The National Research Universal Reactor's re-start date has been pushed back until the end of July. The Atomic Energy of Canada Limited, which runs the NRU reactor, announced the delay on Thursday.

In its previous update, from last week, the AECL predicted the 52-year-old reactor, shut down because of a leaky vessel, would be online by May.

The AECL blames the slow pace on the complexity of the final two repair locations, which require developing and training welders in specific new welding techniques.

According to AECL, they have already fixed eight of the ten weld sites, and are more than half finished. The final two repair sites, the trickiest, represent the remaining 44 percent of work left on the project. Repairs to the first of the two remaining sites should begin next week; repair crews have already begun the initial phase of work, weld development, for the second.

The NRU reactor in Chalk River, Ontario is one of the most important producers of molybdenum-99, the parent isotope of technetium-99, widely used in nuclear medicine. Estimates by the Society of Nuclear Medicine suggest it's responsible for close to 40 percent of world supply of the isotope.

Repairs on the reactor began on December 12 to fix vessel leaks first detected in May.