ИЗСЛЕДВАНЕ ВЛИЯНИЕТО НА ДИСКРЕТИЗАЦИЯТА НА ДЪНОТО НА КОРПУСА НА РЕАКТОРА В МОДЕЛ НА ВВЕР-1000 ЗА КОМПЮТЪРЕН КОД

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1 ЕНЕРГИЕН ФОРУМ 2015 ИЗСЛЕДВАНЕ ВЛИЯНИЕТО НА ДИСКРЕТИЗАЦИЯТА НА ДЪНОТО НА КОРПУСА НА РЕАКТОРА В МОДЕЛ НА ВВЕР-1000 ЗА КОМПЮТЪРЕН КОД ASTECv2.0r3p2 ПО ВРЕМЕ НА ТЕЖКА АВАРИЯ С И БЕЗ ВЪНШНО ОХЛАЖДАНЕ НА КОРПУСА НА РЕАКТОРА Росица Генчева, Антоанета Стефанова и Павлин Грудев INVESTIGATION OF REACTOR VESSEL BOTTOM HEAD DISCRETISATION IN VVER-1000 ASTECv2.0r3p2 MODEL DURING A SEVERE ACCIDENT IN CASE OF IVMR PROCEDURE WITH AND WITHOUT EXTERNAL WATER COOLING Rositsa Gencheva, Antoaneta Stefanova and Pavlin Groudev 1. Abstract This paper presents the results from several calculations made with ASTECv2.0r3p2 and addressed to VVER-1000 reactor type. The main goal of this study is to investigate how the modelling of vessel bottom head discretization in ASTECv2.0r3p2 model influences on the vessel failure in case of severe accident. This investigation was done on the base of previous investigation of the applicability of the In-Vessel Melt Retention (IVMR) strategy with external vessel water cooling for the reactors of VVER-1000/320 type. Some stand-alone calculations have been done with ICARE module of the ASTECv2.0r3p2 computer code with and without external water cooling to predict the heat fluxes from the corium to the vessel and the heat fluxes from the vessel to the outside coolant. 2. Introduction Hypothetically in the case of severe accident the reactor core could be overheated and a mixture of molten materials coming from fuel, cladding and structural materials, and called corium will be formed. This corium can melt through the reactor vessel. The calculations presented hereafter investigate the reactor vessel bottom head failure after one big portion of corium is poured in it. The corium initial parameters are determined after a calculation of a scenario of Large Break LOCA (2 850 mm) with full station blackout (SBO) for VVER-1000 design made with ASTECv2.0r3 [1, 2]. After that these corium parameters and the decay heat power were used as initial conditions for the ICARE stand-alone calculations. The ICARE calculations started at 4910 s. This is the moment of core barrel melting-through and corium relocation into the bottom of lower head vessel. An ICARE model for VVER 1000 vessel without internals and without coolant has been modelled. The cylindrical part of the vessel was modeled as 1 ring and 3 axial segments (summary 3 meshes). Some calculations have been done with different discretization of the vessel bottom head, where: 1. The lower head vessel has been divided into 2 radial rings (MESR 2) and 10 axial segments (summary 20 meshes). 2. The lower head vessel has been divided into 5 radial rings (MESR 5) and 10 axial segments (summary 50 meshes). 3. The lower head vessel has been divided into 9 radial rings (MESR 9) and 10 axial segments (summary 90 meshes). In this case 10 axial segments could be distinguished in the vessel lower head model and in this study we will investigate the radial meshing increasing and decreasing. The basic model with 5 rings is shown at fig.1. The calculations were done with and without IVMR [3] procedure with external water cooling. 1

2 Figure 1: Basic model segmentation of the vessel lower head (elliptical section: segments 1-7, cylindrical section: segments 8-10) 3. Basic assumptions The slump modeling into the lower plenum: The slumps of corium into the lower plenum in the calculations have been performed at the beginning of the calculations (as one big portion). The corium is composed by UO 2, ZrO 2, Zr and Stainless Steel. Initial corium composition is shown in Table 1. Material Mass, t Source UO CORE Zr 15.6 CORE ZrO CORE 34.4 CORE 12.2 elliptic part of barrel Steel 9.0 melted cylindrical part of barrel 12.3 FA-supports 1.94 support grid Table 1: Initial corium composition Melting temperatures of oxides: As eutectic point of the UO2 and ZrO2 it was used in the model 2850 K. It was also assumed that corium arrives from the core to the bottom head with temperature 2800 K. Decay heat: Decay heat time dependence used in the calculations is presented in Table 2: Time Decay heat, Wt (per 1 kg of UO 2 ) Time Decay heat, Wt (per 1 kg of UO 2 ) Table 2: Decay heat history 2

3 4. Results from calculations The results from the calculations without external water cooling are presented in the figures from 2 to 11 below. It could be seen that increasing the number of elements (meshes), at which the vessel lower head is divided, leads to earlier vessel failure. Figure 2: Temperature field at discretization in 2 radial rings Figure 3: Temperature field at discretization in 5 radial rings (50 meshes) Figure 4: Temperature field at discretization in 9 radial rings (90 meshes) The internal and external heat fluxes distribution in height (for the segments from 1 to 10) is presented in figures from 5 to 10. Figure 5: Internal heat flux distribution in case of Figure 6: External heat flux distribution in case of Figure 7: Internal heat flux distribution in case of Figure 8: External heat flux distribution in case of 3

4 Figure 9: Internal heat flux distribution in case of lower head vessel discretization in 9 radial rings (90 meshes) Figure 10: External heat flux distribution in case of lower head vessel discretization in 9 radial rings (90 meshes) The comparison of the maximum heat flux axial profiles for the three calculations is presented at Figure 11. The HF axial profiles are given looking at each external and internal point during the whole time at all elevations. Figure 11: Comparison of bounding curves of maximal heat fluxes There is presented below (Fig. from 12 to14) the results from the basic calculation with vessel lower head modeling by 5 rings and simulation of external water cooling. The calculation was done till 30000s. Figure 12: Internal heat flux distribution in case of external water cooling and lower head vessel discretization in 5 radial rings (50 meshes) Figure 13: External heat flux distribution in case of external water cooling and lower head vessel discretization in 5 radial rings (50 meshes) 4

5 Figure 14:Bounding curve of maximal heat fluxes (basic case with external water cooling) 5. Conclusions In the calculations without water cooling the vessel failure occurs as follows: at sec., at elevation m for the vessel lower head modeling by 2 rings; at sec., at elevation m for the vessel lower head modeling by 5 rings; at sec., at elevation m for the vessel lower head modeling by 9 rings. In the calculation with vessel lower head modeling by 2 rings the maximal heat flux of 1,85 MW/m 2 occurs at 9556 s. It happens at segment 8 between the elevations and In the calculation with vessel lower head modeling by 5 rings (50 meshes) the maximal heat flux of 2.79 MW/m 2 occurs at 7690 s. It happens at segment 7 between the elevations and In the calculation with vessel lower head modeling by 9 rings (90 meshes) the maximal heat flux of 2.91 MW/m 2 occurs at 6950 s. It happens at segment 7 between the elevations and The results from the calculations with external water cooling show that vessel failure doesn t occur. External water cooling is a successful strategy for severe accident management. In the Basic calculation the absolute maximum of heat fluxes (2.84 MW/m 2 ) occurs at s at segment 7 (between elevations and ). 6. Literature [1] H.-J. Allelein, J.P.V. Dorsselaere et al. European Validation of the Integral Code ASTEC (EVITA) First experience in validation and plant sequence calculations, NED235, ,2005 [2] Allelein, H.-J., Neu, K., Dorsselaere, J.P.V., Müller, K., Kostka, P., Barnak, M., Matejovic, P., Bujan, A., Slaby, J., European validation of the integral code ASTEC (EVITA), Nuclear Engineering and Design 221, [3] Theofanous, T.G., Liu, C., Additon, S., Angelini, S., Kymäläinen, O., Salmassi T., 1996 In-vessel Coolability and Retention of a Core Melt, DOE/ID-10460, Vols. I and II. 7. Автори Росица Веселинова Генчева, асист. Aнтоанета Емилова Стефанова, главен асист., д-р Павлин Петков Грудев, доц., д-р Институт за ядрени изследвания и ядрена енергетика, Българска Академия на Науките Teл: (+359 2) roseh@inrne.bas.bg; antoanet@inrne.bas.bg; pavlinpg@inrne.bas.bg. 5