HEAT UP AND COOLING STUDIES OF DEBRIS OF IPHWR

Abstract

220MWe IPHWR comprises of various engineering safety features and an accident management guidelines, which are capable to mitigate the postulated accident scenario and resist the progression and its consequences. Postulated Loss of Coolant Accident (LOCA) has probability less than 10-4/year. In case of LOCA, coolant supply in the fuel channel may be effected due to small break or large break in the header or feeder of the pipe lines. Under postulated LOCA condition in IPHWR, the fuel channel experiences very low sustained flow at the inlet feeder caused by certain breaks. With simultaneous failure of the Emergency Core Cooling System (ECCS) results in degraded convective cooling and fuel temperature inside the fuel channel starts to increases. Stratified flow inside the channel may be established and after long time, fuel channel may be fully voided. At high temperature, creep deformation of the PT may takes place. If the header or feeder break is small, then depressurization will take place in a long time, and pressure inside the PT would be high. At high temperature and high pressure, ballooning of PT would occur. In case of large break LOCA, depressurization would be fast. Low pressure inside PT and fuel bundle weight would cause pressure tube to sag. Deformation of PT, either by ballooning and sagging arrests the temperature rise inside the channel and ultimately transfer the heat to the moderator available as heat sink in the Calandria vessel. With time moderator in the Calandria vessel gets heated up and starts boil off. When pressure inside the Calandria vessel reaches to a specific limit, Over Pressure Rupture Disc (OPRD) opens and depressurization occurs with the expulsion of moderator from Calandria vessel. That may cause few top fuel channels rows to get expose. If the moderator cooling and makeup fails, moderator will boil off continuously. These suspended debris impart loads on the bottom of submerged channels. If load exceed to the threshold value, then all channels may settle down at bottom of the Calandria vessel forming terminal debris. Limited Core Damage Accident (LCDA) differ from Severe Core Damage Accident (SCDA) in respect to availability of moderator which prevent CT to deform. In the present thesis postulated SCDA is considered. An experimental set up is designed and fabricated at IIT Roorkee to investigate the heat transfer phenomenon of a segment of fuel channel. Experiment 1 was carried for SCDA in which moderator in Calandria, boil off and a single channels started to get fully exposed in moderator. Average decay heat for channels is considered and the channels powers are selected as per core power map of 220 MWe IPHWR. Core power map of 220 MWe provide information in which fuel channel having maximum power among the top three rows inside Calandria, generates 1% decay heat is equivalent to 4.03 kW/m, which is lower than the core average 1% decay heat (4.16 kW/m) of the total thermal output. vi Conducting experiment at higher heating power (4.16 kW/m) and low steam flow rate than that would occur when top three channels are exposed, the resulting temperature of the simulated channel will be higher than the actual case. If the temperature is within limit, then the channels in the top three rows will be with in limit. With time, decay heat of the fuel channels decreases following decay curve. Steady state experiments were also carried out for low decay heat varying from 1% to 0.25 %. Steam velocity of test section power was scaled 1:1 with the reactor. Numerical results were simulated on ANSYS Fluent software and validated against experimental results. It is observed that, CT outer surface temperature varies circumferentially from bottom (0°) to top (180°) and obtained a maximum temperature at the contact point. However, opposite trend is observed in for PT, obtained minimum temperature at the contact point and maximum temperature at the top of the PT. Minimum and maximum temperature value of CTs for 1% decay power is 398.4°C and 482°C.The minimum and maximum temperature values of PTs for 1% is 519.6°C and 577°C respectively. After validating results for single exposed test section, numerical work is extended to evaluate the sensitivity of emissivity, which is important parameters to predict the behaviour of exposed fuel channel. After oxidation, Zircaloy emissivity changes from 0.3 to 0.8. Three cases were simulated at 1 % decay heat equivalent to 4.03kW/m. In simulation, symmetry boundary conditions were used to consider the heat transfer effect of neighbouring channels. In first case, emissivity 0.3 is used for fuel channel components (CT, PT and clad). Similarly in second and third case emissivity of 0.5 and 0.8 is used. Results are compared to study the effect of emissivity on temperature of fuel channel. Maximum deviation in temperature is observed at the bottom (0°) of CT is 164.5°C. For PT, maximum deviation is temperature is observed at 170° is 202.7°C. Results are also plotted for circumferential heat transfer and heat transfer coefficient for CT outer surface. After emissivity sensitivity analysis, numerical simulation is further extended for the top three rows of exposed channels, in which maximum power channel at 1 % decay heat, in each row is selected. Emissivity of fuel pin (clad) is assumed to be 0.6. Emissivity of 0.5 and 0.4 is considered for CT and PT. To replicate the array structure of fuel channels, symmetry boundary condition is applied. Two cases are studied when fuel channels are placed at particular pitch of 229 mm as per reactor and fuel channels are in contact with each other. Over all analysis shows that radiation is the dominant mode of heat transfer. Maximum and minimum temperature obtained by CTs (non-contact debris) are 613.2°C and 473.3°C and for CTs (contact debris) are 706.2°C and 496°C. For PTs (non-contact debris) maximum and vii minimum temperature are 720.1°C-572.6°C. Maximum and minimum temperature obtained by PTs (contact debris) are 770.4°C and 642.7°C. Fire fighting water is the addition of moderator through bottom flooding in Calandria to cool the exposed fuel channel. Experiments were performed to investigate the maximum cooling of the exposed channel at 1% decay heat, through bottom flooding water at different feed rate. Vertical water flow rate is calculated and scaled as per reactor data, provided in NPCIL document. Experiment was done for minimum and maximum flow rate at 20°C water temperature. Minimum and maximum bottom flooding rate are calculated as 0.8 cm/min and 2.8 cm/min. In both cases it was observed that CT temperature drops sharply and comes below boiling point of water. Drop in CT temperature through water provides indirect cooling to PT and the fuel pins. After achieving steady state in the experiment with minimum bottom flooding rate of 0.8cm/min, PT temperature near CT-PT contact point (10°) drops to 193.1°C and at the top (180°) to 116.9°C. Similarly for maximum bottom flooding rate (2.8cm/min), the maximum drop in temperature of PT at 10° is 193.8°C and at 180° is 119.2°C. Maximum drop in temperature is observed for the fuel pin, which is closest to CT-PT contact point. For 0.8cm/min flow rate, maximum temperature drop for fuel pin is 93.4°C and for 2.8cm/min is 100.6°C.