Japan nuclear operator aims for cold shutdown in 6-9 months

This is a long time, compare this with the Chernobyl scenario, the following is an excerpt from the IAEA: "Under extremely hazardous conditions, thousands of "Liquidators" worked to contain the remains of the fourth reactor. The shelter surrounding the reactor was completed less than six months after the explosion during peak radioactivity levels. The massive concrete and steel "Sarcophagus", quickly constructed using "arms length" methods, has deteriorated over the years, creating a potentially hazardous situation. Several repairs were made to the current shelter, including the stabilisation of the ventilation stack and reinforcement of the roof. In addition, a plan for the construction of a more secure and permanent structure to be built around the existing Sarcophagus was drafted; work has already begun on the infrastructure of this new shelter. The plan, called the Shelter Implementation Plan, is a project of the Chernobyl Shelter Fund. Both efforts, whose combined expected expenditures over the next eight or nine years exceed $765 million, are administered by the European Bank for Reconstruction and Development."


Also in order to keep cooling the spent fuel and the 3 breached reactors water must be injected continually, and water will always find its way into the ocean. From the latest entries on Fukushima in Wikipedia: "Reports of 13 observations of neutron beams 1.5 km "southwest of the plant's No. 1 and 2 reactors" from 13 March to 16 March raised the possibility that nuclear fission could have occurred after the initial SCRAMing of the reactors at Fukushima Daiichi. "

Neutrons can be emitted from fission -Wikipedia

Neutrons:

"Neutrons also degrade materials; bombardment of materials with neutrons creates collision cascades that can producepoint defects and dislocations in the materials. At high neutron fluences this can lead to embrittlement of metals and other materials, and to swelling of some of them. This poses a problem for nuclear reactor vessels, and significantly limits their lifetime (which can be somewhat prolonged by controlled annealing of the vessel, reducing the number of the built-up dislocations). Graphite moderator blocks are especially susceptible to this effect, known as Wigner effect, and have to be annealed periodically; the well-known Windscale fire was caused by a mishap during such an annealing operation."


In order to achieve an effective fission chain reaction, the neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the nuclear fuel is not sufficiently refined to be able to absorb enough fast neutrons to carry on the fission chain reaction, due to the lower cross section for higher-energy neutrons, so a neutron moderator must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators include graphite, light water and heavy water. A few reactors (fast neutron reactors) and all nuclear weapons rely on fast neutrons. This requires certain changes in the design and in the required nuclear fuel. The element beryllium is particularly useful due to its ability to act as a neutron reflector or lens. This allows smaller quantities of fissile material to be used and is a primary technical development that led to the creation ofneutron bombs.


Wikipedia