Disposal: GRS develops simulation software RepoTREND for analysis of long-term safety

14.02.2017

© istockphoto /dottedhippo

A repository is intended to safely isolate radioactive waste from the environment over a long period of time. To find out whether this can be said to be true of a particular site, scientists carry out a so-called long-term safety analysis. Here, researchers use special simulation codes to predict the complex processes in a repository over this long period of time.

With RepoTREND (Transport and Response of Non-decaying and Decaying contaminants in final REPOsitory), GRS has developed a code which can be used to calculate a possible release of contaminants from the repository. RepoTREND, which is funded by the Federal Ministry of Economics, allows a comprehensive analysis of possible releases from the repository up until the point when they reach man and his environment.

Comprehensive simulation from release to dose received

RepoTREND has a modular design to study the release of the radionuclides in the independent subsystems involved.
One module of RepoTREND simulates possible releases in the near field of the repository in the various host rock types salt, clay and crystalline rock. The near filed subsystem encompasses the underground structure of the repository mine with the waste containers emplaced.  What is simulated here is e.g. the corrosion of waste containers, radioactive decay, the mobilisation of substances, and also the formation of gases or the development of the temperature.
Another subsystem is the geosphere. Here, RepoTREND examines what happens to the radioactive substances that were able to leave the repository system, e.g. as a result of groundwater intrusion. The contaminants enter the deep groundwater, are diluted, retarded or transported - possibly to a surface-near aquifer and thus into the biosphere.

The biosphere is the third subsystem that can be studied with RepoTREND. In order to make predictions, possible exposure pathways of radioactive substances are looked at. The exposure pathway describes how the human body comes into contact with radiation and gives important clues to the calculation of the effective dose of an individual. For example, radioactive substances can enter the human body via food or drinking water ("internal exposure") or have an impact on the body from outside ("external exposure").

RepoTREND provides a holistic view of possible release scenarios across the three subsystems near field, geosphere and biosphere.

Illustration of the different exposure pathways with external exposure (radiation impacts on the body from the outside) and internal exposure (intake of radioactive substances into the human body e.g. via the airways or with food) (Source: GRS)
How data uncertainties are handled

The RepoTREND code package allows not only deterministic analyses, i.e. analyses of precisely defined computational cases which are based on fixed input data, but also statistical analyses. Here, the basic idea is to represent the input data, which have significant uncertainties attached to them, by suitable statistical distributions. With the additional RepoSTAR code, input data are repeatedly determined by roll of dice according to their probability, and then a computation run is performed. The results thus obtained are analysed statistically and allow conclusions to be drawn about the reliability of the analysis.
This is also referred to as an uncertainty analysis. By means of suitable methods, it is possible to determine the sensitivity of the system to variations in the different input data. An analysis of such global character is on the on hand favourable for a general understanding of the repository system. On the other hand, it identifies the input data that have a marked effect on the result and the better knowledge of which would clearly enhance the reliability of the analysis.

Application: RepoTREND for the safety case

RepoTREND is available to scientists, authorised experts and authorities to study the release of contaminants and can thus be used for a safety case. Within the scope of a licensing procedure for a repository, the safety case attests the long-term safe containment of radioactive waste on the basis of scientific criteria. It has to be submitted to the competent authorities for examination.

Find out more

More about RepoTREND
GRS Report „RepoTREND: The code package for the integrated long-term safety analysis of repository systems (in German)“
GRS Repot „RepoSTAR - A code package for the control and evaluation of statistical computation runs with the RepoTREND code package (in German)“