SINGLE AND MULTIPLE CHANNEL SAGGING STUDY UNDER SEVERE HEAT-UP CONDITIONS

Abstract

The Loss of Coolant Accident (LOCA) is a critical initiating event that leads to early degradation of fuel cooling and hence challenges the integrity of a channel. The presence of moderator around the affected channel arrest the temperature rise thus prevents the channel failure and core degradation. These events are termed as Limited Core Damage Accident (LCDA). The severe accidents such as LOCA with the loss of Emergency Core Cooling System (ECCS) and loss of moderator cooling system in the Pressurised Heavy Water Reactors (PHWRs) are termed as Severe Core Damage Accident (SCDA). These are less probabilistic accident events because of a large number of safety features and accident mitigation provisions. However, the progression of these events can lead to severe consequences resulting in severe core degradation. Under postulated circumstances, channels will experience low pressure owing to breach in top-most channels in the Calandria vessel during the early high-pressure event. The channels will experience sagging as they get exposed to void due to moderator boil-off. In the process of moderator boil-off, channels having higher power will sag more as compared to channels with lower power. The Pressure Tube (PT) inside these channels starts to sag first and contact to respective Calandria Tube (CT). After that, the complete channel starts sagging due to longitudinal strain under thermal load and the weight of its component. The sagging nature of the channel is an essential factor that is needed to be studied. Firstly, sagging deformation determines the nature of early debris or suspended debris formation. Secondly, the sagging of channels results in a pull-out force on the Calandria end-shield, and the nature of pull is needed to be investigated. Few experiments are reported in the literature for the study of the deformation of PT under LCDA event for I-PHWR. However, no experimental investigation has been carried out to develop the understanding for progression of channel sagging in low pressure severe heat-up conditions. To address this gap, work has been formulated to understand the deformations aspects of coolant channels of 220MWe Indian PHWR through experiments. It is nearly impossible to perform a full-scale channel sagging experiments in the lab due to inherent difficulties. Therefore, a scaled-down approach for sagging measurement during channel deformation has been implemented. There are specific criteria for geometry, mechanical properties, and applied load on the system that is needed to be considered a priori with their implication to perform scaled-down experiments. These conditions include scaling on length of the core, fuel bundles, lattice pitch, and diameters of PT, CT, and fuel bundles. An approximate reduced geometric scaling of 1:3 was applied on geometric parameters. For getting the desired material behaviour, the PT and CT were fabricated with Zr-2.5wt%Nb and Zr-2 alloys respectively, which are the same zirconium alloys as used for the actual full-length coolant channel. It ensures the same thermo-mechanical properties for both tubes and maintaining the iii iv exact composition in scaled-down channel gives the same material flow characteristics during high temperature sagging. Tungsten weight simulators were used to mimic the weight of fuel bundles to keep the same mechanical stress level as in the prototype channel. Ten such weight simulators were used in the experiments. Herewith, a geometrical scaling of 1=3rd can be achieved on the deformation of a channel. It is needed to evaluate the heating power needed for severe heat-up of a channel, testsection dimensioning, cooling provision on test-section for the experimental facility development. Numerical analyses were performed using the direct-coupled field approach of finite element method to get a preliminary insight into a structural and thermal coupled response on the channel deformation. The outcomes of numerical analysis of channel sagging helped in dimensioning the test apparatus. Consequential analyses of thermal mapping on test apparatus has been made to identify the critical areas, which helped to introduce the precautionary steps in the test apparatus fabrication. A 2D fluid flow analyses of the test apparatus helped to identify the problem of steam accumulation inside the back section. Based upon the findings from a priori numerical studies and scaling calculations, a one-third scaled-down experimental facility was developed and commissioned at Indian Institute of Technology Roorkee, Roorkee. The high-temperature sagging of channel was measured with the help of digital imaging approach, which is based upon the transient recording of sagging phenomena. The recording was performed through the optical ports installed over the different locations at test apparatus. The sagging was thus estimated through the concept of an edge detection method. The present experimental study aims to investigate the sagging of channels under the hypothesis of postulated severe accidents with a large break LOCA and a loss of ECCS as an initiating event with the presumed malfunctioning of moderator cooling system with no make-up water. The experiments have been categorised into two classes; namely, Inert Environment Tests (IETs) and Steam Environment Tests (SETs). IETs were performed into three parts, viz. experiments with a single channel, experiments with two rows of the channel, and lastly, an experiment with three rows of the channel. Whereas, SET was performed with a single channel configuration in the present study. During two rows of channel experiments, lower channel acts as a dummy channel with no power during the run. Similarly, for three rows channel experiment, the lower (last placed) channel acts as dummy channel. Dummy channel concept was conceived to model the assumption of moderator submerged channel during the boil-off stage, thus exposed to less heating. Inert conditions are created by purging a commercial-grade argon gas while steam was supplied from the electric steam boiler. An experiment was also performed with the heating of PT alone to study its deformation in the inert environment. The experimental observations for thermal and structural aspect from PT alone experiment were also studied numerically.