Regulators are putting the design of nuclear reactors slated to be built in the U.S. under additional scrutiny following the March disaster at Japan’s Fukushima Daiichi nuclear plant.

On May 20, the Nuclear Regulatory Commission released a statement that Westinghouse must resolve design issues around the shield building of the AP1000, as well as the peak accident pressures expected within containment. The NRC is expected to conduct additional review of the design at the end of May, and that Westinghouse would submit additional information on the design early in June.

The NRC said that Westinghouse must resolve the issues before it would certify the AP1000 design. Certification was previously expected later this year.  The Shaw Group, Baton Rouge, La., and Westinghouse Corp., Cranberry Township, Pa., are currently in preconstruction for two new AP1000 units near Waynesboro, Ga., and two units near Jenkinsville, S.C.

Gentry Brann, a spokeswoman for Shaw, said that the utilities building the AP1000 will make the final determination on the timeline of constructing the units.

In a related action, the NRC on May 20 also said it has completed its post-Fukushima review of the nation’s nuclear plants. The NRC said of the nation’s 65 operating reactor sites, 12 had Fukushima-related issues that were being resolved: nine had issues dealing with plans for dealing with the effect of flooding or the loss of all of the plant’s AC power; while two had issues with strategies dealing with extreme flooding events.

Two of the more controversial nuclear plants – Indian Point and Diablo Canyon – were included in the list of plants with deficiencies. The NRC said that high pressures could damage equipment necessary to vent hydrogen at Indian Point. Diablo Canyon was found to have only one pump for emergency cooling water and the plant’s six diesel generators were located in the same spot.