PSI - Issue 16

Andrii Kotliarenko et al. / Procedia Structural Integrity 16 (2019) 223–229 Andrii Kotliarenko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

For the first time, the accident with the partial meltdown of the active zone of the reactor vessel took place at the Nuclear Power Plant Three Mile Island, USA (1979). Analysis of the accident results showed that the destruction and movement of molten debris of the core into the lower plenum of the reactor vessel led to its local heating (up to ~ 1100°C in the lower part) and plastic deformation of its wall. A similar situation with the melting of reactor core occurred on power units of the Fukushima-1, the nuclear power plant in Japan, which arose as a result of abnormal earthquakes and led to severe environmental and social consequences. In such situations, due to abnormal operating conditions, a melt is formed in the reactor vessel, which includes a vessel’s internals and uranium (Theofanous et al. (1997)). Maintaining the melt inside the reactor vessel is an essential component of the strategy for managing severe accidents both for operated stations and for new projects of reactors with water under pressure. The criteria for maintaining the melt in the reactor vessel is the absence of melting through the vessel’s wall, the value of the heat flux does not exceed the critical value on the outer wall and the stresses do not exceed the strength limits in the reduced wall thickness due to the partial melting. The fulfilment of the last condition directly depends on the properties of the material, namely on the features of its deformation at operating temperatures and severe accident temperatures. Complex analysis of the thermal parameters and strength criteria of the WWER type reactors are related to the study of the patterns of their heating, creep and fracture during a severe accident. Thermal and strain analysis of the WWER vessels during abnormal operating conditions (Loktionov et al. (1999)) showed that the time to the failure of the vessel is mostly determined by the thermal conditions on the outer wall of the reactor vessel, the minimum residual wall thickness in the melting zone and the value of excess pressure in the vessel. Analysis of the thermomechanical behavior of the WWER vessel during accidents with the failure of the core is based on conducting thermal and strength analysis of the structure. The result of the thermal analysis is definition of the temperature field in the vessel and the assessment of the possibility of its through melting due to the corium heating during the accident. Also relevant is the study of the heating process of the vessel, as well as estimation of the characteristic dimensions of the melting zone of its wall. Both size and change of the melting zone depend on the specific thermal conditions to which the vessel is exposed. Based on the analysis, it can be concluded that numerical simulation requires usage of the actual material properties, which allows determining the level of damage, makes it possible to simulate the process of accumulation of damage and fracture of the vessel material during its deformation. To simulate the creep of 15Kh2NMFA-A steel, it is necessary to use data from high-temperature tension tests of this steel. In the few papers devoted to numerical simulation (Tran et al. (2010); Altstadt and Mossner (2000); Singh et al. (2017)), the material properties are determined according to standard tests, under standardized conditions and loading rate. It should be noted that during a severe accident, the material of the reactor vessel is subjected to loads, deforms at a low rate under certain temperature conditions. Thus, investigation of material properties under such loading conditions, typical for a serious accident, is an urgent task.

Nomenclature Е

Young’s modulus Yield stress Tensile stress Maximum stress

σ 0,2

σ в

 max

δ

Elongation

ψ

Reduction of area

v έ

Loading rate Strain rate