Preparación de un manual de usuario para la realización de análisis de seguridad mediante RELAP

El objetivo principal del presente proyecto es mejorar las condiciones de trabajo y aprendizaje de los usuarios en el uso del código termohidráulico RELAP en lo relativo a la realización de análisis de seguridad. Para ello se ha realizado una caracterización y mejora de las herramientas informáticas disponibles dentro del Grup d’Estudis Termohidràulics, y la posterior preparación de un manual de usuario que incluye el uso de las nuevas herramientas. Las herramientas y procedimientos tratados pertenecen a las siguientes áreas: • Procedimientos de ejecución • Pre y post-proceso de datos • Biblioteca de casos • Codificación y almacenamiento de archivos • Procedimientos de cualificación Posteriormente se ha procedido a realizar una reejecución de actualización con un caso ya contenido en la biblioteca de transitorios para poner en práctica el uso de las nuevas herramientas y procedimientos, además de servir de ejemplo para los demás usuarios. Mediante el trabajo realizado en este proyecto se mejora notablemente el proceso de aprendizaje de ejecución del código y obtención de resultados, y se consolidan los procedimientos a seguir para el almacenamiento y gestión de resultados

Evaluation of TH multi-scale coupling methods in BEPU analysis

The combined use of thermal-hydraulics system with sub-channel codes (TH-TH coupling) in transient analysis provides an integrated tool with the capability of modelling in detail both the core thermal-hydraulic conditions and the system behaviour. Different code coupling methods are often used in the nuclear industry to provide accuracy to the calculations while maintaining a sufficient degree of usage. In some cases, and for specific scenarios, the coupling method can be simplified to the off-line transfer of the the boundary conditions from plant models run by system codes to sub-channel codes (off-line coupling). The off-line coupling approach has been considered valid to evaluate the safety margins for limiting parameters such as the minimum departure from nucleate boiling ratio, which relates the local power with the critical heat flux. The off-line coupling is fast and sufficiently accurate in most situations, however, boundary conditions at the inlet and at the outlet of both solutions will present miss-matchings. In order to overcome the inconsistencies between system and sub-channel codes, advanced on-line coupling methods may be applied (e. g. semi-implicit coupling). These are methods that imply coherent boundary conditions between codes in all domain and are significantly more complex in mathematical terms. The selection of an appropriate coupling method between system and sub-channel codes is crucial. Recent sensitivity studies performed in sub-channel codes have shown a strong influence of the boundary condition's uncertainty to the major figure of merit. The present study aims to evaluate the implications of the coupling methods in safety analysis evaluations in two transients with strong system effects. For this purpose, CTF (a sub-channel code) and RELAP5 (system code) are coupled using two coupling techniques: the off-line coupling, where the boundary conditions are obtained from the RELAP5 model and imposed by table to CTF, and the semi-implicit coupling method, which has been applied to couple CTF and RELAP5, implementing the semi-implicit coupling methodology developed by Weaver in 2002. The selected cases for the study are a complete loss of forced flow and a pressurizer relief valve opening. The models used are, on one hand, a RELAP5 plant model that consists of a full plant model of a generic Westinghouse 3-loop nuclear power plant. On the other hand, a hybrid assembly sub-channel CTF core model has been developed using the same reference data as the RELAP5 model. In recent years, the application of Best Estimate Plus Uncertainty (BEPU) calculations, has gained importance in the scientific community. This method, far from adding conservatism by forcing non-physical conditions to the calculations, tries to take the uncertainties of the calculations into the process. The BEPU method used in the present study is the GRS BEPU method, which implements Wilks' method using order statistics. BEPU analysis can be considered an additional tool for coupling methods comparisons. Results show that base cases and sensitivity analysis present good agreement between the coupling methods with some minor discrepancies. For the first case, the Spearman's rank correlation coefficients and distributions of the BEPU analysis present similar results for the two approaches. On the contrary, the second case shows differences in the evaluation of the figures of merit, which can be explained and correlated to the boundary conditions deviations between codes. This suggests that non-imposed boundary condition values for the off-line coupling method is an important issue to take into account when applying this type of method for system-dependent transients that are extended in time.L'ús combinat de codis termohidràulics de sistema amb codis de subcanal (acoblament TH-TH) en l'anàlisi de transitoris proporciona una eina integrada amb la capacitat de modelar en detall tant les condicions tèrmico-hidràuliques bàsiques com el comportament del sistema complet. Sovint s'utilitzen diferents mètodes d'acoblament de codis a la indústria nuclear per proporcionar precisió als càlculs mantenint la facilitat d'ús dels codis. En alguns casos, i per a escenaris específics, el mètode d'acoblament es pot simplificar com a una transferència de les condicions de contorn des de models de planta executats per codis de sistema a codis de subcanal (acoblament fora de línia). L'enfocament d'aquest acoblament s'ha considerat vàlid per avaluar els marges de seguretat per a paràmetres limitants com la relació del límit d'ebullició nucleada, que relaciona la potència local amb el flux de calor crític. L'acoblament fora de línia és ràpid i prou precís en la majoria de les situacions, però, les condicions de contorn presenten diferències en certs paràmetres. Per tal d'evitar aquestes diferències, es poden aplicar mètodes avançats d'acoblament en línia (per exemple, l'acoblament semi-implícit). Aquests són mètodes que impliquen condicions de contorn coherents entre codis i són significativament més complexes en termes matemàtics. La selecció d'un mètode d'acoblament adequat entre codis de sistema i de subcanal és crucial. Estudis recents sobre sensibilitats realitzats en codis de subcanal han demostrat una forta influència de la incertesa de les condicions de contorn en els resultats. El present estudi pretén avaluar les implicacions dels mètodes d'acoblament en els anàlisis de seguretat en dos transitoris relacionats amb paràmetres de sistema. Amb aquesta finalitat, CTF (codi de subcanal) i RELAP5 (codi de sistema) s'acoblen mitjançant dues tècniques d'acoblament diferents: l'acoblament fora de línia, on les condicions de límit s'obtenen a partir del model RELAP5 i s'imposen per taula a CTF, i el mètode d'acoblament semi-implícit, implementant la metodologia desenvolupada per Weaver l'any 2002. Els casos seleccionats són una pèrdua completa de cabal forçat i una obertura de la vàlvula d'alleujament del pressionador. Els models utilitzats són, d'una banda, un model de planta completa d'una central nuclear genèrica de 3 llaços de Westinghouse realitzat amb RELAP5. D'altra banda, s'ha desenvolupat un model híbrid del nucli en CTF utilitzant les mateixes dades de referència que el model RELAP5. En els darrers anys, l'aplicació dels càlculs Best Estimate Plus Uncertainty (BEPU) ha guanyat importància dins la comunitat científica. Aquest mètode, lluny d'afegir conservadurisme forçant condicions no físiques als càlculs, intenta incorporar al procés les pròpies incerteses dels càlculs. El mètode utilitzat en el present estudi és el mètode GRS BEPU, que implementa el mètode de Wilks mitjançant estadístics d'ordre. L'anàlisi BEPU es pot considerar una eina addicional per a la comparació de mètodes d'acoblament. Els resultats mostren que els casos base i l'anàlisi de sensibilitats presenten una bona coincidència entre els mètodes d'acoblament amb algunes discrepàncies menors. Per al primer cas, els coeficients de correlació de rang de Spearman i les distribucions de l'anàlisi BEPU presenten resultats similars per als dos enfocaments. En canvi, el segon cas mostra diferències en l'avaluació de les magnituds d'interès, que es poden explicar i correlacionar amb les desviacions de les condicions de contorn entre codis. Això suggereix que els valors de les condicions de contorn no imposats per al mètode d'acoblament fora de línia són un tema important a tenir en compte quan s'aplica aquest tipus de mètode per a transitoris dependents del sistema que s'estenen en el temps.Postprint (published version

Methodology for phenomenological code assessment with integral test data

The use of codes in the licensing process requires a rigorous validation process that can be accomplished by means of qualitative and quantitative assessment. In thermal hydraulics, this validation has to be performed at different levels, from separate effects to the integral response of a plant design. Even though the quantitative assessment is preferred, for complex phenomenology involving the behaviour of the whole plant system this approach is difficult and the assessment is usually performed through qualitative expert judgement. In the present article, a methodology is proposed that combines the use of qualitative and quantitative adequacy assessment for the simulation of experiments at integral test facilities. The method makes use of statistical quantification by means of Best Estimate Plus Uncertainty calculations.Peer ReviewedPostprint (published version

Preparación de un manual de usuario para la realización de análisis de seguridad mediante RELAP

El objetivo principal del presente proyecto es mejorar las condiciones de trabajo y aprendizaje de los usuarios en el uso del código termohidráulico RELAP en lo relativo a la realización de análisis de seguridad. Para ello se ha realizado una caracterización y mejora de las herramientas informáticas disponibles dentro del Grup d’Estudis Termohidràulics, y la posterior preparación de un manual de usuario que incluye el uso de las nuevas herramientas. Las herramientas y procedimientos tratados pertenecen a las siguientes áreas: • Procedimientos de ejecución • Pre y post-proceso de datos • Biblioteca de casos • Codificación y almacenamiento de archivos • Procedimientos de cualificación Posteriormente se ha procedido a realizar una reejecución de actualización con un caso ya contenido en la biblioteca de transitorios para poner en práctica el uso de las nuevas herramientas y procedimientos, además de servir de ejemplo para los demás usuarios. Mediante el trabajo realizado en este proyecto se mejora notablemente el proceso de aprendizaje de ejecución del código y obtención de resultados, y se consolidan los procedimientos a seguir para el almacenamiento y gestión de resultados

Preparación de un manual de usuario para la realización de análisis de seguridad mediante RELAP

El objetivo principal del presente proyecto es mejorar las condiciones de trabajo y aprendizaje de los usuarios en el uso del código termohidráulico RELAP en lo relativo a la realización de análisis de seguridad. Para ello se ha realizado una caracterización y mejora de las herramientas informáticas disponibles dentro del Grup d’Estudis Termohidràulics, y la posterior preparación de un manual de usuario que incluye el uso de las nuevas herramientas. Las herramientas y procedimientos tratados pertenecen a las siguientes áreas: • Procedimientos de ejecución • Pre y post-proceso de datos • Biblioteca de casos • Codificación y almacenamiento de archivos • Procedimientos de cualificación Posteriormente se ha procedido a realizar una reejecución de actualización con un caso ya contenido en la biblioteca de transitorios para poner en práctica el uso de las nuevas herramientas y procedimientos, además de servir de ejemplo para los demás usuarios. Mediante el trabajo realizado en este proyecto se mejora notablemente el proceso de aprendizaje de ejecución del código y obtención de resultados, y se consolidan los procedimientos a seguir para el almacenamiento y gestión de resultados

Off-line vs. semi-implicit TH-TH coupling schemes: A BEPU comparison

Several TH-TH code coupling methods are used in the nuclear industry to model the core thermal–hydraulic conditions and the system behaviour. In some cases, the boundary conditions obtained by system codes are applied to sub-channel codes by table (off-line coupling). Even though this approach is in general considered valid, some boundary parameters will present inconsistencies. Alternatively, system and sub-channel codes are coupled using different coupling methods (semi-implicit coupling). Recent studies have shown a strong influence of the bound- ary conditions uncertainty on the sub-channel code results. The present study aims to evaluate the differences produced by the coupling methods by performing a best-estimate plus uncertainties (BEPU) comparison to the following cases: a complete loss of forced flow and a pressurizer relief valve opening. Results show that BEPU analysis presents good agreement with some discrepancies that can be explained and correlated to the boundary conditions deviations between codes.Peer ReviewedPostprint (author's final draft

Linden code validation against NUPEC / PSBT experimental data for void fraction and temperature benchmarks

The subchannel analysis code LINDEN is being developed by China Nuclear Power Technology Research Institute Co. Ltd (CNPRI). The LINDEN code is used in thermal-hydraulics design and safety analysis of pressurized water reactor (PWR) cores. As part of the code development activities, CNPRI commissioned Energy Software Ltd. (ENSO) to conduct an independent assessment of the LINDEN code. The experimental data from Nuclear Power Engineering Corporation (NUPEC) PWR Subchannel and Bundle Tests (PSBT), available through the PSBT benchmark activity, were selected for this validation. The assessment work focused on the void fraction and temperature related benchmarks and was divided into three parts: (1) steady-state void fraction and pressure drop benchmarks, (2) steady-state f luid temperature benchmark, and (3) transient void fraction benchmark. The results presented in this paper correspond to the steady-state parts of the validation work. The assessment of the code comprised the code-to-data comparison as well as the code-to-code comparison. The first one relied on the concepts of accuracy, precision and consistency which can be quantitatively evaluated from statistical indicators and their comparison to the uncertainty of the experimental measurements. The second one consisted in a qualitative assessment against the PSBT benchmark results, with the object of comparing the LINDEN calculated values to other state-of-the-art codes for this type of analysis, and of complementing the code-to-data comparison, which lacked precise information on the experimental uncertainty. The overall conclusion is that the LINDEN calculated values can be considered in good agreement with the PSBT data.Peer ReviewedPostprint (published version

On the scaling of uncertainties in thermal hydraulic system codes

The present work addresses the scaling e ect on safety margins and uncertainties for best estimate plus uncertainty (BEPU) methodologies. The results of an experiment from the OECD/NEA ROSA-2 project at the LSTF facility have been used. LSTF is a mock-up of a PWR reactor which follows a power-to-volume scaling approach. A validated RELAP5 model of the experiment has been upscaled to two di erent scales in order to assess the impact of the di erent uncertainty parameters. This process followed the scaling-up methodology (SCUP) developed at UPC. The comparison of the three calculations at di erent scales is presented as well as an uncertainty quantification following the GRS methodology for propagating uncertainties. The comparison of the propagation of the uncertainties at di erent scales has shown that the influence of the scale on the input parameters is negligible. On the other hand, the safety margins are slightly influenced by the scale of the reactor.Peer ReviewedPostprint (author's final draft

Validation of a BEPU methodology through a blind benchmark activity at the PKL test facility

Validation is a key step in the field of thermal hydraulics and has played a crucial role in the implementation of system codes. These codes are then included in licensing methodologies being those either best estimate plus uncertainties (BEPU) or conservative approaches. However, only a small amount of efforts has been done on the validation of the methodologies as a whole. Such validation would imply the comparison of the bounding limits and the experimental data. Due to the lack of actual data of accidental situations, such validation should be carried out with data from integral test facility (ITF) experiments.The OECD/NEA PKL-4 test programme is investigating safety issues relevant for current pressurizedwater reactor (PWR) plants as well as for new PWR design concepts focusing on complex heat transfer mechanisms under two-phase flow. Within the project, a blind benchmark activity has been launched where an IBLOCA experiment has been simulated by 12 different organizations. The Universitat Politècnica de Catalunya(UPC) participated to the activity with a blind BEPU approach with the main objective to assess the validity of the methodology as a whole. At first, the acceptance criteria for the particular scenario have been evaluated. Secondly, in order to broaden the extend of the validation, the results of the different participants have been interpreted as if they were blind experiments as well. Finally, different output parameters have been considered providing a matrix of results that is used to evaluate the validity of the methodology

On the scaling of uncertainties in thermal hydraulic system codes

The present work addresses the scaling e ect on safety margins and uncertainties for best estimate plus uncertainty (BEPU) methodologies. The results of an experiment from the OECD/NEA ROSA-2 project at the LSTF facility have been used. LSTF is a mock-up of a PWR reactor which follows a power-to-volume scaling approach. A validated RELAP5 model of the experiment has been upscaled to two di erent scales in order to assess the impact of the di erent uncertainty parameters. This process followed the scaling-up methodology (SCUP) developed at UPC. The comparison of the three calculations at di erent scales is presented as well as an uncertainty quantification following the GRS methodology for propagating uncertainties. The comparison of the propagation of the uncertainties at di erent scales has shown that the influence of the scale on the input parameters is negligible. On the other hand, the safety margins are slightly influenced by the scale of the reactor.Peer Reviewe