%X The PBW thermohydraulic analysis is built around the networking of the ESS BILBAO Target Division with the purpose of validate and characterize the PBW System design, in-kind component valued in around 1M€, developed by ESS BILBAO Target Division for the European Spallation Source Target Station. It consists on the CFD and thermal analyses made in addition to the nuclear analysis performed by the neutronic group in order to provide valid inputs to the mechanical analysis.    
The European Spallation Source (ESS) is an under construction 5MW facility which is going to be become the most powerful neutron source. The ESS build is principally composed of the most powerful linear proton beam accelerator, the rotary helium-cooled tungsten target wheel and 22 different neutron instruments for scientific purposes. There are other important elements like multiple laboratories and a super computing data management and software development center. Similarly, to other spallation facilities, there are common elements needed to guarantee the spallation reactions like the heavy metal target, proton accelerator and multiple optics elements to drive the generated neutrons properly to the different instruments. One of these critical components is the separation between the target station and the beam line. For the ESS case, a physical window is required to separate safely both components. The Proton Beam Window (PBW) should be accomplish with strict structural and nuclear requirements. On one hand, the PBW must be thin enough to not interfere with the carefully produced and guided proton beam. On the other hand, the PBW is going to be the first wall of the 5 MW proton beam, hence, heat generation on the window is high enough to need active water cooling.  
Temperature increments for each pulse (14 Hz) should be characterized carefully. For this reason, a Computational Fluid Dynamics model is realized for the PBW cooling channel. This model incorporates the heat generation produced by solving the transport equation for the incident proton beam. These values are provided by the nuclear group using MCNP. Later, the CFD model solves the fluid field coupled to the PBW thermal problem. ANSYS Fluent is used to generate the model. Moreover, a grid of values for the bulk temperatures and film coefficients is exported to ANSYS Thermal as boundary conditions in order to solve the thermal problem of the adjacent PBW bodies conforming the PBW system. Obviously, the heat transport problem is now not coupled to the CFD problem, being able now to perform the thermal analysis to a major number of solids bodies simultaneously.   
Pressure drop must be controlled along the hydraulic system which feeds the different cooling channels of the PBW and adjacent vacuum flanges. For this purpose, CFD models and empirical correlations are used to approximate the pressure losses on the connecting pipes in function of the complexity of each section. Same cooling channels combines different sections with rigid pipes or flex hoses which complicates the analysis. 

 For the CFD model, different cases are studied. Beam pulses are averaged during pulse and relaxation time in order to obtain the steady case heat generation. This case is needed to start the transient case from an approximated value helping to reduce noticeably the computational cost. In the case of thermal problem of the PBW system and shielding, it is not needed the characterize temperatures with much precision. Additionally, operational cases are evaluated too.  
Proton Beam Window maximum temperature is restricted to 60ºC due to the radiation damage which downgrades the structural function of the aluminum. This temperature threshold is linked with the protons energy and the beam current, on this specific component. Helium and other light species are generated due to the proton radiation at higher temperatures these species are diffused forming accumulations that change the component shape (swelling), and in last term, cracking the component. The different analyses show how maximum temperature in normal operation does not exceed 50ºC on the PBW. Temperatures on the stainless steel vacuum flanges and shielding do not affect the PBW but higher temperatures are reached <200ºC.  
Thermal contact resistances are approximated using plastic correlations in order to take account all possible effect that compromise the design. In this case, these temperatures are allowed according to the requirements. Consequently, it is not needed to apply any change on the PBW system and shielding.  
However, the thermohydraulic analysis reveals conflicts on specific components in order to accomplish with minor requirements. Connecting pipes diameters needs to be modified for the current mass flow (0.5 kg/s on the PBW cooling channel) until pressure drop accomplishes with the interface criteria (0.65 bar). Correcting these diameters, the hydraulic system achieves all the requirements. 
From the thermohydraulic point of view, achieving all the commented requirements ensures the correct behavior of the PBW. Then, temperatures could be exported in order to perform further analyses.
%I Industriales
%A Jorge Linde Cerezo
%L upm65533
%D 2020
%K Spallation, neutron, fluid dynamics, cooling system, hydraulic
%T Thermal Analysis of the ESS Proton Beam Window