= Solution
The circumference is the length of the <graphene> wrapping vector:
$$
C=a\sqrt{n^2+nm+m^2},\qquad D=\frac C\pi.
$$
For $(12,12)$,
$$
\boxed{D=\frac{0.246\sqrt{432}}\pi=1.63\,\mathrm{nm}.}
$$
Approximating the closest wall-to-wall separation by the graphite interlayer spacing $d_{002}=c/2=0.3355\,\mathrm{nm}$ gives the nearest-neighbour axis separation in a triangularly packed bundle:
$$
\boxed{s=D+d_{002}\simeq1.96\,\mathrm{nm}.}
$$
This assumes undeformed circular tubes with a graphitic contact gap. Faceting, flattening, adsorbates or pressure can alter a real bundle's spacing.
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