Implementation of a direct perturbation method in MCNP and application to SCALE verification

•77 Heavy-water moderated critical ZED-2 experiments analyzed with MCNP and KENO.•SCALE used to find nuclear data adjustments minimizing MCNP a priori biases.•MCNP patch used to repeat analysis using nuclear data adjustments from SCALE.•Present a detailed code-to-code comparison of of a priori and a postiori keff biases.•SCALE results consistently underestimate impact of proposed corrections by 1–2mk.