As an innovative nuclear fuel assembly, the helical cruciform fuel (HCF) assembly has the characteristics of large specific heat transfer area, short heat conduction path, strong inter-channel mixing and free from the grid spacers. Compared with the traditional cylindrical fuel assembly, the HCF assembly can raise the core power density with compromise on the safety margin. However, the concentrated stress might take place at the location of self-support points, resulting in the plastic deformation and even rupture.

This study aims to analyze the thermal-mechanical behaviors of HCF bundle under steady conditions and accident transitions, so as to obtain the stress and strain of HCF rods, based on which, the integrity of fuel cladding was assessed.

Firstly, a 3×3 typical HCF geometrical assembly model without four rods in corners was constructed and discretized by hexahedral mesh. Then, the steady and transient convective conditions were applied to the outer surfaces of rods to simulate the various working conditions, including single phase, boiling, reactivity insertion accident and loss of coolant accident. Finally, the governing equations for mechanics and heat transfer were established and solved in ANSYS using the thermal and mechanical modules.

The results show that, the maximum von Mises stress and plastic deformation take place at the location where adjacent rods contact, where the stress and strain are determined by both the contact constrain condition and the temperature difference between cladding inner and outer surfaces. However, at the elbow of the blades, the stress and strain are mainly affected by the radial temperature gradient in the cladding material. For the cladding, the plastic deformation is larger while the von Mises stress is smaller under the flow boiling condition compared with these under the single-phase cooling condition. Furthermore, the integrity of fuel cladding can be maintained under the conditions of reactivity insertion and loss of coolant accidents, where the stress and the temperature are lower than the break limit and the zirconium-water reaction temperature, respectively.

From the thermal-mechanical analysis on the HCF assembly, this kind of innovative fuel assembly shows good mechanical performance under normal and accidental conditions.