SADE - Safety analyses for dynamical events

The workflow in setting up BISON simulations and the generation of meshes has been refined at VTT, and the BISON code has been successfully installed to the VTT Linux cluster Potku2.

This preparatory work eases future simulations of advanced fuels and claddings where either the geometry of the fuel is complex or several material layers are present, such as in coated claddings.

The international cooperation such as fuel behaviour part of VTT - Halden Reactor Project in- kind work, participation in working groups OECD/CSNI WGFS and ETSON SAG, as well as the following of CABRI International Project progress is done under this project.

Deliverables in 2018

• FINIX was validated comprehensively for the first time since 2013, and the associated validation report was produced. Two versions of FINIX were packaged during 2018, and a validation report was produced for both.

• A new version of FINIX was published along with the associated user’s manual and code description. The new version contains the cladding oxidation model and the previous coolant model of FINIX was updated and can now model PWR conditions. Another user’s manual was produced after the first validation, where only error corrections and small changes to the code were made.

• The OECD/NEA RIA Fuel Codes Benchmark Phase III was participated in. The activity in 2018 consisted of a sensitivity analysis on irradiated fuel, where the CIP0-1 test performed in the sodium loop of the CABRI reactor was used as the test case.

• A conference article on the application of constrained Gibbs energy minimization to nuclear fuel thermochemistry was prepared and published in TopFuel 2018. Whereas typically Gibbs energy minimization can only be used to calculate the equilibrium composition of a substance, with constrained Gibbs energy minimization, nonequilibrium states can also be calculated. The method was applied to the radiolysis of cesium iodide in the pellet-cladding gap.

• The BISON fuel performance code was used to model a missing pellet surface defect. For the first time at VTT, self-made meshes for BISON were generated and used successfully with BISON. The development allows for more detailed future 3D modelling of nuclear fuel at VTT.

• Previously made measurements were collected into a journal article draft on proton- irradiated cerium oxide and the determination of vacancy cluster behavior with temperature.

• A journal article draft on using the previously developed FRAPTRAN-GENFLO coupling was unfinished at the end of the project due to difficulties in the mechanical modeling in FRAPTRAN. The work will be continued in the future to prepare the journal article.

that can be used for the whole calculation sequence from basic nuclear data to coupled 3D transient analyses. Aim is the tool, which is more accurate and still fast and robust enough for practical safety analysis The developed computational tool set of coupled neutronics, system codes and true-3D thermal hydraulics has been be tested and demonstrated in cases relevant from safety analyses point of view. Objective has been that by the end of the project we have calculated several transients and accidents of real interest. Developing and maintaining our own codes and in-depth understanding of them enables the best possible expertise on safety analyses.

Specific goals in 2018

The project has two main research areas. The objective of the first work package is to enhance the core modelling of VTT’s 3D reactor dynamics codes. In 2018, aim was to finalize and test implementation of axially heterogeneous fuel models and axial discontinuity factors in HEXTRAN so that it can more reliably model transients of reactor cores with modern fuel assemblies. Also modelling of fuel behaviour is enhanced in this work package. In 2018 aim was to update coupling between the reactor dynamics codes and fuel behaviour module FINIX so that up-to-date version of the FINIX can be used for reactor dynamical simulations, and enabling reactivity coefficient modifications and sensitivity studies with HEXTRAN-FINIX code package.

Figure 2.30: Overview of the primary side nodalization of Kalinin-3 VVER-1000 for SMABRE.

The second work package focuses on whole core transient analyses, focusing on cases where mixing in reactor pressure vessel or open core geometry play an essential role. Aim has been to further develop tools that enable more realistic modelling of the transients, and to simulate transients with these improved tools. Modelling and development has had two parallel branches: development of the tools such as internally coupled HEXTRAN-SMABRE that can be routinely used for safety analyses already during the SAFIR2018 program, and modelling of transients with CFD-style codes that have more detailed description. In 2018 aim was to validate internally coupled HEXTRAN-SMABRE for VVER-1000 reactors. One validation case is OECD/NEA Kalinin-3 benchmark problem, switching-off of one of the four operating main

circulation pumps at nominal reactor power. For that case aim was to build HEXTRAN- SMABRE model for Kalinin-3 NPP on the base of existing VVER-1000 reactor models. In addition, application and validation of the fully coupled HEXTRAN-CFD-SMABRE reactor analysis code package has been continued. In 2018, aim was to perform three whole-plant simulations with the new simulation tool and at least in one case use OpenFOAM instead of PORFLO as a CFD solver.

The third work package involves work that supports the project’s research aims and promotes the usefulness of the code system. The work package includes international co-operation and administration work demanded by SAFIR2018 program.

Deliverables in 2018

• Calculation model for axially heterogeneous fuel has been implemented to

HEXTRAN. The new input routines have been tested, and the verification process of the modified HEXTRAN code is nearly complete. The new input syntax and changes to the source code have been reported. The verification process of the modified HEXTRAN continues. Work has been reported in a research report.

• FINIX-0.13.9 has been replaced with structurally considerably different FINIX-0.17.12 in HEXTRAN. Preliminary testing has been completed but validation of the coupling will have to be carried out later. Implementation of the renewed coupling has been described in a research report.

• HEXTRAN-SMABRE was renewed and supplemented with new, internally coupled simulation mode in 2015. In 2018, several modifications have been done to this new code version. In 2018 validation of the new HEXTRAN-SMABRE version including internal simulation mode has been continued with VVER-1000 cases. In 2018, two main validation cases for VVER-1000 reactors have been OECD/NEA/NSC coolant transient benchmark V1000CT-2 concerning hypothetical main steam line break (MSLB) at Kozloduy-5 plant and OECD/NEA/NSC Kalinin-3 benchmark problem, switching-off of one of the four operating main circulation pumps at nominal reactor power. Program modifications as well as validation simulations have been

documented in a research report.

Figure 2.31: Fission power during Kalinin-3 benchmark transient (left) and assemblywise power 50 s after switching of one main coolant pump of working four RCPs versus power at initial state with parallelly coupled HEXTRAN-SMABRE and with HEXTRAN-SMABRE- PORFLO.

• HEXTRAN-SMABRE model for Kalinin-3 VVER-1000 NPP has been created on the base of the existing VVER-1000 reactor models. OECD/NEA/NSC Kalinin-3

benchmark problem, switching-off of one of the four operating main circulation pumps at nominal reactor power, has been simulated using both parallelly and internally coupled simulation mode of the HEXTRAN-SMABRE code. Model report has been completed.

• HEXTRAN-OpenFOAM and OpenFOAM-SMABRE interface routines have been prepared so that OpenFOAM can be used together HEXTRAN and SMABRE instead of the PORFLO code.

• Several transients and accidents have been simulated using fully two-way coupling between HEXTRAN, SMABRE and PORFLO:

o OECD/NEA dynamic benchmark V1000CT-2 - scenario 1 VVER-1000 main steam line break (MSLB)

o OECD/NEA dynamic benchmark V1000CT-2 - scenario 2 VVER-1000 main steam line break (MSLB)

o OECD/NEA Kalinin-3 MCP benchmark

Scenario specific inputs have been prepared for all these cases. Interface routines have been completed for reversed flows. Results have been compared to with those of HEXTRAN-SMABRE.

• OpenFOAM input files were prepared for VVER-1000 pressure vessel. A transient simulation of the OECD/NEA dynamic benchmark V1000CT-2 - scenario was computed with the new analysis framework HEXTRAN-OpenFOAM-SMABRE.

Results were compared with other modelling approaches.

• The project included also participation in the AER working group D meeting in Dresden, Germany, where the presentation was given on development of a VTT’s new nodal code ANTS. Also kick-off meeting of OECD/NEA Rostov-2 benchmark in Lucca Italy was participated.

With CFD Internal coupling Parallel coupling Typical system code nodalization T (K)

Time 56s, min 515.5 K 71.3 s, min. 517.5 K 71.2 s, min. 517.4 K 85.1 s, min 539.1 K P (MW)

Time 56.8 s 72.9 s 72.5 s 85.5s

Figure 2.32. Core inlet temperature at time of minimum temperature, and assembly-wise fission power at time of maximum fission power during the main steam line break of VVER- 1000 reactor with distinct coupling methods between HEXTRAN, SMABRE and PORFLO.

OECD/NEA V1000CT-2 benchmark.