Deep penetration problems have been studied in a very large set of applications; the radi- ation transport simulations have to be as reliable as possible. The results have to be reliable in order to carry out a series of successive calculations such as the dose rate assessment. Estimating dose rates to workers is a basic step of nuclear reactor licensing. The main issue encountered with these kinds of problems is that the simulated particles may not be able to reach the detector regions. This negatively contributes to the estimation of the particle flux, increasing the variance. How can we obtain reliable results quickly? Over the years, the evolution of Monte Carlo codes has been improved with method- ologies to reduce the variance, the uncertainty associated with the results.