Trustworthy numerical simulations are irreplaceable for model-based safety analyses in nuclear waste management. From model conceptualization to model validation, various aspects of confidence building with respect to process models and performance assessment models have to be considered in this context. This is particularly true as soon as significant safety-oriented decisions are involved – and if public participation is at stake.

Therefore, the present issue of SaND explores the following question: "what do we need to trust in models?" The individual papers discuss, from a modeller's perspective, measures to strengthen the confidence in numerical models.

Confidence building in models is a complex issue which has to be examined from different perspectives. Only then can research and development needs be identified. The present "Trust in Models" issue focusses on the following aspects of confidence building:

• technical aspects, such as code verification, the consistency of input data, and other measures of quality measurement;
• safety analysis aspects, such as adequacy of models, handling of uncertainties, and the complexity of demonstration strategies;
• organizational aspects, such as maintenance of competence and the installation of a safety culture.

The individual contributions to this SaND special issue are the results of the workshop series "Trust in Models", which has been organized by BASE annually since 2021.

We invite you to contribute to this format either by commenting on the present manuscripts while public peer review is ongoing or by taking part in one of the "Trust in Models" workshops and/or this special issue of SaND by submitting your own manuscript.

Trustworthy numerical simulations are irreplaceable for model-based safety analyses in nuclear waste management. From model conceptualization to model validation, various aspects of confidence building with respect to process models and performance assessment models have to be considered in this context. This is particularly true as soon as significant safety-oriented decisions are involved – and if public participation is at stake.

Therefore, the present issue of SaND explores the following question: "what do we need to trust in models?" The individual papers discuss, from a modeller's perspective, measures to strengthen the confidence in numerical models.

Confidence building in models is a complex issue which has to be examined from different perspectives. Only then can research and development needs be identified. The present "Trust in Models" issue focusses on the following aspects of confidence building:

• technical aspects, such as code verification, the consistency of input data, and other measures of quality measurement;
• safety analysis aspects, such as adequacy of models, handling of uncertainties, and the complexity of demonstration strategies;
• organizational aspects, such as maintenance of competence and the installation of a safety culture.

The individual contributions to this SaND special issue are the results of the workshop series "Trust in Models", which has been organized by BASE annually since 2021.

We invite you to contribute to this format either by commenting on the present manuscripts while public peer review is ongoing or by taking part in one of the "Trust in Models" workshops and/or this special issue of SaND by submitting your own manuscript.