One of the first things you notice when entering the tour of the reactor containment vessel (CV) is this odd slab of "stuff". This is a section of the CV which was cut out to give the workers better access to the reactor. It also shows how the reactor was protected. The main fear/concern was collisions with other ships. Thus the reactor was armored with a sandwich of 1" steel plate / 2" redwood for about 3 feet. The steel was to provide main protection against collision and the redwood was a shock absorber to diminish the effect of any collision. This was then backed up with about 3 feet of concrete on the inside facing the reactor. Per the nuclear safety engineer who described the work being done, there were a number of slabs like this taken out of the CV. The reactor is now behind watertight doors. Behind this slab, upper left in image, is another watertight door cut into Hold 4 which is being used as a warehouse. Enjoy; thanks for viewing.