PWR low pressure natural circulation and loss of all AC power

First phase of the specific PWR nuclear power plant transient analysis in area of low pressure phenomena, where no special possibilities of interventions exist, has been done. Such transients may be initiated by hypothetical loss of all plant AC power due to different causes during cold or hot shutdown. Complete transient has not been analyzed yet, because of the errors in results calculated by the RELAP5/MOD2 and RELAP5/MOD3 programs. The analysis has shown, that the stable decay heat removal with single phase primary coolant natural circulation is achieved but not preserved in the case where the initial reactor coolant system pressure is 1 bar. After some 5.5 hours into the transient voids start to form in the upper section of steam generators U-tubes and two phase natural circulation appears. On the other hand the stable decay heat removal with single phase primary natural circulation is preserved if initial reactor coolant system pressure is 10 bar. The duration of such stable heat removal is limited by steam generator secondary coolant inventory. RELAP5/MOD2 predictions at 1 bar initial pressure were acceptable despite troubles that appeared after first void formation. RELAP5/MOD3 calculations were performed without any difficulties but results produced are unacceptable due to significant numerical mass error which caused unacceptable primary mass decrease of 8% in the first 10 hours of transient time. Mass error appeared also during the calculations at initial pressure of 10 bar performed by RELAP5/MOD2. It caused significant variations of the secondary coolant mass