Four days passed before concerned parties in Japan were told that mixed plutonium-uranium oxide fuel pellets to be shipped to Japan were improperly checked in Britain, it was learned Wednesday.

According to sources close to the case, the results of an in-house investigation by British Nuclear Fuels Ltd. on the suspected mistake took four days to reach Mitsubishi Heavy Industries Ltd. Kansai Electric Power Co. imports MOX fuel for nuclear reactors in Takahama, Fukui Prefecture, through Mitsubishi Heavy.

In addition, the fax by which BNFL notified Mitsubishi Heavy was not picked up and sent on to Kepco for another two days because it arrived over the weekend, the sources said.