Redistribution of residual stress by thermal shock in reactor pressure vessel steel clad with nickel alloy

Abstract

This paper characterises the residual stress in nuclear reactor pressure vessel steel clad with nickel-based alloy and investigates the interaction between residual and thermal stresses during thermal shock. Residual stress measurements were made on two plates of SA508 Grade 4N steel, clad with Alloy 82 nickel-based alloy. The techniques used to measure the residual stresses were: deep hole drilling, centre hole drilling, and the contour method. One plate was as-welded, the other post-weld heat-treated. The post-weld heat-treated plate was subjected to thermal shock by heating it up and then spraying the surface of the cladding with cold water. The residual stress was measured again afterwards. A finite element simulation was made to investigate the physical mechanisms causing residual stress redistribution during thermal shock. Thermal shock caused significant residual stress redistribution in the cladding due to elastic-plastic interaction between the thermal stress and the cladding residual stress. The results demonstrate that an assessment of the safety of a reactor pressure vessel during thermal shock could be conservative for small surface defects if it is assumed that residual and thermal stresses combine elastically.

Online Determining Heat Transfer Coefficient for Monitoring Transient Thermal Stresses

“…The heat flux at node (4,4,3) was estimated using three formulas (Equations (7)-(9)). The time changes of the heat flux and heat transfer coefficient determined on the sprayed surface at node (4,4,3) are illustrated in Figure 7.…”

“…The four control points at which the measured temperature was compared with the calculated temperature were located as follows: one point on the water-cooled surface; one point at a distance of 11.7 mm from the cooled surface; and two points at a distance of 23.3 mm from the cooled surface. Figure 6 depicts the temperature measured at node (4,4,1) on the rear heated surface, the temperature calculated at node (4,4,2) at distance z = Δz from the insulated surface, and the calculated and experimental temperature values at node (4,4,3) placed on the exposed sprayed surface. The calculated and measured temperature at node (4,4,3) as well as in other points, exhibited a very good coincidence.…”

“…The slab was made of St3S steel with the following dimensions: H1 = 0.8 m, H2 = 0.8 m, and s = 0.035 m. The finite volumes, in the middle of which the temperature is measured or determined, lying in different planes z are marked with different colors (Figures 2-4). With the use of the computational algorithm described above, the temperature at points (4,5,3), (3,4,3), (4,4,3), (5,4,3), and (4,3,3) is found.…”

“…Then, from these equations, the time-dependent temperatures in 13 nodes lying in the plane z = ∆z (Figure 3b The finite volumes, in the middle of which the temperature is measured or determined, lying in different planes z are marked with different colors (Figures 2-4). With the use of the computational algorithm described above, the temperature at points (4,5,3), (3,4,3), (4,4,3), (5,4,3), and (4,3,3) is found. To determine the heat transfer coefficient at node (4,4,3) ( Figures 3c,d), located on the cooled plate surface, the normal component of the heat flux at the same point has been calculated as follows:…”

“…Due to the severe difficulties in determining the actual heat transfer coefficient, especially under transient conditions, simplifying assumptions are made, which, however, reduce the accuracy of the calculations of the stresses. In [4], emergency cooling of the double-layered nuclear reactor pressure vessel was modeled. Due to the large ratio of the reactor diameter to the wall thickness, which was larger than ten, computer modeling as well as experimental studies were carried out on the double-layered plate.…”