Stress Analysis of the Lower Head of Central Measuring Shroud Under Thermal Striping and Thermal Shock Conditions

The central measuring shroud, as an important in-vessel component, provides guidance and protection for control rods and measuring equipment in a sodium-cooled fast reactor (SFR). The lower head of central measuring shroud (LHCMS), which is located above the core outlet, is only 500mm away from the core outlet. Therefore, the LHCMS is affected by the liquid sodium from core outlet for a long period, especially the temperature effects of the following two types. On the one hand, under the operating condition of the SFRs, the uneven distribution of the core power causes the phenomenon of thermal striping, which may cause high cycle fatigue and even initial crack. On the other hand, under the scram condition, the coolant temperature at the core outlet is sharply reduced due to the decrease of the core power, inducing the phenomenon of thermal shock that may cause large thermal stress and low cycle fatigue. Therefore, stress and fatigue analyses of the LHCMS under the thermal striping and thermal shock conditions are very necessary. In the paper, finite element model of the LHCMS was first established, and then according to the temperature curves under thermal striping conditions and thermal shock conditions, the thermal stress of the LHCMS was simulated. The results showed that although the temperature fluctuation outside the LHCMS is severe, the stress caused by thermal striping only slightly fluctuates at 123MPa level, the maximum stress range is 11MPa. Besides, at 20s, there exists the maximum stress difference between thermal striping and thermal shock conditions, the maximum stress caused by thermal shock is about 3 time larger than that caused by thermal striping. According to high cycle and low cycle fatigue analyses, the fatigue damage factor of thermal striping is only 0.0078, while the fatigue damage factor of thermal shock is 3.416, which should provide a reference for the design of the LHCMS.

Proposal for Improving Insulation Installation Practice for Superheated Steam Line

Insulation is widely used in process plants to reduce heat loss of process fluids in piping and pressure vessels. However, insulation is often not installed around Normally No Flow (NNF) line pipe. In a refinery plant, a steam leak incident happened due to a through-wall crack, which was found around the connection between an insulated superheated steam line with insulation and an uninsulated safety valve line. The through-wall crack was identified to be a fatigue crack initiated at the inner surface by fractography. An unsteady Computational Fluid Dynamics (CFD) analysis was performed to identify mechanism of the through-wall crack initiation. Based on the observation of fractography and the CFD analysis, it is inferred that the through-wall crack was induced by a high cycle thermal fatigue phenomenon, so-called thermal striping, due to incomplete mixing of hot and cold fluids. Many thermal striping incidents in nuclear plants and process plants have been reported. In view of the above fact, it is suggested that conventional insulation installation practice for NNF line pipe, in particular superheated steam line, may cause cracks due to thermal striping around the connection between main superheated steam pipe and branch dead-end leg. In this paper, a convenient guideline for insulation installation is proposed for a dead-end leg of superheated steam line to prevent cracks caused by thermal striping. The guideline can be used to judge the necessity of insulation installation, based on degree of superheat of steam.