Long-term core cooling

If scram fails, the core melts regardless of other successes. Scram can succeed but the core melts by failure of the core cooling. Slow melt occurs if the long-term core cooling fails. Sequences 5, 9, and 10 have missing nodes to indicate inapplicability of ECl, C02, and ECR for the cases shown. 

The PDHR system is employed for both the hot stand-by and long-term core cooling modes. This system can operate at full reactor coolant system pressure and places the reactor in the long-term cooling mode immediately after shutdown. 

The boronated water for safety injection is provided by the refiielling water storage tank (RWST) (1500 m ). After low level is reached in the RWST, the safely injection system is switched to recirculation, and the long term core cooling is provided. 

Increases the post LOCA containment flood up level, thereby maintaining and increasing the long-term core cooling safety margins. 

Long-term core cooling is achieved by active means by pumping water from reactor cavity to the core through heat exchangers. 

Since the drain piping is located at the bottom of the pressure vessel, this accident is the most severe one with respect to core flooding. The results show that the accumulator injection system can keep the water level above the top of the core for one day after initiation of the LOCA. Although the active component of the FLS is not activated in the present analysis to show the performance of the ACC, the FLS is supposed to be activated to maintain long-term core cooling by injecting water from the suppression pool, even one day after initiation of the LOCA. For isolation of the reactor, the passive isolation condenser (IC) is introduced for core cooling whereas the RCIC system driven by the steam turbine is eliminated. 

The VVER 440/213 has many of the enhanced reactor safety features of the new generation of VVER-440 reactors, in particular a large reserve of water for effective long-term core cooling. This is accomplished by using six horizontal steam generators that are connected by six isolatable loops to the reactor. 

The Action Plans do not explicitly address the requirement that the core can be maintained at an acceptable temperature for an extended period of time after ECS initiation. WSRC indicated that preliminary experimental work on maintaining long-term core cooling has been completed. More experimental work is planned to see if the flow can be safely reduced. This is an open item. 

The acceptance criterion for the resolution of USI A-43, (Reference 1) is that the containment spray and safety injection systems (including the containment sump) must be capable of performing their intended safety functions (in accordance with the requirements of GDC 38), including long-term core and/or containment building cooling. 

The insulator material is yttria-stabilized zirconia, and the ceramic insulator is cladded by the inner and outer liner tubes. The insulator-liner mbe assembly is attached to the fuel assembly, rather than the pressure tube, and is replaced after three fuel cycles. The insulator size and geometry are determined by the requirement that the fuel channel concept incorporates the ability to maintain core components below melting temperature even under accident scenarios that require long-term passive cooling. 

The core geometry remains coo I able for long term cool ing... 

The core is re-flooded in some tens of seconds (when the fuel reaches its worst conditions in the transient) then, the core cools steadily. The operators then initiate the long-term cooling procedure. 

A mixture condenser of the indicative volume of about 1500 m, of which 500 m are initially occupied by borated water at 2000 ppm boron and the rest by nitrogen. The function of this component is the collection of the fluid discharged by the primary system and the confinement of the fission products contained in it, the dissipation of its thermal energy to the outside environment and the formation of an additional reserve of water for the long-term cooling of the core, that is beyond the 10 hours (by natural or forced circulation, using the low power pumps, according to the... 

The function of the SCS pumps is to provide flow through the SCS heat exchangers and reactor core for normal plant shutdown operation or for long term post-accident core cooling. During normal power operation, the SCS pumps are isolated from the RCS by redundant motor-operated valves in series. 

Emergency core cooling system and long-term supply of cooling water... 

Evaluation of Long Term Post Accident Core Cooling... 

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