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An example of the innovative thinking behind the IRIS safety-by-design approach is given by the handling of small break LOCAs, which historically has been the one most plaguing to PWRs.
The IRIS approach is to limit and eventually stop the loss of coolant from the vessel rather than relying on active or passive systems to inject water into the RV.
This is accomplished by taking advantage of the following three features of the design:
1) The large coolant inventory in the reactor vessel;
2) An emergency heat removal system (EHRS) employing the steam generators to remove heat directly from inside the RV thus depressurizing the RV by condensing steam, rather than by discharging mass;
3) The compact, small diameter, high design pressure containment that assists in limiting the blowdown from the RV by rapidly equalizing the vessel and containment pressures. In fact the IRIS small spherical containment can be designed to 4-5 times the current containment pressure, at the same shell thickness.
Referring to the Figure, for a pictorial representation of the accident sequence, after the LOCA initiation, the reactor vessel (RV) depressurizes and loses mass to the containment vessel (CV) causing the CV pressure to rise (Blowdown phase).
The mitigation sequence is initiated with the reactor trip and pump trip;
the EHRS is actuated to depressurize the primary system by condensing steam on the steam generators (depressurization without loss of mass);
and finally the ADS is actuated to assist the EHRS in depressurizing the RV.
The containment pressure is limited by the Pressure Suppression System and the reduced break flow due to the EHRS.
At the end of the blowdown phase the RV and EV pressure become equal (pressure equalization) with a CV pressure peak <8 barg and the break flow stops.
The coupled RV/CV system is then depressurized (RV/CV depressurization phase) by the EHRS (steam condensation inside the RV exceeds decay heat boiloff).
In this phase the break flow reverses since heat is removed not from the containment, but directly inside the vessel: the CV pressure is thus reduced following the RV depressurization as steam from the containment is condensed inside the pressure vessel.
As the containment pressure is reduced, a portion of suppression pool water is pushed out through the vents and assists in flooding the vessel cavity.
The depressurization phase is followed by the long term cooling phase (RV and CV pressure reduced to <2 barg in <12 hours), during which the gravity makeup of borated water from both suppression pool and RV cavity are available as required.
Since decay heat is directly removed from within the vessel, the long term break flow does not correspond to the core decay heat, but in fact it is determined only by the containment heat loss.





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