Analysis of in-vessel accident progression in VVER1000 NPP during SBO accident with external reactor vessel cooling method

 of 
the segments slightly decreased after the 
depressurization, they, however, started to 
DOAN MANH LONG et al.
9
increase right after appearance of core debris in 
lower plenum. Generally, the first five 
segments were imposed thermal load more 
than others which led to higher evolution of 
their temperatures compared to others. Since 
20000 seconds, temperatures of the first five 
segments were over 1000oK which initiated the 
thermal strain of the lower head vessel at the 
segments. The evolutions of thermal strain 
fraction at 9 segments are displayed in Fig. 12. 
Among the first five segments, the 3rd segment 
seemed to be imposed the highest heat hence
its temperature was the strongest increase. Due 
to experience of the highest thermal load, the 
evolution of elastic strain of the 3rd segment 
was the sharpest increase and reached to the 
value of 0.18 at 21938 seconds (6 hours) since 
initiation of the accident, marked the failure of 
lower head vessel.
Fig. 12. Elastic strain of 9 segments
Table IV. Mass of debris in lower plenum
Debris Mass (kg)
UO2 80600
Zr 18150
ZrO2 8800
Steel 31000
Oxide of steel 2750
Fig. 10 indicates the relocation of core 
fuels started from 10864 seconds and stopped 
at 15000 seconds, and together with other 
components such as ZrO2, Zr and oxides of 
steel was stable after 15000 seconds. 
Meanwhile, the evolution of steel debris kept 
on going due to the collapse of supporting 
structure in lower plenum and stabilized after 
20000 seconds. The results showed all core 
fuels relocated to lower plenum and according 
to time relocation of core fuels the decay heat 
power deposited in debris bed with elimination 
of decay power of volatile gases varied from 
32 MW to 29 MW. At the time of lower head 
failure, also end of simulation time, total mass 
of debris components, predicted by MELCOR
v1.8.6, are listed in Table IV.
A code-to-code comparison for the main 
events of in-vessel accident progress between 
MELCOR code in this work and ASTEC in [14] 
was presented in Table V. The comparison 
shows the considerable differences of results 
obtained in both codes. The prediction of 
ASTEC on relocation of core melt to lower 
plenum was at 9705 seconds which is 1141 
seconds earlier than MELCOR prediction 
which was at 10846 seconds. As a result, the 
failure of lower head vessel predicted by 
ASTEC was at 20886 seconds which is 1052 
seconds earlier that MELCOR prediction 
which was at 21938 seconds. However, 
MELCOR results predicted a larger mass of 
corium in lower plenum than ASTEC results 
which were 131 tons and 73.3 tons respectively. 
The differences in the time of main 
events were caused by considerable differences 
in calculation models in two codes. Besides the 
reactor trip assumption of ASTEC also 
contributed to differences in predictions when 
it was assumed to happen at the same time with 
initiation of SBO accident which is unrealistic. 
ANALYSIS OF IN-
10
In addition, the difference in model of core 
debris relocation to lower plenum caused a 
significant difference in prediction of mass of 
UO2 relocating to lower plenum. In MELCOR 
code, the relocation of core debris was 
triggered when the lower core support plate 
was failed by both mechanical and thermal 
load, and when the lower core support plate in 
a ring failed and lost capability of supporting, 
its entire structure and all material resting on it 
would totally collapse and relocate to lower 
plenum. Meanwhile, ASTEC code [16] did not 
predict the mechanical failure of the lower core 
support plate but only its thermal failure, and 
the core debris only relocated to lower plenum 
by melting the plate and crossing through the 
plate hole to lower plenum. In this case, 
results of MELCOR simulation showed the 
lower core support plate in five rings 
completely collapsed, therefore all core fuel 
relocated to lower plenum. In case of ASTEC, 
the mass of core debris relocating to lower 
plenum depended on the size of the hole in the 
plate hence somehow the size was not large 
enough for all core fuel relocating to lower 
plenum in this scenario. 
Table V. MELCOR and ASTEC main events
Main events Time (second)
MELCOR ASTEC
Initiation of accident 0.0 0.0
Reactor tripped 1.6 0.0
Opening of steam-dump to atmosphere valves (BRU- A) 8.0 45.0
Begin of core uncovery 2300 -
Heating up of the core 4000 8767
Total core uncovery 4100 -
Steam dry-out 8000 3800.0
Closing of BRU-A valves 8000 7200
Initiation of primary depressurization 9727 10341
Begin of water injection from HACCs 9910 10514
Stop of HACCs injection 9990 17545
First material slump in lower plenum 10846 9705
Failure of lower head vessel (CREEP-RUPTURE) 21938 20886
Total mass of corium in lower head (tons) 131 73.33
B. Deployment of IVR strategy with ERVC
In this study, the external vessel cooling 
strategy was adopted in order to cool down 
the hot debris from external surface of the 
VVER1000 lower head vessel by injecting 
water into the cavity. The water injection was 
initiated when the vapor temperature in 
reactor core exceeded 650oC. The water level 
in reactor cavity was maintained at the height 
of cold leg. Until now, there has not been 
official design of ERVC for VVER1000 
reactor. Therefore, the study proposed a 
simplified scheme of ERVC strategy for 
VVER1000 reactor which included a cooling 
channel formed by reactor vessel and a frame 
structure, and an unlimited water resource. 
Fig. 13 demonstrated a nodalizational scheme 
of the ERVC in MELCOR code. 
DOAN MANH LONG et al.
11
The SBO accident was re-simulated with 
deployment of ERVC strategy. The results 
showed the performance of ERVC strategy did 
not affect the in-core accident progress. The 
evolution of the accident remained the same as 
the scenario without implementing ERVC 
strategy. A comparison of main events between 
two SBO scenarios with and without deploying 
IVR strategy is presented in Table VI. 
The initiation of ERVC strategy was 
simultaneously happened at 9727 seconds with 
the primary depressurization when vapor 
temperature in reactor core exceeded 650oC. 
Fig. 14 demonstrated water level in reactor 
cavity and the water level reached the height of 
cold leg at 12500 seconds, then it was 
maintained at the level. Figs. 15-17 displayed 
the temperature of 9 segments of VVER1000 
lower head wall in two both scenarios. The 
solid lines and the dash lines present the results 
of the scenario without ERVC strategy and 
with ERVC strategy respectively. The figures 
indicate the decrease of temperature of lower 
head walls in both cases happened right after 
the initiation of depressurization at 9727 
seconds. However, in case of implementing 
ERVC (dash line) the temperature of lower 
head at 9 segments decreased deeper. 
Fig. 13. A nodalization for external reactor vessel 
cooling strategy
Fig. 14. Water level in cooling channel and cavity
Table VI. Main events of in-vessel accident progression
Main events Time (seconds)
With IVR Without IVR
Initiation of accident 0.0 0.0
Reactor tripped 1.6 1.6
Begin of core uncovery 2300 2300
Total core uncovery 4100 4100
Initiation of primary depressurization 9727 9727
Start of cavity flooding 9727 9727
Begin of water injection from HACCs 9910 9910
Stop of HACCs injection 9990 9990
Start of oxidation 10000 10000
Start of fuel cladding failure 10300 10300
Failure of lower core plate 10846 10846
Failure of lower head vessel (CREEP-RUPTURE) 27341 21938
ANALYSIS OF IN-
12
Fig. 15. Temperature of segments 1-3
Fig. 15 and Fig. 16 indicate the 
temperature at segments 1-6 started to increase 
at 15000 seconds, and it strongly increased at 
17000 seconds. Meanwhile, Fig. 17 shows the 
temperature of lower head wall at segments 7-9 
was efficiently cooled and kept at low 
temperature below 600oC. Together with the 
increase of temperature of segments 1-6, the 
heat transfer between lower head vessel and 
water drastically increased as well, which 
caused the vibration of water level in cooling 
channel (Fig. 14). Fig. 16 shows the temperature 
of segments 4-6 was kept under 1000oK (727oC). 
Fig. 16. Evolution of temperature of segments 4-6
However, Fig. 15 indicates the 
temperature of segments 1-3 increased beyond 
1000oK, among them the temperature of 
segments 1 and 3 were beyond 1000oK at 
24000 and 26000 seconds respectively which 
caused the occurrence of thermal strain of the 
segments. Fig. 18 displayed the evolution of 
thermal strain fraction of all segments. The 
results show the thermal strain of lower head at 
segment 1 reached the value of 0.18 at 27341
seconds which marked the failure of 
VVER1000 lower head reactor vessel.
Fig. 17. Evolution of temperature of segments 7-9
Fig. 18. Evolution of thermal strain of segments
DOAN MANH LONG et al.
13
IV. CONCLUDING REMARKS
In this paper, the in-vessel accident 
progress during the Station Black-Out (SBO) 
accident combined with externally vessel 
cooling was analyzed for VVER1000 reactor 
by using MELCOR v.1.8.6 code. Some 
conclusions were given as following: 
Under SBO accident, only water 
from four hydro-accumulators could not 
prevent the collapse of reactor core and 
relocation of core debris to lower plenum. The 
collapse of reactor core marked by failure of 
fuel cladding was occurred at 10300 seconds 
(2.86 hours), the appearance of first core debris 
in lower plenum was at 10846 seconds (3.01 
hours), and all core fuels relocated to lower 
plenum at 15000 seconds (4.2 hours). The 
decay heat power deposited in debris bed 
locating in lower plenum was estimated from 
32 to 29 MW with taking the elimination of 
decay power of volatile gases into 
consideration. All the outcomes would be 
significant information for further study on 
thermal behavior of debris bed/molten pool in 
lower plenum and thermal response of 
VVER1000 lower head vessel as well;
In the base case without 
implementing IVR strategy, the VVER1000 
lower head vessel was failed at 21938 seconds 
(6.09 hours) due to thermal creep-rupture. 
Meanwhile, in the case of IVR strategy 
deployment, the results indicated the strategy 
could not prevent VVER1000 lower head 
vessel from failure caused by thermal creep-
rupture and only prolonged the existence of 
VVER1000 lower head vessel for 5437 
seconds (1.5 hours) compared to the base case; 
The flat shape of VVER1000 lower 
head vessel also raises a concern about the 
occurrence of failure at low position when the 
critical heat fluxes at the low positions were 
low as seen in results of ULPU [7]. The 
results of the study also showed the failure of 
VVER1000 lower head vessel happened at 
low positions, therefore, it is necessary to 
optimize geometry of cooling channel for 
VVER1000 reactor; 
In addition, the comparison on the 
main phenomena during accident progress 
between MELCOR and ASTEC suggested it is 
necessary to take further investigation on in-
vessel accident progression in case of SBO 
accident by other severe accident codes in 
order to set up a range of key parameters for 
the bounding configuration of molten pool in 
VVER1000 lower plenum and specify decay 
heat power deposited molten pool as well. 
ABBREVIATIONS
AMM Accident Management Measures
BRU-A A kind of atmospheric dump 
valves of steam generators
CC Cold Collector
CL Cold Leg
CP Cold Part
CV Control Volume
ERVC External Reactor Vessel Cooling
HACCs Hydro-ACCumulators
HC Hot Collector
HL Hot Leg
HP Hot Part
IVR In-Vessel melt Retention
LBLOCA Large Break Lost of Coolant 
Accident
LP Lower Plenum
MSH Main Steam Header
NPP Nuclear Power Plant
ANALYSIS OF IN-
14
PORV Pilot-Operated Relief Valve
PRZ PRessuriZer
SBO Station Black-Out
SG Steam Generator
SL Steam Line
UP Upper Plenum
ACKNOWLEDGEMENT
The authors are grateful for financial 
funding of Ministry of Science and Technology
and Vietnam Atomic Energy Institute. This 
work was performed under ministerial project 
on 
05/01/2018.
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