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Found a better explanation:

"The extremely high destructive power of such a beam imposes an external dump [...].
Each beam dump absorber consists of a 7m long segmented carbon cylinder of 700mm diameter, contained in a steel cylinder, comprising the dump core. This is water cooled, and surrounded by about 750 tonnes of concrete and iron shielding. The dump is housed in a dedicated cavern at the end of the transfer tunnels."

https://lhc-machine-outreach.web.cern.ch/components/beam-dump.htm