Faulty reactor instrumentation has led Atomic Energy of Canada Limited to push back the return-to-service date of the National Universal reactor by up to two weeks, the group said Wednesday.

"During the fuel reloading, it was determined that a specific set of reactor instrumentation, important to safe operation, was not responding as expected," the AECL said in a statement.

The Chalk River, Ontario-based NRU reactor is expected to be up and running again by August 10 or 15, the AECL said. The originally planned return-to-serve date was July 31.

The AECL warned that "unforeseen operational issues" could always further extend the re-start date.