7 tons per hour, which could lead to higher reactor temperatures. The highly radioactive water halts work on restoring the cooling pumps and other powered systems to reactors 1–4. TEPCO confirms finding low levels of plutonium in five samples during 21 and 22 March. Enriched levels of Plutonium-238, relative to Plutonium-239 and Plutonium-240, at two of the sites in the plant (solid waste area and field) indicate that contamination has occurred at those sites due to the "recent incident". Nonetheless, the overall levels of plutonium for all samples are about the same as background Pu levels resulting from atmospheric nuclear bomb testing in the past. However, these weapons did not use fuel that has decayed significantly. Such nuclear accidents as this one are a potent source of biologically accumulating isotopes of transuranic elements such as curium and americium which are incorporated into bones, complexed into the Ca/PO4 hydroxide. Tuesday, 29 March TEPCO continues to spray water into reactors 1–3 and discovers that radioactive runoff water is beginning to fill utility trenches outside the three reactor buildings. The highly radioactive water in and around the reactor buildings continues to limit progress of the technicians in restoring the cooling and other automated systems to the reactors. Wednesday, 30 March At a news conference TEPCO Chairman Tsunehisa Katsumata announces that it is unclear how the problems at the plant will be resolved. An immediate difficulty is the removal of large quantities of radioactive water in the basement buildings, but also salt build up inside the reactors, from using seawater for cooling, will need to be removed. Building concrete walls to enclose the reactors in a shield is being considered, as had been done at Chernobyl. The Environmental Protection Agency (EPA) finds traces of radioactive iodine in milk in the United States. The amount is "well below levels of public health concerns". Thursday, 31 March Workers pump radioactive water from a utility trench near reactor 1 into a storage tank near reactor 4. Water in the condensers for reactors 2 and 3 is shifted to outside storage tanks so that the condensers can remove more contaminated water from inside the reactors. The world's largest concrete pumping truck is shipped from the United States to Fukushima. The truck has been slightly modified to be able to pump cooling water initially, then will later possibly to be used to pump concrete for any eventual permanent containment