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

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Steam Explosion Resistance of an Internal Core Catcher of a Nuclear Reactor Pressure Vessel Structures under Shock & Impact VI, C.A. Brebbia & N. Jones (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-820-1 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. Structures under Shock & Impact VI, C.A. Brebbia & N. Jones (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-820-1 164 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 & Impact VI, C.A. Brebbia & N. Jones (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-820-1 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 Structures under Shock & Impact VI, C.A. Brebbia & N. Jones (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-820-1 166 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 & Impact VI, C.A. Brebbia & N. Jones (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-820-1 Structures Under Shock and Impact \ 7 167 V.
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