Citations in Space Viking indicate that collapsium can withstand nuclear
explosions.

The resulting question is whether collapsium absorbs the heat or
reflects the heat.

The next question is whether collapsium radiates away the heat from the
outer shell or conducts the heat and thermal radiation takes place on
both the outer and inner surfaces of a collapsium-shielded vessel.

Thermal radiation into a vessel might not be too pleasant for the crew.