On March 11th 2011 at 14:46 an extremely high magnitude earthquake struck the East coast of Japan. One minute later, at Fukushima Daiichi, all reactors started scram as design operation. Later on, due to the struck of tsunami AC power, provided by diesel generators, stopped and Residual Heat Removal (RHR) system failed to be activated in all the operating units. The follow-up of the transients highly depended on the plants' availability of DC power and on valve operability. DC power given by batteries provides energy for valve operation of those safety systems which do not directly necessitate AC power, that is to say: Isolation Condenser (Unit 1), RCIC and HPCI (Unit 2 and 3). At Unit 3 measurements of the two main parameters from Tokyo Electric Power COmpany (TEPCO) are available until the hydrogen explosion which occurred on March 14th. However, measurement data are often incoherent between activation of SRVs, water level and core pressure, frequently missing and likely to become more and more imprecise following the accident progression due to measurement tools degradation. Severe accident codes such as SAMPSON are therefore crucial tools to provide information about the accident, in order to fill the incompleteness of the available measurements, give prediction of the current plant conditions and enhance understanding of the events. The analysis of Unit 3 is here presented through the severe accident code SAMPSON until termination of HPCI. At this time the core is predicted to be always abundantly covered by water and providing high confidence about the availability of emergency systems like RCIC and HPCI for Unit 3. The present analysis represents an effort to reconstruct the accident progression and show the ability and improvements of the modules employed in SAMPSON for severe accident analysis.
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