After the loss of coolant accident (LOCA), the safety injection system injects water into the reactor coolant system (RCS), and the residual heat rejects from the break. The containment spray system is operating in recirculating cooling mode to ensure that the containment is cooldown. This state must be maintained for several months. After the accident, in order to respond the design extension conditions (DEC) of failure of two containment spray pumps or two low pressure safety injection pumps, the design of the original H4 connections was improved, and the H4 procedure (loss of containment spray pumps or low pressure safety injection pumps) was developed. H4 procedure demands to put into service 2 permanent (one for each train) interconnections of containment spray system and safety injection system, called “H4 connections”. Through the design improvement of the H4 connections, the mutual backup function of safety injection and containment spray can be realized implemented. The manual valves of the H4 connections in the original design were changed to electric valves, which ensured the accessibility of operator and avoided the radiation of high radioactivity level to operator after the accident. In addition, the improved H4 connections enable mutual backup of safety injection and containment spray in the early stage after the accident to be implemented, which fully improves the ability to respond to accidents and safety design level of the nuclear power plant (NPP). This also makes it possible to intervene early after the accident.

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