The SAWA/SAWM strategy was integrated into the BWROG EPG/SAG Rev.4 after a previous analysis on the Mark-I containment of EPRI, extending it to all BWR designs, including those with Mark-III containment, in which the implementation of this strategy has not meant any impact on the diversity of injection means and its location (RPV or containment), previously existing capacity and linked to the containment flooding strategy, but about the benefits associated with the control of that injection.
Cofrentes NPP, which has a Mark-III containment, has updated its emergency procedures in 2023 to the criteria of the BWROG generic procedures, including the SAWA/SAWM strategy. In this process, specific analyses have been realized with the MAAP5 severe accident simulation code to analyze the impact of the strategy in all application operational configurations, strongly conditioned by the availability of a filtered containment venting system (FCVS) for the containment pressure control. The FCVS was installed in Cofrentes NPP after the Fukushima Dai-ichi accident, although it not required by the NRC in USA Mark-III containments (SECY-16-0041).
The SAWA/SAWM strategy in Cofrentes NPP gives priority to direct injection into the vessel for the cooling function of the debris and establishes two reference flow rates, the initial one, greater to absorb the energy from the chemical reactions and the cooling to saturation conditions, process that can last more than 24 hours due to the narrowness of the pedestal and, the second, smaller and associated with the stabilization of the debris by absorbing its residual heat in the long term. The overflow level drywell– suppression pool marks the transition from one flow to another, which guarantees sufficient water inventories on the debris, both from a thermo-hydraulic and radiological point of view and preserves the operation of the drywell and containment pressure control systems in case of integrity over both cubicles.
For those configurations with the equipment hatch open and, therefore, without containment and drywell integrity, limit flooding to overflow level allows you to fulfill the function of heat extraction from the debris and it also limit the dose released to the outside without loss of inventory through the openings.
Compared to a direct containment flooding strategy, the SAWA/SAWM strategy generates a more intense initial containment pressure response without hardly affecting the stabilization times of the debris, conditioned by the geometry of the pedestal.
The analyzes conducted with the MAAP5 code validate the SAWA/SAWM strategy through different paths and aspirating from both external and internal sources, admit delays in its implementation, a relevant criterion for scenarios with short-term damage to the core, and justify reductions in the reference flow rates as the onset of core damage is delayed.
After the Fukushima Dai-ichi accident, the NRC established, through order EA-13-109, requirements to guarantee containment venting capacity in a severe accident for BWR designs with Mark-I and Mark-II containment. NEI 13-02 provided different methods to comply with the requirement, including the controlled water supply strategy in severe accidents (SAWA/SAWM). This strategy, accepted for the NRC, is considered a reliable method of preserving the venting capacity of the containment for a period long enough to reestablish reliable alternative methods of removing residual heat from containment and the non-need for have a direct vent from the drywell.
The SAWA/SAWM strategy was integrated into the BWROG EPG/SAG Rev.4 after a previous analysis on the Mark-I containment of EPRI, extending it to all BWR designs, including those with Mark-III containment, in which the implementation of this strategy has not meant any impact on the diversity of injection means and its location (RPV or containment), previously existing capacity and linked to the containment flooding strategy, but about the benefits associated with the control of that injection.
Cofrentes NPP, which has a Mark-III containment, has updated its emergency procedures in 2023 to the criteria of the BWROG generic procedures, including the SAWA/SAWM strategy. In this process, specific analyses have been realized with the MAAP5 severe accident simulation code to analyze the impact of the strategy in all application operational configurations, strongly conditioned by the availability of a filtered containment venting system (FCVS) for the containment pressure control. The FCVS was installed in Cofrentes NPP after the Fukushima Dai-ichi accident, although it not required by the NRC in USA Mark-III containments (SECY-16-0041).
The SAWA/SAWM strategy in Cofrentes NPP gives priority to direct injection into the vessel for the cooling function of the debris and establishes two reference flow rates, the initial one, greater to absorb the energy from the chemical reactions and the cooling to saturation conditions, process that can last more than 24 hours due to the narrowness of the pedestal and, the second, smaller and associated with the stabilization of the debris by absorbing its residual heat in the long term. The overflow level drywell– suppression pool marks the transition from one flow to another, which guarantees sufficient water inventories on the debris, both from a thermo-hydraulic and radiological point of view and preserves the operation of the drywell and containment pressure control systems in case of integrity over both cubicles.
For those configurations with the equipment hatch open and, therefore, without containment and drywell integrity, limit flooding to overflow level allows you to fulfill the function of heat extraction from the debris and it also limit the dose released to the outside without loss of inventory through the openings.
Compared to a direct containment flooding strategy, the SAWA/SAWM strategy generates a more intense initial containment pressure response without hardly affecting the stabilization times of the debris, conditioned by the geometry of the pedestal.
The analyzes conducted with the MAAP5 code validate the SAWA/SAWM strategy through different paths and aspirating from both external and internal sources, admit delays in its implementation, a relevant criterion for scenarios with short-term damage to the core, and justify reductions in the reference flow rates as the onset of core damage is delayed.
