Liquid metal heat pipes for fusion application

Fusion Engineering and Design 42(1998)373–379Liquid metal heat pipes for fusion applicationA.Makhankov a,*,A.Anisimov a ,A.Arakelov b ,A.Gekov b ,N.Jablokov a ,V.Yuditskiy b ,I.Kirillov a ,V.Komarov a ,I.Mazul a ,A.Ogorodnikov a ,A.Popov baEfremo 6Institute ,St .Petersburg 189631,RussiabRSC ‘Energia ’,Korolyo 6,Moscow Region 141070,RussiaAbstractLiquid metal heat pipes (HP)possess excellent heat transfer characteristics and are now widely used in space and atomic industries as very attractive fully autonomous devices.However,their application in magnetic field devices is restricted due to very scare data on the influence of magnetic field on heat transfer parameters of liquid metal HP.The main objective of the present experiments is to determine the maximum surface heat flux removed by HP in magnetic field.Specially designed tubular sodium heat pipe has the following features:capillary structure which consists of metallic felt in the evaporation zone and a perforated screen in the condensation zone;the structural material is niobium alloy.Experimental set-up consists of a vacuum chamber placed between the poles of an electromagnet which can ensure magnetic field strength up to 1.5T in the vacuum chamber.The direction of the applied magnetic field is transverse to the liquid metal flow in the heat pipe.The set-up provides a possibility to incline the vacuum chamber up to 45°relative to the horizontal axes.The dependence of critical heat flux of the heat pipes as a function of magnetic field strength up to 1.5T has been studied.The concept of fusion reactor plasma facing components cooled by liquid metal heat pipes is presented.©1998Elsevier Science S.A.All rights reserved.1.IntroductionDivertor of future thermonuclear reactor is a sacrificial part of the reactor specially designed to shift the great part of plasma interaction problems from the main tokamak vessel into remote vol-ume.Therefore all divertor plasma facing compo-nents (PFC)are exposed to extremely high and quite peaked particle and heat fluxes that lead to erosion of PFC armour and excessive stresses in structure respectively.The lifetime of divertor PFC and divertor as a whole in such operational conditions will be rather limited.The estimatedlifetime for ITER divertor is about 1000–3000full shots depending on the scenario.That is much less than required,i.e. 30000shots planned for ITER.The replacement of the divertor results in a lot of problems associated with full stop of the reactor for several months and burial of nuclear waste.From an economical point of view,this procedure is very expensive,and fusion reactor can become economically unattractive.The arrangement of the heat removal system is critical for any high energy device and strongly influences its design.In this respect,liquid metal heat pipes (HP)are very attractive due to their excellent heat transfer properties and,particu-larly,their full autonomy.Heat flux in the vapour*Corresponding author.0920-3796/98/$19.00©1998Elsevier Science S.A.All rights reserved.PII S0920-3796(98)00216-6A.Makhanko6et al./Fusion Engineering and Design42(1998)373–379 374channel of HP can reach hundreds MW m−2for sodium and lithium HP at nearly isothermal tem-peratures along the direction of the heatflow [1,2].Therefore,effective thermal conductivity of HP is very high and exceeds the thermal conduc-tivity of copper and silver by\104times.How-ever,commonly designed HPs handle rather moderate(51MW m−2)surface heatfluxes in spite of very high heatflux in the vapour channel (\200MW m−2for lithium heat carrier)[2]. During the last30years,heat pipes have been widely used in nuclear and space industries in operational conditions similar to those in toka-mak:high heatfluxes and severe neutron radia-tion.Wide experience in manufacturing and understanding is achieved.Nowadays,heat pipe is a rather cheap and reliable device and can operate over a wide temperature range from100to 2500°K depending on heat carrier.To overcome the problem of full replacement of the divertor in the case of failure,many ap-proaches of in situ repairs have been suggested. The concept of divertor PFC cooled by HP is suggested in this paper.A similar approach for the First Wall has been proposed by Kovalenko et al.[3].The main advantage of the proposed con-cept is the absence of direct contact of armour with permanent heat sink which allows the re-placement of the damaged element separately. Incident heatflux is removed by HPs and trans-ferred by thermal radiation through the gap be-tween HP walls and permanent heat sink.To evaluate the feasibility of proposed concept in the fusion environment(concentrated heatflux and magneticfield),specially designed heat pipes were manufactured and tested to examine the dependence of critical heatflux on magneticfield.2.Heat pipe designHeat pipe is a fully autonomous device consist-ing of closed volume with inner surface covered by the porous structure of wick.The working fluid evaporates in the heated area,and the va-pour moves towards the heat removal zone and condenses there.Capillary forces in the wick push thefluid back towards the evaporation area.Heat transfer occurs due to subsequent processes of evaporation and condensation.The process is nearly isothermal with a small temperature drop. The physical properties of the workingfluid,espe-cially surface tension,latent heat of evaporation and pressure of saturated vapour greatly influence the heat transfer properties of HP.Lithium and sodium are most suitable for removal of high heat fluxes[4],and have operational temperature ranges from1000to1600°C and from500to 1000°C,respectively.In the liquid metal HP,electrically conducting liquidflows from the condensation zone to the evaporation zone through the capillary structure with the velocities of several cm s−1.As the magneticfield strength reaches6T in fusion environment,we should investigate the influence of such afield on the energy transfer parameters of the heat pipe.Theoretical analysis[5]shows that the influence of the magneticfield can be compensated for by proper choice of capillary structure.However,this analysis concerns heat flux in the vapour channel,and in the case of plasma facing application the main requirement on HP is the capability of removing concentrated surface heatflux with relatively moderate heat flux in the vapour channel.In order to study HP performance in magnetic field,four tubular sodium heat pipes(Fig.1, Table1)especially designed to remove concen-trated heatflux have been manufactured.There exist two main causes for limiting surface heat flux removed by HP:the boiling of superheated (metastable)heat carrier in the evaporation area and the drying of the wick structure due to mis-balance of the evaporation rate and the liquid phase return rate.Thefirst effect does not depend on the magneticfield and is strongly affected by the purity of the heat carrier and pore size.On the contrary the second one is influenced by the mag-neticfield through the additional pressure drop in the liquid phase due to MHD resistance.To reduce the influence of the above mentioned ef-fects,different wick structures have been applied in the evaporation and the condensation areas. Application of metallic felt wick in the evapora-tion structure is the best choice since it is resistant to rapid drying of the wick so this type of wickA.Makhanko6et al./Fusion Engineering and Design42(1998)373–379375Fig.1.Tested liquid metal heat pipe.operates more stably than the mesh structure. Even in the case of partial drying of the wick due to vapour bubbles in larger pores,the remaining part of the wick can ensure heat removal.How-ever,such structure possesses rather high hy-draulic resistance,so it is not reasonable to use it in the condensation area.In order to reduce the hydraulic resistance and compensate unfavourable influence of magneticfield,wick with compara-tively large cross section forfluidflow is applied here.It consists of a perforated screen placed coaxially to the HP walls with the gap of0.5mm. Such a complex wick structure allows the removal of the concentrated heatflux in the evaporation area and at the same time has comparatively low hydraulic resistance in the remaining parts.In order to facilitate heat transfer from the HP to heat sink,the outer tube wall is covered with black coating with emissivity of 0.85.3.Test without magneticfieldAll the heat pipes were tested without magnetic field in horizontal position and showed the fol-lowing values of maximum absorbed heatflux (critical heatflux)at wick structure in evaporation area: 3.75, 4.2,4,29, 5.3MW m−2.Increasing power input above these levels leads to failure of HP detected from rapid rise of temperature in the evaporation area and simultaneous temperature drop in the condensation zone.The reason for failure is the drying of the wick in the evaporation area.The best results have been achieved on the HP type2with molybdenum felt wick in the heated area.The heat pipe of type1was tested at different angles relative to horizon with the heated area below the condensation area.It can be seen in Fig.2that even the slightest inclination consider-ably enhances( 20%)the critical heatflux since gravity facilitates liquid metal to return to the evaporation area and increases stability against boiling.Testing at higher inclination angles was not performed due to restricting capability of the testing device at this stage of investigation.Table1Heat pipe properties1HP type2Heat-transportfluid Na Na16.2Heat-transportfluid16.2mass,gr.Structural material of NbNbcondensation areaStructural material of MoNbevaporation areaWick of condensation Perforated Perforatedscreen screenareaMaterial NbNbThickness,mm0.150.15150930150930Hole diameter,m m350930Step between holes on350930 triangle mesh,m m0.590.1Gap between screen0.590.1and inner HP wall,mmWick of evaporation Nb alloy felt Mo feltarea1Number of tested pipers3A .Makhanko 6et al ./Fusion Engineering and Design 42(1998)373–379376Fig.2.Dependence of HP critical heat flux on gravity.The experimental procedure was as follows.After the preliminary heating of the HP up to 200°C by means of the additional heater,heat flux was increased step by step and the thermal response of the HP was measured.When steady state was established,the heat flux was increased to the next step.This procedure was repeated until HP failure occurred,registered as a rapid temperature rise at the evaporation zone and a temperature drop at the condensation zone.A magnetic field was applied when HP isothermal steady state of 450°C at the condensation zone was established.The experiments in the presence of a magnetic field have been carried out with HP of type 1.The working temperature of HP was in the range of 650}700°C.The total power transferred by HP is measured by a calorimeter through the enthalpy increase and thermocouples through the thermal radiation law.The incoming heat flux in the evap-oration area was evaluated as:q in =4·Q tot /(y ·d 2)where Q tot is the total transferred power and d is the diameter of heated area of the evaporation zone (see Fig.1).The values of incoming heat flux estimated from enthalpy increase and from ther-mal radiation law differ by no more than 5%.The results of these experiments are shown in Fig.4.It can be seen that critical heat flux decreases with increasing magnetic field strength.The possi-ble reason lies in the increase of pressure drop in4.Test in magnetic fieldThe experimental apparatus for investigation of HP properties in magnetic field consists of a vac-uum chamber placed between the poles of an electromagnet with maximum magnetic field strength of 1.5T.The main parts of the apparatus are shown in Fig. 3.The HP heating system consists of a resistive graphite heater with a maxi-mum heat flux of 10MW m −2.The HP equipped with thermocouples is surrounded by an air cooled sheath.An additional heater makes it pos-sible to heat the incoming air flow up to a temper-ature of 300°C and serves for regulation of HP working temperature and preliminary HP heating.The outer HP wall and the inner sheath wall are covered by a black coating with emissivity 0.85.In order to measure the temperature of the inner sheath wall two thermocouples are mounted on it.Fig.3.Scheme of set up for investigation of HP performance in magnetic field.A.Makhanko6et al./Fusion Engineering and Design42(1998)373–379377Fig.4.Dependence of HP critical heatflux on magneticfield.the liquid phase due to MHD effects.However,a reduction of critical heatflux in magneticfield of 1.5T is only 18%and probably can be compen-sated by HP inclination(see Fig.2).5.Divertor cooled by heat pipesOn the basis of the current understanding of the HP performance,the following principles for the application of HPs in tokamak environment can be postulated:1.For the most loaded components,it is desir-able to use favourable orientation of HP rela-tive to horizon,so that condensation zone should be higher than evaporation zone in order to facilitate heat carrier recirculation and provide stability against boiling in the evaporation wick structure.2.The path forfluid return should be as short aspossible.3.HP should be oriented in such a way thatmagneticfield is parallel to liquid metalflow in the parts with maximumflow rate in the evap-oration zone.One of the possible realizations of the divertor cooling system,taking into account some of the above mentioned principles is presented in Fig.5. The incident heat is removed in two steps by specially arranged HPs and further by thermal radiation through the vacuum gap to the main permanent heat sink.The main idea of this con-cept is to avoid direct contact of armour with permanent heat sink.Since there is no brazed or welded joint,any damaged element can be re-placed separately.The PFC surface is assembled from the ar-moured HPs of thefirst level that absorb surface heatflux and transfers it to the opposite side.The heat pipe of the second level carries the heat away and transfers it to the permanent heat sink through side wall thermal radiation.Thefirst level heat pipe serves to remove highly peaked heat loads and not only transfer the heat to the oppo-site side wall but spread it along the HP towards the area with smaller heatflux.Therefore,the distribution of the heatflux carried away by the second level HP will be nearly uniform.Due to remarkable isothermal properties of HP,nearly the whole structure excluding the armour has a uniform temperature,which reduces global stresses in the divertor structure.In this design,it is possible to replace each element separately in the case of damage.Since radiative heatflux from HP to the permanent heat sink is sufficiently low, i.e. 0.1MW m−2,stainless steel can be used as a heat sink material.The dimensions of the element depends on the HP working temperature and the average heat flux onto the plasma facing surface of the diver-tor.An example for the ITER divertor is pre-sented in Table2.It should be noted that for the ITER divertor,only lithium heat pipe can be used for cooling the most loaded components withA.Makhanko6et al./Fusion Engineering and Design42(1998)373–379 378peak heat loads of15MW m−2.Additional experiments are required to confirm the ability of lithium HP to remove concentrated heatflux of15 MW m−2.6.ConclusionsFrom experimental study on HP behaviour in magneticfield,it has been shown that criticalFig.5.Divertor on the base of heat pipes..A.Makhanko6et al./Fusion Engineering and Design42(1998)373–379379Table2Parameters of divertor cooling systemC or WMaterial of plasmafacing armourModeratelyDivertor components Highly loadedloaded compo-componentsnentsPeak heatflux15MW m−23MW m−21MW m−21MW m−2 Average heatfluxNaHP heat carrier Li700Heat pipe working1100 temperature,°CW or VW or VHP structural mate-rial0.8 Emissivity of HP and0.8heat sink wallsDimensions(see Fig.5)22a,cm1b,cm1c,cm1110l,cm25f,cm0.80.80.04Heatflux to perma-0.1nent heat sink,MW m−2SS SS Permanent heat sinkstructural material rather low( 3.5MW m−2)for cooling the most loaded ITER plasma facing components with peak heatflux of 15MW m−2.However, sodium HP can be successfully applied for moder-ately loaded,i.e.53MW m−2,reactor elements, e.g.diagnostic elements,moderately loaded diver-tor parts and so on.Application of lithium heat pipe with higher heat transfer performance proba-bly can allow the use of HP for cooling all high heatflux reactor elements.The presented divertor concept based on HPs allows the reduction of stresses in structure due to thermal gradients and the separate replacement of the damaged elements without disturbing the per-manent heat sink associated with cutting of the cooling tubes.References[1]M.N.Ivanovsky,V.P.Sorokin,I.V.Iagodkin,PhysicalPrinciples of Heat Pipes,Atomizdat,Moscow,1978. [2]P.I.Bystrov,D.N.Kagan,G.A.Krechetova,E.E.Shpil-rain,Liquid–Metal Coolants for Heat Pipes and Power Plants,Hemisphere,New York,1990,p.272.[3]V.Kovalenko,V.Khripunov, A.Antipenkov, A.Ulianov,Heat-pipes-basedfirst wall,Fusion Eng.Des.27 (1995)544–549.[4]H.Bolt,W.Kohlhaas,R.Duwe,A.Gervash,J.Linke,I.Mazul,Heatflux experiments on heat pipes for plasma facing application,18th Symp.on Fusion Technology, Karlsruhe,Aug.22–26,1994,pp.383–386.[5]G.A.Carlson,M.A.Hofmann,Heat pipes in the mag-netic-field environment of a fusion reactor,Trans.ASME (1972)282–288.surface heatflux depends on magneticfield rather weakly( 20%)in the range0–1.5T.The unfa-vourable influence of magneticfield can be com-pensated for by proper HP inclination relative to the horizon.Critical heatflux for sodium HP is.。