XIX-4.5.1. Severe accidents

Compared to the standard PWR, safety is improved by the elimination of initiating events based on a specific design, as mentioned previously (optimum between safety, economy and human interface): large breaks on the primary circuit, reactivity insertion accident by rod ejection. However, the hypothetical case of a core meltdown is manageable in the following manner:

• Core meltdown: corium cooling should be ensured by reactor vessel pit flooding, because the core power density is small, and the large grace delay before an hypothetical core meltdown reduces the decay heat when the corium enters the lower plenum.

• Hydrogen risk: the Reactor Vessel Compartment atmosphere is inerted (cf. Figure XIX-5) to prevent hydrogen combustion like BWRs.

XIX-4.5.2. Management of the design extension condition

For SCOR, the transients in the design extension conditions are practically eliminated.

• H1 (total loss of the heat sink): SCOR concept is based on several independent decay heat removal loops (RRP) ready to operate in passive mode, having their heat sink either on pools with a limited availability of several hours, or on air cooling tower whose availability is almost infinite.

• H2 (total loss of feed water of the Steam Generator): the decay heat is removed by systems located on the primary circuit with a large redundancy (16 x 25%). There is no need of safety auxiliary feed water system.

• H3 (total loss of the power supplies): natural convection is possible on all the decay heat removal systems with the integrated heat exchangers, from the primary circuit to the heat sink.

• H4 (loss of the containment spray or loss of the low pressure safety injection): SCOR has no containment spray, because the containment is a pressure suppression containment type. The low pressure safety injection has a less significant role than in standard PWRs, because of the large primary inertia, the suppression of the large LOCAs and the strong effectiveness of decay heat removal systems.

• ATWT (Anticipated Transient Without Trip): SCOR has two independent shutdown systems. These transients will be treated individually as for the standard PWRs. The management of this complementary condition is eased owing to the always negative and higher moderator temperature coefficient than that of standard PWRs.

• Multi steam generator tubes rupture and non-isolable containment: the discharge of the Steam Generator is carried out in a dedicated pool.

• Failure of High Pressure Safety Injection: no HPSI is foreseen in SCOR.

XIX-5. Containment

The compactness of the primary circuit of SCOR leads to design of a pressure suppression containment similar to BWRs. The containment building in Figure XIX-5 consists of two physically separate compartments. The lower compartment is the reactor containment. The upper compartment is the mainly building again external hazard to protect the secondary side. The two compartments of the containment building are:

• The compartment of the primary side (or primary containment) is located under the reactor vessel-SG mating surface. It contains the vessel and all its primary pipe connections. Its volume is small. A pressure suppression device, as in BWR, controls the pressure. This compartment is inerted to manage the hydrogen risk.

• The compartment of the secondary side (or secondary containment) houses the steam generator. It is not inerted because it has no contact with the primary circuit when the vessel is closed.

XIX-6. Conclusions

SCOR is a simple compact PWR operating under low pressure. All the components are located inside the vessel, including the decay heat removal systems. The reactor has only one SG acting as the vessel head. The soluble boron free core operates with a low power density.

The configuration offers significant safety improvements over traditional loop-type PWRs (generation II) in achieving safety by design. The SCOR design eliminates the large LOCA. The decay heat removal system located very close to the core is very efficient. Calculations made with the CATHARE code have shown that, for the most penalizing accidents of the Design Basis Conditions, the core remains safely cooled with only four out of sixteen passive decay heat removal systems. For the most penalizing LOCA, a Low Pressure Safety Injection with small flow is required for a short time, one hour after the beginning of the transient.

The compactness of the reactor leads to the use of a pressure suppression containment type. It is inerted to prevent the hydrogen risk and, in case of a hypothetical core melting, the corium is cooled by pit flooding.