Thermal Contact Resistance and Ambient Temperature Effects on the Cooling of Mo 99 Plate Targets inside the Hot Cell

Transcription

1 Theral Contact Resistance and Abient Teperature Effects on the Cooling of Mo 99 Plate Targets inside the Hot Cell Abou Elaaty Talal, Reactors departent, Atoic energy authority, Egypt ABSTRACT The Cooling of Low Enriched Uraniu (LEU) irradiated fuel p late targets that used for Molybdenu -99 productions, after their extraction fro the nuclear reactor core require a good degree of interest. This is to satisfy two iportant criteria s, the first keeping the radiation dose liits within the safety liits to protect the worker and the second to keep the integrity of the plate itself for easy handling and anoeuvring at the different stages. The cooling processes of the plate targets are passing through different odes, by forced convection cooling using water coolant inside the core during irradiation, by natural convection cooling using water coolant inside the reactor ain tank during the decay period and finally by air convective and radiation inside the hot cell during their loading. In this study, a steady state atheatical odel was developed to study the effect of theral contact resistance and the abient air teperature inside the hot cell on plate targets cooling by free convection during an abnoral condition of lose of electrical power supply, which eans no ventilation syste, is available. In this siulation, it assued that three targets were unloaded on the table inside the cell in the for of pile, one target on the top of the other, as a purposed huan error. The axiu pile teperature variation with target power ratio (target power in hot cell relative to target axiu power during irradiation) was analyzed and calculated. Also the probable teperature variation against different power ratios for one free vertical plate target hold by the Tele-anipulator, exposed to air free convection and theral radiation on its both sides, during the loading process was estiated in this study. This study was clarified that, the contact resistance and the abient teperature have an essential effects on the targets plate cooling inside the hot cell as will be clarified in that work. Key words: Mo 99, Plate targets, hot cell INTRODUCTION The target used for Mo-99 production is a aterial containing uraniu- 235 that is designed to be irradiated in a nuclear reactor. The target is designed to satisfy several requireents: First, it ust be properly sized to fit into the irradiation position inside the reactor. Second, it ust contain a sufficient aount of U-235 to produce the required aount of Mo-99 when it is irradiated. Third, it ust have good heat transfer properties to prevent over-heating (which could result in target failure) during irradiation. Fourth, the target ust provide a barrier to the release of radioactive products, especially fission gases, during and after irradiation. Fifth, the target aterials ust be copatible with the cheical processing steps that will be used to recover and purify Mo-99 after the target is irradiated as indicated in Coittee on Medical Isotope Production without Highly Enriched Uraniu (2009). To eet these criteria, targets are fabricated in a wide variety of shapes and copositions to eet the needs of individual Mo-99 producers. Targets ay be shaped as plates (Figure 1), pins, or cylinders. Target copositions include uraniu etal, uraniu oxides, and alloys of uraniu, nearly always with aluiniu. Metallic targets are typically encapsulated in -375-

2 aluiniu or stainless steel to protect the cheically reactive uraniu etal or alloy and to contain the fission products produced during irradiation. This encapsulation is referred to as the target cladding. Soeties an interediate barrier aterial such as aluiniu or nickel is used to separate the cladding fro the U-235 target aterial. FIG. 1 CNEA s high-density LEU-aluiniu dispersion targets. These targets have been used since 2002 to produce Mo-99 in Argentina. The target is approxiately 15 c in length. SOURCE: Courtesy of Pablo Cristini, CNEA, Argentina, Coittee on Medical Isotope Production without Highly Enriched Uraniu (2009) 1.2. Irradiation of Targets in a Nuclear Reactor Mo-99 is produced in the uraniu-bearing targets by irradiating the with theral neutrons. Soe of the U-235 nuclei absorb these neutrons, which can cause the to fission. The fission of the U-235 nucleus produces two but soeties three lower-ass nuclei referred to as fission fragents. Approxiately 6 percent of these fission fragents are Mo- 99 atos. Nuclear reactors provide an efficient source of theral neutrons for Mo-99 production. This is why all ajor Mo-99 producers irradiate their targets in nuclear reactors. The aount of Mo-99 produced in a target is a function of irradiation tie, the theral neutron fission cross section for U-235, the theral neutron flux on the target, the ass of U- 235 in the target, and the half-life of Mo-99. For typical reactor theral neutron fluxes on the order of neutrons per square centietre per second, irradiation ties of about 5 to 7 days are required to achieve near-axiu Mo-99 production in the targets, Coittee on Medical Isotope Production without Highly Enriched Uraniu (2009). Beyond these irradiation ties, the aount of Mo-99 produced in the targets approxiately balances the aount of Mo-99 being lost to radioactive decay, so further irradiation is not productive. Even at axiu production, only about 3 percent of the U-235 in the target is typically consued. The reaining U-235 along with the other fission products and target aterials are treated as waste. Techniciu-99 (99Tc) has a 6 hour half-life and eits a gaa ray when it de-excites. Attached to various cheicals, it can be followed by its gaa eissions through the body and thereby can be used to exaine the functioning of various organs. Its short half-life and lack of beta radiation iniizes unnecessary radiation doses. It is derived fro olybdenu-99 ( 99Mo), which has a half-life of 2.7 days and decays into 99Tc. 99Mo is adsorbed onto the surface of a bed of sall aluina particles in generators fro which the 99Tc decay product is drawn off in solution

3 1.3. Contact resistance Contact resistance is priarily caused by the iperfect contact between two surfaces due to the presence of icroscopic asperities of engineering surfaces. The real area of contact for such surfaces is only a sall fraction ( 1-2%) of the noinal contact area even at very high pressure, Bowden and Tabor (1950). In spite of that, ost of the heat through the interface flows through the actual contact spots, as the theral conductivity of these contact spots is uch higher than that of the surrounding gap. That reason causes constriction of heat flux lines through the bulk solid aterial in the vicinity of the contact interface, which leads to constriction resistance at all of the contacting asperities on a surface. A finite eleent analysis was carried out in order to investigate the role of theral contact resistance on heat anageent within a siple central processing unit (CPU)/heat sink assebly. A special attention is paid in assessing the effect of surface roughness characteristics, echanical and theral properties of the contacting bodies, applied contact pressures and the use of theral interface aterials on the axiu teperature experienced by the CPU. Two classes of theral interface aterials are shown in Figure 2: (a) phase-change aterials and (b) acrylicor silicone-based tapes are considered. The results clearly reveal that plastic deforation of icro-contacts (prooted by high contact pressures and lower icro-hardness levels) and the use of theral interface aterials which eliinate (high theral resistance) icro -gaps can significantly lower the overall CPU/heat sink theral contact resistance and facilitate heat anageent. It is also shown that the retention of asperity icro-contacts and good wetting of the ating surfaces by the theral interface aterial are critical for achieving an effective reoval of the heat generated by the CPU, Grujicic et. al (2005). Fig. 2. A scheatic of two contacting bodies: (a) in direct contact and (b) separated by a theral interface aterial. The arrows denote the heat flow across the interface The general theory adopted for the prediction of theral contact resistance was odified by using the guarded hot plate ethod for theral conductivity easureents. A good estiation of theral resistance with a variety of interface aterials was predicted. The effects of pressure, aterial hardness, surface roughness and theral properties of interface aterial on the theral resistance between two sooth steel surfaces were studied, Wolf and Schneider (1998). A predictive odel for estiating theral contact resistance between two noinally flat rough surfaces was developed and experientally validated. In that study, the actual area of contact for each contact spot was calculated, also the effects of constriction resistance and gas gap conductance between the contacting surfaces were calculated by Vishal Singhal et.al. ( 2005). generally the theral contact conductance or resistance is of interest in any fields including internal cobustion and superconductors

4 2. Theral odel A steady state, one diensional axial heat flow odel is built to siulate the in pile of three irradiated Mo-99 plate targets rested on a table inside the hot cell as a result of an operator error during the handling and transporting process of the targets, as shown in Figure 3. The table is siulated as a four diagonal strips each with length L d and cross sectional area W d t t, while the table leg length is L leg and leg cross sectional area is A leg Fig. 3 Three plate s in-pile for on a table The in pile siulation of the three irradiated plate targets is carried out for different power ratios (plate decay power to plate noinal powe r during irradiation) to calculate the axiu probable teperature inside each plate to ensure the liit of blistering of the Aluiniu cladding ( 400 o C) for safe handling inside the hot cell without releases of radioactive nuclides or to avoid deforation during the transportation to the process plant. The theral resistances for heat transfer through each plate includes the theral resistance of the clad, the theral resistance of the fuel eat and the contact resistance between each two contact plates or between the botto plate surface and the stainless steel table. A convective and theral radiation boundary condition was considered for the upper plate so; the siulation was carried out at two abient teperatures to evaluate the abient teperature effects. During the noral procedures of handling of the irradiated plate targets inside the hot cell, holding the plate by the tele-anipulator tool free of contact with the other plates or with the table, only expose to convective and theral radiation boundary condition on both faces, the plate target siulation during that situation is carried out also in this study. The upper plate steady state energy balance equation; R. Qgen. 2 δ al k al.a T1 δ k.a T2 Qrad Qconv TCC al al The interediate plate energy balance equation; T 2 T 1 T 2 T R. Q 3. gen al al.... kal A k A TCCalal kal A k A TCCalal The third plate energy balance equation; T 3 T T 3 T R. Q 2. inv gen al L d L leg al k al A k A TCC al ss k ss W d t t k ss A leg k al A k A TCC al al (1) (2) (3) The upper plate boundary condition; T1 Ts1 Qrad Qconv 0.0 al. 2.. kal A k A (4) -378-

5 The energy balance equation for a free vertical plate; R. Q gen 2. Q rad 2. Qconv 0.0 (5) Heat transfer by free convection; The Rayleigh nuber is defined as follows 3. A 2 g T. s 1 Ra Ts1 Tinv.. P (6) For a horizontal plate, Nusselt nuber can be calculated as follows [6]; ( ) if Ra ( ) Nu = 0.54Ra Ra "error" otherwise if Ra (7) For a vertical plate, Nusselt nuber can be calculated as follows [6]; 0.387Ra Nu (8) / Pr9 16 h = Nuk L (9) Qconv.. A. (10) h T s 1 Heat transfer by theral radiation; Qrad.. A. T 4 4 s 1 T inv (11) The conductance between two Aluiniu plates (TCC al-al ) or between Aluiniu and stainless steel (TCC al-ss ) were adopted according to pressure and surface roughness as shown in references [5] and [6]. RESULT AND DISCUSSIONS The previous syste of equations through 1 to 5 was nuerically calculated by using the MATHCAD software to deterine the axiu teperature in each plate fro the table surface. Two leading paraeters were supposed in that siulation, the first paraeter was the axiu power that could be generated inside the irradiated Mo-99 plate target during the reactor full power operation (Q gen = 10 Kwatt for theral neutron flux of nv), while the second paraeter was the power ratio paraeter (R= decay power/noinal power). Different power ratios (0.16, 0.18, 0.20 and 0.22) were tested during the siulation to ensure closeness fro the cladding blistering liit ( 400 o C) at the purposed case study (Q gen = 10 Kwatt). For a different axiu generated powers (Q gen ) inside themo-99 plate target, there were anther different power ratios should be tested to attain the cladding blistering liit. Figure 4 shows the in pile siulation of three plates at a power ratio of 0.16% with and without contact resistance consideration for two different environental teperatures (25 o C and 50 o C) to siulate the purposed winter or suer conditions inside the hot cell. In this -379-

6 siulation, no ventilation syste is available inside the hot cell, so a free convection and theral radiation boundary conditions were considered. 330 Teperature, o C Plates, Tep. distribution % Power ratio = 0.16% (16Watt/plate) = 50 o C, with contact resistance = 50 o C, without contact resistance = 25 o C, with contact resistance = 25 o C, without contact resistance Distance fro table surface, Fig.4 teperature variation along the in-pile vertical direction For no contact resistance consideration, the teperature distribution of the in pile three plates is approxiately flat with approxiately constant value of o C at an abient teperature of 25 o C. This flat constant value was raised to 321 o C when the abient teperature was increased to 50 o C. For contact resistance consideration, the plate in contact with the table recorded a iniu teperature of oc, a gradual increasing is teperature with distance was observed while oving upward. The axiu teperature was attained at the interediate plate eat with a value of o C, hence, a gradual decreasing in teperature with distance was observed but with little slope till reaching the upper plate surface the recorded teperature value was o C. A siilar trend but with higher values was noticed if the contact resistance was taken into consideration at higher abient hot cell teperature of 50 o C. The plate teperature in contact with the table recorded o C while a axiu teperature of o C was recorded at the interediate plate eat and o C at the upper plate surface. For this siulation, at a power ratio of 0.16%, all the recorded teperature were below the cladding blistering liit (400 o C).Different siulation steps were perfored at different higher power ratios as shown in Fig. 5a and Fig.5b, looking for the power ratio at which the liit of blistering is reached. The results showed the sae trend as in Fig. 4 but with higher values because of the increase in power ratio

7 360 Teperature, o C Plates, Tep. distribution % Power ratio =.18 (18Watt/Plate) = 50 o C, with contact resistance = 50 o C, without contact resistance = 25 o C, with contact resistance = 25 o C, without contact resistance Distance fro table surface, Fig 5a teperature variation along the in-pile vertical direction 390 Teperature o C Plates, Tep. distribution Power ratio =0.2% (20 Watt/ plate) = 50 o C, with contact resistance = 50 o C, without contact resistance = 25 o C, with contact resistance = 25 o C, without contact resistance Distance fro table surface, Fig 5b teperature variation along the in-pile vertical direction For any siulations, the axiu in pile teperature was recorded against the power ratio as shown in Fig.6. This axiu teperature was occurred at the interediate plate. For an abient teperature of 25 o C, the contact resistance effect ade this axiu teperature very close to the blistering liit at a power ratio of 0.22%. For an abient teperature of 50 o C, without the contact resistance effect the blistering liit was exceeded at a power ratio lies between 0.21% and 22% while with contact resistance consideration this blistering liit is occurred between 0.2% and 0.21%

8 Max. teperature o C Plates ax. teperature = 50 o C, with contact resistance = 50 o C, without contact resisitance = 25 o C, with contact resistance = 25 o C, without contact resistance Tep. Liit = 400 o CFunction Plot % Power Ratio Fig.6 axiu teperature variation with power ratio % of heat transfer Conduction Convection Thera Radiation Fig.7 percentage of the aount of heat transfer by different echaniss Figure 7 shows the average percentage of the aount of heat transfer by different echaniss through the siulations at different power ratios. The aount of heat transfers by conduction though the table legs showed the heights rate of heat transfer, about 60%. While the heat transfer by free convection averagely recorded 30% and the reainder 10% for the heat transfer by theral radiation during this siulation. Solution of equation (5) for one plate under convective and theral radiation boundary conditions on both sides inside the hot cell gave a good estiation for the axiu expected surface teperature of the plate. This siulation was carried out at two different abient teperatures also as shown in Fig.8. The plate surface teperature increases with the increase in power ratio. The increase in abient teperature increases the expected plate surface teperature. The difference in plate surface teperature for the two abient teperatures (25 o C, 50 o C) was approxiately constant at a value of 22.1 o C. In coparing a one holding plate ( not in contact with the table) axiu surface teperature and the three in-pile plates (laid on table) axiu teperature for the purposed power ratio, it was shown a great difference in teperature can lead to blistering liit for the in-pile plates

9 220 Surface Teperature, o C One plate free in hot cell = 50 o C =25 o C % Power Ratio Fig.8 Free plate surface teperature variation with power ratio CONCLUSION Generally, transferring the irradiated Mo-99 plate targets fro the water to the abient air inside the hot cell expose the to a poorer cooling due to the change in ediu theral properties. Keeping the Mo-99 plate target in a vertical position during the handling is better than rest it horizontally on a table. The hot cell operator not allowed putting the plates in the in-pile for because the contact resistance between plates and the high abient teperature could raise the plate teperature to the blistering teperature liit. The ventilation syste inside the hot cells plays as a strong factor for liiting the plate teperature rise, because it supplies the cell with forced cooled air, that ake the ode of plate cooling is a forced convection instead of free convection and prevent the hot cell teperature rise. Noenclature A Plate surface area 2 A leg Table leg cross section Area 2 h Heat transfer coefficient W/ 2. o C k Theral conductivity W/. o C L leg Leg length L d Table diagonal strip length Nu Nusselt nuber P perieter Pr Prandtl nuber Q gen Heat generation per plate W Q conv Convective heat flow W Q rad Irradiative heat flow W R Plate power ratio Ra Rayleigh nuber T Teperature o C TCC Conductance W/K t Table thickness W t Table diagonal strip width Greek sybols al aluiniu cladding thickness -383-

10 fuel eat thickness α Theral diffusivity 2 /s ν Kineatic viscosity 2 /s eissivity factor σ Stefan-Boltzann constant W/ 2.K 4 Subscripts al al-al al-ss ss S1 inv aluiniu cladding aluiniu to aluiniu contact aluiniu to stainless steel contact stainless steel Meat of fuel Upper plate eat Interediate plate eat lower plate eat Upper plate surface environent REFERENCES 1. Bowden, F. P., Tabor, D., The Friction and Lubrication of Solids, Oxford University Press,London. 2. Coittee on Medical Isotope Production without Highly Enriched Uraniu,2009. Medical Isotope 3. Production without Highly Enriched Uraniu, National Research Council, ISBN: , 220 pages, 6 x 9, 4. Grujicic, M., *, Zhao,C.L., Dusel, E.C., The effect of theral contact resistance on heat anageent in the electronic packaging, Applied Surface Science 246, Incropera, F. C., Dewitt D..P.,,1996. Fundaentals of Heat and Mass Transfer,4th ed., John Wiley&Sons, New York.. 6. Wolf, E. G., Schneider, D. A., Prediction of theral contact resistance between polished surfaces, International Journal of Heat and Mass Transfer 41, Singhal, V., Litke, J. P., Black, F. A., Gariella, V.,2005. An experientally validated theroechanical odel for the prediction of theral contact conductance, International Journal of Heat and Mass Transfer 48,