Critical Reflections about Doel3 & Tihange2

Integrity reactor vessels Doel 3 and Tihange 2

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5 crack ceiling displacement ( µ m) 10

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radius from centre of crack (mm)

Figure 13: Displacement of the crack wall as result of an internal pressure of 3796 bar, from the center to the edge of the crack.

create such a high density of cracks present in the high crack zones of the Doel3 lower shell. In order to obtain a first estimate the amount of hydrogen needed to generate the flaws in the high flaw density material, it is assumed that 41 flaws of a mean diameter of 17mm are present in 1 dm 3 steel. The flaw is caused by accumulating hydrogen at a hydrogen trap (impurities, segregation zones) in the vessel wall material and at a certain pressure, the material cracks. During cracking of the material, the pressure in the crack drops until the material stress intensity factor (SIF) drops below the threshold value and the crack gets its final dimensions. For a crack of 17 mm diameter, the internal pressure inside the flaw equals for are circular shaped flake σ = π 2 K I c p π a , [ 4 ] in which K I c = 40 MPa / p m is the stress intensity factor at room temperature and a = 8.5 mm is the half diameter of the flaw. To estimate the volume of such a crack, a linear elastic finite element simulation has been carried out. Figure 11 presents the axisymmetric 2D-FEM model of a circular crack of 17mm diameter in the center of a block steel of 400mm diameter and 800mm high, such that the crack can be considered as if it is present in an infinite medium. Only the bottom is fixed, all the other boundaries are free. Inside the crack, the pressure is introduced which should remains directly after the cracking, in this case 3796 bar. The resulting stress distribution around the crack is shown in figure 12, where the stress concentration at the crack tip is visible. Figure 13 presents the resulting displacement of the crack upper boundary, from which the volume can be estimated. The volume of such a crack amounts 0.00525ml. The normal volume H 2 from 41 cracks at 3796 bar pressure amounts then 818ml, which is 13 times higher than the 61ml H 2 which

R.Boonen & J.Peirs

May 18, 2017