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Analyses of Complex Sequences, Severe Accidents, and Limiting Events in Critical Systems for Transmutation

SAS-SFR and SAS-LFR are designed to perform deterministic analyses of severe accidents in sodium (SFRs) or lead/lead-bismuth (LFRs) cooled fast reactors with mixed oxide fuels, which are foreseen for transmutation in fast systems. Detailed mechanistic models describe both the steady state operation and accident conditions caused e.g. by protected and/or unprotected loss of coolant flow accidents or reactivity insertion accidents. The initiation phase of an accident is modelled, including coolant heating and boiling, fuel heat-up, melting and pre-failure in-pin fuel motion. Consequences of clad failure or melting and subsequent core materials relocation (fuel, clad, fission gases, coolant) in the coolant channel and, after loss of integrity of the fuel pin, in a broken-up configuration are simulated. The calculations with SAS-SFR are stopped, when a long-term sub-critical core state is reached or when the hexcan of one subassembly group looses its integrity.

For simulation of a reactor core, a single channel represents the behaviour of one or more subassemblies simulating a representative fuel pin, its associated coolant inventory, and the respective hexcan structure. For an adequate representation of a whole core, a large number of channels must be employed. The heat transfer in pin and structure is performed with a quasi two-dimensional heat conduction model, where the radial heat conduction in pin and structure is coupled with the one-dimensional axial coolant flow. Reactivity feedback effects associated with the behaviour of the different core materials are taken into account by first order perturbation theory (point kinetics), including fuel and clad heat up (axial expansion and Doppler effect), coolant heating and boiling, and clad and fuel motion.

SAS-SFR has been extensively qualified with a large variety of results from various experiments. The test programmes covered a wide range of pin and test conditions: pre-irradiations from 0 to 12 at% burn-up, solid and hollow fuel pellets, transient power insertion starting at nominal cooling conditions up to extensive clad melting conditions with power transients from slow ramps (1 % of nominal power per second) up to pulses with a half-width of a few ten milliseconds only.

The code system SAS-SFR is the result of long-term cooperation between scientists from the KIT/INR (Germany), CEA, IRSN (France) and JAEA (Japan). The development started in the early eighties, taking the SAS4A code developed at ANL (USA) as a basis, but it became actually a new code frame due to a large number of improvements. Numerous coding errors and inconsistencies, found in the original code version, were corrected, existing models were extended and new ones were introduced, and the experimental database for qualification of the updated and the newly developed models was broadened. Due to this extensive work, requiring much manpower over the last twenty years, progress has been reached in two aspects: the new code could be applied with sufficient reliability for the simulation of rather different core and plant designs, and it had a stable performance irrespective of the application. This development work is continued and organised in a close cooperation between KIT (Germany), CEA/IRSN (France) and JAEA (Japan).

Presently, SAS-SFR is used at KIT in the framework of research and development activities, investigating safety issues related to innovative sodium cooled critical systems. These activities are mostly imbedded in research programmes supported by the European Commission in the 7th Framework Programme. The priority of interest in these cooperative programmes from KIT point of view is to evaluate options for transmutation of actinides with the objective of a reduction of the radiotoxicity of the final long-term repository for long lived fission products.


[1]  Krüssmann, R., Ponomarev, A., Pfrang, W., Schikorr, M., Struwe,, D.: Comparison of Results of SAS-SFR Calculations of the CP-ESFR Working Horse and Optimized Core Designs during the Initial Phase of an ULOF Accident. International Conference on Fast Reactors and Related Fuel Cycles: Safe Technologies and Sustainable Scenarios (FR13). 4–7 March 2013, Paris, France, organized by the International Atomic Energy Agency (IAEA).
Perez-Martin, S. et al.: Safety Analysis of a Sodium-Cooled Fast Reactor with Transmutation Capabilities. European Nuclear Conference ENC 2012, Manchester. United Kingdom 9–12 December 2012; Transactions at http://www.euronuclear.org/events/enc/enc2012/transactions.htm.
[3]  Perez-Martin, S, Ponomarev, A., Krüssmann, R., Pfrang, W.: Importance of Fuel Thermo-mechanical Properties in an ULOF Transient of a Sodium-cooled Fast Reactor Used for Minor Actinides Transmutation. Proceedings of the ICAPP 2013, Jeju Island, Korea, April 2013.