Washington (CNN) -- Nuclear experts at the Union of Concerned Scientists said again Thursday there are many challenges ahead and few options left to Japanese workers trying to ease the crisis at the Fukushima Daiichi plant.
"The fact they have to handle two spent fuel pools and three reactor cores with kid gloves, (they) don't have any margin for error," said David Lochbaum, a nuclear engineer and director of the group's Nuclear Safety Project.
"It's hard to say whether things will get better or things will get worse because they have so many challenges to face on so many places that it's going to be difficult to be 100% right all five times. So it is a bad situation over there."
At the damaged plant, engineers began injecting nonflammable nitrogen into the No. 1 reactor Thursday to counter a buildup of potentially explosive hydrogen. That work continued even after a strong aftershock rattled much of central Japan shortly before midnight, the plant's owner, the Tokyo Electric Power Company, reported early Friday.
Hydrogen buildup is a symptom of overheated fuel rods in the cores of the reactors, which plant workers have been struggling to keep under control since the earthquake and tsunami. The nitrogen injections are aimed at displacing oxygen in the reactor shell, reducing the possibility of an explosion, a chance Tokyo Electric called "extremely low."
"Now that there has been so much venting, over weeks, to try to control the pressure buildup in the containment, that nitrogen is not in the containment building as much as it needs to be. So, they're trying to re-inert the containment or refill the containments with nitrogen to guard against a hydrogen buildup and also oxygen buildup," said Lochbaum.
The scientists could not ever recall another time that nitrogen had been injected into a nuclear containment vessel.
"After the 1979 accident at Three Mile Island, the hydrogen built up to the point where it did explode about 10 hours after the initial event. ...They had no control over it," Lochbaum said. "Because of that, in this country, we started putting in hydrogen recombiners, hydrogen inerting, hydrogen igniters and so on, to try to control it, but I don't know of anybody who has actually gotten into this situation and tried to put in nitrogen after the fact or to restore the nitrogen that was there originally," he added.
In Japan, a hydrogen explosion blew the roof and upper walls off the No. 1 reactor building two days after the quake, and another blast two days later blew apart the No. 3 building. A suspected hydrogen explosion is believed to have damaged the No. 2 reactor on March 15 as well.
The scientists expressed concern about the viability of the containment structures in light of Thursday's strong aftershocks, saying the plant was designed to withstand seismic forces but that was before the hydrogen explosions and addition of water.
"It's hard to judge how those structures will last during the aftershocks or new earthquakes. It's just too hard to speculate," Lochbaum said.
The scientists told reporters on a telephone conference call there is a limited amount of equipment to deal with the emergency.
Lochbaum said, "They're so far beyond where emergency procedures and preplanning have done that they're basically having to jerry-rig solution paths or potential solution paths for situations that were never anticipated.
"The other problem that's complicating it is nobody ever anticipated an accident on more than one reactor or more than one spent fuel pool at a site," Lochbaum said.
"The assumption all along was we'd have a problem at one reactor or one spent fuel pool, and we'd use the equipment from the others to help deal with the situation. Now you've got problems across the board, and you don't have that supplemental equipment that you can shift from one unit to the other to try to help you out. Everybody needs help, and there's just not enough equipment to deal with that."
The Union of Concerned Scientists is an independent nuclear-industry watchdog group that focuses on safety issues.