Study and implementation of neutronic-thermohydraulic coupling methods for detailed analysis of transients for safety margins optimization
- Autores: Adrian Sabater Alcaraz
- Directores de la Tesis: Diana Cuervo Gómez (codir. tes.), Carolina Ahnert Iglesias (codir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Nuria García Herranz (presid.), José Cesar Queral Salazar (secret.), Santiago Sánchez Cervera Huerta (voc.), Rafael Miró Herrero (voc.), Francisco Álvarez Velarde (voc.)
- Programa de doctorado: Programa de Doctorado en Energía Sostenible Nuclear y Renovable por la Universidad Politécnica de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: Archivo Digital UPM
- Resumen
Understanding the performance of a nuclear reactor is a complex task that covers different physical fields as the neutron behavior or fluid dynamics. These fields are not isolated, and there is a feedback among them.
Along nuclear engineering history, different codes have been developed to analyze each of the physics in which the problem can be split in an isolated way. An example of this are the neutronic or the thermal-hydraulic codes. To study the performance of the reactor in a realistic way, it is necessary to take into account the interaction among the physics, or in other words, the codes have to be coupled.
Several methodologies or models can be applied to develop a coupled system. A common way to classify the coupling is defining two types communication: internal and external. The external couplings are developed employing an external platform to exchange data among the codes. The external platform adapts the information from the source code to the requirements of the target code. The internal couplings do not require an external structure to exchange information. A set of subroutines are developed to allow the coupling. These subroutines are designed for specific codes. Modifying the code implicates to modify these subroutines.
The multi-physics platforms integrate several codes developed to analyze different fields of the physics involved in the nuclear reactor. These codes are coupled in different ways to supply a more realistic view of the performance of the system. Integration of the codes into these platforms may become a complex task depending on the code structure that was initially developed to be executed stand-alone.
At present, different kinds of multi-physics platforms exist that has been developed in the past, like SEANAP, or are being developed now, like NURESIM, CASL, MOOSE, etc. One of these platforms, NURESIM, is the result of the work performed by different European organizations through three European projects being the last one called NURESAFE. The work described in this Thesis was developed partially as part of the NURESAFE project but can also be considered as one of the applications of the product of this international collaboration.
As a result of previous European projects, the in-house UPM neutronic code, COBAYA, was integrated in the NURESIM platform although not yet with all its capabilities. Through the work performed for the last project, NURESAFE, the code was able to fully use both solvers included on it to perform either nodal or Pin-by-Pin diffusion calculations.
The Thesis has been carried out in the framework of the mentioned project with the objective of taking advantage of the capabilities provided by the multi-physics platform to coupled COBAYA with different codes and to use different spatial discretizations. This has allowed to study deeply the effects that different types of spatial discretizations or coupling schemes may produce in the results obtained for different transient scenarios that are related with the plant safety.
Regarding the first topic, spatial discretization analysis, the Thesis has focused on analyzing the differences obtained when nodal or pin cell diffusion analysis are carried out in the neutronic field in combination with average channel or subchannel thermal-hydraulic calculations. Within the NURESIM European Platform, the data exchange and the execution are controlled by an external system, the computational platform SALOME. Each one of the codes coupled in the platform creates its own mesh or meshes to store the fields in order to exchange the information with the other codes. SALOME interpolates the fields to exchange among the several meshes created by each code. This capability of the platform has allowed to study the effect exerted by different combinations of meshes for the neutronic and the thermal-hydraulic solvers. This effect has importance in safety analysis where it is a common approach that both calculations differ in the node size definition what has an impact in the safety variables values obtained like fuel or clad temperature.
Concerning the other aspect studied, the effect that the temporal coupling can also have on the results obtained for the safety variables, the external coupling used with COBAYA code allows implementing different temporal coupling schemes without significant modifications in the codes. This type of study was firstly proposed in a previous European project, NURISP, that was also devoted to the development of the European platform but the study was not possible because the integrated codes did not fulfill at that moment all the requirements to allow the definitions of different temporal schemes. Therefore, it has been with the work of this Thesis when one of the objectives of this series of EU projects has been achieved.
The transient analyses have been performed employing four different coupling schemes. Three of them are improvements of the Operator Splitting method, an explicit coupling method that is widely used for temporal code coupling in multiphysics platforms where implicit coupling is not possible. The fourth one is the extrapolated method, a semi-implicit method that was previously used by the COBAYA code for an internal coupling with COBRA-III and COBRA-TF codes but that has not been used yet in the external coupling within the NURESIM platform.
In conclusion, this Thesis has made use of the capabilities included in the NURESIM European Platform through the coupled system COBAYA/COBRA-TF to carry out detailed analysis regarding safety variables for the plant operation. Further studies can be performed using other codes integrated in the Platform to capitalize the efforts made by the organizations involved.
