Fukushima Daiichi Nuclear Power Station Unit 1: Blunt Force Impact Damage Inside Primary Containment

The Mar. 11, 2011, Tohoku-Chihou-Taiheiyou-Oki Earthquake and Tsunami precipitated losses of coolant and fuel meltdowns in three reactors at Fukushima Daiichi Nuclear Power Station. Unit 1 was the station's first to incur a meltdown.

Yesterday, the nuclear power station's operator Tokyo Electric Power Co. (TEPCO) released a video of the first endoscopic exploration inside Unit 1's primary containment vessel (PCV) (TEPCO press release with the title "Punching an Access Hole at the Penetration (X100B Penetration) of Unit 1 PCV at Fukushima Daiichi Nuclear Power Station", dated Sep. 27, 2012). TEPCO's video can be downloaded here. General Electric's Mark I primary containment systems are composed of a pear-shaped drywell housing the reactor pressure vessel (RPV) and a doughnut-shaped, water-filled suppression chamber, also known as wetwell. TEPCO's video below explores the inside of the drywell (courtesy: SimplyInfo.org).

Note that the upper part of the drywell is filled with dense steam. Water must be boiling at the bottom of the primary containment.

Furthermore, the dominant color of the drywell's inner surface is grimy black and not brown as observed in Unit 2 (see the video in my post with the title "Fukushima: Fuel Meltdowns & Cold Shutdown" published online Feb. 15, 2012), indicating combustion either by explosion or fire.

18:05 minutes into the video blunt force impact on the drywell becomes visible. A large piece of sharp-edged debris can be seen deposited adjacent to exposed rebar of a reinforced concrete wall structure. The impact suggests that the location was struck by a heavy object, perhaps the object nearby, either falling from above or projected against the drywell wall like a missile.

A high-pressure steam jet exiting from a small RPV breach, also known as small breach Loss of Cooling Accident (LOCA), could have blasted chunks off the RPV or its piping, turning the chunks into projectiles that impacted the drywell.
Addendum

• During the past week, TEPCO undertook another video camera foray into the primary containment vessel (PCV) of Unit 1 (TEPCO's press handout with the title "Investigation Results of the Inside of Unit 1 PCV at Fukushima Daiichi Nuclear Power Station" dated Oct. 15, 2012). The company released three videos of the exploration.
  The two pictures below, courtesy of Simplyinfo.org, were captured between 37 and 44 minutes in the hour-long video.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  The pictures show three types of damage: • the PCV floor fractured (first picture; top left corner), • scattered metal shards (both pictures), suggesting piping shattered in a blast caused by excess interior pressure, and • extreme force impact fragmented large structures and components (bottom picture)(10/17/2012).