overheating caused a reaction between the water and the zircaloy, creating hydrogen gas. On 12 March, leaking hydrogen mixed with oxygen exploded in Unit 1, destroying the upper part of the building and injuring five people. On 14 March, a similar explosion occurred in the Reactor 3 building, blowing off the roof and injuring eleven people. On 15 March, there was an explosion in the Reactor 4 building due to a shared vent pipe with Reactor 3. Core meltdowns in units 1, 2, and 3 The amount of damage sustained by the reactor cores during the accident, and the location of molten nuclear fuel ("corium") within the containment buildings, is unknown; TEPCO has revised its estimates several times. On 16 March 2011, TEPCO estimated that 70% of the fuel in Unit 1 had melted and 33% in Unit 2, and that Unit 3's core might also be damaged. As of 2015 it can be assumed that most fuel melted through the reactor pressure vessel (RPV) and is resting on the bottom of the primary containment vessel (PCV), having been stopped by the PCV concrete. In July 2017 a remotely controlled robot filmed for the first time apparently melted fuel, just below the reactor pressure vessel of Unit 3. TEPCO released further estimates of the state and location of the fuel in a November 2011 report. The report concluded that the Unit 1 RPV was damaged during the disaster and that "significant amounts" of molten fuel had fallen into the bottom of the PCV. The erosion of the concrete of the PCV by the molten fuel after the core meltdown was estimated to stop at approx. in depth, while the thickness of the containment is thick. Gas sampling carried out before the report detected no signs of an ongoing reaction of the fuel with the concrete of the PCV and all the fuel in Unit 1 was estimated to be "well cooled down, including the fuel dropped on the bottom of the reactor". Fuel in Units 2 and 3 had melted, however less than in Unit 1, and fuel was presumed to be still in the RPV, with no significant amounts of fuel fallen to the bottom of the PCV. The report further suggested that "there is a range in the evaluation results" from "all fuel in the RPV (none fuel fallen to the PCV)" in Unit 2 and Unit 3,