Serpent/SCF pin-level multiphysics solutions for the VERA Fuel Assembly benchmark

Highlights

• The VERA problem #6 is solved using Serpent + Subchanflow codes.

• A pin-by-pin coupling scheme was developed to provide further coupling criteria.

• Preliminary verification is held and convergence criteria aspects are analyzed.

• Results for global and axially detailed parameters are presented and discussed.

• A brief analysis of impact of main considerations in the coupling is included.

Abstract

Continuous improvement in nuclear industry safety standards and reactor designers’ and operators’ commercial goals represent a push for the development of highly accurate methodologies in reactor physics. Those methodologies are usually oriented to improve the prediction capabilities of specific reactors’ parameters under steady state and transient scenarios by the use of coupled state-of-the-art calculation codes. As a result, an important effort to develop highly accurate multiphysics approaches for nuclear reactor analysis is being held by diverse organizations and stakeholders around the world. Under this framework, the European Research and Innovation project McSAFE is a coordinated effort started in September 2017 with the objective of moving Monte Carlo (MC) stand-alone and coupled solution methodologies to become valuable tools for industry like applications for LWRs of generations II and III. As part of this project several developments in coupling strategies for transient and steady-state calculations are being developed, implemented and tested. Such capabilities assessment, together with the imperative verification and validation process are carried out inside McSAFE in a graded approach by the detailed comparison with high-quality benchmarks. As part of this approach, this work presents the main results obtained for the Fuel Assembly (FA) coupled Neutronic-Thermalhydraulic VERA Benchmark (namely VERA problem #6) proposed by the CASL consortium. For this purpose a pin-by-pin coupling scheme using the Serpent 2 MC code (developed by VTT, Finland) and the subchannel code SUBCHANFLOW (developed by KIT, Germany) was developed. Results are obtained and compared with diverse available published data from several similar high-fidelity calculation projects, both in terms of integral parameters and detailed axial profiles inside the FA at pin or coolant subchannel level. In addition, main additional key aspects of the problem are also presented and discussed. The results obtained and presented with the proposed approach show a very good agreement with available results from several similar high-fidelity projects, which encourages to continue to more complex problems and provides relevant insights for further developments and implementations.

Introduction

The continuous improvement in nuclear industry safety standards and reactor designers’ and operators’ commercial goals provides a driving force to the development of highly accurate methodologies in reactor physics. The global trend is usually oriented to improve the prediction capabilities of specific reactor’s parameters under steady state and transient scenarios by the use of coupled state-of-the-art calculation codes. During the past years an important effort was observed worldwide in collaborative projects aimed to develop high accurate multiphysics approaches for nuclear reactor analysis (CASL, 2010, Gaston et al., 2015, HPMC, 2011).

Under this framework, the European Research and Innovation project McSAFE (NUGENIA association, 2017, Luigi Mercatali et al., 2017) is a coordinated effort held by 12 institutions from 7 different countries around the EU and an extended community of users around the world that aims to tackle this demand. This McSAFE project represents a continuation of past efforts (HPMC, 2011, Ivanov et al., 2013, Ivanov et al., 2015), developed as proof of concept of main challenges. As a result, this project, which has started in September 2017, has the main objective of moving Monte Carlo (MC) stand-alone and coupled solution methodologies to become valuable tools for industry like applications for LWRs of generations II and III. As part of this project several developments in coupling strategies for transient and steady-state calculations are being developed, implemented and tested.

Specific MC code goals under McSAFE include full core LWR calculations, both including depletion and Thermal Hydraulic (TH) feedback in a full core approach. Moving towards these high-fidelity calculation approaches states a challenge not only from the multi-physics point of view, but also from several aspects related to computational requirements and massive parallelization (Ivanov et al., 2013, Ferraro et al., 2018). In addition, the statistical behaviour of MC codes also play a key role both regarding convergence aspects and statistical noise amplification.

To tackle those issues, the capabilities assessment together with the imperative verification and validation process are carried out inside McSAFE in a graded approach by the detailed verification with high-quality numerical benchmarks in a first stage, together with a foreseen comparison with experimental data in a second stage. Consequently, the present work develops the coupled Neutronics-TH Fuel Assembly problem stated in the VERA Benchmark (namely VERA problem #6 (Godfrey, 2014)) proposed by the CASL consortium (CASL, 2010) using the available tools and methodology.

For this purpose a pin-by-pin coupling scheme using the Serpent 2 MC code (developed by VTT, Finland (Leppänen et al., 2013)) and the subchannel code SUBCHANFLOW (referred here also as SCF, developed by KIT, Germany (Imke et al., 2012)) was developed, together with the analysis of main convergence issues.

The MC plus subchannel TH coupling approach was already been proposed in previous developments (Daeubler et al., 2015) in a master-slave approach, which encouraged to develop a detailed analysis for a high fidelity benchmark, as proposed in the present work.

For this present work, an external coupling is proposed to solve a high-fidelity benchmark based in Multi-physics Files (as developed in Valtavirta (2017) and is described in Serpent developer team (2018)). This decision was held in this case due to the stated workpath is oriented to develop the main criteria and strategies for the following coupling strategies to be implemented, namely externally supervised API approach, which departs from the traditional master-slave approach developed in former works (Daeubler et al., 2015). In addition, this decision allows to easily test and implement diverse concepts such as convergence criteria, relaxations schemes and temperature averaging schemes.

As a result, this work proposes the VERA problem #6 solution, where both global parameters (such as reactivity, average power profiles, integrated pin power distributions and outlet coolant temperatures) and local parameters (such as axial power profiles and axial coolant temperatures for specific rods and channels respectively) are analyzed.

It should be regarded that despite no reference results are provided in benchmark specifications (Godfrey, 2014) for Problem #6, other high-fidelity projects provide reported values in open publications (Aviles et al., 2017, Kochunas et al., 2014, Wilderman et al., 2015), which are used in this work to verify main results obtained. It is important to note here that the scope of this work is not aimed to reproduce a specific results from a third party project, but to present the main aspects and insights of the proposed approach.

In addition, other key aspects regarding convergence criteria are also to be analyzed in order to assess the global behaviour of the coupled problem and to provide the basis for more complex problems analysis to be further developed. The main issue to consider in this point is the fact that the statistical nature of MC codes leads to a potential amplification of statistical noise, which has to be analyzed in advance (Valtavirta, 2017). Furthermore, as far as the average running time of MC codes is several orders of magnitude higher than those from TH subchannel codes, a wise choice of relaxation schemes between iterations is compulsory.

Dealing with this combined problem represents a challenging issue, where obtaining a general solution usually requires the application of highly-advanced techniques (Aufiero and Fratoni, 2017) outside of the scope of this work. For such reason, an holistic approach is proposed, which consists of a three steps procedure:

• Estimate the propagation of MC power error in the coupled convergence criteria.

• Adjust the MC convergence and coupling convergence criteria based on step 1.

• Test the convergence using step 2 together with a relaxation of TH fields.

Using this approach, results were obtained without dealing with unrealistic convergence criteria and also avoiding unstable behaviour of the coupled solution iteration. In addition the insights gained are valuable for the coupling approaches and criteria to be further developed under McSAFE project.

Finally, the impact of different fuel temperature averages when mapping TH fields is also briefly discussed. This additional study is deemed to provide a raw estimation of the effect in main calculated parameters, within the scope of a high-fidelity goals.