For the purposes of fission fast reactors analysis, the presence of an intense neutron flux within the core leads to the need of neutron kinetic and thermal hydraulic (NK/TH) coupled simulations to study operating and accidental conditions which may interest the nuclear power plant (NPP). The capability to perform such calculations is one of the main open topics in the Generation IV (GEN IV) fast reactors development. In the framework of a collaboration between Idaho National Laboratory (INL, USA) and the Sapienza Department of Astronautical, Electrical and Energy Engineering (DIAEE), the nuclear engineering group of DIAEE is a contributor for the developing of PHISICS (Parallel and Highly Innovative Simulation for INL Code System), a reactor analysis toolkit to perform advanced neutron transport calculations. PHISICS is capable to work coupled with RELAP5-3D®, the reference thermal hydraulic code for transient analyses in nuclear reactors, developed by INL. Thanks to the proposed coupling, symmetric and non-symmetric transients can be studied by using detailed 3D thermal hydraulic and 3D transport (or diffusion) neutronic models. A validation of this coupled approach is the object of this proposal and will be performed through the comparison with the results of a loss of flow without scram (LOFWOS) test performed in 1986 in the American facility of Fast Flux Test Facility (FFTF) during the passive safety demonstration program. In 2017, the International Atomic Energy Agency (IAEA) selected LOFWOS Test #13 for an international benchmark (CRP I32011 in the framework of IAEA Nuclear Power Programme). DIAEE will participate to this benchmark.
The safety analysis plays a crucial role on determining reactor design principles and components. So far, accident analyses were successfully performed by using system thermal-hydraulics codes. In case of complex problems involving thermal-hydraulics together with other disciplines, such as reactor physics (neutron diffusion/transport), TH system codes are usually not enough. Coupled codes can improve accident analysis capabilities with reasonable increase in cost and calculation requirements and with moderate additional user effort. The PHISICS code is geared to maximizing accuracy for a given availability of computational resources. This is obtained by implementing several algorithms and meshing approaches which the user can choose from in order to optimize his computational resources and accuracy needs. The coupling with RELAP5-3D© has been available starting from 2013, providing a brand new instrument for advanced core design coupled simulations. Verification and validation of this coupling approach can be performed by means of existing experimental data, such as the ones provided by IAEA international benchmark involving FFTF LOFWOS Test #13. The expected result of the activity described in this proposal is the demonstration of the PHISICS/ RELAP5-3D© coupled codes reliability on simulating all the TH-NK transients of interest, which will be analyzed during the reactor design, especially in order to maximize their safety by reducing the uncertainty boundaries related to the methodology approximations. The innovation proposed is focused on the validation of a best platform for the TH-NK transient analysis for fast reactors, with the maximization of the accuracy in the neutronic calculation maintaining an acceptable calculation time. From the tools point of view, PHISICS (with 3D transport solver) allows the non-symmetric transient calculation and the possibility to simulate an unlimited number of energy groups, obtaining a very accurate simulation of fast reactor core and an advancement in the TH-NK simulation state of the art.