Scoping calculations of an in-vessel steam explosion. Castem-Plexus computations

International audienceSteam explosion is considered as a potential risk for PWR nuclear power plants in case of a severe accident. The loss of coolant can cause the degradation of the core and its melting. By falling down in the water remaining in the lower plenum, the corium transfers fastly its energy to the water which vaporizes. This paper presents parametrical computations of an in-vessel steam explosion, regarding the possible lower head vessel ruin resulting from a large central core degradation. The mechanical consequences of the steam explosion on the lower head are foreseen by the CASTEM-PLEXUS fast dynamic software, estimating approximately the thermodynamic data.A 2D-axisymmetric representation is adopted because of the vessel symmetry. The vessel lower head is considered as an elastoplastic hemispheric shell without penetrations and coupled with coolant. Four parameters are examined: the corium location, the energy transferred from the corium to the water, the energy transfer kinetics and the water constitutive law.During the explosion, we observe the propagation of a pressure wave from the corium zone towards the vessel and the core. The wave reflects against the vessel and the core support plate and then is sent again to the corium zone. After the wave passing, the water vaporizes massively and escapes in the free space in the core centre

Simulation of the MARA 10 test representing a core disruptive accident

In the case of a Hypothetical Core Disruptive Accident in a Liquid Metal Fast Breeder Reactor, the core of the nuclear reactor is assumed to have partially melted, and the interaction between molten fluid and liquid sodium creates a high-pressure gas bubble in the core. The violent expansion of this bubble loads and deforms the reactor vessel and the internal structures, thus endangering the safety of the nuclear plant. The MARA 10 experimental test simulates a HCDA in a 1/30-scale mock-up schematising simply a reactor block. The vessel is filled with water, topped with an air blanket. The test is fired using an explosive charge. This paper presents the numerical models implemented in the EUROPLEXUS code, and a numerical solution of the test. The evolution of the fluid flows and the deformations of structures are analysed in detail to understand the progress of the explosive phenomenon.JRC.G.5-European laboratory for structural assessmen

Assessment of the hydrodynamic loads due to a LOCA in a 3-loop PWR. Castem-Plexus computations

International audienceThe safety analysis of the PWR plants requires to assess the consequences of a hypothetical Loss Of Coolant Accident in the whole primary circuit and the reactor. This paper is dedicated to the study of the depressurization phase immediately following the break opening and to the evaluation of the mechanical consequences of the transient on the inner structures of a 3-loop PWR.The primary circuit and the reactor are represented with a pipe-model. A guillotine rupture is applied to one of the cold legs, just downstream the pump. The geometric model and the hydraulic conditions were described in [1]. This paper only deals with the analysis of the results calculated with the CASTEM-PLEXUS code. We suppose that the LOCA occurs in a reactor working at nominal rating. A first calculation is carried out at nominal rating for 2~s in order to validate the numerical model. The validation is obtained by comparing the computed pressures, volumic flows and pump working conditions with the estimated nominal rating ones. As the computation converges around the initial data (approximating the nominal rating), then the model is correct and can be used for the LOCA computation.A second calculation is performed during 500~ms, in accidental operation, from the previous initial conditions. The results are globally and locally scrutinized thanks to whole circuit drawings at a given time and local curves showing the evolution of a variable versus time. The analysis deals with the pressures and the void fractions in the complete circuit, and with the local pressure differences in the reactor.We observe a pressure drop with three phases: - from 0 to 2~ms: a pressure drop at the break, - from 2 to 100~ms: a general pressure loss in the whole circuit, - after 100~ms: a slower diphasic pressure decrease in the primary circuit.From the break extremities, two depressurization waves propagate, in opposite directions, through the whole circuit. The wave passing is characterized by a pressure fall until the saturation pressure at 8.2~MPa. Both waves superimpose at the higher plenum outlet and in the non broken loop steam generators. In the core area, we note horizontal pressure differences on the core barrel that can reach 1.2~MPa. The vertical gaps don't exceed 0.5~MPa. The wave coming from the reactor side break causes a flow rate increase whereas the wave coming from the pump side break brings about a flow rate decrease including a direction change in some places

Hydrodynamic Loads on a PWR Primary Circuit due to a LOCA - Pipe Computations with the CASTEM-PLEXUS Code

International audienceThe hydrodynamic loads due to a LOCA are computed with the CASTEM-PLEXUS code for a PWR primary circuit. An hydraulic model of the complete circuit (pipes, pumps, steam generators and reactor) is performed here assuming rigid pipes. CASTEM-PLEXUSbeing able to carry out FSI, the future work will take into account this effect

Interpretation with the Europlexus code of the MARA8 experiment simulating a hypothetical Core Disruptive Accident

International audienceIn the case of a Hypothetical Core Disruptive Accident (HCDA) in a Liquid Metal Fast Breeder Reactor, the interaction between the fuel and the liquid sodium leads to the formation of a bubble of gas at high pressure in the core of the reactor. The violent expansion of this bubble loads and deforms the vessel and the internal structures. The MARA8 mock-up is a small-scale experiment representing a simplified reactor. A Hypothetical Core Disruptive Accident is simulated in the mock-up. The mock-up includes a flexible vessel with a flexible roof. The vessel is filled with water, topped with an air blanket. The test is fired using an explosive charge. The mock-up is well-instrumented in order to be able to compare the experimental results with numerical computations.Up to now, several numerical simulations have been carried out with different computer codes. To conclude the study, the last model has been improved and a complete analysis of the accident has been performed. This paper presents a numerical simulation of the MARA8 experiment with the fast-dynamics code EUROPLEXUS and the comparison of the numerical results with the experimental ones. The explosion is simulated using a specific sodium-argon-bubble constitutive law describing the three-component mixture. Coupled fluid-structure computations are carried out to simulate the internal fluid and the external structures modeled by shells.The computation shows the propagation of a shock wave from the explosive zone towards the external structures. Water vaporizes in the middle of the mock-up and the air layer, initially located below the roof, is crashed in the top corner of the mock-up. The vessel base and the roof move away and two bulges appear in the lateral wall of the vessel.The comparison of the results computed by EUROPLEXUS with the experimental results and previous numerical results carried out with the codes SIRIUS, CASTEM-PLEXUS and EUROPLEXUS shows that the current results are in good agreement with the experiment but some discrepancies are still present

Screening calculations on the vessel lower head behaviour due to an in-vessel steam explosion

International audienceThe mechanical consequences of a steam explosion on a PWR lower plenum vessel are estimated through a parametric study regarding the corium location, the kinetics to transfer the corium energy to the water and the water constitutive law

Description of the MARS small scale replica of a Fast Breeder Reactor and of the numerical models used to simulate a Hypothetical Core Disruptive Accident in the test-facility

In case of a Hypothetical Core Disruptive Accident (HCDA) in a Liquid Metal Reactor, the interaction between fuel and liquid sodium creates a high pressure gas bubble in the core. The violent expansion of this bubble loads the vessel and the internal structures, whose deformation is important. The experimental test MARS simulates a HCDA in a small scale mock-up containing all the significant internal components of a Fast Breeder Reactor. The test-facility is filled with water topped by an argon blanket and the explosion is generated by an explosive charge.This paper describes the MARS test-facility and presents two numerical modelings used to simulate the test with the CASTEM-PLEXUS code. In the first model, the main internal structures are described with a classical shell model. The second model represents, in addition, the heat exchangers and pumps by means of a porosity model homogenizing the components with the surrounding fluid

Qualification of the numerical simulation of a Hypothetical Core Disruptive Accident on the MARA 10 test-facility

In case of a Hypothetical Core Disruptive Accident (HCDA) in a Liquid Metal Reactor, the interaction between fuel and liquid sodium creates a high pressure gas bubble in the core. The violent expansion of this bubble loads the vessel and the internal structures, whose deformation is important. The experimental test MARA10 simulates a HCDA in a mock-up schematizing simply the reactor block of a Fast Breeder Reactor (external vessel and roof as well as the main internal structures of the reactor). The vessel is filled with water, topped with an air blanket. The test is fired using an explosive charge.This paper presents a simulation of the MARA10 test with the fast dynamics code CASTEM-PLEXUS. A specific HCDA constitutive law was implemented in this code to simulate this kind of explosion. The external structures are represented by shells. Coupled fluid-structure computations are carried out.The bubble gas expands in the central zone confined by the internal structures and afterwards it escapes in the rest of the mock-up by the narrow space between the ACS and the shield. Water vaporises out of the radial shield, below the diagrid and in the ACS. The air layer is compressed under the roof by the water pushed upwardsby the explosion.Very soon, the diagrid moves down, the radial shield goes away and the ACSbase goes up. Then the vessel base moves down and the lateral wall goes away, thus creating a lower bulge. The ACS lateral wall buckles, the ACS top goes up, thus pulling up the rest of the roof. Finally, an upper bulge creates in the upper part of the vessel lateral wall and the ACS base changes curvature.After a brief presentation of the MARA 10 test-facility, this paper is focused on the numerical model, the analysis of the results computed by the code CASTEM-PLEXUS and a comparison with the experimental results and previous numerical results computed by SIRIUS and CASTEM-PLEXUS

Numerical simulation of a Hypothetical Core Disruptive Accident in the MARA8 mock-up with the CASTEM-PLEXUS computer code

International audienceIn case of a Hypothetical Core Disruptive Accident (HCDA) in a Liquid Metal Reactor, the interaction between fuel and liquid sodium creates a high pressure gas bubble in the core. The violent expansion of this bubble loads the vessel and the internal structures, whose deformation is important. The experimental test MARA8 is a simple small scale representation of a reactor. It simulates a HCDA in a mock-up closed by a flexible vessel and a flexible roof. The vessel is filled with water, topped with an air blanket. The test is fired using an explosive charge.This paper presents a simulation of the MARA8 test with the fast dynamics code CASTEM-PLEXUS. A specific HCDA constitutive law was implemented in this code to simulate this kind of explosion. The computed results are explained by means of an analysis of the pressure, gas fraction, fluid velocities, displacements, strains and stresses

Computation of the MARS test simulating a Hypothetical Core Disruptive Accident in a small scale replica of a Fast Breeder Reactor

International audienceIn case of a Hypothetical Core Disruptive Accident (HCDA) in a Liquid Metal Reactor, the interaction between fuel and liquid sodium creates a high pressure gas bubble in the core. The violent expansion of this bubble loads the vessel and the internal structures, whose deformation is important. The experimental test MARS simulates a HCDA in a small scale mock-up containing all the significant internal components of a Fast Breeder Reactor. The test-facility is filled with water topped by an argon blanket and the explosion is generated by an explosive charge. his paper presents a numerical simulation of the test with the CASTEM-PLEXUS code. The top closure is represented by massive structures. The main internal structures are described by shells while the peripheral massive structures are taken into account with a pressure loss because their geometry is too complicated to mesh them