The latest development in the Fukushima saga is probably one of the more ominous to date. Yomiuri reports that radioactive Zirconium 95 has been found after samples were taken near the water outlet. Google translated: "Zirconium is used for nuclear fuel cladding, the cladding melts some of the spent nuclear fuel was hot cooling water is lost, possibly mixed with sea water flowing into the large drainage There. TEPCO am on March 23, collected about 330 m south from the water at the point of outlet. Zirconium-95 concentration was 0.23 becquerels per cubic centimeter. Atomic Energy Research Institute of Kinki Sugiyama Wataru teachers (of nuclear safety), "The evidence that melting in the heat of the fuel cladding, said first find. Will come from a spent fuel storage pool at," he said." Shortly thereafter NHK spokesman admitted that this is why large amounts of radiation are leaking into the environment, making attempts to control the situation 'very challenging'. If indeed the fuel rod zirconium casing is coming off, it means that the risk for recriticality could be increasing.
A typical solid nuclear fuel rod includes a zirconium alloy tube or “cladding” encasing a single column of uranium fuel pellets. The cladding tube is smaller in diameter than your index finger, and is about 12 feet long. The uranium pellets are each about the size of the tip or your pinky finger, with the energy equivalent of 17000 cubic feet of natural gas, 1780 pounds of coal or 3.5 barrels of oil. The pellets are stacked in the tube with allowance for pellet expansion during fission and heating of the uranium. Once the uranium pellets are loaded into the cladding tube, zirconium end caps are welded in place to form a complete loaded fuel “rod.”
The cladding, pellets and even an individual virgin rod are not hazardous to handle alone, however, multiple loaded rods in close proximity will begin a spontaneous fission reaction. The rods are thus maintained in a non-critical, i.e., a non-fissioning, state during storage or transport by either substantial separation between rods or by control rods or other moderators suitable to absorb neutrons in a more compact rod arrangement.
The fuel rods are then arranged in “bundles” or “fuel rod assemblies”, e.g., 14×14 or 17×17 arrays, which are then inserted into the core with a number of control rods being retractable from the bundle to initiate fission and insertable into the bundle to stop fission. Many rod bundles are oriented vertically in the reactor core with a substantial flow of water passing upward through the bundles to convey the fission reaction heat to a steam turbine for generation of electricity.
The zirconium cladding serves to hermetically isolate the uranium pellets and accumulated fission byproducts from exposure to the water flow in the core or cooling tank or to the atmosphere. The thin-walled cladding is transparent to radiation but is naturally affected by the high heat stresses and heat loading in the core. The rods are preemptively retired after a finite core cycle, 18 months to several years, to maintain cladding integrity even though only a small fraction of the uranium is “spent.” This finite core cycle is also limited by accumulation of fission byproducts, particularly nuetron absorbers, inside the fuel rod. A retired or spent nuclear fuel (“SNF”) rod is placed in a water cooling tank for an initial cool-down period during which the more highly radioactive (shorter half-life) isotopes rapidly decay. During this period, the rapid decay still generates substantial decay radiation and heat, albeit only a small fraction of the fission radiation and heat that is generated during reactor operation. After this initial cool-down period, the slower decay of the remaining longer-half-life isotopes generates a moderate amount of decay radiation and heat, which is readily absorbed by a concrete “dry cask” during long-term storage.
A typical nuclear plant can have hundreds of active fuel rod bundles in each core, thousands of SNF rods in short-term cool-down tanks and fuel from tens of thousands of SNF rods in long-term dry cask storage. The cooling tanks at the compromised Fukushima Daiichi nuclear plant collectively house around 11,000 SNF rods with a portion of those housed in the cooling tanks above reactors 1-4.
Water in the cool-down tanks acts as a neutron moderator, radiation shield and coolant, so long as the water level around the rods in the tank is maintained. If the SNF rods are left exposed and uncooled long enough, rapid oxidation (often called “burning”) and extreme heat stress can eventually compromise the cladding, expose the uranium, generate hydrogen, and release fission byproducts. Unmoderated and uncooled SNF rods can produce sufficient radiation and heat that even brief close proximity worker exposure is unacceptable. Should the cooling tank levels drop too low for too long, it could be challenging to restore the cooling tank water levels from a safe distance.
Hopefully, the cooling tank water levels at the Fukushima Daiichi nuclear plant will be restored and the situation stabilized soon.
And if that was not enough, Kyodo reports that high-level radiation is now suspected to be leaking from the core of reactor #3, implying reactor damage.
High-level radiation detected Thursday in water at the No. 3 reactor's turbine building at the crisis-hit Fukushima Daiichi nuclear plant appears to have originated from the reactor core, the government's Nuclear and Industrial Safety Agency said Friday.
But no data, such as on the pressure level, have suggested the reactor vessel has been cracked or damaged, agency spokesman Hidehiko Nishiyama emphasized at an afternoon press conference, backing down from his previous remark that there is a good chance that the reactor has been damaged. It remains uncertain how the leakage happened, he added.
A day after three workers were exposed Thursday to water containing radioactive materials 10,000 times the normal level at the turbine building connected to the No. 3 reactor building, highly radioactive water was found also at the No. 1 and No. 2 reactors' turbine buildings.
The latest development in Japan's worst nuclear crisis raises the risk of more workers being exposed to radioactive elements, hampering their efforts to restore the plant's crippled cooling functions that are key to putting the crisis under control.
The three workers were transferred to the National Institute of Radiological Sciences in Chiba Prefecture Friday afternoon, after two of them were taken Thursday to a Fukushima hospital for possible radiation burns to their feet, the plant operator Tokyo Electric Power Co. said.
Following the incident, the nuclear regulatory agency ordered the utility known as TEPCO to improve radiation management at the power station, located about 220 kilometers northeast of Tokyo.
Nishiyama said the high-level radiation is suspected to have come from the reactor, where overheating fuel rods are believed to have partially melted.
He said further verification is needed to find out how the radioactive water reached the underground site where the workers were exposed. Huge volumes of water have been poured into the reactor as well as its apparently boiling spent fuel pool since they lost their cooling functions.
The government, which has set the exclusion zone covering areas within a 20 kilometer radius of the Fukushima plant, meanwhile, encouraged residents within a 30 kilometer radius of the power station to voluntarily leave, while the official directive is for them to stay indoors.
The Nuclear Safety Commission of Japan, a government panel, recommended voluntary evacuation as the release of radioactive materials from the plant is expected to continue for some time.
Despite the partial halt of restoration work due to the technicians' radiation exposure, TEPCO on Friday began injecting freshwater into the No. 1 reactor core, as it prepares to inject freshwater into all the troubled three reactor cores and four spent fuel pools, instead of seawater currently used.
Lastly, for those who believe this is reporting merely an attempt to induce panic, here is Bloomberg on the same topic.
Reactor Core May Be Breached, Leaking Radiation at Plant
Japan’s nuclear regulator said one reactor core at the quake-damaged Fukushima Dai-Ichi power plant may be cracked and leaking radiation.
"It’s very possible that there has been some kind of leak at the No. 3 reactor," Hidehiko Nishiyama, a spokesman at the Japan Nuclear and Industrial Safety Agency said in Tokyo today. While radioactive water at the unit most likely escaped from the reactor core, it also could have originated from spent fuel pools stored atop the reactor, he said.
Repair work at the site of the worst nuclear disaster since Chernobyl has been plagued by explosions, fires and leaks of toxic material. Workers using fire engines have streamed 4,000 tons of water on the No. 3 reactor, five times more than any of the other five units, according to the government.
Two plant workers were hospitalized yesterday with radiation burns after stepping in the water, which was found to have radiation levels 10,000 times higher than water used in reactor cooling, Nishiyama said earlier today.
Tokyo Electric Power Co., the plant operator, said it found eight different radioactive materials in the water of the turbine building basement, where the men were attempting to connect a power cable. The materials are made through a process of fission, and include cobalt and molybdenum-99, a spokesman for the power utility said.