On the 4th of April, 2023, Tepco released the reactor 1 investigation report that describes the damage scale inside the base of reactor 1. This investigation was implemented one week before the data release. This research was aimed to complement the previous research carried out last year.
The connected data of two investigations from last year and this year revealed the significant parts of the concrete base of reactor 1 have already disappeared. The base is a ring shaped structure made of concrete with 1.2m thickness. From the bottom to 1m height, the iron reinforcing bars were exposed to the outside. This means, the concrete base was likely to be evaporated at over 2,800 celsius degrees up to 0.6m thickness. However, it is not clear why the iron bars were not affected by the heat.
On the bottom of the inside of the base, debris was observed to be accumulating with up to 1m thickness, which is assumed to be the mixture of molten fuel and internal structures of the reactor container vessel. Some parts of the control rod facility were also seen lying on the layered debris, however the rest of the control rod facility is still unstably hanging in the vessel. Even though most parts of the base still can have a half thickness, based on Tepco’s estimate, the concrete is “broken though” at a 60 degree angle around the open door of the base with only iron bars supporting the whole base. In case another major quake hits Fukushima, there’s a risk that the entire base collapses. Tepco admitted this possibility, yet stated it wouldn’t sink more than 30cm due to the exterior structure. However, if the large part of the control rod facility hanging in the vessel falls onto the molten debris, it may crack the debris into small pieces and trigger a recriticality accident according to Tepco. The Nuclear Regulation Authority of Japan demanded Tepco to draft countermeasures but Tepco states the probability of such an accident is significantly low.
