Boron dilution accidents

The initial design of French NPPs takes into account that a certain number of accidents can possibly occur during shutdown conditions (LOCA with RHRS connected, homogeneous boron dilution accident, etc.). Only a few years after commissioning the first unit of the industrial series of NPPs in France (TRICASTIN 1), the first Hl-2 (total loss of the heat sink) and H3-2 (Blackout) procedures specific to RHRS-connected states (post-TMI actions) appeared. The first incidental procedure I-RRA2 (Malfunction of RHRS), which results in the introduction of the standby RHRS was implemented at the same time. The first technical specifications related to cold shutdown modes were subsequently implemented since 1988. 

Redundancy and diversity have been required for those alarms that detect boron dilution accidents. 

The chemical and volume control system is also isolable from the reactor coolant system in the event of a boron dilution accident. In this case, the redundant valves are closed, tripping the make-up pumps, and/or aligning the suction of the makeup pumps to the boric acid tank (see Section 9.3.6.4.5.1 of Reference 6.1). 

No soluble boron Soluble boron-free core Boron dilution accident (BDA) Physically eliminates BDA... 

Rod drop (in BWRs), rod ejection (in PWRs) or boron dilution accidents (in PWRs) ... 

Reactor start-up is easily performed by continuous boron dilution. The injection of pure water for boron dilution is performed by a pump which can be operated only when the MCP is operating. This design provides a fail-safe mechanism for avoiding the possibility of reactivity insertion accidents. 

Major accidents, not limited to the specific reactor type (e.g. post-Chernobyl boron dilution concerns for PWRs) ... 

Therefore, accidents which take place during low power and shutdown conditions (LPS) have been under extensive study all over the world for several years. Results have shown that the risk of an accident initiation during the shutdown and refueling phase is high. Important contributors to risk are boron dilution, loss of residual heat removal with the reactor cooling system in reduced inventory conditions, loss of primary coolant, loss of off-site power, fires and human errors. 

Reactivity insertion accident (RIA) since the innovative hydraulic control drive for the FSS and the adjust and control system is located inside the RPV, the control rod ejection accident is avoided only inadvertent control rod withdrawal transients are postulated. Two scenarios considering FSS success and FSS failure with SSS actuation were modelled, assuming a conservative hypothesis. The results of simulation show that safety margins are well above critical values (DNBR and CPR - critical power ratio) and no core damage is expected. Moreover, as there is no boron in the coolant, boron dilution as a reactivity-initiating event is precluded. 

Reactivity events are a specific category due to their specific issues and consequences. Reactivity accidents can lead to a local or a full core criticality. Examples like boron dilution, unintentional withdrawal of control rods or refuelling errors may be considered in the SLP PSA. International experience has shown that many such events occurred at NPPs, and their frequencies is high, though the consequences are low (recoveries are possible in many of those events). Some phenomena, like unborated slug of water entering the core and its consequences, are still being analysed. 

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