Research on the Main Structure of the Central Detector of Jiangmen Underground Neutrino Observatory (JUNO)

The Central Detector of Jiangmen Underground Neutrino Observatory (JUNO) is a further exploration of neutrino after Daya Bay neutrino experiment. It will measure and rank the mass of three kinds of neutrino. The central detector is the core component of JUNO, which is used to capture the neutrino produced by nuclear reactor or other ways. The overall shape of the central detector is spherical, which is divided into inner layer structure and outer layer structure. The inner layer structure is a PMT shell with a diameter of 35. 4 m (inner diameter), which is used to hold 20 000 tons of liquid scintillation for capturing neutrinos; the outer structure is not only used as the support carrier of PMT (photomultiplier tube), but also provides support for the inner PMMA's sphere; the whole central detector structure is through the column supported at the bottom of the pool.
After the comparison and selection of multiple schemes, the center detector selects the single-layer steel reticulated shell as its main structure scheme. It is composed of a single-layer reticulated shell and an internal PMMA's sphere which are formed by the warp and weft braiding of the outer steel members (bolt connection between the components). The PMMA's sphere is connected with the outer shell through a strut, and the lower part of the shell is connected with the pool bottom through the column. The materials used in the central detector must be low background (low radioactivity), such as PMMA's sphere and stainless steel. All materials in contact with liquid flash and pure water shall meet the compatibility requirements. Therefore, s31608 stainless steel, PMMA's sphere and A4-70 stainless steel bolts are used as the main material forms of the structure. The section form of H-beam is selected, and the buoyancy of structural members is considered by reducing the weight of water from that of stainless steel. In this paper, in order to accurately calculate the force on the PMMA's sphere, the gravity of the PMMA's sphere, the pressure on the inner surface and the pressure on the outer surface are separately considered in the form of load calculation, so as to consider the influence of the thickness of the PMMA's sphere on the buoyancy. The internal and external liquid level difference refers to the height of liquid flash liquid level in the upper chimney higher than the external pure water level. This liquid level difference will add a constant uniform pressure on the inner wall of the PMMA's sphere, partially offset the internal and external pressure difference of some spheres, and improve the force of strut, but has little effect on the total buoyancy. In the scheme calculation stage, two calculation models are considered, one is the calculation model of single-layer reticulated shell without considering the effect of PMMA's sphere; the other is the overall calculation model of single-layer reticulated shell considering the interaction with PMMA's sphere.
The main structure of the detector in JUNO is designed to support the inner PMMA's sphere with a single-layer steel shell, and the inner and outer glass is filled with non-uniform density liquid, which makes the structural stress more complex, especially the simmulating and loading of buoyancy. In addition, due to the brittleness and low reliability of PMMA, the safety margin of structural design is required to be very high, mainly to control the internal force of strut, glass stress and stability of single-layer reticulated shell. Through a large number of program calculations, the detector structure is optimized based on the control index from the aspects of steel consumption, strut number, strut internal force, lattice shell internal force and plexiglass stress. The static performance, stability, seismic analysis, nodal finite element analysis, single point failure and tolerance analysis of the optimized structure are carried out, and the performance of the detector structure is improved from various aspects Detailed analysis shows that the calculation indexes meet the requirements.