The reactors were very hot, though(they operate at about 55o degrees Fahrenheit) and it takes a while to remove that heat. In addition -and this is the real problem even though the chain reaction is stopped, heat is still generated in the fuel by the natural decay of the radioactive elements present This heat is why the plant engineers needed to find a way to keep pumping water into the reactor core -it stays so hot that the water can boil off, exposing the fuel rods. And if the fuel rods are exposed, even for a short time(as happened at at least two of the reactors ) they become damaged and radioactivity is released The engineers took what seemed at the time like a desperate step, flooding the reactors with sea water laced with boric acid, a step that permanently disabled them. Pumping and Venting The japanese authorities have provided few details of how they are doing that Nuclear engineers and executives with experience at other reactors say that the Japanese are most likely using several pumper engine firetrucks in a row, with each increasing the pressure of the water and pushing it through a hose to the next truck At the Fukushima Daichi nuclear power plant, maximum pressure is needed to force water into overheated reactors in which a significant proportion of the water has turned into very hot steam at very high pressure The pumping process has been fraught with problems -at least one engine ran out of fuel at Reactor No. 2 on March 14 for several hours, interrupting the pumping of sea water during this time, because no tanker truck was immediately available te refuel it. Temperature and pressure climbed in the reactor as a result, and may have contributed to damage to the fuel rods and to an explosion at the base of the primary containment building Experts say that when the sea water reaches the inside of the reactor it is turning to steam. The reactor is still so hot that the sea water they are pumping in is just intended to replace the amount that boils off. In other words, in the best of situations they are just managing to keep the reactor covered with waterThe reactors were very hot, though (they operate at about 550 degrees Fahrenheit), and it takes a while to remove that heat. In addition — and this is the real problem — even though the chain reaction is stopped, heat is still generated in the fuel by the natural decay of the radioactive elements present. This heat is why the plant engineers needed to find a way to keep pumping water into the reactor core — it stays so hot that the water can boil off, exposing the fuel rods. And if the fuel rods are exposed, even for a short time (as happened at at least two of the reactors), they become damaged and radioactivity is released. The engineers took what seemed at the time like a desperate step, flooding the reactors with sea water laced with boric acid, a step that permanently disabled them. Pumping and Venting The Japanese authorities have provided few details of how they are doing that. Nuclear engineers and executives with experience at other reactors say that the Japanese are most likely using several pumper engine firetrucks in a row, with each increasing the pressure of the water and pushing it through a hose to the next truck. At the Fukushima Daiichi nuclear power plant, maximum pressure is needed to force water into overheated reactors in which a significant proportion of the water has turned into very hot steam at very high pressure. The pumping process has been fraught with problems — at least one engine ran out of fuel at Reactor No. 2 on March 14 for several hours, interrupting the pumping of sea water during this time, because no tanker truck was immediately available to refuel it. Temperature and pressure climbed in the reactor as a result, and may have contributed to damage to the fuel rods and to an explosion at the base of the primary containment building. Experts say that when the sea water reaches the inside of the reactor, it is turning to steam. The reactor is still so hot that the sea water they are pumping in is just intended to replace the amount that boils off. In other words, in the best of situations, they are just managing to keep the reactor covered with water