What is Radiation
Radioactive decay (also known as nuclear decay, radioactivity or nuclear radiation) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.....
To make a long story short, radioactivity in minetest is an emittance of harmful radiation. It can be provoked by uranium ore, uranium blocks and products of a melted nuclear reactor.
Radiation Resistance of Blocks
Radiation resistance represents the extent to which a material attenuates radiation passing through it; i.e., how good a radiation shield it is. This is identified per node type. For materials that exist in real life, the radiation resistance value that this system uses for a node type consisting of a solid cube of that material is the (approximate) number of halvings of ionising radiation that is achieved by a meter of the material in real life. This is approximately proportional to density, which provides a good way to estimate it. Homogeneous mixtures of materials have radiation resistance computed by a simple weighted mean. Note that the amount of attenuation that a material achieves in-game is not required to be (and is not) the same as the attenuation achieved in real life.
The rate at which damage is caused by a radioactive block depends on the distance between the source and the player.
Protecting from Radiation
Radiation is attenuated by the shielding effect of material along the path between the radioactive block and the player. In general, only blocks of homogeneous material contribute to the shielding effect: for example, a block of solid metal has a shielding effect, but a machine does not, even though the machine's ingredients include a metal case. The shielding effect of each block type is based on the real-life resistance of the material to ionising radiation, but for game balance the effectiveness of shielding is scaled down from real life, more so for stronger shield materials than for weaker ones. Also, whereas in real life materials have different shielding effects against different types of radiation, the game only has one type of damaging radiation, and so only one set of shielding values.
Using Uranium to Protect from Uranium
The best way to use uranium as shielding is in a two-layer structure, of uranium and some non-radioactive material. The uranium layer should be nearer to the primary radiation source and the non-radioactive layer nearer to the player. The uranium provides a great deal of shielding against the primary source, and the other material shields against the uranium layer. Due to the damage threshold mechanism, a meter of dirt is sufficient to shield fully against a layer of fully-depleted (0.0%-fissile) uranium. Obviously this is only worthwhile when the primary radiation source is more radioactive than a uranium block.
When constructing permanent radiation shielding, it is necessary to pay attention to the geometry of the structure, and particularly to any holes that have to be made in the shielding, for example to accommodate power cables. Any hole that is aligned with the radiation source makes a "shine path" through which a player may be irradiated when also aligned. Shine paths can be avoided by using bent paths for cables, passing through unaligned holes in multiple shield layers. If the desired shielding effect depends on multiple layers, a hole in one layer still produces a partial shine path, along which the shielding is reduced, so the positioning of holes in each layer must still be considered. Tricky shine paths can also be addressed by just keeping players out of the dangerous area.