Manual section on radioactivity

Manual section on radioactivity

manual.md

@@ -672,6 +672,144 @@ As with the chest of the basic Minetest game, each chest type comes
 in both locked and unlocked flavors.  All of the chests work with the
 pneumatic tubes of the pipeworks mod.
 
+radioactivity
+-------------
+
+The technic mod adds radioactivity to the game, as a hazard that can
+harm player characters.  Certain substances in the game are radioactive,
+and when placed as blocks in the game world will damage nearby players.
+Conversely, some substances attenuate radiation, and so can be used
+for shielding.  The radioactivity system is based on reality, but is
+not an attempt at serious simulation: like the rest of the game, it has
+many simplifications and deliberate deviations from reality in the name
+of game balance.
+
+In real life radiological hazards can be roughly divided into three
+categories based on the time scale over which they act: prompt radiation
+damage (such as radiation burns) that takes effect immediately; radiation
+poisoning that becomes visible in hours and lasts weeks; and cumulative
+effects such as increased cancer risk that operate over decades.
+The game's version of radioactivity causes only prompt damage, not
+any delayed effects.  Damage comes in the abstracted form of removing
+the player's hit points, and is immediately visible to the player.
+As with all other kinds of damage in the game, the player can restore
+the hit points by eating food items.  High-nutrition foods, such as the
+pie baskets supplied by the bushes\_classic mod, are a useful tool in
+dealing with radiological hazards.
+
+Only a small range of items in the game are radioactive.  From the technic
+mod, the only radioactive items are uranium ore, refined uranium blocks,
+nuclear reactor cores (when operating), and the materials released when
+a nuclear reactor melts down.  Other mods can plug into the technic
+system to make their own block types radioactive.  Radioactive items
+are harmless when held in inventories.  They only cause radiation damage
+when placed as blocks in the game world.
+
+The rate at which damage is caused by a radioactive block depends on the
+distance between the source and the player.  Distance matters because the
+damaging radiation is emitted equally in all directions by the source,
+so with distance it spreads out, so less of it will strike a target
+of any specific size.  The amount of radiation absorbed by a target
+thus varies in proportion to the inverse square of the distance from
+the source.  The game imitates this aspect of real-life radioactivity,
+but with some simplifications.  While in real life the inverse square law
+is only really valid for sources and targets that are small relative to
+the distance between them, in the game it is applied even when the source
+and target are large and close together.  Specifically, the distance is
+measured from the center of the radioactive block to the abdomen of the
+player character.  For extremely close encounters, such as where the
+player swims in a radioactive liquid, there is an enforced lower limit
+on the effective distance.
+
+Different types of radioactive block emit different amounts of radiation.
+The least radioactive of the radioactive block types is uranium ore,
+which causes 0.25 HP/s damage to a player 1 m away.  A block of refined
+but unenriched uranium, as an example, is nine times as radioactive,
+and so will cause 2.25 HP/s damage to a player 1 m away.  By the inverse
+square law, the damage caused by that uranium block reduces by a factor
+of four at twice the distance, that is to 0.5625 HP/s at a distance of 2
+m, or by a factor of nine at three times the distance, that is to 0.25
+HP/s at a distance of 3 m.  Other radioactive block types are far more
+radioactive than these: the most radioactive of all, the result of a
+nuclear reactor melting down, is 1024 times as radioactive as uranium ore.
+
+Uranium blocks are radioactive to varying degrees depending on their
+isotopic composition.  An isotope being fissile, and thus good as
+reactor fuel, is essentially uncorrelated with it being radioactive.
+The fissile U-235 is about six times as radioactive than the non-fissile
+U-238 that makes up the bulk of natural uranium, so one might expect that
+enriching from 0.7% fissile to 3.5% fissile (or depleting to 0.0%) would
+only change the radioactivity of uranium by a few percent.  But actually
+the radioactivity of enriched uranium is dominated by the non-fissile
+U-234, which makes up only about 50 parts per million of natural uranium
+but is about 19000 times more radioactive than U-238.  The radioactivity
+of natural uranium comes just about half from U-238 and half from U-234,
+and the uranium gets enriched in U-234 along with the U-235.  This makes
+3.5%-fissile uranium about three times as radioactive as natural uranium,
+and 0.0%-fissile uranium about half as radioactive as natural uranium.
+
+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.
+
+Almost any solid or liquid homogeneous material has some shielding value.
+At the low end of the scale, 5 meters of wooden planks nearly halves
+radiation, though in that case the planks probably contribute more
+to safety by forcing the player to stay 5 m further away from the
+source than by actual attenuation.  Dirt halves radiation in 2.4 m,
+and stone in 1.7 m.  When a shield must be deliberately constructed,
+the preferred materials are metals, the denser the better.  Iron and
+steel halve radiation in 1.1 m, copper in 1.0 m, and silver in 0.95 m.
+Lead would halve in 0.69 m if it were in the game, but it's not, which
+poses a bit of a problem due to the drawbacks of the three materials in
+the game that are better shielding than silver.  Gold halves radiation
+in 0.53 m (factor of 3.7 per meter), but is a bit scarce to use for
+this purpose.  Uranium halves radiation in 0.31 m (factor of 9.4 per
+meter), but is itself radioactive.  The very best shielding in the game
+is nyancat material (nyancats and their rainbow blocks), which halves
+radiation in 0.22 m (factor of 24 per meter), but is extremely scarce.
+
+If the theoretical radiation damage from a particular source is
+sufficiently small, due to distance and shielding, then no damage at all
+will actually occur.  This means that for any particular radiation source
+and shielding arrangement there is a safe distance to which a player can
+approach without harm.  The safe distance is where the radiation damage
+would theoretically be 0.25 HP/s.  This damage threshold is applied
+separately for each radiation source, so to be safe in a multi-source
+situation it is only necessary to be safe from each source individually.
+
+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.
+
 electrical power
 ----------------
 
@@ -1349,6 +1487,5 @@ This manual needs to be extended with sections on:
     *   sonic screwdriver
 *   liquid cans
 *   wrench
-*   radioactivity
 *   frames
 *   templates