// Calculate energy of blast (3.5 Mj/m3 is an average for most hydrocarbons)
$E = [VCVo:m3] * 3.5e6;
// Calculate scaled pressure
$Ps1 = [ePd:Pa] / [PA:Pa];
// Calculate scaled distance (for class 10)
$r1 = max(0.4096 * pow($Ps1, -0.859), 0.6829 * pow($Ps1, -0.414));
// Calculate explosion radius
$r = $r1 * pow($E / [PA:Pa], 1/3);
// Return
return $r;

See equations 5.2 - 5.3 and figure 5.8A in section 5.5.1 (p. 5.35-5.38).