Workers at the stricken Fukushima Daiichi nuclear power plant are onto Plan C in their bid to stop highly radioactive water from gushing directly into the Pacific Ocean through a cracked concrete shaft, a Japanese nuclear official said Monday.

Neither of the first two attempts to fill up the 20-centimeter (8-inch) crack outside the No. 2 reactor's turbine building -- on Saturday by pouring in concrete, and then Sunday by using a chemical compound mixed with sawdust and newspaper -- has been successful [...]

As they mull other ways to cut off the leak at its source, workers will install a silt fence along a damaged sea wall surrounding the plant, Hidehiko Nishiyama of Japan's nuclear and industrial safety agency said Monday. The aim of this screening, which is usually used to halt erosion at construction sites, is to prohibit the spread of radioactive particles into the sea.

Workers also have injected a dye tracer into the water to allow them to track the dispersal of such particles [...]

In some cases, authorities don't even know how much radiation is getting out.

After some high-profile errors while offering regular radiation measurements on seawater, groundwater and the air, little such new information has been released since Thursday. One reason is that the dosimeters being used don't go above 1,000 millisieverts per hour, Junichi Matsumoto, an executive with the plant's owner Tokyo Electric Power Company, told reporters Sunday [...]

The amount of emitted radiation, be it into the water or air, is also unknown. Authorities do believe there had been at least a partial meltdown of nuclear fuel -- thanks to intense heat, at one point, topping 2,700 Celsius (4,800 Fahrenheit) in the No. 1 reactor and 1,800 Celsius (3,200 Fahrenheit) in the Nos. 2 and 3 reactors, according to an analysis from Areva, one of the world's top nuclear energy companies based in France.