22.08.2015   DESIGN FEATURES TO ACHIEVE. DEFENCE IN DEPTH IN SMALL AND. MEDIUM SIZED REACTORS

Summary of performance in design basis accidents

Table IV-3 gives a comparison of the progression of typical design basis accidents between a standard PWR and the SCOR.

The calculations performed for the SCOR show that all transients could be adequately managed in a passive way (in the vessel, in the RRP loop, and in the heat sink) with only 4 out of 16 RRP loops, no matter what the heat sink is: a pool or an air cooling tower. This represents a redundancy of 16 times 25%. RRP operation is compatible with an active or passive mode, whatever the primary pressure or temperature. As the in-vessel heat exchangers of the RRP loop are located very close to the core, and thanks to the flow bypass of the venturi, the RRP are operational in a two phase flow mode (primary side), in the case of a small primary water inventory. Long term cooling may be ensured in a totally passive mode due to the RRP with an air cooling tower. A safety injection at 2.0 MPa with a small flow rate is needed only one hour after the beginning of the biggest possible LOCA, that is, a double break of the pressurizer line (2 x 50 mm). In the event of a steam generator tube rupture, the steam released from the safety valves of the secondary circuit is condensed in a dedicated pool. No steam is released to the atmosphere.

TABLE IV-3. DESIGN BASIS ACCIDENTS IN STANDARD PWRS AND IN SCOR [IV-1]

Initiating event

Transient progress in standard PWRs

Transient progress in SCOR

NPP blackout

-Natural convection in the primary circuit — An external electricity source (diesel) is required for the systems involved (seal pump, safety injection, etc.)

-Heat sink effective for a few hours

-Natural convection in the primary circuit — Very few systems involved (diesels with a reduced power or a battery)

-Infinite autonomy of the RRP systems with an air heat sink

Steam line rupture

-Risk of recriticality

-High pressure safety injection (HPSI) with borated water required

-No risk of recriticality — Not need for safety injection

LOCA

-Possible early core exposure, depending on the break size

-Demand for safety injection systems of three types: HPSI, hydro-accumulators, and low pressure safety injection (LPSI)

-Possible demand for a fast safety injection (depending on the break size)

-Long term cooling by LPSI (active system) required

-No early core dewatering (at least for 1.5 hours after the transient start with no RRP operation) — Safety injection of only one type — LPSI — is needed, with a small flow rate

-No demand for immediate LPSI operation — Long term cooling provided by the RRP systems in a passive mode

Steam

generator tube rupture

-Risk of a primary water release through the broken steam generator — Request for safety injection disturbs the transient management — Delicate management of the decreasing pressure is required to prevent the secondary water without boron from flowing into the primary circuit through broken tubes of the steam generator

-No steam release to the atmosphere (steam is condensed in a pool)

-Cooling by RRP systems; no need for safety injection

-Primary coolant has no soluble boron; therefore, no risk of dilution by the secondary coolant