Pressurized water reactors passive emergency systems

Emergency core cooling system consists of two trains. Each train meets the single failure principle. System includes high and low pressure sub-system. High-pressxue subsystem includes passive (hydro-accumulators) and active pumps and water storage tanks) features for water injection in reactor. Low-pressure sub-system ensures returning a condense accumulated in containment, into the reactor by recirculation pumps. 

The system of passive heat removal is actuated, if an emergency arises, by opening the valves on the pipelines for discharge of water from the heat exchangers in response to signals from the automatic control system, as well as directly by the action of the working medium (signal from pressure or coolant level variations in the reactor). 

II - Active channel of RHRS through SG 12 - Passive channel of RHRS through SG 13 - Condensation subsystem for emergency pressure suppression in the protective shell 14 - Active channel of RHRS through primary/third circuit heat exchanger 15 - Passive channels of ECCS 16-System for reactor caisson fill-up with water... 

Regarding the control of accidents within the design basis (Level 3 in Table 4), all water cooled SMRs rely on certain inherent safety features (such as the reactor vessel penetrations located in the upper, steam part of the reactor pressure vessel to ensure that the leakage rate is low and the core is not uncovered in loss of coolant accidents ) and incorporate various combinations of passive and active systems. Most of the designs target an increased reliance on passive systems, as benefiting from smaller reactor size. In different water cooled SMRs, passive systems shoulder the functions of back up or main shutdown systems, emergency core... 

The reactor trips due to low pressure in the pressurizer. It is assumed that the emergency core coolant system does not operate during the transient. It is also assumed that there is not auxiliary feed water to the steam generators and no steam dump to the condenser. Only the accumulators participate in the sequence, since they are passive elements their discharge is only produced when vessel breaches, as, until this moment, the primary pressure is over the setpoint of the accumulator valves. Under these conditions the transient leads to core damage and melt-down. 

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