than the other condensers,
determined the heat loss through external structures and the depressurisation
rate in the long-term.Figure 11
Comparison of pressure evolutions calculated from base case and calculated without external structures, MASP0.For the final
analysis of all MASPn experiments, it was decided to simulate all structures, but to decrease the area of the junction behind the lower condenser. This leads to a decrease of gas flow
and, consequently, the heat transfer at this part of the facility. The area of
the junction was selected in such a way that the calculated depressurisation
rate due to heat loses, that is, MASP0 test, comply with the measured results
best. The characteristic length of all structures was assumed 0.01 m. In the case of the
MASP0 experiment, pressure reaches ~2.2 bar at ~500 seconds and stays a
bit over 2 bar at ~4000 seconds (Figure12), that is, results are similar to the experimental.
When this MASP0 test simulation was performed and acceptable agreement with
measured depressurisation rate was achieved (Figure 12), simulation of MASP1 and MASP2 experiments was
performed to check the influence of the sprays. The same modelling assumptions
and conditions as in MASP0 test simulation were used. Sprays were modelled with
previously described assumptions. Mass flow rate of the droplets and water
temperature in the nozzle were set according to specification (Table 1). Results of the calculations showed that at the end
of the spray phases obtained pressure values are similar to the experimental
ones in both tests, but in the beginning of the spray phase the pressure
decreases too slow compared to the experiment and, accordingly, too fast in the
second part of the phase. The performed analyses show that more modelling
efforts and clear definition of initial and boundary conditions of experiments
are required even for lumped parameter codes in order to perform a detailed and
reliable modelling of the experiments. In the future, the calculations of tests
will be performed by modelling M5 experiment, which preceded MASPn tests. When
the M5 experiment is simulated and calculated stratification conditions fit the
measured values then the detailed analysis of sprays on the destruction of
atmosphere stratification could be estimated.Figure 12
Pressure evolutions calculated with decreased junction area behind lower condenser.


## 5. Conclusions

The analysis of
MASPn experiments was performed using the lumped-parameter code COCOSYS. The
influence of different experimental parameters and modelling assumptions was investigated in order to determine the
most important parameters that influence the depressurisation rate during the MASP0
experiment performed in the frames of spray benchmark in MISTRA test facility.The performed
analysis showed that a clear definition of the initial and boundary conditions,
including developed gas stratification, of the experiment is required in order
to develop a nodalisation, which could be used for simulation of experiments.None of the
considered modelling parameters (loss coefficient in the junctions,
characteristic length of structure) had major influence on the calculated
results.Variation of the
atmosphere stratification conditions could not reproduce the measured
depressurisation rate.The performed
simulations of MASPn experiments and obtained results show that the calculated
heat losses through the external walls behind the lower condenser installed in
the MISTRA facility determines the long-term depressurisation rate.



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*Source: 896406-2008-10-28.xml*