- Email: [email protected]
Calculation of LOFT L3-5 experiment with CATHARE
Nuclear Engineering and Design 102 (1987) 177-181 North-Holland, Amsterdam
177
CALCULATION OF LOFT L3-5 EXPERIMENT WITH CATHARE F. B A R R E , Ch. C H A U L I A C and B. N O E L CE.4-Centre d'Etudes Nucl$aires de Grenoble, D$partement des R$acteurs ~ Eau, Service d'Etudes Thermohydrauliques, Post Box 85X, 38041 Grenoble Cedex, France
This paper presents some results obtained in calculating the LOFT L3-5 experiment (2.5% cold leg break with pumps off) with CATHARE. A basic calculation with detailed meshing is first presented. Then sensitivity tests are shown on a simplified geometry. These tests include better description of the test facility and improvements in CATHARE physical laws.
1. Introduction
The CATHARE code is a best estimate code for simulation of PWR loss-of-coolant accidents (large break, small break) and transients. It is developed in Grenoble by the French CEA *, EDF ** and FRAMATOME ***. It can describe any topology by associating modules representing different components of a circuit. The basic module takes into account a one dimensional, two fluid six equation model with mechanical and thermal non-equilibrium. It can be associated with different sub-modules such as pump, steam generator, wall 1D or 2D heat conduction, reactor kinetics, fuel pin, accumulator... Other modules available are: capacity module (two nodes model with 2 subvolumes and 1 level, low velocities and thermal non equilibrium), tee module and boundary condition module. The space discretisation uses the sta~ered mesh and donor cell technique. Time discretisation is fully implicit. The assessment strategy is based on two activities: - Qualification which deals with separate effects tests. More than 15 different test sections and more than 110 tests were calculated in this program. The following topics were studied: critical flow, blowdown of heated or unheated test sections, boil-off, reflood, SG behaviour (primary and secondary side) in small break LOCA conditions. - Verification which performs calculation of integral
* CEA = French Atomic Energy Commission. ** EDF= French utility. *** FRAMATOME = French vendor.
test: LOBI, PKL, LOFT, BETHSY are under way at the moment. ROSA IV is planned for the future. In this strategy, qualification is the first step during which physical correlations are checked. The Second step, verification, uses those correlations in more complex geometry and boundary conditions. If any discrepancies are identified and can be attributed to physical models, the general way is to return to separate effect tests (if they exist) in order to understand the physical phenomena in more simple experimental conditions so that correlations can be improved. The first test calculated in the verification program was LOFT L3-5. This experiment simulates a 4 in-diameter break on a four-loop PWR. The break is located on the intact loop cold leg between the primary coolant pumps and the reactor vessel. It is a pump off test with pump trip at break opening. Calculations of L3-5 were performed in two steps. First, a basic calculation was realized with detailed meshing of the test facility. Second, geometry and meshing of the test facility were simplified, agreement with former calculation was checked and sensitivity studies were performed. The results of those calculations are presented here. 2. Basic calculation ("A" calculation)
In this first calculation detailed description of the test facility is looked for (fig. 1). 30 elements are used: 15 pipes (using the basic module) for primary loops, downcomer, core and core-bypass; 7 capacities for lower and upper plena, vessel bottom head and steam generator channel heads; 4 tees for Reflood Assist Bypass Valve (RABV), pressurizer and break branching;
0 0 2 9 - 5 4 9 3 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
178
/ Calculation
F. Barre et aL
of L O F T L 3 - 5 experiment with CA T H A R E " COLD{LEG DEI;WSITY (W(]/~)
t
600
•
" HOT LEG DEhSITY [
'
(kq/m3)
1
600
~'"- " . .... .-"-~.~
400. "....
i.!,', \ ,i,~.,.. ,~,% I T,'MF ~o 400 &~o'~,,,:/~X~
!,~
riME|
zoo 400 ~ ' ~ 1 1 ]
Fig. 3. Density in hot and cold leg in "A" and "B" calculations. - experiment, - . . . . . A calculation, - . . . . . B calculation.
400
1 Steam-generator (secondary side calculated with O-D module in which temperature and mass flow-rate of the feedwater and heat transfer coefficient on secondary side are imposed); 3 boundary conditions. The total n u m b e r of meshes is 188. Results of that calculation are shown in figs. 2 and 3. The calculated primary pressure is not far from the experimental one but a small increase in calculated pressure is observed between time 100 and 300 s. It does exist in experiment and this discrepancy seems to be due first to a too low heat exchange at steam generator and second to a too low vapour flow rate at the break. Difference between calculated and experimental pressure in steam generator secondary side is significant. Before time 100 s, it can be explained by two reasons: a bad knowledge about experimental steam leakage through Steam Flow Control Valve (SFCV) and contradiction between the experimental SFCV opening set point (7.11 MPa) and the measured secondary pressure (opening set point < 7 MPa). After time 100 s differences can be explained by an error in the input data deck (auxiliary feedwater temperature is 460 K instead of 300 K used in the calculation). Flow rate is overestimated up to 400 s. This is mainly due to an excessive density in cold leg.
200.
3. Simplification of geometry and meshing
Fig. 1. LOFT diagrams for calculations.
SECONDARY PRESSURE
PRIMARY PRESSURE (MPo)
(MPo) 8.
,~
TIME
• 2oo. ,too. 690 l,)
7¸
. 200
.
4oo.
60o
TIME .I,)
DENSITY IN BREAK LINE {kq/m ~) BREAK FLOW RATE (.kq/s) 16 600
, k~ : ,
II .:-. ""'" :1"..."" 2,, ,
200
400
....
600
4.
($)
,
. .-
..
:"
,
: ~(,~E
Fig. 2. Results of "A" and "B" calculations. - experiment, - . . . . . A calculation, . . . . . . B calculation.
Main tation of As it effect in
differences between former and new representhe test facility are shown in fig. 1. was noticed that dead legs had a very small previous calculation, they are here much sim-
F. Barre et al. / Calculation of LOFT L 3 - 5 experiment with CATHARE
plified: the RABV line is now suppressed and a capacity module is used to modelize dead steam generator and dead pump. The number of capacities is decreased: vessel bottom head and lower plenum are now described together in one capacity and the steam generator channel heads are calculated with the axial basic module. This permits to decrease the number of elements involved in calculation. In this new representation meshing is decreased from 188 to 117 nodes, but the same meshing is maintained in the break pipe. A new calculation was performed with this new representation and it was checked that no significant difference appeared between former and new representation. As the mean time step was increased in a factor of two and CPU time decreased in the same order of magnitude it was decided to use this new representation for sensitivity tests. According to the lacks noticed in the basic calculation the aims of those tests were to handle the more sensitive factors permitting to: - decrease break flow rate before time 400 s, - increase secondary pressure after time 200 s, - change the levels in downcomer and upper plenum in order to improve density calculation in hot and cold active legs. A good primary pressure calculation was expected to be a consequence of the above improvements. As L3-5 main events occur before time 600 s it was decided to stop those sensitivity tests at that time.
4. First sensitivity test ("B" calculation) The differences between "A" and "B" calculations are the following: - The two bypasses which experimentally exist between upper plenum and downcomer are separately modelized: The first one is located at the level of hot and cold legs. The second one opens a communication between upper plenum and downcomer tops [1]. As stratification occurs very early in those capacities, these two bypasses have different behaviour throughout the transient. Particularly the second one brings vapour from upper plenum to downcomer (and then to the break) very early in the transient. This is expected to fulfil at least the first of the objectives listed above (section 3). In the former calculation only the first bypass was modelized. In this sensitivity test the same mass flow rate is assumed to flow between the two capacities but it is divided between the two bypasses, half on each one. - The second difference introduced in this sensitivity
179
test deals with C A T H A R E physical laws. Revision 2 of CATHARE (used in former calculation) calculated heat transfer between wall and fluid in forced convection conditions with the Colburn correlation. In this correlation the length dimension used in Nusselt number is the hydraulic diameter. In the qualification process of this revision, calculation of the Patricia GVI experiment (condensation on the primary side of a steam generator) showed that heat exchange coefficient is underestimated at high void fraction [2]. In order to better fit the experimental results, it was necessary to use a more realistic length dimension (twice the film thickness) in Nusselt number. This modification leads to a higher heat transfer coefficient on primary side. It is expected to induce an increase of secondary pressure after 200 s. - Other improvements introduced here are corrections of errors in input data deck: auxiliary feed water temperature, SFCV opening set point, steam leakage through SFCV before time 100 s (section 2). Results are shown in figs. 2 and 3. Second bypass between upper plenum and downcomer permits more vapour to flow earlier towards the break. It decreases mass flow rate at the break and draws it nearer to the experimental value: calculated mass flow rate gets into the experimental uncertainty after time 80 s. Density in break line is also corrected predicted after time 120 s. Secondary pressure is better estimated. After time 150 s it is slightly overestimated (up to 0.2 MPa) but the experimental uncertainty is about 0.12 MPa. Primary pressure calculation is also improved. Increasing vapour mass flow rate at the break and heat transfer at steam generator (from primary to secondary side) results in eliminating pressure increase noticed between times 100 and 300 s in former calculation. The pressure slope is right after time 3 seconds. Nevertheless three problems still remain: - cold leg density is underestimated for a large time, - density in break line drops too early in the transient, - hot leg density is slightly overestimated and drops too late in the transient. The first two items are related to calculation of liquid and vapour entrainment through the break tee towards the break under stratified conditions in cold leg pipe. This will be discussed in section 6 ("D" calculation). The last item is studied in the next section.
5. Second sensitivity test ("C" calculation) Improvement in the hot leg density prediction is here looked for. In the hot leg, the flow is stratified and the
F. Barre et al. / Calculation of L O F T L 3 - 5 experiment with CA T H A R E
180
(MPo)
SECONDARY PRESSURE (MPo) 8
J
7
, ,L~O , 400
600
~o,
,~o, 6oo ,t,,
,(=)l
Fig. 4. Results of "C" and "D" calculations: Primary and secondary pressure, - experiment, - . . . . . C calculation, . . . . . . D calculation. density is directly related to fluid level in upper plenum. Among other parameters, this level is a result of void fraction in the core. And particularly void fraction profile depends on interfacial friction. In the Revision 2 of CATHARE the interracial friction correlation for bubble-slug flow is derived from the Zuber-Findlay drift flux model with some adjustments • DENS'ITYIN'BREAK'LINE (kq/m 3)
BREAK FLOW RATE (k¢/s) •
600
~t
400
12
~
B-
J
:~ •-
•.
, 200
,', i
, ( %,
200
In calculation "B", the prediction of mass flow rate at the break is good, but this result is due to compensating errors: - stagnation conditions are wrong (cold leg density too low), - liquid entrainment and vapour pull-through at the break Tee is not correctly calculated. At the moment, any phase separation effect does not exist in the Tee module of CATHARE. An experimental program is now underway on the Super Moby Dick test facility (Grenoble) in order to study two-phase flow in Tees with stratified flow in the
1/3
entra~ j J liquid
HOT'LEG D(.N$1TY (kq/rr#) ,
COLD"LEG DENSITY kq/m 3)
~.1.~.~ .. .' , . lmk~ttrflt.. .~.gii.'='¢:.
~
aJ
600,
400,
TIME B,OOI,~
6. Third sensitivity test ("D" calculation)
TIME (s)
TIME 400 . 600 .Is]
~o ~
found necessary for qualification of the Dadine experiment. This correlation applies very well to many different experiments selected in the qualification program of CATHARE. Nevertheless some problems raised about calculation of swell level for boil-off experiments. Although the swell level was correctly calculated in tubular test sections, large discrepancies were noticed for rod bundles (G2, Ersec, Patricia GV2). A systematic research for different rod bundle test sections [3] led to a new correlation for interfacial friction factor in rod bundles. So, as a result of the qualification process, this second sensitivity test uses this correlation for the core (added to all the changes chosen for first test). Results are shown in figs. 4 and 5. A core void fraction decrease is observed which lowers fluid level in upper plenum• This helps hot legs to empty faster and improves hot leg density prediction.
200
• QO
0.5
/ / /~/
/
vapour pull-through
(VPT)
111 TI ME
2,0o 4o,0 ,~
Fig. 5. Results of "C" and "B" calculations. - experiment, - . . . . . C calculation, • . . . . . D calculation.
I
I
~=~ "-CIO
0.5
Fig. 6. Phase separation in Tee module.
F. Barre et al. / Calculation of L O F T L 3 - 5 experiment with CA THA R E
main pipe. It will permit better understanding of the physical phenomena involved in the Tees and lead to a phase separation model. However, as this model is not yet available, the experimental results of L3-5 about density in cold leg and break line were here taken into account in order to derive a phase separation relation in terms of void fraction at the secondary pipe (a3) versus void fraction at the main pipe (a0) (fig. 6). This simple data reduction shows quite clearly the effect of liquid entrainment and vapour pull-through. The aim of this third sensitivity test was mainly to improve cold leg calculation. So the part of the curve corresponding to vapour pull-through (broken line) was not kept and a 3 = a 0 was used for a 0 < 30%. Nevertheless it must be noticed that use of the VPT part of the curve would improve calculation of primary pressure and density in break line before time 150 s and increase discrepancy about break flow for the same period of time. Finally " D " calculation uses the following changes: - all conditions of test "C", - solid line of fig. 6 as phase separation model, - more fine meshing of the break line. The results are shown in figs. 4 and 6. The difference with the previous calculations is that, for a given density in the cold leg, flow rate at the break is now less important. This permits more liquid to accumulate in the cold leg and increases cold leg density. Break flow rate and cold leg density are now very well calculated. Other parameters are as well predicted as in previous calculation except for hot leg density which drops a little later. This last discrepancy is probably connected to the value of mass flow rate flowing through the upper bypass between upper plenum and downcomer (value of 3.3% in liquid steady state conditions was arbitrarily chosen because it is not measured): difference between tests "A" and "B" shows that the introduction of that bypass does not change much the initial value of hot leg density but delays greatly density drop. So, flow leakage through that bypass seems to be the most sensible parameter. Finally, some small discrepancies still exist between experiment and calculation but they are not much im-
181
portant: secondary pressure is slightly overestimated but experimental leak through SFCV is not well known; primary pressure and density in break line are underestimated before time 150 s but they could be easily improved by taking into account vapour pull through law (fig. 6). The threshold of vapour entrainment occurs at about 20% of void fraction.
7. C o n c l u s i o n
Better understanding of L3-5 was made possible through different sensitivity tests. Sensitive geometrical or physical parameters have been pointed out: • The two bypasses between upper plenum and downcomer have to be modelized. • Rod bundle interracial friction derived from G2, Ersec and Patricia GV2 experiments improves hot leg density calculation. • Right treatment of phase separation in the break Tee improves cold leg density calculation. The sensitivity study about geometrical parameters shows that the code does not predict in itself good trends: the user's effect can be important. The study of the influence of physical parameters provides an illustration of the French assessment philosophy: in particular, the return to separate effect tests (if they are available) when lacks in physical models are suspected. Finally, this paper shows that CATHARE is an interesting tool for LOCA analysis. Knowledge acquired through L35 study will be used for PWR LOCA calculations.
References
[1] D.L. Reeder, LOFT system and test description NUREG/CR-0247, TREE-1208 (July 1978). [2] Code CATHARE 1 - Version 1, Dossier de qualification de la rrvision 2 Tests Patricia GV1, Note TT/EM/84-22 (June 1984). [3] D. Bestion, Interracial friction determination for the ID-6 equations two fluid model used in the CATHARE Code, European Two-Phase Flow Group Meeting, Southampton, June 1985.
177
CALCULATION OF LOFT L3-5 EXPERIMENT WITH CATHARE F. B A R R E , Ch. C H A U L I A C and B. N O E L CE.4-Centre d'Etudes Nucl$aires de Grenoble, D$partement des R$acteurs ~ Eau, Service d'Etudes Thermohydrauliques, Post Box 85X, 38041 Grenoble Cedex, France
This paper presents some results obtained in calculating the LOFT L3-5 experiment (2.5% cold leg break with pumps off) with CATHARE. A basic calculation with detailed meshing is first presented. Then sensitivity tests are shown on a simplified geometry. These tests include better description of the test facility and improvements in CATHARE physical laws.
1. Introduction
The CATHARE code is a best estimate code for simulation of PWR loss-of-coolant accidents (large break, small break) and transients. It is developed in Grenoble by the French CEA *, EDF ** and FRAMATOME ***. It can describe any topology by associating modules representing different components of a circuit. The basic module takes into account a one dimensional, two fluid six equation model with mechanical and thermal non-equilibrium. It can be associated with different sub-modules such as pump, steam generator, wall 1D or 2D heat conduction, reactor kinetics, fuel pin, accumulator... Other modules available are: capacity module (two nodes model with 2 subvolumes and 1 level, low velocities and thermal non equilibrium), tee module and boundary condition module. The space discretisation uses the sta~ered mesh and donor cell technique. Time discretisation is fully implicit. The assessment strategy is based on two activities: - Qualification which deals with separate effects tests. More than 15 different test sections and more than 110 tests were calculated in this program. The following topics were studied: critical flow, blowdown of heated or unheated test sections, boil-off, reflood, SG behaviour (primary and secondary side) in small break LOCA conditions. - Verification which performs calculation of integral
* CEA = French Atomic Energy Commission. ** EDF= French utility. *** FRAMATOME = French vendor.
test: LOBI, PKL, LOFT, BETHSY are under way at the moment. ROSA IV is planned for the future. In this strategy, qualification is the first step during which physical correlations are checked. The Second step, verification, uses those correlations in more complex geometry and boundary conditions. If any discrepancies are identified and can be attributed to physical models, the general way is to return to separate effect tests (if they exist) in order to understand the physical phenomena in more simple experimental conditions so that correlations can be improved. The first test calculated in the verification program was LOFT L3-5. This experiment simulates a 4 in-diameter break on a four-loop PWR. The break is located on the intact loop cold leg between the primary coolant pumps and the reactor vessel. It is a pump off test with pump trip at break opening. Calculations of L3-5 were performed in two steps. First, a basic calculation was realized with detailed meshing of the test facility. Second, geometry and meshing of the test facility were simplified, agreement with former calculation was checked and sensitivity studies were performed. The results of those calculations are presented here. 2. Basic calculation ("A" calculation)
In this first calculation detailed description of the test facility is looked for (fig. 1). 30 elements are used: 15 pipes (using the basic module) for primary loops, downcomer, core and core-bypass; 7 capacities for lower and upper plena, vessel bottom head and steam generator channel heads; 4 tees for Reflood Assist Bypass Valve (RABV), pressurizer and break branching;
0 0 2 9 - 5 4 9 3 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
178
/ Calculation
F. Barre et aL
of L O F T L 3 - 5 experiment with CA T H A R E " COLD{LEG DEI;WSITY (W(]/~)
t
600
•
" HOT LEG DEhSITY [
'
(kq/m3)
1
600
~'"- " . .... .-"-~.~
400. "....
i.!,', \ ,i,~.,.. ,~,% I T,'MF ~o 400 &~o'~,,,:/~X~
!,~
riME|
zoo 400 ~ ' ~ 1 1 ]
Fig. 3. Density in hot and cold leg in "A" and "B" calculations. - experiment, - . . . . . A calculation, - . . . . . B calculation.
400
1 Steam-generator (secondary side calculated with O-D module in which temperature and mass flow-rate of the feedwater and heat transfer coefficient on secondary side are imposed); 3 boundary conditions. The total n u m b e r of meshes is 188. Results of that calculation are shown in figs. 2 and 3. The calculated primary pressure is not far from the experimental one but a small increase in calculated pressure is observed between time 100 and 300 s. It does exist in experiment and this discrepancy seems to be due first to a too low heat exchange at steam generator and second to a too low vapour flow rate at the break. Difference between calculated and experimental pressure in steam generator secondary side is significant. Before time 100 s, it can be explained by two reasons: a bad knowledge about experimental steam leakage through Steam Flow Control Valve (SFCV) and contradiction between the experimental SFCV opening set point (7.11 MPa) and the measured secondary pressure (opening set point < 7 MPa). After time 100 s differences can be explained by an error in the input data deck (auxiliary feedwater temperature is 460 K instead of 300 K used in the calculation). Flow rate is overestimated up to 400 s. This is mainly due to an excessive density in cold leg.
200.
3. Simplification of geometry and meshing
Fig. 1. LOFT diagrams for calculations.
SECONDARY PRESSURE
PRIMARY PRESSURE (MPo)
(MPo) 8.
,~
TIME
• 2oo. ,too. 690 l,)
7¸
. 200
.
4oo.
60o
TIME .I,)
DENSITY IN BREAK LINE {kq/m ~) BREAK FLOW RATE (.kq/s) 16 600
, k~ : ,
II .:-. ""'" :1"..."" 2,, ,
200
400
....
600
4.
($)
,
. .-
..
:"
,
: ~(,~E
Fig. 2. Results of "A" and "B" calculations. - experiment, - . . . . . A calculation, . . . . . . B calculation.
Main tation of As it effect in
differences between former and new representhe test facility are shown in fig. 1. was noticed that dead legs had a very small previous calculation, they are here much sim-
F. Barre et al. / Calculation of LOFT L 3 - 5 experiment with CATHARE
plified: the RABV line is now suppressed and a capacity module is used to modelize dead steam generator and dead pump. The number of capacities is decreased: vessel bottom head and lower plenum are now described together in one capacity and the steam generator channel heads are calculated with the axial basic module. This permits to decrease the number of elements involved in calculation. In this new representation meshing is decreased from 188 to 117 nodes, but the same meshing is maintained in the break pipe. A new calculation was performed with this new representation and it was checked that no significant difference appeared between former and new representation. As the mean time step was increased in a factor of two and CPU time decreased in the same order of magnitude it was decided to use this new representation for sensitivity tests. According to the lacks noticed in the basic calculation the aims of those tests were to handle the more sensitive factors permitting to: - decrease break flow rate before time 400 s, - increase secondary pressure after time 200 s, - change the levels in downcomer and upper plenum in order to improve density calculation in hot and cold active legs. A good primary pressure calculation was expected to be a consequence of the above improvements. As L3-5 main events occur before time 600 s it was decided to stop those sensitivity tests at that time.
4. First sensitivity test ("B" calculation) The differences between "A" and "B" calculations are the following: - The two bypasses which experimentally exist between upper plenum and downcomer are separately modelized: The first one is located at the level of hot and cold legs. The second one opens a communication between upper plenum and downcomer tops [1]. As stratification occurs very early in those capacities, these two bypasses have different behaviour throughout the transient. Particularly the second one brings vapour from upper plenum to downcomer (and then to the break) very early in the transient. This is expected to fulfil at least the first of the objectives listed above (section 3). In the former calculation only the first bypass was modelized. In this sensitivity test the same mass flow rate is assumed to flow between the two capacities but it is divided between the two bypasses, half on each one. - The second difference introduced in this sensitivity
179
test deals with C A T H A R E physical laws. Revision 2 of CATHARE (used in former calculation) calculated heat transfer between wall and fluid in forced convection conditions with the Colburn correlation. In this correlation the length dimension used in Nusselt number is the hydraulic diameter. In the qualification process of this revision, calculation of the Patricia GVI experiment (condensation on the primary side of a steam generator) showed that heat exchange coefficient is underestimated at high void fraction [2]. In order to better fit the experimental results, it was necessary to use a more realistic length dimension (twice the film thickness) in Nusselt number. This modification leads to a higher heat transfer coefficient on primary side. It is expected to induce an increase of secondary pressure after 200 s. - Other improvements introduced here are corrections of errors in input data deck: auxiliary feed water temperature, SFCV opening set point, steam leakage through SFCV before time 100 s (section 2). Results are shown in figs. 2 and 3. Second bypass between upper plenum and downcomer permits more vapour to flow earlier towards the break. It decreases mass flow rate at the break and draws it nearer to the experimental value: calculated mass flow rate gets into the experimental uncertainty after time 80 s. Density in break line is also corrected predicted after time 120 s. Secondary pressure is better estimated. After time 150 s it is slightly overestimated (up to 0.2 MPa) but the experimental uncertainty is about 0.12 MPa. Primary pressure calculation is also improved. Increasing vapour mass flow rate at the break and heat transfer at steam generator (from primary to secondary side) results in eliminating pressure increase noticed between times 100 and 300 s in former calculation. The pressure slope is right after time 3 seconds. Nevertheless three problems still remain: - cold leg density is underestimated for a large time, - density in break line drops too early in the transient, - hot leg density is slightly overestimated and drops too late in the transient. The first two items are related to calculation of liquid and vapour entrainment through the break tee towards the break under stratified conditions in cold leg pipe. This will be discussed in section 6 ("D" calculation). The last item is studied in the next section.
5. Second sensitivity test ("C" calculation) Improvement in the hot leg density prediction is here looked for. In the hot leg, the flow is stratified and the
F. Barre et al. / Calculation of L O F T L 3 - 5 experiment with CA T H A R E
180
(MPo)
SECONDARY PRESSURE (MPo) 8
J
7
, ,L~O , 400
600
~o,
,~o, 6oo ,t,,
,(=)l
Fig. 4. Results of "C" and "D" calculations: Primary and secondary pressure, - experiment, - . . . . . C calculation, . . . . . . D calculation. density is directly related to fluid level in upper plenum. Among other parameters, this level is a result of void fraction in the core. And particularly void fraction profile depends on interfacial friction. In the Revision 2 of CATHARE the interracial friction correlation for bubble-slug flow is derived from the Zuber-Findlay drift flux model with some adjustments • DENS'ITYIN'BREAK'LINE (kq/m 3)
BREAK FLOW RATE (k¢/s) •
600
~t
400
12
~
B-
J
:~ •-
•.
, 200
,', i
, ( %,
200
In calculation "B", the prediction of mass flow rate at the break is good, but this result is due to compensating errors: - stagnation conditions are wrong (cold leg density too low), - liquid entrainment and vapour pull-through at the break Tee is not correctly calculated. At the moment, any phase separation effect does not exist in the Tee module of CATHARE. An experimental program is now underway on the Super Moby Dick test facility (Grenoble) in order to study two-phase flow in Tees with stratified flow in the
1/3
entra~ j J liquid
HOT'LEG D(.N$1TY (kq/rr#) ,
COLD"LEG DENSITY kq/m 3)
~.1.~.~ .. .' , . lmk~ttrflt.. .~.gii.'='¢:.
~
aJ
600,
400,
TIME B,OOI,~
6. Third sensitivity test ("D" calculation)
TIME (s)
TIME 400 . 600 .Is]
~o ~
found necessary for qualification of the Dadine experiment. This correlation applies very well to many different experiments selected in the qualification program of CATHARE. Nevertheless some problems raised about calculation of swell level for boil-off experiments. Although the swell level was correctly calculated in tubular test sections, large discrepancies were noticed for rod bundles (G2, Ersec, Patricia GV2). A systematic research for different rod bundle test sections [3] led to a new correlation for interfacial friction factor in rod bundles. So, as a result of the qualification process, this second sensitivity test uses this correlation for the core (added to all the changes chosen for first test). Results are shown in figs. 4 and 5. A core void fraction decrease is observed which lowers fluid level in upper plenum• This helps hot legs to empty faster and improves hot leg density prediction.
200
• QO
0.5
/ / /~/
/
vapour pull-through
(VPT)
111 TI ME
2,0o 4o,0 ,~
Fig. 5. Results of "C" and "B" calculations. - experiment, - . . . . . C calculation, • . . . . . D calculation.
I
I
~=~ "-CIO
0.5
Fig. 6. Phase separation in Tee module.
F. Barre et al. / Calculation of L O F T L 3 - 5 experiment with CA THA R E
main pipe. It will permit better understanding of the physical phenomena involved in the Tees and lead to a phase separation model. However, as this model is not yet available, the experimental results of L3-5 about density in cold leg and break line were here taken into account in order to derive a phase separation relation in terms of void fraction at the secondary pipe (a3) versus void fraction at the main pipe (a0) (fig. 6). This simple data reduction shows quite clearly the effect of liquid entrainment and vapour pull-through. The aim of this third sensitivity test was mainly to improve cold leg calculation. So the part of the curve corresponding to vapour pull-through (broken line) was not kept and a 3 = a 0 was used for a 0 < 30%. Nevertheless it must be noticed that use of the VPT part of the curve would improve calculation of primary pressure and density in break line before time 150 s and increase discrepancy about break flow for the same period of time. Finally " D " calculation uses the following changes: - all conditions of test "C", - solid line of fig. 6 as phase separation model, - more fine meshing of the break line. The results are shown in figs. 4 and 6. The difference with the previous calculations is that, for a given density in the cold leg, flow rate at the break is now less important. This permits more liquid to accumulate in the cold leg and increases cold leg density. Break flow rate and cold leg density are now very well calculated. Other parameters are as well predicted as in previous calculation except for hot leg density which drops a little later. This last discrepancy is probably connected to the value of mass flow rate flowing through the upper bypass between upper plenum and downcomer (value of 3.3% in liquid steady state conditions was arbitrarily chosen because it is not measured): difference between tests "A" and "B" shows that the introduction of that bypass does not change much the initial value of hot leg density but delays greatly density drop. So, flow leakage through that bypass seems to be the most sensible parameter. Finally, some small discrepancies still exist between experiment and calculation but they are not much im-
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portant: secondary pressure is slightly overestimated but experimental leak through SFCV is not well known; primary pressure and density in break line are underestimated before time 150 s but they could be easily improved by taking into account vapour pull through law (fig. 6). The threshold of vapour entrainment occurs at about 20% of void fraction.
7. C o n c l u s i o n
Better understanding of L3-5 was made possible through different sensitivity tests. Sensitive geometrical or physical parameters have been pointed out: • The two bypasses between upper plenum and downcomer have to be modelized. • Rod bundle interracial friction derived from G2, Ersec and Patricia GV2 experiments improves hot leg density calculation. • Right treatment of phase separation in the break Tee improves cold leg density calculation. The sensitivity study about geometrical parameters shows that the code does not predict in itself good trends: the user's effect can be important. The study of the influence of physical parameters provides an illustration of the French assessment philosophy: in particular, the return to separate effect tests (if they are available) when lacks in physical models are suspected. Finally, this paper shows that CATHARE is an interesting tool for LOCA analysis. Knowledge acquired through L35 study will be used for PWR LOCA calculations.
References
[1] D.L. Reeder, LOFT system and test description NUREG/CR-0247, TREE-1208 (July 1978). [2] Code CATHARE 1 - Version 1, Dossier de qualification de la rrvision 2 Tests Patricia GV1, Note TT/EM/84-22 (June 1984). [3] D. Bestion, Interracial friction determination for the ID-6 equations two fluid model used in the CATHARE Code, European Two-Phase Flow Group Meeting, Southampton, June 1985.