Recently, an approach of multi codes and multi-scale analysis is widely applied to study core thermal hydraulic behavior such as void fraction prediction. Better results are achieved by using multi codes or coupling codes such as PARCS and RELAP5. The advantage of multi-scale analysis is zooming of the interested part in the simulated domain for detail investigation.  Therefore, in this study, the multi codes between MCNP5, RELAP5, CTF and also the multi-scale analysis based RELAP5 and CTF are applied to investigate void fraction in hot channel of VVER-1000/V392 reactor. Since VVER-1000/V392 reactor is a typical advanced reactor that can be considered as the base to develop later VVER-1200 reactor, then understanding core behavior in transient conditions is necessary in order to investigate VVER technology. It is shown that the item of near wall boiling,  in RELAP5 proposed by Lahey mechanistic method may not give enough accuracy of void fraction prediction as smaller scale code as CTF.

Performance of  Passive Heat Removal through Steam Generator (PHRS-SG) of VVER-1200/V491 reactor presented in Safety Analysis Report for Ninh Thuan 1 shows that in case of long term station black out (SBO),  VVER-1200/V491 reactor can be cooldown and remained in safety mode at least 24 hours based on PHRS-SG performance. Anyway, long term station blackout along with small break in main coolant pipe of VVER-1200/V491 is assumed to be happening as an extension design condition that needs to be investigated. This study focuses on investigation of SBO along with different size of small break of LOCAs with expectation of finding the range of break size that the reactor is still kept in safety mode during 24 hours. During the investigation, some indicators for fuel damage such as the timing of HA1 actuation or mass of coolant inventory discharged are introduced as necessary information contributed to Severe Accident Management Guideline (SAMG).

CTF is a version of the widely used COBRA-TF code with capability of 3D simulation for core sub channel thermal hydraulics behavior. Recently, CTF is reviewed and the consideration of CTF to predict void fraction in PWR sub channel conditions such as subcooled region still  need more investigation. Due to the fact that the Chen’s correlation of heat transfer coefficient  is developed for relatively low pressure and high quality conditions associated with forced convection vaporization, and is not strictly valid for  PWR operation conditions, so that, in this study,  some runs of single channel in the benchmark based on NUPEC PWR Sub channel and Bundle Tests (PSBT) are used to investigate  void fraction prediction by CTF  in subcooled region and also to verify  some remarkable notice of CTF from other authors. The goal of the study is to evaluate deviation for CTF void fraction prediction in PWR sub channel conditions.

Recently, CTF, a version of COBRA-TF code is reviewed to validate its simulation models by several experiments such as Castellana 4x4 rod bundle, EPRI 5x5 bundle tests, PSBT bundle tests and TPTF experiment. These above experiments provide enthalpy, mass flux (Castellana), temperature (EPRI) and void fraction (PSBT, TPTF) at exit channel only. In order to simulate PWR rod bundle flow behavior, it is necessary to review CTF with more experiment in high pressure condition and it is found that the ENTEK BM facility is suitable for this purpose. The ENTEK BM facility is used to simulate Russia RBMK and VVER rod bundle two phase flow with pressure at 3 and 7 MPa and it gives measured void fraction distribution along the channel. This study focus on two points: (a) accuracy assessment between CTF’s void fraction distribution predictions versus experiment void fraction distributions and (b) investigation of void fraction prediction uncertainty from propagation of input deviations caused by measured accuracy.