Reactor vessels nozzle cracking

Nuclear Reactor with a Hole in the Head On March 6,2002, personnel repairing one of the five cracked control rod drive mechanism (CRDM) nozzles at Davis-Besse Nuclear Plant, Oak Harbor, Ohio, discovered extensive damage to the reactor vessel head. The reactor vessel head is a dome-shaped structure made from carbon steel housing the reactor core. The reactor vessel head is placed such that it can be removed when the reactor is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods inside the reactor vessel. Reactor operators withdraw control rods from the reactor core to start the operation of the plant and insert the control rods to shut down the operation of the reactor. 

No external primary loop piping. The use of a natural circulation driven integrated reactor vessel eliminates the potential for hot and cold leg breaks, pressurizer surge line nozzle cracking and breaks, reactor coolant pump suction line breaks, and reactor coolant pump seal breaks. 

Cyclic temperature stresses at questionable points in the reactor pressure vessel and steam generators were explored experimentally. Mockups were fabricated for the testing of the pressure-vessel nozzle joints and the stainless-steel-to-Zircaloy bolted joint inside the pressure vessel. In each test the temperature was cycled from approximately 250°F to approximately 600°F in 1/2 hr and cooled back to 250°F in 1/2 hr. After 100 cycles the joints were found to be sound. The main steam generators were also cycled in similar tests. Several tube joints cracked open during the first 50 cycles. They were repaired and the heat exchangers were subjected to an additional 10 cycles before final acceptance. 

All weldments in a reactor contain high residual stress and, unless they are suitably stress-relieved, they are focal points for environmental cracking. However, the circumferential welds by which nozzles are attached to the vessels are particularly vulnerable. The radius of a dished head also contains residual stress from the forming operation, and the top head is often the first point of failure when volatile species (e.g., traces of hydrogen or ferric chloride) are involved (Fig. 12.5) [7]. External jackets can cause problems, unrelated to the process, that arise from the use of cooling water or steam in the jacket which causes external SCC of the vessel projjer. 

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