The bolts used to secure covers on metal containers for transporting low-level radioactive waste by sea might be failing, Nuclear Fuel Transport Co. disclosed over the weekend.

The containers carry drums filled with radiation-tainted clothes and equipment discarded by nuclear power plant workers. The items are mixed with cement and sealed inside the drums before making the voyage to a storage facility in the village of Rokkasho, Aomori Prefecture.

The Tokyo-based company — the only one in Japan transporting that kind of waste — said that none of the five bolts found so far broke during shipping and that there had been no environmental impact. The Land, Transport, Infrastructure and Tourism Ministry has ordered the firm to halt all transport operations until safety measures can be confirmed.

The problem was first discovered in February, but Nuclear Fuel Transport failed to report it to the ministry for more than four months after judging it to be a “peculiar case,” the ministry said Saturday.

The first case involved the discovery of one broken bolt among four used to secure the lid of a single container. It was found during a check of empty containers at the company’s storage facility in Rokkasho, it said.

Last Monday, it found another broken bolt while preparing to transport waste from Kansai Electric Power Co.’s Mihama nuclear power plant in Fukui Prefecture, the company said.

When another broken bolt was found Thursday at the Rokkasho facility, the company conducted further checks and found two more there, it said.

There are 3,300 such containers, made between 2011 and 2014, around the country. It will take a while to check them all and for transport operations to resume, domestic media reports said.

All of Japan’s commercial nuclear reactors are idle and must pass new stringent safety checks adopted in the wake of the triple core meltdown at the Fukushima No. 1 nuclear plant in 2011 before going back online.

For years, Japan shipped spent nuclear fuel to France and Britain for reprocessing because its commercial reactors generated nearly a third of its electricity needs before the Fukushima disaster.