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The 20 kton liquid scintillator detector of the Jiangmen Neutrino Underground Observatory (JUNO), which is currently under construction in Southern China, has a vast potential for new insights into various fields of (astro-)particle physics. Stringent limits on the liquid scintillator radiopurity are required for several physics goals of JUNO. For both 232Th and 238U, a radiopurity of 10−15 g/g is required for reactor antineutrino measurements, 10−16 g/g for solar neutrino measurements. An independent detector, the Online Scintillator Internal Radioactivity Investigation System (OSIRIS), will be used to ensure these limits are kept. This report will present OSIRIS and its sensitivity to 232Th and 238U in detail. OSIRIS allows an online radiopurity evaluation of the scintillator during the months-long filling of JUNO. The design of OSIRIS is optimized for tagging fast 214Bi-214Po and 212Bi-212Po coincidence decays in the decay chains of 238U and 232Th, respectively. The coincident decay signatures and their rates offer a potent background rejection as well as a direct translation into 238U-/232Th-abundances in the scintillator. OSIRIS will also be able to measure the levels of 14C in the scintillator, down to a 14C/12C ratio of 10−17 at 90% confidence level. Furthermore, the level of 210Po and a possible contamination by 85Kr can be determined. To achieve its goals, OSIRIS features a water-submerged 20 ton liquid scintillator target monitored by 76 intelligent photomultiplier tubes (iPMTs). The novel design of the iPMTs allows a triggerless readout scheme with high signal quality. A single computer is sufficient to process the data stream into events for further analysis. The timing and charge calibration of the iPMTs will be performed with Laser-and LED-based systems. The energy and vertex reconstructions will utilise height-adjustable radioactive sources within the liquid scintillator. [ABSTRACT FROM AUTHOR]