A hazard analysis is in progress to determine the operation mode of the ventilation system in the ATLAS hall in case of an Argon spill. Two risk scenarios have been investigated so far. In the first, the behaviour of an Argon gas pool is calculated for different ventilation strategies. In the second, the behaviour of Argon gas leaking from the bottom part of the detector is studied for different flows. The description of the study, results and conclusions are presented

Gustavsson, L

Peón-Hernández, G

CERN Document Server

ORGANISATION EUROPEENNE POUR LA RECHERCHE NUCLEAIREEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHLaboratoire Europeen pour la Physique des ParticulesEuropean Laboratory for Particle PhysicsTECHNICAL NOTE ST-98-038G. Peon & L. GustavssonARGON SPILL IN THE HALL OF THE ATLASEXPERIMENTAbstractA hazard analysis is in progress to determine the operation mode of the ventilation system in theATLAS hall in case of an Argon spill. Two risk scenarios have been investigated so far. In the rst,the behaviour of an Argon gas pool is calculated for dierent ventilation strategies. In the second, thebehaviour of Argon gas leaking from the bottom part of the detector is studied for dierent ows. Thedescription of the study, results and conclusions are presented.Geneva, October 1998Argon gas in the hall of the ATLAS experiment 2Contents1 Introduction 32 Finite Volume Model 33 The rst approach: Behaviour of an Argon gas pool 43.1 Inuence of the distance from the oor of the grilles . . . . . . . . . . . . . . . . . . . . . . . 43.2 Inuence of the air supply and extraction ows . . . . . . . . . . . . . . . . . . . . . . . . . . 43.3 Stopping the main ow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 The second approach: Behaviour of a ow of Argon gas coming from the detector 74.1 Leakages and ventilation parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Appendix: Illustration of two Argon extraction processes 9Of an Argon gas pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9For a leak of 91080m3/h of Argon gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11CERN ST/CVArgon gas in the hall of the ATLAS experiment 31 IntroductionThe ATLAS detector is one of the new experiments to be built at CERN for the LHC accelerator. In theexperimental area there are Dewar vessels for liquid Argon. A hazard analysis is in progress to determinethe conguration of the ventilation system and the operation mode in case of an Argon leak into the cavern.As the rst approach, the behaviour of a pool of Argon gas at ambient temperature with a depth of 1.6meters was analysed. The second approach considers a hypothetical leak of Argon in the ATLAS detectorwhich would provoke a ow of ARGON gas into the cavern. The studies were simplied into two dimen-sional cases and the results are presented in this document. It involves mass transfer (two mixing uids) andtransient analysis. The analysis has been carried out with the aid of the Computational Fluid Dynamics(CFD) package Star-CDR. The geometry and boundary conditions were completely parameterised in orderto easily change the positions, dimensions and air-ow of the supply and extraction grilles.It should be pointed out that the results presented hereafter can only be used for comparison among thedierent cases and the values obtained cannot be considered as absolute values.Animated gif les illustrating the behaviour of Argon with time can be accessed from the WWW page:http://wwwcn.cern.ch/~gpeon/ATLAS/Main.html2 Finite Volume ModelA two dimensional model of the ATLAS experimental hall is presented in Fig. 1. The air coming from thediusion grilles are represented by the lower arrows and the uppermost ones represent the air coming froma neighbouring cavern. The crosses represent the extraction grilles.Figure 1: 2D model of the ATLAS experimental hall.CERN ST/CVArgon gas in the hall of the ATLAS experiment 43 The rst approach: Behaviour of an Argon gas poolIn the present section the behaviour of a 1.6m deep Argon gas pool at ambient temperature is studied fordierent positions of the diusion grilles and dierent air supply and extraction ows. Results are given after3 minutes from the beginning of the extraction process and the values correspond to the Argon concentrationat 1.6m from the oor, near the detector, near the wall of the hall at the right of the detector and half distancebetween these two points, Fig. 2.Figure 2: Measurement points of Argon concentration3.1 Inuence of the distance from the oor of the grillesFour dierent distances from the oor of the diusion grilles are studied. In these four cases the supplyair into the cavern and the extraction at the top of the hall (main ow) are 45000m3/h. The ows comingfrom the neighbouring cavern and extracted at the oor level (secondary ow) are 16000m3/h. In the Tablebelow, the calculated Argon concentration is presented for dierent distances from the oor of the diusiongrilles.Table 1: Calculated Argon concentration (%) for dierent distances from the oor of the diusion grilles.Distance from the oor (m) 6.5 4 2 0Near the wall 42 58 72 0Between wall and detector 17 15 22 28Near the detector 2 3 4 103.2 Inuence of the air supply and extraction owsThe behaviour of the same Argon pool is investigated under doubled main and secondary ows, 90000m3/hand 32000m3/h respectively. Results are presented in the next table:CERN ST/CVArgon gas in the hall of the ATLAS experiment 5Table 2: Calculated Argon concentration with doubled main and secondary ow and for dierent distancesfrom the oor of the diusion grilles.Distance from the oor (m) 6.5 4 2 0Near the wall 64 78 79 0Between wall and detector 3 1 15 0Near the detector 0 0 3 273.3 Stopping the main owThe operation mode having the main ow brought to zero is studied for two secondary ows and the resultsare presented in the next table.Table 3: Calculated Argon concentration for the case of main ow = 0Secondary ow (m3/h) 16000 32000Near the wall 34 22Between wall and detector 16 5Near the detector 1 03.4 ConclusionsA 2D nite volume model of the Atlas experimental hall was made to analyse the inuence of the distance fromthe oor of the air supply grilles and the air ow rate in a 1.6m high Argon gas pool at ambient temperature.The analysis of the results points out that similar Argon concentrations are attained after 3 minutes andat 1.6m from the oor for the cases having the diuser at the oor level, the main ow of 90000m3/h andthe secondary of 32000m3/h, and for the case having the main ow of 0 and the secondary of 32000m3/h.The Argon concentration in the model however looks dierent, Fig. 3 and Fig. 4.It can be observed that the Argon concentration is, in general, lower for the case where the main ow is90000m3/h, the secondary 32000m3/h and where the diusion grilles are at the oor level.CERN ST/CVArgon gas in the hall of the ATLAS experiment 6Figure 3: Main ow 90000m3/h, secondary ow 32000m3/h. Diusion grilles at the oor levelFigure 4: Main ow 0, secondary ow 32000m3/hCERN ST/CVArgon gas in the hall of the ATLAS experiment 74 The second approach: Behaviour of a ow of Argon gas comingfrom the detectorIn case of a leak of liquid Argon in the internal part of the detector, part of the liquid will evaporate as itpasses through the components of the detector at room temperature. The proportion and amount of liquidand gas leaving the detector depend on the size and position of the leak and will change with time.4.1 Leakages and ventilation parametersIn this section, the Argon concentration in the hall was calculated assuming that all the liquid Argon isevaporated from leaks of 1l/s, 10l/s and 30l/s. The gas ow rate at room temperature produced by theleaks are 3036m3/h, 30360m3/h and 91080m3/h respectively. The parameters for the ventilation used inthe analysis correspond to those leaving the lowest Argon concentration in the previous section: main ow90000m3/h, secondary ow 32000m3/h and diusion grilles at the oor level.The results after 3 minutes from the beginning of the owing process are presented in Fig. 5, 6 and 7.4.2 ConclusionsIt can be observed that after 3 minutes from the beginning of the leakage, only the case of 91080m3/h ofArgon has high Argon concentration in the whole hall whereas in the other two cases the Argon volume isrestrained in the central pit.Figure 5: Argon concentration for an Argon gas ow rate of 3036m3/hCERN ST/CVArgon gas in the hall of the ATLAS experiment 8Figure 6: Argon concentration for an Argon gas ow rate of 30360m3/hFigure 7: Argon concentration for an Argon gas ow rate of 91080m3/hCERN ST/CVArgon gas in the hall of the ATLAS experiment 9Appendix: Illustration of two Argon extraction processesa.- Of an Argon gas poolFor illustration, Fig. 8 - 10 show the Argon concentration in the entire model for dierent times after thebeginning of the extraction process. The boundary conditions are the main ow of 90000m3/h, secondaryow of 32000m3/h and diusion grilles at the oor level.Figure 8: Argon concentration. Initial conditionsCERN ST/CVArgon gas in the hall of the ATLAS experiment 10Figure 9: Argon concentration after 1 minute from the beginning of the extraction processFigure 10: Argon concentration after 3 minutes from the beginning of the extraction processCERN ST/CVArgon gas in the hall of the ATLAS experiment 11b.- For a leak of Argon gasFor illustration, Fig. 11 - 13 show the Argon concentration in the entire model for dierent times after thebeginning of the extraction process. The boundary conditions are the main ow of 90000m3/h, secondaryow of 32000m3/h, diusion grilles at the oor level and the leak of Argon gas is of 91080m3/h.Figure 11: Initial conditions, no Argon gas in the hallCERN ST/CVArgon gas in the hall of the ATLAS experiment 12Figure 12: Argon concentration after 1 minute from the beginning of the leakageFigure 13: Argon concentration after 3 minutes from the beginning of the leakageCERN ST/CV

Argon spill in the hall of the ATLAS experiment

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