EE WAS REMOVING SCAFFOLD FROM NO. 10 FLASH TANK ALONG W/REST OF CREW. THE SCAFFOLD INSIDE THE TANK IS LARGE & THE CREW WORKED ALL DAY REMOVING IT. AT THE END OF THE DAY WHEN THE JOB WAS COMPLE TE, EE FELT DISCOMFORT IN HIS LOWER BACK. HE WENT TO MEDICAL & TOLD THE NURSE. MON. 10/21/02 HE RETURNED TO MEDICAL & THE DR CHECKED HIM OUT & PRESCRIBED SOME MEDICATION. HE RETURNED TO WORK.