The effect of spatial fluctuations of the effective beta fraction on the reactor transient characteristics is investigated in this paper. A detailed 3-D effective delayed neutron fraction core model has been developed and applied for the transient analysis of the reactor BR2, taking into account the detailed 3-D power and burnup distributions in the core. Due to the fuel depletion, the effective beta fraction is changing in time and forms a complex 3-D distribution profile in the reactor core. Two detailed 3-D effective beta core models have been developed using a standard MCNP method and a new MCNP based method. The new method is alternative to the standard MCNP approach (1-kp/kp+d) and it is based on tally calculations of fission integrals and actual delayed neutron fraction, βm, for each fissile isotope m in a fuel type k. The standard 3-D effective beta model is characterized with high spatial fluctuations, caused by MC statistical errors in small calculation meshes, while the model developed with the new MCNP method has a smoothed spatial profile. The new MCNP method has been applied also for estimation of the effective delayed photoneutron fraction in the beryllium reflected BR2 reactor core. The new method has been validated on measurements of the delayed neutron and delayed photoneutron fractions in the BR02 mock up criticality facility. The different effective beta models are tested on sensitivity of the reactor response to power, temperature and energy release distributions. The study is performed for protected and unprotected reactivity insertion transients, assuming zero reactivity feedback coefficients.