Control Knobs from Three Mile Island Unit 2
These three knobs are from the Three Mile Island (TMI) Unit 2 Control Room. The one on the left side of the photo controlled Pressurizer Spray Line Valve RC-V-1 (a motor operated 2.5" globe valve). The purpose of the spray system is to allow cool water into the pressurizer thereby reducing the water temperature and pressure.
The function of the other two knobs is not known.
Size: ca. 2-3" across
To a large extent, the accident at TMI 2 was due to a malfunction of the pressurizer's pilot-operated relief valve (PORV). The following diagram (produced by the Nuclear Regulatory Commission) indicates the PORV but not the spray line valve.