Chunkdata=temporary data fetched for report like for showing page Y of Z. If you can'r restore ,  repair_allow_data_loss will probably clean out (some of) the Chunkdata table as the clustered index is affected
Read backup/restore techniques for RSTempDB: https://docs.microsoft.com/en-us/sql/reporting-services/install-windows/backup-and-restore-operations-for-reporting-services
About chunkdatatable: https://blogs.msdn.microsoft.com/sqlcat/2013/09/20/report-server-catalog-best-practices/