PhD thesis

Improvement and Validation of a Computer Model for the Thermo-mechanical Fuel Rod Behaviour during Reactivity Transients in Nuclear Reactors

Pic Holt
Dummy of a pressurized water reactor fuel assembly

PhD student:
Lars Holt
Dr. U. Rohde (HZDR), Dr. P. Van Uffelen, Dr. A. Schubert (JRC-ITU Karlsruhe), Dr. M. Seidl (E.ON Kernkraft), Prof. Dr. R. Macián-Juan (Technical University Munich)
Reactor Safety


Over the last years, with increasing availability of fast multi-processor computers, multi-physics coupling between transient simulation codes for nuclear reactors became a more and more important topic. In the past, the main focus was on the coupling of thermo-hydraulic system codes and reactor dynamics codes. Current developments concentrate on the coupling of fuel performance codes with the thermo-hydraulic system and/or the reactor dynamics codes. On the one side fuel performance codes have a simplified thermo-hydraulic model, on the other side the thermo-hydraulic system codes and reactor dynamics codes include often only a simplified fuel rod model. Thence features of the different codes could be integrated in one larger code system. This kind of coupling scheme is expected to result in a better modeling of the thermo-mechanical fuel rod behavior, e.g. for a reactivity initiated accident (RIA) analysis in nuclear power reactors.

Fuel performance codes like TRANSURANUS can take into account burn-up levels > 50 MWd/kgU and related phenomena, e.g. the high burn-up fuel structure (HBS). The occurrence of the HBS is related to such phenomena as increase of fuel porosity, Xe depletion in the fuel matrix and decrease of the grain size. For reactor safety limits, the behaviour of high burn-up rods is investigated in several experiments like for reactivity initiated accident (RIA) in the ongoing OECD/NEA Cabri Water Loop Project.


The reactor dynamics code DYN3D developed at HZDR is widely validated for RIA. Recent developments of the TRANSURANUS fuel performance code developed at the ITU in Karlsruhe focus on an extension of the application for RIA, especially for high burn-up rods. For the validation of these developments, TRANSURANUS calculations will be performed in the frame of an OECD RIA benchmark. Moreover, experiments from the ongoing OECD/NEA Cabri Water Loop Project will be calculated and analysed with TRANSURANUS. Earlier experiments showed an important influence of the HBS on the fuel rod behaviour during RIA. A part of the work is therefore to refine the HBS modelling in TRANSURANUS itself, in collaboration with the JRC-ITU in Karlsruhe.

To put together the modelling of thermo-mechanical fuel behaviour during a RIA with calculation of reactor power and thermal hydraulics, a general coupling interface will be developed for the fuel performance code TRANSURANUS. This coupling interface is proposed to be used for RIA analysis in combination with the reactor dynamics code DYN3D, although it is developed to be generally applicable for other code couplings with the TRANSURANUS code. After successful testing, the thermo-mechanical fuel behaviour during RIA will be analysed in detail for the code system DYN3D-TRANSURANUS compared to DYN3D as standalone running code.

Project partner:

European Commission Joint Research Centre, Institute for Transuranium Elements, Karlsruhe


The project is funded by E.ON Kernkraft within the knowledge preservation program in nuclear safety research.