CRP on Natural Circulation Phenomena, Modelling and Reliability of Passive Safety Systems that Utilize Natural Circulation September, 2007, IAEA, Vienna Modelling of natural circulation phenomena in VVER-440 reactors P. Matejovič, M. Bachratý

VVER-440/V213 design 2 nd generation of soviet design PWRs of medium power incorporated most of design requirements of PWRs built at the same time (e.g. 3 x 100% ECCS) constructed in Russia (2), Ukraine (2), CEC (12+2), Finland (2) 2 units still under construction (Mochovce 3&4 NPP) rather inefficient, but robust and conservative design with large T-H safety margin Loviisa NPP upgraded for severe accidents (IVR, PARs)

VVER-440 geometry of primary system: Natural circulation is influenced by: horizontal SG => driving head for the natural circulation is rather small six loops configuration loop seals in both, hot and cold legs large primary and secondary side coolant inventories

PACTEL facility, new configuration Volumetric scale 1:305; Basic elevations preserved in full scale; Reduced number of loops (3 instead of 6); Widely used for LOCAs, SG boil-off, etc.;

“Large” diameter PACTEL steam generator

VVER-440 horizontal SG

VVER-440 SG – horizontal cross-section SG tubing

Natural circulation during boil-off transients Main Goals of LOF-10:  to study study the SG behavior in VVER-440 reactor geometry during a loss-of-feedwater transient.  to test the ability of thermal-hydraulic computer codes to analyse this kind of phenomena

Rows [-] Heat transfer area [m2] Heat transfer area versus number of rows for VVER-440 steam generators

LOF-10 boil-off experiment:  the experiment started from steady-state conditions with forced circulation in single loop (1000 s)  RCP was switched off and the FW injection was interrupted  Core heating power was 75 kW, what corresponds to 1.7% power of the reference reactor

RELAP5-3D nodalization of the PACTEL used for LOF FW

hotcollectorhotcollector coldcollectorcoldcollector Calculated natural circulation flow pattern in SG tubing at t = 2000 s

Conclusions:  reasonable results were obtained with RELAP5-3D  necessary condition: sufficiently fine nodalisation should be used with large number of SG tube layers  practical limitations: 6-loop arrangement with 78 layers of heat exchange tubes in 1 SG = > compromises are necessary

An example of improperly designed facility