This paper presents a nuclear data sensitivity and uncertainty analysis of the effective delayed neutron fraction b eff for critical and subcritical cores of the MYRRHA reactor using the continuous-energy Monte Carlo N-Particle transport code MCNP. The b eff sensitivities are calculated by the modified k-ratio method proposed by Chiba. Comparing the b eff sensitivities obtained with different scaling factors a introduced by Chiba shows that a value of a = 20 is the most suitable for the uncertainty quantification of b eff . Using the calculated b eff sensitivities and the JENDL-4.0u covariance data, the b eff uncertainties for the critical and subcritical cores are determined to be 2.2 ± 0.2% and 2.0 ± 0.2%, respectively, which are dominated by delayed neutron yield of 239 Pu and 238 U. *