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Hydraulic transient is a flow situation where the velocity of the flow and the pressure change rapidly with time in pipes networks filled with fluid. Water hammer is one of the consequences that may occur accidentally in the reactors cooling system. The pressure level can exceed the limits given by the manufacturer, and it leads to the failure of the pipes integrity or nuclear fuel elements fixation in the core. Cooling system involves many components, and when a flow control component changes abruptly its status (for example, a valve closing or pump trip), it causes rapid changes in velocity and propagates pressure pulses in short time. The cooling system is equipped with flapper valve and flywheel which overcomes the transient issues. A pump or check valve may be stopped or closed abruptly and may cause bad impacts on the system which is simulated with soft trip using flywheel and without flywheel in case of its failure for any accidental reasons. In addition the abrupt valve closure is deemed as a major issue in the system if occurred. AFT Impulse 5.0 software is used to simulate and study the transient of each case. The simulation appears that the reactor cooling system may suffer from surge that may occur during the sudden closure of check valve but in other hand it can be avoided by regular maintenance of flapper valve to open in timely manner.

The design of a nuclear reactor core requires basic thermal-hydraulic information concerning the heat transfer regime at which onset of nucleate boiling (ONB) will occur, the pressure drop and flow rate through the reactor core, the temperature and power distributions in the reactor core, the departure from nucleate boiling (DNB), the condition for onset of flow instability (OFI), in addition to, the critical velocity beyond which the fuel elements will collapse. These values depend on coolant velocity, fuel element geometry, inlet temperature, flow direction and water column above the top of the reactor core. Enough safety margins to ONB, DNB and OFI must-emphasized. A heat transfer package is used for calculating convection heat transfer coefficient in single phase turbulent, transition and laminar regimes. The main objective of this paper is to study the possibility of power upgrading of WWR-S research reactor from 2 to 10 MWth. This study presents a one-dimensional mathematical model (axial direction) for steady-state thermal-hydraulic design and analysis of the upgraded WWR-S reactor in which two types of plate fuel elements are employed. FOR-CONV computer program is developed for the needs of the power upgrading of WWR-S reactor up to 10 MWth.

Heat transfer deterioration (HTD) phenomena in supercritical water is one of the most thermal hydraulic critical issues for supercritical reactors. The onset of heat transfer deterioration and bouncy effect have been studied in details, however limited work has been done on the mechanism of HTD occurrence. In this work, three dimensional analysis of velocity and temperature boundary layers under heat transfer deterioration conditions is conducted. The effect of thermo physical water properties changes inside the fluid boundary layer and its effect on the heat transfer coefficient is studied. Analogy between HTD and critical heat flux condition in subcritical boiling is observed and reported. Under HTD conditions, clotting or sudden stopping of the flow inside the tube is predicted due to the severe variation in density and other thermo physical properties in the tubes radial direction.

In this work, the closure flow values effect on the penetration depth of radioactive water inside the core chimney of a typical MTR reactor is studied numerically using commercial CFD software ANSYS FLUENT. The closure flow values within the chimney are allowed to change from the reactor design value of 50 m3/h to no flow value. The flow stream function is presented for each value of the closure flow showing the penetration depth for each closure flow. The stream functions show the flow structure within the core chimney. The penetration depth of the radioactive water increases with decreasing the value of the closure flow. The stream function contours reveals that for a closure flow greater than 1 m3/h, no radioactive water penetrates from the chimney to the pool.