  The Primordial Disk of small icy planetesimals, once located at 15-30 AU from
the Sun, was disrupted by giant planet migration in the early Solar System. The
Primordial Disk thereby became the source region of objects in the current-day
Kuiper Belt, Scattered Disk, and Oort Cloud. I present the thermophysics code
"Numerical Icy Minor Body evolUtion Simulator", or NIMBUS, and use it to study
the thermophysical evolution of planetesimals in the Primordial Disk prior to
its disruption. Such modelling is mandatory in order to understand the
behaviour of dynamically new comets from the Oort Cloud, as well as the
activity of Centaurs and short-period comets from the Scattered Disk, that
return pre-processed to the vicinity of the Sun. I find that bodies in the
midst of the Primordial Disk with diameters ranging 4-200 km lost all their CO
ice on time-scales of order 0.1-10 Myr depending on size, through a combination
of protosolar and long-lived radionuclide heating. CO and other hypervolatiles
therefore require a less volatile host for their storage. I consider two
possible hosts: amorphous water ice and CO2 ice. Because of the high luminosity
of the protosun, some Primordial Disk bodies may have sustained significant
crystallisation, CO:CO2 segregation, and CO2 sublimation in the uppermost few
tens of meters. I discuss how this may affect coma abundance ratios and distant
activity in dynamically new comets.
