Probabilistic performance assessment for radioactive waste disposal: A simplified biosphere model

The uncertainties in the considered parameter values give rise to uncertainties in the calculated annual doses of about one order of magnitude for Np and Pu and of about two orders of magnitude for Cs and Tc. The distributions of the annual doses resulting from a unit concentration in the well water can be described by Weibull distribution. In probabilistic performance assessment codes, the biosphere model may be substituted by concentration-to-dose conversion factors defined by these Weibull distributions. A first advantage of this substitution is the reduction of the computer time needed for the execution of the codes. A very interesting advantage is that a better estimate of the contribution of the uncertainties in the biosphere parameters to the total uncertainties in the calculated annual doses is obtained and that it allows easy evaluation of the relative contributions from the near field, the far field and the biosphere to the total uncertainty in the calculated annual doses.