( Interim Report)SAFETY EVALUATION OF INDIAN NUCLEAR POWER PLANTS POST FUKUSHIMA INCIDENT

An unprecedented earthquake of magnitude 9 (Richter scale) followed by a
Tsunami  of height much larger than the value considered in design of Fukushima
Dai-ichi Plant had hit north eastern part of Japan on March 11, 2011.
There are 13 nuclear power plants, all of Boiling Water Reactor type located in the
affected zone.  Six of the units are located at Fukushima Dai-ichi (3 under operation
and 3 under shutdown), 4 at Fukushima Dai-ini and 3 at Onagawa all operating.
The severe earthquake resulted in the disruption of the grid resulting into nonavailability of offsite power.  All the operating plants were automatically shut down
on sensing the earthquake. The decay heat removal system started functioning
normally as per design requirements. The Tsunami which hit the affected area about
half an hour later,  resulted into submergence of the emergency power supply
systems at Fukushima Dai-ichi leading to total loss of on-site power supply, termed
as station blackout condition.  The decay heat removal could not be resumed, which
ultimately resulted in the fuel assemblies getting uncovered. This led to overheating
of the fuel. Metal water reaction between zirconium and water resulted in
generation of hydrogen.  In the process, the reactor containment vault pressure
increased and reached upto almost two times the design pressure.  It was decided to
vent the reactor containment vault to prevent damage to it. During the
depressurization, hydrogen and steam leaked into the secondary containment
resulting in hydrogen explosion.  Spread of radioactivity necessitated evacuation of
public in the nearby areas extending up to 20-30  Kms to prevent exposure of the
public. The event was initially rated as Level 5 on the International Nuclear Event
Scale. The rating was subsequently revised to Level 7. However, the total
radioactivity released during this incident was about 10% of that released during the
Chernobyl accident in 1986. There was also degradation in the cooling provisions
of spent fuel pool in Unit-4 resulting in spent fuel getting uncovered.  The situation
is still evolving.

Download the Summary of Reporthttp://www.npcil.nic.in/pdf/Final_Report_Four_TFs_combined_report.pdf