Structure design and compression experiment of the supporting node for JUNO PMMA detector

Jiangmen Underground Neutrino Observation is going to build a huge polymethyl methacrylate (PMMA) detector to hold 20,000 tons of liquid scintillator to capture neutrinos in a 700-m underground cave. This PMMA detector has a spherical shape with an inner diameter of 35.4 m and is supported by an outside stainless steel structure through 590 supporting nodes. The maximum compression force applied to these supporting nodes would be about 150 kN when the detector is running. This paper focuses on the design and validation of the PMMA supporting node and compares the effects of two gaskets (round gasket and ring gasket) on the supporting node stress and ultimate compression load. An innovative PMMA supporting node structure is first proposed, and a 1/4 symmetric model with the material nonlinearity and frictional contact boundary is established in the finite element analysis (FEA). The FEA results show that the principal stress of the structure is less than 3.5 MPa and the Mises stress is less than 5.5 MPa. The stress and deformation at the groove of the supporting node using the ring gasket are smaller than that using the round gasket. The compression experiments of the supporting node using two types of gaskets were conducted to study the effect of gaskets on the ultimate compression load of supporting node. The ultimate compression load of the supporting node with ring gasket is larger than 900 kN, which is six times of design load. In a comparison of experimental results with FEA, the maximum difference is 15.78%, demonstrating the validity of FEA results. Through the material test of PMMA and experiment of the PMMA supporting node, it is known that PMMA is a brittle material and it is very sensitive to sharp corner defects that should be avoided in the design of PMMA structure.