Inverse square law for radiation damage
Material-dependent radiation shielding

Radiation is attenuated exponentially by passing through shielding
material.  Radiation resistance values are assigned to all bulk-material
nodes, and the radiation damage ABM traces the path of each radiation ray
to count up the shielding.  The relative radiation resistance values are
essentially real, but the effectiveness of all shielding is scaled down
by a factor of about 70 for game purposes.  Strength of the existing
radiation sources is increased by varying amounts to compensate for
shielding.  Uranium block and ore, both usable as shielding, are made
slightly radioactive, the latter only very slightly.
Grace period before reactor meltdown

The meltdown check now doesn't trigger meltdown immediately on reactor
structure being compromised.  Instead, there's a grace period of up to
100 s, during which the reactor can be repaired.  The check doesn't just
look at whether the structure is damaged at all: it looks at how damaged
it is, counting the number of faulty nodes.  The amount of damage is
integrated over time, and the grace period is actually 100 node-seconds,
so greater damage causes meltdown more quickly.  If the active core is
dug then it melts down immediately, preventing the tactic of digging
the core to avert meltdown.

Incidentally move the meltdown check into its own ABM, from the
technic_run callback, so that it applies even when the reactor is not
connected to a switching station.
Audible siren to warn of reactor meltdown

The siren sounds a "danger" tone continuously while it is active and
damaged, such that meltdown is imminent.  It sounds a one-off "clear"
tone if it has been sounding "danger" and the danger has passed, either
because the structure is repaired or because the reactor has become idle.
Make outer layer of reactor structure optional

As the layers of reactor structure now have a practical purpose,
in attenuating the modelled radiation from the core, it is no longer
necessary to make so much of it mandatory in order to motivate players
to build it.