Doel 3 - Tihange 2 / German RSK Evaluation & Reply

Based on the design data, all transients were evaluated for their stresses on flakes. Accordingly, loss-of- coolant accidents (LOCAs) are the leading transients for distances s < 20 mm from the surface, cool-down transients leading for 20 mm < s < 30 mm and heat-up transients for greater depths up to 120 mm. For LOCAs, a rotationally symmetric, abrupt thermal shock with cooling to the feed temperature of the emergency cooling system at the inner surface of the RPV is assumed in a simplified 1D analysis with a conservatively high heat transfer coefficient. In a multi-step procedure to demonstrate the acceptability of the flakes, the first step was to calculate the maximum acceptable size of circular flaws using 3D finite element (FE) analyses as a function of their diameter 2a, distance s to the surface, local RT NDT at the flaw position, and angle α to the surface according to the provisions of the ASME XI code, also taking into account safety factors √10 and √2 required therein. This resulted in 24 curves for the acceptable size 2a acc as a function of the distance s for four different tilt angles α (10°, 20°, 30° and 45°) and in each case six different values of local RT NDT. For every flaw with the parameters s, RT NDT and α, the acceptable size 2a acc can thus be determined by interpolation. In a first step, all flaws were classified as “harmless” that are smaller than half the acceptable size. This concerns 99.75% of all registered flaws or groups of flaws in the case of Doel-3 forged rings [12] and 99.7% of the flaws or groups of flaws in Tihange-2 [13]. The remaining 0.25% or 28 flaws or groups of flaws at Doel-3 and 0.3% or 11 flaws/groups of flaws at Tihange-2 were subjected to a more detailed but less conservative assessment. These are groups of flaws, with only one exception each of a near-surface crack. For this analysis, the identical load assumptions were made and the individual flaws modelled as largest possible ellipses instead of circles, fitting into the same rectangular UT boxes. In addition, closely spaced flaws were no longer grouped into one combined flaw but modelled using an extended finite element model (XFEM) accounting for their interactions. All flaws modelled accordingly meet the same acceptance criteria as above with a considerable safety margin. The maximum value for 2a/2a acc is 0.32 for Doel-3 [12] and 0.52 for Tihange-2. The flaw indication concerned there is a single crack very close to the inner surface. All other analyses for Tihange-2 result in values of less than 0.2 for 2a/2a acc [13]. In a further step, the calculated curves of the stress intensity factor K I (T) during the transients for flaws considered as enveloping were compared to the acceptable fracture toughness at the lower shelf according to the regulations, applying the respective safety factor SF= √10 or SF= √2 . In most cases, the stress intensity factor K I (T) max is below the lower shelf of the curve of the applicable regulations, i.e. the value of the acceptable fracture toughness at low temperatures, divided by SF. Thus, these loads are acceptable regardless of the actual value of RT NDT . In the few cases in which K I (T) max exceeds the lower shelf of the curve K IR (T - RT NDT )/SF, the distance of the maximum K I (T) max to the curve K IR (T - RT NDT )/SF defined as a margin on the temperature axis. For Doel-3, the smallest margin for a single flake near the surface (s = 3.6 mm) was determined to be ΔT = 80K. All other values are between ΔT = 105 K and 190 K [12]. For Tihange-2, loads exceeding the lower shelf of the applicable curve were only calculated for two flakes. The margins for these flakes are 110 K and 130 K, respectively [13].

RSK/ESK Secretariat at the Federal Office for the Safety of Nuclear Waste Management

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