Tokyo Electric Power Co. confirmed that some spent fuel rods in one of the reactors at the Fukushima Daiichi nuclear power plant are damaged.

Kyodo News reports that the company analyzed the water in the spent fuel pool of reactor No. 4. The water had iodine-131 and cesium-137 at much higher than expected levels. TEPCO said it measured 220 becquerels per cubic centimeter of iodine-131, 88 becquerels per cubic centimeter of cesium-134 and 93 becquerels of cesium-137. Such levels are up to 100,000 times normal, according to the Japanese Nuclear and Industrial Safety Agency.

There are a total of 1,331 fuel rods in the pool, and 204 that were unused. The fuel rods contain the uranium oxide, which generates neutrons and produces heat. To keep the rods cool, they have to be stored in water for about five years, until the amount of heat they produce decreases enough for them to be stored in steel casks.

In reactor No. 4, some of the rods in the spent fuel pool were relatively new, because the reactor was undergoing maintenance. Such new fuel rods generate much more heat than older ones.

When the cooling systems failed, the water eventually started boiling away. With no water, the heat generated by the fuel rods damaged their casings, which is made of a zirconium alloy. Exposed to heat and steam the alloy releases hydrogen as well as the decay products of uranium. If the water level drops below the top of the fuel rods the casings can deform, crack or even melt.

Many outside experts had already said that it was likely the fuel rods were damaged because of the presence of iodine and cesium isotopes in the areas around the plant. A tell-tale sign that rods are damaged is the presence of iodine-131, because its half-life is only about eight days. Cesium-134 has a half-life of about two years and cesium-137's half-life is about 30 years.

TEPCO said the fuel rods may also have been damaged by the hydrogen explosion that blew off part of the roof and upper walls of the reactor building, as well as the falling steel frame.

TEPCO workers poured thousands of tons of water into the plants to fill the spent fuel pools and keep the rods there cooled down, as well as to cool the reactor cores.