Nuclear Casktop [nuclear shield plug] damaged in Fukushima reactor # 3 [most deadly MOX fuel reactor]

The shield plug is a more than 1 and 1/2 meters [4.5 feet] thick plug composed of 1/2 meter [~ 1.5 foot] thick steel surrounded by a meter or more of reinforced concrete. It is the principle neutron shield of a reactor. Probably, it is too dangerous to completely remove the neutron shield plug in the case of a Mox fuel meltdown. That it is damaged is a bad sign, possibly signifying that the hydrogen explosion may have derived from inside the reactor core itself and not from the surrounding containment vessel. The black soot plutonium containing bucky balls found many tens of miles from the reactors may have come from the core of this reactor. 8kg of plutonium can kill from two million to several billion people depending on who you believe. Melted cores are also a potent source of deadly neutrons. Neutrons kill or injure humans and animals, damaging their organs and DNA and rendering them radiactive in themselves, not unlike the devilish neutron bombs. When the Japanese attempt to remove the cores, every worker within a kilometer or more could die from unshielded neutrons. 

http://en.wikipedia.org/wiki/Neutron_radiation

Health hazards and protection[edit]

In health physics neutron radiation is considered a fourth radiation hazard alongside the other types of radiation. Another, sometimes more severe hazard of neutron radiation, is neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters, including the body tissues of the workers themselves. This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide. This process accounts for much of the radioactive material released by the detonation of a nuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations, as it gradually renders the equipment radioactive; eventually the hardware must be replaced and disposed of as low-level radioactive waste.

Neutron radiation protection relies on radiation shielding. Due to the high kinetic energy of neutrons, this radiation is considered to be the most severe and dangerous radiation to the whole body when exposed to external radiation sources. In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei, so hydrogen-rich material is more effective than iron nuclei. The light atoms serve to slow down the neutrons by elastic scattering, so they can then be absorbed by nuclear reactions. However, gamma radiation is often produced in such reactions, so additional shielding has to be provided to absorb it. Care must be taken to avoid using nuclei which undergo fission or neutron capture that results in radioactive decayof nuclei that produce gamma rays.

Neutrons readily pass through most material, but interact enough to cause biological damage. The most effective shielding materials are hydrocarbons, e.g.polyethylene, paraffin wax or water. Concrete (where a considerable amount of water molecules are chemically bound to the cement) and gravel are used as cheap and effective biological shields due to their combined shielding of both gamma rays and neutrons. Boron is an excellent neutron absorber (and also undergoes some neutron scattering) which decays into carbon or helium and produces virtually no gamma radiation, with boron carbide a commonly used shield where concrete would be cost prohibitive. Commercially, tanks of water or fuel oil, concrete, gravel, and B₄C are common shields that surround areas of large amounts of neutron flux, e.g.nuclear reactors. Boron-impregnated silica glass, high-boron steel, paraffin, and Plexiglas have niche uses.

Because the neutrons that strike the hydrogen nucleus (proton, or deuteron) impart energy to that nucleus, they in turn will break from their chemical bonds and travel a short distance before stopping. Such hydrogen nuclei are high linear energy transfer particles, and are in turn stopped by ionization of the material through which they travel. Consequently, in living tissue, neutrons have a relatively high relative biological effectiveness, and are roughly ten times more effective at causing biological damage compared to gamma or beta radiation of equivalent radiation exposure. Neutrons are particularly damaging to soft tissues like the cornea of the eye.