Structural Design of the Central Detector for the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory （JUNO） is a next-generation large-scale liquid scintillator detector led by China. It aims to precisely determine the neutrino mass ordering and conduct cutting-edge research across multiple fields using a 20000-ton liquid scintillator target. To meet the stringent requirements of the central detector regarding its massive scale， ultra-high precision， and extremely low radioactive background， this project innovatively proposed a novel two-layer nested detector configuration consisting of a single-layer stainless steel reticulated shell and an acrylic sphere. This configuration utilizes pure water instead of white oil as the shielding medium and replaces the traditional closed spherical shell with a reticulated shell， achieving a cost-effective leap from kiloton-scale to ten-thousand-ton-scale liquid scintillator detectors. This paper focuses on the key structural design technologies of the central detector. 1） To address the mechanical challenges of the acrylic sphere under immense buoyancy， a load effect adjustment method was proposed. By optimizing the elevation difference between the internal and external liquid levels and developing a spatial hinge whose axial stiffness can be adjusted to regulate the strut forces， the internal forces of the struts were strictly maintained within safe limits. 2） To resolve the issue of overall structural stability， a hinged-rigid conversion joint was designed to effectively enhance the anti-rotational stiffness of the acrylic sphere. Nonlinear analysis demonstrated that the structure exhibited a high stability coefficient. 3） The challenge of connecting heterogeneous materials with vastly different properties （acrylic and stainless steel） was overcome by developing a high-strength hinged joint with an embedded stainless steel ring. Full-scale model tests verified that its ultimate bearing capacity was more than six times the design load. 4） A refined model encompassing the surrounding rock， the structure， and the fluid was established. A two-stage analysis method was employed to conduct seismic response analysis considering fluid-structure interaction （FSI） in underground spaces， confirming the seismic safety of the system. Finally， a replacement filling method for the liquid scintillator was proposed. By synchronously controlling the liquid level difference and medium displacement， the structural safety throughout the entire filling process was ensured.