UPDATE 2-Suncor Alberta refinery hit by another outage

* Second unplanned shutdown in less than a month

* Company still rebuilding fuel inventory

* Shell suffers unspecified upset at refinery, upgrader (Adds Shell declining comment)

CALGARY, Alberta, Aug 13 (Reuters) - Suncor Energy Inc's (SU.TO) refinery in Alberta was hit by a second unplanned outage in less than a month, complicating efforts to rebuild inventory after its retail network suffered severe shortages during the peak of the summer driving season.

The 135,000 barrel a day Edmonton refinery, which was owned by Petro-Canada before Suncor's takeover of PetroCan at the start of this month, was restarting on Thursday after undergoing a controlled shutdown of some units the day before, spokeswoman Sneh Seetal said.

It was forced to take the units off line when a hydrogen supplier suffered production problems, Seetal said.

"Our third-party supplier was able to restart its hydrogen production plant overnight, and so the refinery is now in the process of restarting," Seetal said.

She said it will take some time before production returns to normal, but did not give the volume of production that was affected.

Meanwhile, Royal Dutch Shell's (RDSa.L) 98,000 barrel a day refinery and 155,000 barrel a day oil sands upgrader near Edmonton also had unspecified "process issues" on Wednesday, according to a message on a community hotline.

But a spokeswoman from the oil major's London office declined to say if the problems had any impact on production, citing a policy of not commenting on day-to-day operations.

Suncor is still working to replenish its Alberta inventory after a severe storm cut power to the plant on July 18, leading to shortages of fuel at gas stations across the province. The situation was made worse by high demand during Canada's August civic holiday long weekend.

Imperial Oil Ltd (IMO.TO), the other major refiner in the region, also suffered a shutdown due to the storm, contributing to the tight supply situation.

Suncor was forced to arrange for alternative supplies, which included shipping in fuel by rail from its Montreal refinery.

"Over the weekend we were able to return almost all of the sites, with the exception of two in Calgary, to regular operations," she said. (Reporting by Jeffrey Jones; editing by Peter Galloway)