Based on the dosimetries on the SGTS filters at 1F4, it is presumed that venting gases flowed from the 1F3 reactor building through the SGTS line(s) into the 1F4 reactor building. On the other hand, venting gas flow through the SGTS lines is assumed to be unlikely because different trends were observed in the dosimetries on the SGTS filters at 1F3, and the dosimetry downstream of the valve connected with the stack (② in the figure) was significantly low. However, the possibility of venting gas flow should be further examined because the dosimetry of the filter in the center was the highest, and the containment pressure did not exceed the design pressure for a long time.