1. Conclusions 1.The CFD simulation demonstrated that the heat removal systems provide sufficient cooling capacity to prevent fuel overheating, the maximum temperature reached by the fuel (83.94 ˚C) was smaller to the allowable temperature limit (90 ˚C). 2.The temperature of the hottest spot in the reactor core is about 8.5 ˚C above the average core temperature. 3.The neutronic calculations demonstrated that the reactor is able to produce 370 six-day curies of 99 Mo in 5 days operation cycles. 4.The discharge burn-up after 10 years without taking into consideration the reactivity feedbacks introduced in the solution by the volumetric expansion of the fuel solution reach 24.79 GWday/tonU. 5.The consumption rate of fissile material 235U is insignificant (0.46 g of 235U / 5 day-batch) and the plutonium buildup in 10 years operation is 0.03 Kg. 6.A total volumetric expansion of the fuel solution due to thermal expansion of the fuel solution and the void volume generated by radiolytic gas bubbles of 0.001832 m 3 was obtained. 7.The reactivity feedback introduced in the solution by the volumetric expansion of the fuel solution (-2660 pcm) for the fresh fuel, represents the 46% of the planned initial reactivity reserve in the core 5800 pcm, it provokes a reduction in the burn-up achievable in the reactor form 24.79 GWday/tonU to 12.61 GWday/tonU. 8.The conducted studies contribute to demonstrate the feasibility of using AHR for the production of medical isotopes, however further studies are needed to confirm these results and contribute to development and demonstration of their technical, safety, and economic viability. Thermal-hydraulics results After evaluating the heat removal systems for the ARGUS designed thermal power (20 kWth), the uniform volumetric heat generation rate was increased to 75 kW, for this value, the average temperature in the core reach 120.52 ˚C. This value is above the design limit of temperature of 90 ˚C. To solve this problem, the number of coiled cooling pipe inside the core was increased from one to five, for this new configuration the maximum fuel temperature in the core reach 83.94 ˚C. Neutronic results OPTIMIZATION STUDY AND NEUTRONIC AND THERMAL-HYDRAULIC DESIGN CALCULATIONS OF A 75 KWTH AQUEOUS HOMOGENEOUS REACTOR FOR MEDICAL ISOTOPES PRODUCTION Daniel Milian Pérez, Daniel E. Milian Lorenzo, Carlos A. Brayner de Oliveira Lira, Lorena P. Rodríguez Garcia, Manuel Cadavid Rodríguez, Jesús Salomón Llanes, Carlos R. García Hernández Instituto Superior de Tecnologías y Ciencias Aplicadas, InSTEC, Cuba Universidade Federal de Pernambuco, UFPE, Brasil Tecnología Nuclear Médica Spa, TNM, Chile Abstract: 99m Tc is the most common radioisotope used in nuclear medicine. It is a very useful radioisotope, which is used in about 30-40 million procedures worldwide every year. Medical diagnostic imaging techniques using 99m Tc represent approximately 80% of all nuclear medicine procedures. Although 99m Tc can be pro duced directly on a cyclotron or other type of particle accelerator, currently is almost exclusively produced from the beta-decay of its 66-h parent 99 Mo. 99 Mo production system in an Aqueous Homogeneous Reactor (AHR) is potentially advantageous because of its low cost, small critical mass, inherent passive safety, and si mplified fuel handling, processing and purification characteristics. In this paper, an AHR conceptual design using Low Enriched Uranium (LEU) is studied and optimized for the production of 99 Mo. Aspects related with the neutronic behavior such as optimal reflector thickness, critical height, medical isotopes production and the reactivity feedback introduced in the solution by the volumetric expansion of the fuel solution due to thermal expansion of the fuel solution and the void volume generated by radiolytic gas bubbles were evaluated. Thermal-hydraulics studies were carried out in order to show that sufficient cooling capacity exists to prevent fuel overheating. The neutronic and thermal- hydraulics calculations have been performed with the MCNPX computational code and the version 14 of ANSYS CFX respectively. The neutronic calculations demonstrated that the reactor is able to produce 370 six-day curies of 99 Mo in 5 days operation cycles and the CFD simulation demonstrated that the heat removal systems provide sufficient cooling capacity to prevent fuel overheating, the maximum temperature reached by the fuel (83.94 ˚C) was smaller to the allowable temperature limit (90 ˚C). The temperature of the hottest spot in the reactor core is about 8.5 ˚C above the average core temperature. SHINE Subcritical ADS AHR, United States MCNPX Benchmark Critical experiments performed at the Russian Research Center “Kurchatov Institute” in 1980-1981 [12] were used. The estimated uncertainty in experimental data for the Keff is ±0.0050 for all configurations. Table 1. Configurations of Benchmark Critical Experiments. Table 2. Results of calculations with the MCNPX and MCU code. Keff results for configurations 1 and 4 are in the estimated uncertainty for the experimental data. In configuration 2 and 3 calculated Keff values range from about 1.7% to 2.5% around the benchmark Keff values. CFD Benchmark Figure 8. Keff vs. reflector thickness. No. Uranium Concentration (gU/liter) Vessel Thickness (cm) Critical Volume (liters) Solution Density (g/cm 3 ) 1263.3 ±0.50.5 ±0.0128.50 ±0.031.3373 ±0.0013 2382.2 ±0.80.5 ±0.0118.40 ±0.031.4945 ±0.0014 3382.2 ±0.80.3 ±0.0117.60 ±0.031.4945 ±0.0014 4505.0 ±1.00.5 ±0.0115.55 ±0.031.6527 ±0.0015 No. MCNPX (This paper)MCU [12] Keff ± standard deviation 10.99840 ±0.00014 0.9855 ±0.0013 20.98330 ±0.00014 0.9807 ±0.0013 30.97488 ±0.00014 0.9777 ±0.0013 41.00285 ±0.00014 0.9735 ±0.0014 Figure 2. Geometrical model of the reactor on the Visual Editor of the MCNPX. ParameterValue Fuel solutionUranyl sulphate Enrichment19.8 % Uranium concentration ∼ 390 g/L Inner core diameter30.5 cm Reactor height65.6 cm Reactor vesselStainless steel Vessel thickness0.5 cm Reflector (radial)Graphite Solution Density1.4382 g/cm 3 Solution Volume25.5 L Thermal Power75 kW Solution Height44 cm Reactivity Reserve5800 pcm Fuel rechargeAfter 10 years AHR conceptual designs based on the ARGUS reactor LEU configuration, Russian Federation Solution Height (cm) Keff ± standard deviation 15.60.72559 ±0.00082 25.60.92988 ±0.00083 30.60.98423 ±0.00082 31.00.98807 ±0.00082 32.00.99512 ±0.00080 33.01.00324 ±0.00081 34.01.00967 ±0.00082 35.61.02102 ±0.00083 40.61.05027 ±0.00082 Figure 10. Accumulation of 99 Mo for 528 hours of reactor operation. MIPS Medical Isotope Production System, Babcock and Wilcox, United States MIPR Medical Isotope Production Reactor, China LOPO - 1943-1944HYPO - 1944-1949SUPO - 1951-1974HRE 1 and 2 - 1952-1958ARGUS - 1981-2015 AHR Conceptual Design Table 3. The reactor core parameters. Figure 3. AHR core geometries used in the CFD thermal-hydraulics study. One and five coiled cooling pipes models. Figure 3. CFD mesh used in the thermal-hydraulics study of the one coiled cooling pipe model. Figure 4. AHR regions used in the CFD thermal-hydraulics study of the one coiled cooling pipe model. Mesh Skewness ∼ 0.25 Mesh Orthogonal Quality ∼ 0.85 Figure 6. Temperature profile in the central YZ plane of the AHR core. Figure 7. Temperature profile in the central XY plane of the AHR core. Figure 5. Volumetric temperature distribution in the AHR core with a uniform volumetric heat generation rate of 75 kW. As the AHR conceptual design heat removal systems are based on the ARGUS reactor, the first step was evaluating the heat removal systems for the ARGUS designed thermal power (20 kWth). This initial study even though does not constitute a validation task, it contributes to the verification of the developed detailed model of AHR for thermal-hydraulics studies. The volumetric temperature distribution in the AHR core for a uniform volumetric heat generation rate of 20 kWth is presented in Figure 1; the average fuel solution temperature was 75.16 ˚C and the maximum temperature in the core (81.91 ˚C) was located on the top of the fuel solution and is below the design limit of temperature of 90 ˚C. The fuel solution temperature should be below 90 ˚C for avoiding the solution boiling. Figure 1. Volumetric temperature distribution in the AHR core with a uniform volumetric heat generation rate of 20 kW. Table 4. Keff vs. fuel solution height. Figure 9. Keff vs. fuel burn-up. Figure 11. Accumulation of 89 Sr, 133 Xe, 131 I, 132 I and 133 I for 528 hours of reactor operation. Figure 12. Keff evolution for the reactor without bubbles and with bubbles of 1 mm of diameter. The calculated production of 99 Mo using MCNPX is 370 six-day curies in 5 days operation cycles. Others aspects taken into consideration were the 235 U consumption and the accumulation of 239 Pu. Only the 15.4% of the 235 U initially charged was consumed with a consumption rate of 0.46 g of 235 U / 5 day-batch. 239 Pu presents a linear growth proportionally to the time of burnup, reaching only 0.03 Kg at the end of the burnup cycle.