Natural-circulation loops passive safety systems

The AP600 passive safety system includes subsystems for safety injection, residual heat removal, containment cooling, and control room habitability under emergency conditions. Several of these aspects are in existing nuclear plants such as accumulators, isolation condensers as natural-circulation closed loop heat removal systems (in early BWRs), automatic depressurization systems (ADS - in BWRs) and spargers (in BWRs). 

Natural circulation driven main coolant system. A passive system for normal core heat removal. A 1/3-scale integral system test loop has been constructed and operated at Oregon State University in conjunction with INEEL. It includes all of the primary side and secondary side passive safety systems. Initial testing completed. Current status Primary side stability tests being developed Secondary side control logic under development Neutronic feedback tests being developed. 

In many passive safety systems natural circulation is an essential part of the system. The main aim is to use density differences in a pool or in a closed loop to transfer thermal energy from a source to a heat sink without using other energies than gravitational energy. This process can take place with or without phase changes. 

The reactor is designed for a near-zero reactivity bum-up swing such that the safety rod system is vested with minimal positive reactivity at Beginning of Life (BOL) full power. A safety rod scram system provides a first line of defence for reactivity initiators. Moreover, passive reactivity feedbacks and passive self adjustment of natural circulation flow could maintain reactor power to flow ratio in a safe operating range even with failure to scram this safe passive response applies for all out-of-reactor vessel initiated events, i.e., for any and all events communicated to the reactor through the flibe intermediate loop. Periodic in situ measurements would be made to confirm the operability of these passive feedbacks. 

The design concept of JSFR, Japan s Generation IV reactor, was reviewed. It is a loop type and is characterized, in terms of safety, by a self-actuated (passive) RSS, a re-criticality free core design, and a natural-circulation DHRS. It is also characterized, from the viewpoint of economy, as a two-loop heat transport system, integrated IHX/ pump component, and others. 

The MBIR reactor configuration is typical for SFR with three loops with a secondary sodium loop (Tretiyakov and Dragunov 2012). MBIR safety features include a passive removal of decay heat in the primary loop by natural circulation, physical separation of the primary and secondary systems to cancel out the possibility of radioactive sodium leakage, and a fuel core catcher inside the reactor vessel. The operations and controls of MBIR also have safety features built in, such as automated process control systems to decrease the chances of operator error. CDF and LRF are estimated at 9.8 X 10 per reactor year and 6.1 x 10 per reactor year, respectively (Tretiyakov et al., 2014). The main design characteristics of the MBIR reactor are given in Table 12.4. 

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