Nuclear regulators have revealed that a leak of heavy water from a research reactor in September temporarily halted a clinical study into an advanced cancer therapy.
The reactor at the Kyoto University Research Reactor Institute in Kumatori, Osaka Prefecture, was shut down for a month but has since resumed operation.
The leak occurred even though the reactor passed the stricter safety guidelines adopted after the 2011 Fukushima nuclear crisis.
The clinical study involved boron neutron capture therapy, which uses neuron radiation generated from the reactor. It kills only cancerous cells by injecting patients with boron and then projecting neutrons. Requiring no surgery, it has been called "a therapy of the next generation."
The institute discovered the reactor was leaking heavy water from a pipe after an alarm went off Sept. 20 in a system that monitors tritium, suspending the clinical study, according to the Nuclear Regulation Authority.
The NRA confirmed there had been no harmful effects on patients or staff, and that no radioactive material escaped from the facility.
The university apologized to the patients involved in the study for the inconvenience caused by the shutdown.
The NRA said the leak involved about 100 milliliters of heavy water, and that the density of radioactive materials was very low and did not violate safety rules.
The institute restarted the clinical research on Oct. 24 after fixing the leaky pipe.
The reactor began operating in 1964 with a capacity of 5 megawatts. It has been used for more than 500 case studies involving the therapy in cooperation with medical institutions since the 1970s.
It was deactivated for a regular check in 2014, following the Fukushima disaster. The institute restarted it Aug. 29 and clinical research resumed two days later.