LBE-Water interaction in LIFUS V facility under different operating conditions A. Ciampichetti, D. Bernardi - ENEA T. Cadiou - CEA N. Forgione – Università di Pisa D. Pellini - KIT International DEMETRA Workshop Berlin, March 4th, 2010

Introduction The XT-ADS and EFIT reactors’ heat exchangers / steam generator modules are designed to be placed in direct contact with the heavy liquid metal in the main vessel. Since the likelihood of a pipe break is not negligible, the interaction between the secondary coolant and LBE represents an important concern for such a configuration. In fact, the consequences might have a strong impact on safety, design and maintenance of these reactors. Consequences The peculiarities of the heavy liquid metals (such as high thermal conductivity, high density and low surface tension) determine their gift to interact with water energetically, thus producing vapour at high pressure. The interaction leads to pressures waves propagation which might damage the surrounding structures, causing an escalation of the accident. The seriousness of the consequences is determined by the injection pressure and flow rate, the vapour production rate and the intervention of safeguards.

Description of work An experimental study focused on LBE/water interaction aimed at assessing physical effects and possible consequences relating to this kind of interaction has been performing in ENEA through LIFUS 5 plant. The main parameters for carrying out the experiments have been selected taking into account the XT-ADS primary heat exchanger design and the indications obtained from the pre-test activity performed with SIMMER code. The parameters considered have been: - system geometry; - lead temperature; - temperature and pressure of the water The modelling activity with SIMMER code have been performing in CEA and University of Pisa.

LIFUS 5 Facility The main LIFUS 5 components are: the reaction vessel S1, containing at the bottom the water injection device. Its volume is 100 l the expansion vessel S5, connected to S1 through four tubes. Its volume is 10 l the pressurised water vessel S2 the safety vessel S3 the liquid metal storage vessel S4 Instrumentation: S1 is equipped with water-cooled high precision piezometric pressure transducers, which allow to achieve very low time constants. A number of K-type thermocouples are also present. A fast DAQ system with a dedicated software acquires the main test parameters in different positions of the system. LIFUS 5 has been designed to simulate LOCA accidents and to operate in a wide range of conditions (pressure up to 200 bar, initial LM temperature up to 500 °C)

LIFUS 5 Facility

Experimental campaigns Three experimental campaigns for DEMETRA have been completed and the last one will be completed in March 2010.

Test n.1: experimental results Pressure evolutions in the reaction-expansion vessel First phase: the pressure reaches a maximum of about 65 bar. Second phase: pressure decreases in S1 because of the free flow of gases into S5 is not balanced by an equivalent injection of water. Third phase: there is a further pressure increase in both S1 and S5 due to the further water vaporization. A maximum value of about 80 bar has been reached. Fourth phase: pressure become stable at 70 bar due to the reverse flow-rate. Liquid metal temperature: 350 °C Water injection pressure: 70 bar Water temperature: 235 °C (sub-cooling of 50 °C) Test performed with the expansion vessel S5

Test n.1: modelling SIMMER model of LIFUS5 Pressure evolutions in the interaction vessel (S1) and expansion vessel (S5) S1 S5

Test n.2: experimental results Pressure evolution detected in gas phase followed the same trend as in LBE but the first sharp peak was not present. Liquid metal temperature: 350 °C Water injection pressure: 6 bar Water temperature: 130 °C (sub-cooling of 28 °C) Test performed without the expansion vessel S5 Temperature evolution detected in three vertical positions.

Test n.2: modelling /1

Test n.2: modelling /2 SIMMER III domains developed by PISA University Comparison between SIMMER simulations and experimental results Pressure evolution Temperature evolution University of Pisa

Experimental and SIMMER results of Test n.2 were used to support the assessment of the accidental scenario of “heat exchanger tube rupture” considered as reference accident in the safety analysis of the ICE activity. Test n.2: experimental results SIMMER domain for Test n.2 Safety activity for ICE /1

ICE simulations with SIMMER have shown that a double rupture of a HX tube produces a fast pressurisation of CIRCE main vessel that strongly overcomes the design value. ICE design team has developed another solution for the HX based on the adoption of water at 1 bar and double wall tubes Safety activity for ICE /2 University of Pisa

In order to simulate the possibility to discharge the vapour/liquid metal mixture outside the steam generator module during a tube rupture accident, the reaction system of Lifus5 facility has been modified. A discharge line directly connecting the reaction vessel S1 with the safety vessel S3 has been designed and constructed. Modification of LIFUS5

LIFUS5: old and new configuration

Test n.3: experimental results /1 Pressure evolution detected by the different transducers placed in S1 Liquid metal temperature: 350 °C Water injection pressure: 40 bar Water temperature: 235 °C (sub-cooling of 15 °C) Possibility to discharge in S3

Test n.3: experimental results /2 Pressure Temperature PT close to the water injectorPT in the flange of S1 TCs in front of the water injector

Features of the domain Simplified geometry: cylindrical coordinates (r-z) with 30 radial and 39 axial meshes. In LIFUS 5 there is a strong asymmetry in the geometry Assumptions 1.The overall volume of the main elements is conserved 2.The injector and vent pipe are placed coaxially with the reaction vessel S1 3.The flow area of the various pipes is conserved Test n.3: modelling /1 In the reaction vessel S1, U tubes are represented by 12 “no calculation” regions The strong asymmetry due to U tube shape and position cannot be adequately accounted for U tubes are simulated through annular elements which conserve the overall volume SIMMER III Model (2D): Geometry University of Pisa

Test n.3: modelling /2 Comparison between calculated and experimental pressure in S1 vessel SIMMER III overestimates the maximum value of the pressure in the reaction vessel S1 even though the trend is in agreement especially for the pressurization Comparison between calculated and experimental temperature in LBE region: top thermocouple University of Pisa

SIMMER IV Model (3D): Geometry 3-D model of LIFUS 5 facility performed with SIMMER IV code Simplified geometry: Cartesian coordinates (x- y-z) with 20 meshes along x, 15 along y and 18 along z The correct position of the water injector and of the vent tube is preserved Test n.3: modelling /3 University of Pisa

Test n.3: modelling /4 The 3D version of the code is able to evaluate the first pressure peak associated with the impact between the water jet and the “rigid surface” of the liquid metal. A better agreement concerning the maximum values reached with respect to SIMMER III results even though some discrepancies still remain. University of Pisa

Test n.3: modelling /5 Test Total mass of injected water [kg] SIMMER III1.66 SIMMER IV1.13 Experimental data1.60 Mass Flow Rate: Comparison between SIMMER III and SIMMER IV calculation results University of Pisa

Possible consequences The pressure evolution detected during Test n.3 has shown a first sharp peak and a following slower pressurisation of the system. In reactor scale the latter event can be avoided using adequate countermeasures (e.g. rupture disks, stop and check valves on the steam generator tubes). The first sharp peak reached about 15 bar in the pressure transducer closest to the water injector and smaller values in the other pressure sensors. This peak is originated from the impact of the water jet on the liquid metal and it could be a threat for the integrity of the surrounding tubes. Further tests are necessary to investigate this issue.

Conclusions Experimental results concerning the simulation of water large leaks in LBE have been obtained under different operating conditions. Maximum pressure values higher than the water injection pressure have been detected during Test n.1 and 2. The experimental results provided helpful data to prove the capability of the SIMMER code to simulate such an accident. SIMMER III was able to reproduce the interaction between a water jet and LBE, even though the 2D feature of the code has represented a limitation in reproducing the LIFUS 5 results. A new activity with SIMMER IV (3D) has been recently launched and is giving promising results. Test n.2 was used to support the safety analysis of ICE. In this case simulations showed that a double rupture in the LBE–pressurized water shell heat exchanger leads to a fast pressurisation of CIRCE main vessel that strongly overcomes the design value. Considering this warning, the ICE design team has adopted another solution with water at atmospheric pressure. LIFUS 5 has been modified in order to simulate the possibility of discharging the vapour/liquid metal mixture outside the steam generator module during a tube rupture accident as it might happen in lead cooled reactors. After that, Test n.3 was carried out in the operating conditions fixed for the heat exchanger of XT-ADS. Test n.3 has shown a first sharp pressure peak and a following slower pressurisation of the system. In reactor scale, the first peak could be dangerous for the surrounding structures while the following pressurisation can be avoided with adequate safeguards.