The Development of PVC Extrusions for the 14,000 Ton NOvA Neutrino Detector
The NOvA Neutrino Experiment is building a one of a kind self-supporting plastic structure, potentially the largest ever built. The PVC structure serves as a neutrino detector and is composed of 28 individual blocks that measure 51 feet high by 51 feet wide by 7 feet deep. The PVC needed for each block weighs approximately 400,000 pounds. The primary parts in the detector construction are 51-foot, 16-cell PVC extrusions. These extrusions form the basis of the detector modules which are laminated together in a crossed pattern to form the individual blocks and then filled with mineral oil. The hydrostatic loading on the extrusions results in complex loading of the laminated structure.
In addition, the mass of the contained mineral oil is 1.7 times the PVC mass, thereby adding to the mechanical stress. The self-supporting nature of the detector places important structural requirements on both the PVC formulation and the extrusions. Similarly, the need for leak-tight seals along with block assembly requirements imposes narrow geometric tolerances on extrusions whose large size is pushing the manufacturing limit. Furthermore, due to the method of detecting neutrinos, another fundamental requirement of the extrusions is that it possesses exceptionally high reflectivity over a particular wavelength range. This requirement places additional restrictions on the components of the PVC formulation. Taken altogether, the PVC extrusions have to maintain important reflectivity characteristics, provide structural support to the detector, and meet relatively tight geometric requirements for assembly.
In order to meet these constraints, a custom PVC formulation had to be created. One of the main features of the PVC compound was exceptionally high loading of anatase TiO2 (to meet the reflectivity requirements). This fact combined with a large and complex extrusion profile presented difficulties in developing a PVC formulation. The final formulation and extrusion process were developed after an extensive R&D program which included industrial partners. We present a brief description of the purpose and requirements of the physics detector leading to the complex PVC and extrusion requirements, summarize the R&D process and discuss the lessons learned.