Simulation and optimization design of fuel rod in pressurized water fuel assemblies

https://doi.org/10.1016/j.nucengdes.2020.110856Get rights and content

Highlights

  • Simulation for three types nuclear fuel elements.

  • Study on the mechanics behavior of the nuclear fuel cladding.

  • Find out the optimal design of the dual-cooled annular fuel element.

Abstract

Since the working environment of the pressurized water reactor (PWR) fuel element is complex, the analysis and optimization of its performance involve many physical phenomena, such as thermic, mechanics and so on. In order to analyze and optimize the performance of PWR fuel element, the column fuel element (CFE), singular-cooled annular fuel element (SCAFE) and dual-cooled annular fuel element (DCAFE) are simulated based on the finite element analysis software called ABAQUS. For the DCAFE, the inner diameter is modified, and the concept of “normalized volume parameter” is proposed to eliminate the influence of volume change on the performance of the fuel element, and to accurately describe the thermodynamic performance of the fuel element under different inner and outer diameters. The results show that: The DCAFE has advantages of low von Mises stress, small radial displacement and low temperature. When the ratio of inner and outer diameters is about 0.67, thermodynamic properties are the best and the optimal size is achieved.

Introduction

At present, PWR type nuclear power units are widely used in the world running nuclear power units. The nuclear energy is mainly supplied by PWR nuclear power units and this trend is not expected to change significantly when considering the new projects planned (Huang and Zhang, 2012). Since the working environment of PWR nuclear fuel elements is complex, a variety of physical phenomena, such as thermal mechanical coupling, nuclear-force coupling, irradiation swelling, damage of fuel pellets and cladding due to defects in manufacturing process need to be comprehensively analyzed and considered. Therefore, investigation of PWR fuel elements has important engineering significance.

The commonly used PWR fuel element is composed of fuel pellets and cladding. There is a certain gap between pellets and cladding to prevent the cladding damage caused by excessive expansion of pellets. In recent years, a lot of investigations have simulated the CFE and summarized the trends in some important parameters (such as temperature, stress, etc.) during the working process of fuel elements. Considering the axisymmetric characteristics of the fuel element (Williamson, 2011), used a 2D axisymmetric model to simulate the fuel rod behavior under steady and transient conditions. In order to obtain more accurate results, (Goldberg et al., 2019, Newman et al., 2009, Williamson et al., 2012) built a three dimensional model to investigate the fuel behavior under the coupling of multiple physical fields. (Spencer et al., 2016) investigated the effect of pellet surface defects in fuel on the thermal and mechanical response of the cladding. These investigations showed that the CFE had a uneven stress and displacement distribution in the pellet and cladding. Even more, severe stress concentration occurred at the contact of two pellets. In the working process, because of the excessive central temperature of fuel pellets and the excessive displacement of the cladding due to the interaction with the cladding, it is necessary to replace the fuel element regularly (Zohuri, 2017).

In order to improve fuel efficiency, MIT proposed the idea of DCAFE and started related researches (Kazimi, 2006, Kazimi, 2007, MORRA, 2004). In recent years, the feasibility and properties investigation of annular fuel element application in the PWR nuclear power station has been carried out successively. (Hejzlar and Kazimi, 2007, Yang and Bo, 2011) analyzed the feasibility of DCAFE in practical engineering application. They found a significant reduction of the cost of power plants and increase in safety margins by using this fuel element. (Kim et al., 2011) investigated the mechanical design issues and resolutions of DCAFE in real nuclear power units and proposed a fuel element design and assembly scheme. (Rhee et al., 2013) investigated the rod-inserted sintering of annular pellets and verified the feasibility of fuel element preparation. (Shin et al., 2012, Zaidabadi et al., 2017) assessed the thermal hydraulic performance of DCAFE. These investigations indicated that the annular fuel increased the thermal margin up to a point. (Deng et al., 2016b, Xia et al., 2019) developed analysis code for annular fuel element to investigate the thermal–mechanical behavior in DCAFE. (Deng et al., 2016a, Yang et al., 2009) investigated the mechanism on heat split phenomenon in DCAFE and analyzed the influence on fuel elements due to the heat split. (Ansarifar and Ebrahimian, 2016, Ebrahimian and Ansarifar, 2016) used nano fluid as coolant and investigated its effects on heat transfer characteristics. These investigations indicated that the nano fluid could decrease the temperature of the fuel center and the safety margins increased accordingly. However, most analyses of annular fuel elements focus on the feasibility and thermal mechanical behaviors of DCAFE. There is little investigation on the optimization design of annular fuel elements.

In this paper, the finite element simulation method is used to establish the calculation model of column fuel, singular-cooled annular fuel and dual-cooled annular fuel. The effect of multiple physical phenomena, such as irradiation swelling, heat conduction on the thermal mechanical behaviors of the fuel elements has been considered. The distribution of von Mises stress, radial displacement and temperature of three types of fuel elements have been investigated. In order to eliminate the influence of DCAFE volume effect, the normalized von Mises stress, radial displacement and temperature have been obtained by using “normalized volume parameter”. Furthermore, by finding out the optimal design size of DCAFE, this paper provides a reference for improving the thermodynamic performance of fuel elements in practical engineering application.

Section snippets

Basic equations

In this paper, the thermal mechanical behavior of the fuel element is analyzed by solving three basic equations: energy conservation equation, mechanical equilibrium equation and constitutive equation. The expressions are as follow:

Fuel pellets will release a large amount of heat under operation condition. The heat will be carried away by the coolant through the cladding and the gas between pellets and cladding. The energy conservation equation can be expressed as:ρcpTt+q-S=0q=-kGrad(T)

ABAQUS simulation

In this paper, the finite element analysis software ABAQUS is used to simulate (Hibbett, 2014). ABAQUS is one of the commonly used finite element analysis software in simulation, which has powerful modeling and analysis ability, especially in dealing with nonlinear problems. This paper deals with thermodynamics, contact, material properties with temperature, and has strong nonlinear characteristics. Therefore, ABAQUS is used for analysis and calculation. The CFE and annular fuel element are all

Thermodynamic performance analysis of three fuel elements

Corresponding finite element results are obtained by ABAQUS calculation and analysis. The fuel cycle is a long-time process (at least six months). It is not meaningful and realistic to compare the thermal mechanical performance of CFE, SCAFE and DCAFE in the whole fuel cycle. Therefore, in this paper, we select the moment when the cladding and pellets come into contact with each other. Using ABAQUS finite element simulation, the contact is evidenced by an abrupt change in the von Mises stress

Conclusions

In this paper, three types of fuel element have been simulated and investigated in ABAQUS. Considering the coupling of multi-physical phenomena, such as thermodynamic, fuel swelling, the von Mises stress, radial displacement and temperature distribution under normal operation condition are obtained. The results indicate that among three fuel elements, the DCAFE has the best thermodynamic performance. The SCAFE and CFE have relatively poor thermodynamic performance. The DCAFE has a low level in

Funding

This work was supported by the National Natural Science Foundation of China [11232015, 11572355].

CRediT authorship contribution statement

Ze Xu: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Writing - original draft. Yulan Liu: Funding acquisition, Investigation, Methodology, Project administration. Biao Wang: Funding acquisition, Investigation, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (29)

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