The Project is based on the use of Western technology, software and hardware received in the frame of TACIS 91 - 1.3. Project Accident analysis.
The objectives were:
- To define a set of accident scenarios to act as design basis accidents for protection system of VVER-440/230;
- To perform necessary accident analyses;
- To evaluate the possible addition of new protection signals.
The six following signals were evaluated:
- Signal 1: Low SG level for Emergency Protection (EP) actuation;
- Signal 2: Low SG level for Emergency Feedwater (EFW) pump actuation;
- Signal 3: High hot leg temperature;
- Signal 4: High pressurizer level;
- Signal 5: Low primary pressure;
- Signal 6: Low pressurizer level.
The following main conclusions were found concerning proposed signals:
The low steam generator liquid level signal for EP actuation was found to improve the safety of the plant. In all scenarios of the loss of all four Main Feedwater (MFW) pumps where the new EP signal was considered, it was the first EP signal, which appeared during the transient. In the loss of one MFW pump transient the new signal was not the first to come. However, it can be considered the only signal, which indicates the event. At the same time it gives redundancy for the present to defend against the loss of the heat sink.
Therefore, the implementation of this signal is recommended.
The low steam generator liquid level for Emergency Feedwater (EFW) pumps actuation was found to have a positive impact on the analysed event. The new signal makes the steam generator liquid level to decrease more slowly than without it. After approximately 20 min. one EFW pump is capable to remove the decay heat and restore SG level. The uncovering of the feed water distribution header is also delayed. Therefore, the implementation of this signal is recommended.
The high hot leg temperature signal for EP actuation enhances safety margins. Installation is therefore recommended.
The high pressurizer level signal for EP actuation does not improve the safety margins of the plant since for the evaluated transient the existing signal high primary pressure is reached earlier.
The low primary pressure signal for EP actuation was found to increase reliability of reactor scram for pressurizer valve stuck open transient as well as for Loss of Coolant Accident (LOCA) transient; conclusions of project TACIS-91 1.3 are confirmed and installation of this new signal is recommended.
Signal low pressurizer level acts as additional signal for LOCA transient, confirming TACIS 91-1.3 proposal; nevertheless this signal is not considered mandatory due to additional considerations on reliability of measurements.
In addition 5 computer systems were delivered to the Beneficiary, including a range of relevant consumables and interfaces.
The planned outputs were achieved. At the end of this project, the project partner was already preparing recommendations to the NPPs in respect of important as well as not mandatory signals. In order to qualify the results of the licensing analyses performed by the certified Russian codes, Western codes were transferred according to the objective. The Beneficiary received essential know-how in respect of the Western practice related to the design of emergency protection signals.
This know-how resulted in new ideas, which lead to the possible addition of new protection signals for VVER 230 substantiated by the performed analyses.
Thus the realisation of the objectives can be considered as good.