In the TOF experiment, a high-energy proton beam is sent on a lead target, which releases an important amount of heat inside. In order to avoid that the target temperature rises over the lead melting point, a cooling system was designed. Starting from the heat distribution generated inside the target by the proton beam, steady and unsteady simulations of the target thermal behavior were carried out. These simulations were performed with different ways of cooling and in the various beam configurations foreseen for the experiment. It will be explained how these simulations helped to design a suitable water cooling system for the target. The hydraulic circuit providing the cool water will be briefly described as well

Gasser, D

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHORGANISATION EUROPÉENNE POUR LA RECHERCHE NUCLÉAIRECERN - ST DivisionPresented at the 4th ST WorkshopChamonix, France, 30 January - 2 February 2001CERN-ST-2001-01430 January 2001TARGET COOLING FOR THE TOF EXPERIMENTD. GasserAbstractIn the TOF experiment, a high-energy proton beam is sent on a lead target, which releases animportant amount of heat inside. In order to avoid that the target temperature rises over thelead melting point, a cooling system was designed. Starting from the heat distributiongenerated inside the target by the proton beam, steady and unsteady simulations of the targetthermal behavior were carried out. These simulations were performed with different ways ofcooling and in the various beam configurations foreseen for the experiment. It will beexplained how these simulations helped to design a suitable water cooling system for thetarget. The hydraulic circuit providing the cool water will be briefly described as well.21 INTRODUCTIONThe design of innovative Accelerator-Driven Systems for incineration of nuclear waste, energyproduction and radio-isotope activation for medical applications as well as many other subjects inNuclear Physics, requires an experimental determination of neutrons cross sections, elastic andinelastic across the whole Mendeleiev table.The TOF facility aims at study systematically and with excellent resolution, neutron crosssections of almost any element.In the TOF experiment, a proton beam is sent on one side of a lead target. The spallationmechanism, taking place inside the lead, produces a neutron source on the other side of the target. Theneutrons are then canalized through a vacuum pipe to an experimental area located 200 m downstreamof the target.Because of its high energy, the proton beam releases an important amount of heat inside the leadtarget. In order to avoid that the target temperature rises over the lead melting point, a cooling systemfor the target was designed.After a short description of the experiment (target, proton beam, heat power released), it isexplained how the target cooling system was developed. The hydraulic circuit providing the cooledwater for the target is briefly described at the end.2 THE LEAD TARGET IN TOF EXPERIMENT2.1 Description of the target and the proton beamThe target is a cube with an alcove on one of its face (seeFigure 1). The proton beam goes along the Z-axis and hits the lead inside the alcove. A neutron sourceis then produced on the opposite side.Looked from the top, the proton beam has a 10° angle relative to the Z-axis, in the YZ plane.The energy of the beam is 20 GeV/c.The proton beam consists of a sequence of bunches sent onto the target with a certain period oftime, called a supercycle. The time length of a supercycle is 14.4 seconds. Each bunch contains 7.1012protons and is emitted during a very short time of 6 nanoseconds. Two beam configurations areforeseen:• 1 bunch per supercycle (i.e. 1 bunch every 14.4 s),• 4 bunches per supercycle (i.e. 4 bunches every 14.4 s at the time 0, 1.2, 2.4 and 3.6second).3Figure 1: target geometry2.2 Heat released inside the targetA neutronic code allows to calculate the energy released inside the target by the proton beam. Thisenergy Q is given in Gev/cm3/protons. Knowing the number of proton per bunch (np = 7.1012), thecorresponding volumetric heat power P (in W/m3) released inside the target can be calculated as:tnpQP6919 1010106.1 ∗∗⋅∗∗=−(1)Where 1.6.10-19 is the energy in Joules of 1 eV, the factor 109 converts the Gev in eV and thefactor 106 converts the cm3 in m3.P can be averaged, in first approximation, over one super-cycle (t = 14.4 s) or given as aninstantaneous figure for each bunch (t = 6.10-9s).It can be seen in Eq. (1) that the ratiotnp 19106.1 −⋅∗corresponds to the beam intensity Ib,which yields :1510⋅⋅= IbQP (2)3 DEVELOPMENT OF THE TARGET COOLING3.1 First solutions foreseenThe very first solution foreseen to cool down the target was to use ambient air free convection.Therefore, a thermal simulation of the target was performed with StarCD1 considering an ambienttemperature of 20o C at the lead surface and a heat transfer coefficient with the air of 5 W/m2/K whichis a standard value for free convection. The volumetric heat power P applied inside the target isaveraged over one super-cycle (Eq. (1) with t=14.4s). This means that the simulation is done with asteady state approach, which is enough to get a first estimation of the temperature figures. Thecalculation was performed in the configuration of four bunches per super-cycle (np=4*7.1012); thiscorresponds to a total power released inside the target of about 3200W.The temperature distribution arising from this simulation is shown on Figure 2 on a YZ planecutting the center of the target. The beam impact is located on the left side in the center of the alcove.The temperature iso-contours show clearly the 10 degrees inclination of the beam. Moreover, the iso-contours are almost perpendicular to the lead surface because of the small heat exchange with air. Thehottest spot is located inside the target a few centimeters after the impact. The temperature inside thetarget remains in the range between 200 and 315o C which is too much knowing that the lead meltingpoint is around 320o C. Indeed, the lead may get soft even around 200o C, which is not compatiblewith the experiment. The free convection cooling is therefore not enough.1 StarCD is a commercial Computational Fluid Dynamic code, developed by Computational Dynamics Ltd.4Figure 2: temperature distribution inside the target (air free convection cooling)Another simulation was done with an air forced convection around the target (i.e. heat transfercoefficient of 20 W/m2/K). The temperature figures arising from this simulation were still too high.3.2 Dimensioning of the water coolingThe coolant finally chosen for the target is demineralised water which, thanks to its important heatcapacity, is able to remove the rather big amount of heat generated inside the lead (3200 W). Thepractical solution is to immerse the target in a tank full of water and to provide a circulation inside,with two main streams, one along the side with the alcove (proton side) and one along the oppositeside (neutron side). The reason for this is that on the first simulation (Figure 2), it can be noticed thatthese two sides are the hottest ones and has to be cooled down with the best efficiency in comparisonwith the other faces of the target. As it can be seen on Figure 3, five holes are dig on the target tank, oneach side of the two walls perpendicular to the Yaxis. Pipes are connected to the holes in order tobring the water into the tank through the inlet holes and bring the water out through the outlet holes,and this with a constant flow rate.In order to provide a good heat transfer coefficient along the two main streams, the water flowrate has to be high enough in order to ensure a turbulent flow. This flow is assumed as an internal oneinside a pipe of rectangular section L x w, where L is the length equal to the target height (H=80 cmseeFigure 1) and w is the width equal to the water thickness between the target wall and the tank.The influence of the alcove, on the proton side is neglected in first approximation. The width w is 3cm on the proton side and 5 cm on the neutron side. These thickness figures were chosen mainly forphysical reasons, the water acting as a moderator for the neutrons as well.5Figure 3: water cooling of the targetThe Reynolds number of the flow along the two main streams is given by :νDhV ⋅ (3)where ν is the water kinematic viscosity (1.1 10-6 m2/s2) , V the velocity and Dh the hydraulicdiameter given by ( )wLwL+⋅⋅⋅24. In order to have a well established turbulent regime for both streams,the Reynolds number has to be over 6000, which give V=0.125 m/s on the proton side (w = 3 cm) andV=0.075 m/s on the neutron side (w=5 cm). This makes a total flow rate q on the two sides of 6 l/s.The heat transfer coefficient h between lead and water can then be estimated from nusseltnumber, i.e.:DhNuh λ⋅= where λ is the water heat conduction coefficient (0.598 W/m/K). TheNusselt number is given by the following formula [1]:( )ÿÿ      −⋅ÿ     ⋅+⋅−⋅=1Pr827.11Pr1000Re8325.0ffNu (4)Where Pr is the Prandtl number and f the friction factor at the lead water interface estimatedfrom the following formula [2]:629.074.57.3log25.0ÿ     +⋅∆=ReDhf (5)where ∆ is an average roughness equal to 0.046 mm.h is estimated to about 1000 W/m2/K for both streams, which is a good figure and should ensurean efficient heat transfer at the lead/water interface.3.3 Simulation of the water coolingIn order to check that the water cooling solution is working, StarCD simulations of the target cooleddown by two water streams were performed. The model (mesh and boundary conditions) used forthese simulations is shown on Figure 4. The model takes into account the fluid (water) and the solid(lead) with the heat transfer between both media.The black cells correspond to the two water streams cooling down the proton and neutron sides,with the inlet velocity in yellow ; the inlet temperature is set to 30oC which is the figure than can beachieved in summer ; in green are represented the two water streams outlets. The two water streamsare longer than the target for simulation purpose, in order to get a permanent flow regime all along thesides of the target. Moreover, the alcove is taken into account in this model, it generates in thesimulation a flow vortex taking place inside the alcove.In blue are represented the four remaining sides of the target cooled down by air free convectionwith a heat transfer coefficient of 5 W/m2/K and an ambient temperature identical to inlet watertemperature (30oC). This model does not fit to the exact reality because the four sides are actuallycooled by water since the whole target is immersed in water. But it has to be considered that the waterflow velocity is very low along these four sides with a small heat transfer coefficient. This situation isapproximated as an air free convection cooling. So, the simulation is done in the worst possiblesituation in order to have a kind of safety margin.Figure 4: mesh and boundary conditionsThe two first simulations were done on a steady case (volumetric power P averaged over onesuper-cycle), for the two beam configurations: one bunch and four bunches per super-cycle. Thetemperature distributions inside the target, arising from these two simulations are shown on Figure 5and Figure 6 on a YZ plane. The maximum temperature reached inside the target is 56oC for one7bunch per super-cycle and 135oC for four bunches per super-cycles. These two figures remain farabove the melting point. At 135oC, the target might get softer in the center but this phenomenon islimited to the center of the cube and should not affect its stability. On the lead surface, the temperaturedo not rise over 70oC thanks to the important heat transfer coefficient (1000 W/m2/K) provided by thewater flow. The lead surface should not get soft at such a temperature.Figure 5: temperature distribution inside the target with water cooling (steady case, 1 bunch per super-cycle).Figure 6: temperature distribution inside the target with water cooling (steady case, 4 bunches per super-cycle).8The two previous simulations, performed in a steady case, gave a good evaluation of thetemperature figures.But the heat power is actually released instantaneously at each bunch (during 6 nanoseconds),which means that the target heating is unsteady. This unsteady effect should have a minor influence onthe temperature distribution in comparison with the steady case. Nevertheless, two unsteadysimulations were carried out for the two beam configurations. The simulation were performed overone super-cycle (14.4 s), the heating power being applied during 6 nanoseconds (eq. (1) with t=6.10-9second):• at time 0s for the first beam configuration,• at times 0, 1.2, 2.4 and 3.6 s for the second beam configuration.In fact, the simulations were done over several super-cycles. Indeed, the target first gets hotterthrough a transient regime and then reaches a permanent unsteady regime. In order to reach faster thisregime, the initial conditions applied were those arising from the steady simulation. The permanentunsteady regime was reached when, for one super-cycle, the temperature distribution inside the targetat time 0 and 14,4 s were identical. The results of these two simulations show that the temperature iso-contour have the same shape as in the steady case and that the lead surface do not rise over 70oC,except on a very small area, around the beam impact point, where the temperature reached 100oC onlythe time of a bunch (6 nanoseconds).Figure 7 shows the time evolution of the maximum temperature reached inside the target duringone super cycle, in the case of four bunches (the time axis scans two super-cycle). It can be seen thatthe temperature reaches 175oC at the fourth bunch, but the average value over one super-cyclecorresponds to the maximum temperature got in the steady case (see Figure 6).Figure 7: time evolution of the maximum temperature in the target (4 bunches)The TOF experiment has been commissioned in the configuration of one bunch per super cycle.Some thermocouples were installed inside the lead, and especially one in the center of the target.Figure 8 compares the time evolution of the temperature measured at this point, with the estimationdone by StarCD. What is worth comparing between the two curves are the temperature amplitude ateach bunch and the average temperature level over one super-cycle. The temperature amplitude9measured by the thermocouple is about 6oC and the one estimated by the calculation is 4oC. Theaveraged temperature level estimated by StarCD is higher of 15oC in comparison with themeasurement, which makes sense since the simulation is done in an unfavorable case (see section 3.3)with an overestimated thermal charge on the target. Nevertheless, it must be mentioned that themeasurement where done in winter when the water temperature provided by the cooling system isabout 20oC, in summer, this temperature is about 30oC, which should reduce of 10oC the shift of 15oCobserved between calculation and measurement.Figure 8 : Time evolution of the temperature in the center of the target (1 bunch) – Comparison with measures.4 OUTSIDE HYDRAULIC CIRCUIT FOR THE TARGET COOLINGA hydraulic circuit, feeding the target tank with cooled demineralised water, was built in the TOFexperimental zone. The principle of this circuit is described on Figure 9. The secondary loop coolsdown the target, and the heat is removed by a heat exchanger, which is connected to the primary waternetwork. This network is itself connected to the cooling towers. In the secondary loop, the variablevelocity pump ensures a constant flow rate (6 l/s) through a PID loop controlled by a set pointmeasured by the flow transmitter (FT). On the primary circuit, the regulating valve (RV) ensures aconstant temperature at target tank inlet through a PID loop controlled by a set point measured by thetemperature transmitter (TT).10Figure 9: cooling circuit principle5 CONCLUSIONA cooling system was designed for the target. It consists of providing a flow circulation inside a tank,where the whole target is immersed. It was shown how the simulations of the target thermal behaviorhelped to define the system, mainly the flow rate figure and the way of ensuring the flow circulationaround the target.The measurements of the temperature reached inside the target during the experiment confirmedthat the simulation did not give an underestimation of the temperature loads, which would have led towrong a design.The target and the tank were installed in the TOF experimental zone; an outside hydrauliccircuit, with a heat exchanger, feeding the tank with cool water was built as well.REFERENCES[1] Fundamentals of Heat and Mass Transfer, Franck P. Incropera, David P. DeWitt, Wileyedition, fourth edition[2] Mechanics of Fluids, Irving H. Shames, Mc Graw-Hill international edition, third edition

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