Steam Explosion Resistance of an Internal Core Catcher of a Nuclear Reactor Pressure Vessel

Steam explosion resistance of an internal core catcher of a nuclear reactor pressure vessel

D. Aquaro & E. Fontani Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione

Pisa University, Italy

Abstract

This paper deals with a feasibility design of a core catcher structure that could mitigate the consequences of the core meltdown in a commercial light water reactor. This activity has been performed in the framework of the "Concerted

Action" IVCRS (In Vessel Core Retention Strategies), financed by the European Union in order to assess the safety of the next generation of nuclear power plants. These plants will be designed to support the consequences deriving from a core meltdown accident and from the phenomena that accompany it. Steam explosion is considered as a potential risk in the hypothesis of a severe accident occurring in a Pressurised Water Reactor nuclear power plant. The loss of coolant, which can occur in the case of a pipe break, provokes the degradation of the core geometry, its coolability and then its melting. The molten core falls down in the vessel to the lower hemispherical part rapidly transferring its energy to the water remaining in the lower plenum, which vaporises. In order to mitigate the consequences of a severe accident, a core catcher device, able to contain and to cool the molten core, has been designed. The same structure is analysed here as energy dissipators to prevent the reactor pressure vessel lower head failing in the case of steam explosion. In this paper, emphasis is placed on the structural aspects of the problem. Thermal fluid dynamics is treated macroscopically. This conservative approach allows us to overcome the actual uncertainties in the heat transfers mechanism between molten core and water. The results of a simulation, conducted with a finite element code, shows that the implementation of an internal core catcher could prevent the Reactor Pressure Vessel lower head from failing.

164 Structures Under Shock and Impact VI

1. Introduction

The Three Mile Island (TMI) accident, occurred in March 1979 to a Pressurised Water Reactor (PWR), showed the importance of analysing the Severe Accident, of the core meltdown, until then considered incredible. In the meltdown accident scenario, the molten fuel falls down in the Reactor Pressure Vessel (RPV) lower head leading to its probable melt-through and rupture due to high thermal

gradients. The melt-through is followed by expulsion of the melt material as small particles or droplets into the containment cavity. The large quantities of chemical and thermal energy may produce a rapid increase of the containment temperature and pressure. Up to now, two different strategies are followed in order to mitigate the

consequences of the core melting accident: ex-vessel or in-vessel strategies. The ex-vessel core retention strategy aims to cool the molten core by means of high heat transfer surface in the cavity of the containment. The in-vessel strategy introduces a particular device, named core-catcher, inside the RPV in order to cool the molten core, preserving the vessel structural integrity.

The "Concerted Action IVCRS, financed by the European Union, has the main purpose to identify the feasibility of the in-vessel core retention and to define a research strategy devoted to design satisfactory innovative solutions for the future Nuclear Power Plants (NPPs).

The University of Pisa, partner of the IVCRS, elaborated an original solution of core-catcher. A thermal analysis has been performed [1] showing the corium coolability in a steady state condition after that the decay power is decreased to 0.1% of the NPP full power. This paper presents a second step of the core- catcher performance analysis. By falling down in the water remaining in the

lower plenum, the molten core may transfer fastly its energy to water which vaporises. This phenomenon, known as Steam Explosion, is relevant in the nuclear safety analysis because the propagation wave, generated by the vaporisation, could reach the structures causing their damage. The core catcher device has been designed to prevent the RPV rupture.

2. Steam explosion mechanism

When the molten core enters in contact with the water (Figure 1) steam is formed very quickly at the interface between the two liquids during a short transient. It is worldwide accepted that the steam explosion scenario can be divided in the

following stages: 1. Premixing: during this phase (0.1-1 s) the corium is divided in particles of about 10 mm in diameter. A vapour film which separates the particles from the coolant limits the heat transfer; 2. Fragmentation: during this phase (few milliseconds) the vapour film is

destabilised and locally heat transfer increase and pressurisation occur. As a consequence corium and coolant interaction leads to fine fuel fragmentation (10-500 urn); 3. Propagation: during this phase (few milliseconds) very fast heat exchange

Structures Under Shock and Impact VI 165

occurs between corium and water leading to very high pressure spikes;

4. Expansion: during this phase (few milliseconds) damage of the surrounding structures is produced by the kinetic energy of the corium slug. Because of the strong uncertainties in the laws governing the phases n° 1, 2 and 3, it is impossible to perform an analytical analysis of these phases. The heat

transfer mechanism between the molten fuel and the coolant depends strongly on the dimension of the corium particles. In the following a conservative approach has been utilised. The phases n° 1, 2 and 3 have been supposed to occur instantaneously by means of an isochoric process. This approach determines a more high evaluation of the pressure peak.

2.1 Description of the assumed accident

The assumed scenario, before the steam explosion occurrence, is similar to the one occurred during the TMI accident. The primary event is a Loss Of Coolant Accident (LOCA), which produces a depressurisation until 1 MPa. The water in

the lower plenum is saturated and the liquid level is 1.6 m above the RPV bottom.

Water participating to Ac explosion (V*)

Molten core participating to the explosion (Vr)

Figure 1: Hypothesised scenario of a steam explosion. Because of the degradation of the core coolability, a 60 % core meltdown has

been assumed. Due to a different composition the following corium characteristics are estimated: A. A corium of 160 Mg and an initial corium temperature of 2527°C;

B. A corium of 204 Mg and an initial corium temperature of 2000°C. During the expansion phase of a steam explosion in a PWR, some of the

explosion pressure may be relieved to the downcomer volume. This phenomenon is known as downcomer venting. Additional relief is possible if the resulting forces are of sufficient magnitude to fail the lower head. The remaining energy is converted into upward-directed kinetic energy. The materials, still found on the core plate, are accelerated in the RPV causing their impact on the upper internal

166 Structures Under Shock and Impact VI structures [2]. In this analysis the downcomer venting has been accounted by imposing a constant pressure at the top of the downcomer. No impact between core plate and RPV upper internal structures has been simulated.

2.2 Energy release evaluation

The energy released to the fluid, due the water vaporisation, has been evaluated assuming that the explosion occurs instantaneously. In the lower plenum (Figure 1), a mass Mf of molten fuel has been supposed to come in thermal equilibrium with a mass M«, of water by means of an isochoric process [3], as shown in

Figure 2. The resulting thermodynamic state 2 for the corium-water mixture has been obtained from the following equations:

M^(w, — u^) = Mj\Cj\Tj.i -TjJ+Lj.] (1) where Lf= 270 kJ/kg is the latent heat of fusion of the fuel and Cf= 0.5 kJ/kg °C is the heat capacity. The expansion phase (from point 2 to point 3) has been simulated by means of an isentropic expansion. Due to the very fast transient, no heat transfer between the water participating to the explosion and the saturated water has been assumed. Only the water has been supposed to participate to the expansion where the resulting thermodynamic state 3 has been obtained by a politropic transformation.

As it is shown in Figure 1, the explosion has been supposed to originate from a region of spherical shape located in the lower plenum. The sphere radius R has been evaluated by conserving the water mass participating to the explosion.

Isentropicexpan;

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Entropy [ J /(kg°C)]

Figure 2: Thermodynamic evaluation of the energy release.

Several values of molten core mass and water rates participating to the explosion have been selected considering 1% and 0.1% of the molten core falling down in the lower plenum, respectively. The fuel-water rates have been evaluated by varying the water volume fraction % (eqn (2)) from 0.81 to 0.95. Values less than 0.81 determine an extent of the region at high pressure very small. Values greater than 0.95 determines a pressure very low.

Structures Under Shock and Impact \ 7 167

V. (2)

The initial conditions for a parametric study of the expansion phase have been calculated using water state equation highly accurate for pressure up to 3000

MPa [4]. The results are reported in Table 1, where p is the pressure, p is the water density, u is the water internal energy, y is the constant for the politropic expansion and R is the radius of the sphere where the water participating to the explosions (Figure 1) is located.

Table 1: Initial conditions (point 2) for the expansion phase. X CoriumA 1% Corium B 1% Corium A 0.1% Corium B 0.1% p = 905.63 MPa p = 989.92 MPa /? = 194.40 MPa /? = 239.36 MPa p = 789.70 kg/m' p = 822.18kg/m' p = 338.29 kg/nr* p = 402.49 kg/nf 0.81 w = 3413.97 kJ/kg w = 3331.07kJ/kg u = 3528.33 kJ/kg M = 3452.02 kJ/kg 7=1.335 y= 1.361 7=1.162 7=1.172 R = 0.568m # = 0.617m R = 0.350m # = 0.363m p = 77 1.94 MPa p = 836.82 MPa /? = 156.36 MPa /? = 193.86 MPa p = 847.2 lkg/m' p = 875.95kg/m' p = 464.25 kg/W p = 529.04 kg/m' 0.88 M = 2526.60 kJ/kg w = 2461.92kJ/kg w = 261 4.48 kJ/kg w = 2540.84 kJ/kg 7=1.360 y=1.388 7=1.128 7=1.144 R = 0.665 m R = 0.724 m # = 0.377m R = 0.397m p= 413.16 MPa p = 398.84 MPa p = 50.73 MPa /? = 87.23 MPa p = 884.46 kg/m' p = 885.14kg/nf p = 670.8 kg/m' p = 718.70 kg/m' 0.95 u= 1538.60 kJ/kg u= 1508.62 kJ/kg «=1565.14kJ/kg w= 1531.32 kJ/kg y=1.303 y= 1.298 7=1.048 7=1.079 R = 0.901m # = 0.991m R = 0.458m ^ = 0.493m

3. Description of the core catcher device

The core catcher device could ensure the corium coolability and the structural

integrity of the RPV lower head bottom by collecting the melting material in a crucible.

Figure 3: Modifications to the PWR vessel to allow the core catcher insertion.

The original solution elaborated by the University of Pisa consists of an internal

168 S core catcher (Figure 3) located inside the RPV, made of a multilayered structure composed of several refractory and insulator materials externally lined with stainless steel. The modifications of the RPV, shown in Figure 3, are necessary to introduce the core catcher device in the lower plenum.

4. Numerical simulation

The finite elements analyses have been performed using the MSC.Dytran structural code [5]. MSC.Dytran is a three-dimensional analysis code for the analysis of the dynamic and non-linear behaviour of structures and fluids.

Lagrangian and Eulerian solvers are available to enable modelling of both structures and fluids and their interaction [6]. Due to the symmetry of the structures only a quarter of the RPV has been modelled. The meshes for the fluid and the vessel are reported in Figure 4.

DC top

Corium-water mixture

Fluid mesh Vessel mesh Core catcher mesh

Figure 4: Eulerian and Lagrangian meshes.

4.1 Fluid simulation

In the MSC.Dytran code, the corium interaction with the water during the explosion is not modelled. The simulation starts when the thermal equilibrium between the corium and water has been reached (point 2 in Figure 2). MSC.Dytran allows to simulate many fluids without mass and energy exchange. In this work vapour and water have been modelled as separate fluids without interactions as the condensation. The equations of state for these fluids have been assigned by polynomial interpolation of the pressure as a function of density and internal energy. The corium water mixture has been simulated taking into account that the corium is solid after it has transferred its energy to the water. At those pressure and temperature (see Table 1) the water can be assumed to behave as perfect gas

Structures Under Shock and Impact VI 159

during a politropic expansion.

Downcomer, lower plenum and vacuum in the core region have been modelled with 1503 8-nodes solid elements. The mesh is showed in Figure 4. In order to validate the results of the simulations, the spatial convergency has been verified by using several meshing, the greatest having a total number of elements of 19380.

4.2 Vessel simulation

The RPV has been simulated by 228 4-nodes shell elements. The Key-Hoffs shell elements formulation [5], which is able to consider very large strain, has been used.

Mesh with more elements have been used, a comparison between meshing with 228 elements and 1245 element is showed in Figure 5. An elosto-plastic behaviour for the A533-B carbon steel material has been assumed. This material fails for a 20% plastic strain.

4.3 Core catcher and refractory simulation

The multilayer structure of the core catcher device has been simulated by coupling the shell elements of the external stainless steel liners with the solid elements of the refractory (Figure 4). Considering an elosto-plastic behaviour of the material consistent with the von

Mises yield model, the assumed failure criterion is based on a maximum value of the effective plastic strain, fixed equal to 20 %. The refractory materials have been simulated with 3D solid elements (Figure 4), using the Krieg and Key model [5]. It uses an isotropic plasticity theory and the response of the material to deviatoric stress and hydrostatic stress is completely

uncoupled, as it is shown by eqn (3):

OX^^) = ^-(^+^P + ^/), (3)

being p the pressure, J^ the second invariant of the stress deviation tensor and £, constants. The yield surface in principal stress space is a surface of revolution centred about the hydrostatic pressure line.

5. Main results

The transient analysed described the expansion phase (transformation from the point 2 to the point 3 in Figure 2). The steam explosion transient lasts about 20 ms. In the following the main results of the simulation with and without core

catcher are reported.

5.1 Reactor Pressure Vessel response without the core catcher

For all the cases analysed, afirs tpressur e peak has been observed in the lower plenum (Figure 8 relative to the case with 1% of the corium B and % equal to

170 Structures Under Shock and Impact VI

0.95). The high pressure region moves from the lower plenum centre to the

vessel wall (velocity vectors in Figure 5). Small part of the corium-water mixture expands in the void part created by the core relocation. This pressure wave is reflected by the core upper plate and determines a second pressure peak in the lower plenum at about 8 ms, This second peak is very low and it is not visible in Figure 8. Only the first pressure wave is relevant for its consequences on the

vessel walls. In the Table 2 the time at which the RPV falls are reported. In all the cases where 1% of molten core is supposed falling down in the lower plenum the vessel fails. No failure is predicted if the corium is reduced to the 0.1%.

Table 2: Time (in ms) at which the vessel fails.

Corium Corium Corium Corium % Al% B 1% A 0.1% BO.1% 0.81 3.74 1.78 NO NO 0.88 2.00 1.76 NO NO 0.95 1.90 1.76 NO NO

The deformed shape of the RPV wall is reported in Figure 5. The point of maximum deformation is located at the bottom of the lower hemispherical part.

During the transient all the hemispherical part reaches the yield stress and at the end of the explosion this entire zone fails.

Plastic Strain 228 shell elements 1245 shell elements

Figure 5: Main results without core catcher.

In Figure 9 the stress values in the shell elements at different location are reported. These are relative at the most severe case with 1% of the corium B and X equal to 0.95. In all the analysed location the yield stress is reached, but failure

occurs only in the lower plenum bottom.

5.2 Reactor Pressure Vessel response with the core catcher

The same transients have been analysed on the modified vessel with the insertion of the core catcher device. The core catcher device prevents the pressure wave from reaching the vessel

Structures Under Shock and Impact \ 7 171 wall (Figure 8). The cerium-water mixture expands in the lower plenum and goes up in the downcomer (velocity vector in Figure 7).

The internal core catcher liner (deformed shape reported in Figure 6) fails 1-2 ms after the beginning of the expansion phase. In all the analysed cases no failure of the external liner and supports has been noticed. The refractory material is compressed (Figure 6) carrying the loads due to the pressure wave.

As it is shown in Figure 6 and Figure 7, the maximum von Mises stress on the RPV wall is situated above the core catcher support (point A) and it is noticed when the pressure wave expands in the downcomer (pressure peak in Figure 8). Due to the reduced pressure, this wave causes the yielding of the wall but the equivalent plastic strain is always less than 5% (Figure 7).

Stainless steel lyner Reactor Pressure Vessel

Figure 6: Structures deformed shape with the core catcher.

Maximum velocity 1110 m/s

Maximum plastic strain 4.32%

Von Miscs equivalent stress Fluid velocity

Figure 7: Main results with core catcher.

Equivalent stress [MPa] Equivalent stress [MPa] 88888 88888

21 Pressure [MPa] Pressure [MPa] Pressure [MPa] OQ' o-»tow*>.tna5^cocDo oooooooo

Equivalent stress [MPa] Equivalent stress [MPa] 88888 88888

O §. O M i I

p vo

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Structures Under Shock and Impact VI 173

6. Conclusions

Structural behaviour of the RPV in the case of an assumed steam explosion during a severe accident has been analysed. In order to perform a conservative study of the vessel and core catcher device, this has been conducted assuming that all the corium energy is transferred to the water.

Parametric thermalhydraulic calculations have been necessary to evaluate the supposable scenario at the beginning of the expansion phase. This phase has been simulated with the finite element code MSC.Dytran by coupling fluids and structures.

The results of calculations indicate that the RPV fails if 1% of the molten core is supposed to react with the water and that the best vessel part in demand is the lowest point of the shell located on the symmetry axis. The same transient have been analysed on the modified vessel after the core catcher device introduction. In this case the core catcher can carry the pressure

wave due to the explosion and the RPV preserves its structural integrity. This work, conducted in the frame of the Concerted Action "In-Vessel Core Retention Strategy", investigated on the possibility of to mitigate the consequences of an in-vessel steam explosion by means of energy dissipators. The obtained results reflect the expected trends that the implementation of an

internal core catcher prevents the RPV lower head from failing.

References

[1] Aquaro, D., Forasassi, G., Harghel, C., Thermal Fluid Dynamic Analysis of a Core-Catcher inside a Nuclear Power Plant Pressure Vessel in a Core

Meltdown Scenario, 17* UIT National Heat Transfer Conference, vol. 2, pp. 773-783, Ferrara, 1999 [2] Theofanous, T. G., Najafi, B., Rumble, E., An Assessment of Steam Explosion Induced Containment Failure. Part I: Probabilistic Aspects, Nuclear Science and Engineering, vol. 97, pp. 259-281, 1997.

[3] Amarasooriya, W. H., Theofanous, T. G., An Assessment of Steam Explosion Induced Containment Failure. Part III: Expansion and Energy Partition, Nuclear Science and Engineering, vol. 97, pp. 296-315, 1997. [4] Haar, L., Gallagher, J. S., Kell, G. S, NBS/NRC Steam Tables, Hemisphere Publication Corporation, New York, 1984.

[5] MSC.Dytran Version 4.7 Users Manual, The MacNeal-Schwendler Corporation, U.S.A, 1999. [6] Aquaro, D., Fontani, E., Forasassi, G., Harghel, C., Vezzani, M., Available Computational Tools Models for Structural and Thermal Fluid Dynamic Analyses of the Core Catcher, Concerted Action on In-Vessel Core

Retention Strategy (IVCRS Project), Pisa, 1999.