In-Vessel Retention of corium through External Reactor Vessel Cooling（IVR-ERVC）is an ongoing technique for the extraction of heat out of the reactor pressure vessel.The efficiency of heat transfer and critical heat flux are dependent on the optimal design of the external flow channel,working fluids,and the boiling conditions.The experimental and theoretical study on flow and heat transfer characteristics of boiling DI（deionized）water as a coolant has been executed on ULPU-IV based test facility built at Beijing Key Laboratory of Passive Safety Technology for Nuclear Energy for IVR-ERVC.The water as a coolant is allowed to flow from the reservoir through the extermal 90° curved channel at a flow rate of 0.129m3s-1 under natural circulation with inlet subcooling temperature of 20K and 80K.The heat flux（301.204kWm-2）is supplied perpendicularly to the convex wall of the flow channel owing to the square cross-section with hydraulic diameter and radius of curvature of 0.040m and 0.476m respectively.The two-phase boiling model of computational fluid dynamics（CFD）is incorporated for the execution of numerical results.The increase in the degree of subcooling from 20K to 80K enhanced two folds of heat transfer coefficient（HTC）which consequently embellished heat dissipation out of the surface and reduced heating wall temperature.The flow and heat transfer characteristics of two-phase fluid have been discussed by considering velocity,void fraction and temperature distribution of the heating wall.The bubble departure frequency increased with the increase in the degree of subcooling and the sliding motion of bubbles on the inclined surface played a vital role in the enhancement of heat transfer.The computed results agreed well with the experimental results.The mean percentage error（MPE）is calculated as 29.21%.Insertion of flow field modulators i.e.fins,grooves,etc.within the channel are considered as promising factors for maximizing heat dissipation out of the vessel via flowing fluid,but on the other hand,their insertion may impose excessive back pressure on the channel which may lead it to be ruptured.So,it is necessary to analyze the pressure effect while designing the flow channel in order to overcome associated safety issues.By keeping this in view,the present study has been executed in order to analyze the two-phase pressure gradient through the curved channel.Two-phase（steam-water）pressure gradient calculation through the 90° square,even cross-sectional,curved channel occupying 40mm of hydraulic diameter and 476mm of curvature radius has been deliberated by conferring Homogeneous Flow Model（HFM）as well as Separated Flow Model（SFM）for the computation of combined effect of frictional,acceleration and gravitational pressure gradients.The calculated pressure gradient with respect to exit steam quality（x）was validated by the experimental results conducted at ’ Beijing Key Laboratory of Passive Safety Technology for Nuclear Energy ’.The experimental and calculated results were in good agreement within 0<x<0.030 under ’bubble flow regime’having RMSE of 0.389 and 1.822 with HFM and SFM respectively.As the efficacy of IVR-ERVC is strongly correlated with the CHF on the lower head of the reactor pressure vessel（RPV）.The high limit of critical heat flux（CHF）prevents the RPV from failure by extracting much heat out of the vessel through the circulation of working fluid in an external flow channel.In the present study,the deionized water is allowed to flowthrough a curved flow channel having a radius of curvature of368mm and hydraulic diameter of 40mn with a square cross-section.The heat flux of 1500kWm-2 is applied perpendicular to the convex wall of the channel CHF on the wall of the flow channel has been predicted by incorporating CFD based two-phase（liquid-vapor）boiling model The investigations on spatial variation of pressure,temperature,and velocity and heat flux acquired as a result of numerical simulations have been discussed in detail.The CHF and average HTC is predicted as 1.798e+04kWm-2 and 38.98lkWm-2K-1 respectively.The numerical calculations are performed to predict CHF during subcooled boiling of deionized water through 90° curved flow channel with a square cross-section.Two types of flow regimes have been observed at low and high flow velocities.In case of low flow velocities,the continuous accumulation of vapors formed a vapor blanket on the heating surface which consequently reduced heat dissipation out of the surface and triggered CHF due to drying out of liquid between vapor blanket and heating wall.On the other hand,coolant circulation at higher velocities hindered the formation of vapor blanket and allowed subcooled liquid for rewetting.The consistent rewetting led to the formation of small and separate vapor blankets.The enhancement in CHF has been observed with an increased degree of subcooling and by increasing flow velocity in the present study.The three-phase（water-liquid,water-vapor and nanoparticles）numerical simulations have also been done on CFD with the model of ULPU-IV facility of Beijing Key Laboratory by considering water and four water based-nano fluids as a working fluid viz.Graphene-water,Alumina-water,Titania-water,and Silica-water.The supplied heat flux is 1500kWm-2,and the coolant is allowed to flow at 0.02m3s-1 under natural circulation with an inlet temperature of293K.These nanofluids have been analyzed on the basis of thermo-physical properties by incorporating particle diameter of 85nm within the range of volume concentration（1-3 vol.%.）.The effeetive thermal conductivity,HTC and heating wall temperature have been examined by taking each nano fluid as a coolant at different volume concentration and the enhancement ratio is evaluated with respect to water（liquid-vapor mixture）alone as a coolant.The stability analysis by comparing sedimentation velocities of the aforementioned nanofluids and cost analysis on the basis of a recent survey are also done.It has been evaluated that the incorporation of nanoparticles in water showed a remarkable result in the enhancement of effective thermal conductivity,HTC and in reducing wall temperature as compared to water coolant.It has been observed that increasing volume concentration imparted negative impact on heat transfer phenomena in case of Alumina-water,Titania-water and Silica-water as a coolant from 1 vol.%to 3 vol.%whereas in case of Graphene-water the enhancement of heat transfer occurred from 1 vol.%to 2 vol.%but at 3 vol.%reduction in heat transfer was observed due to agglomeration and sedimentation of particles.It has been concluded that the water-based graphene nanofluid outperformed other working fluids at an optimum concentration of 2 vol.%due to its high heat extraction efficiency and profound stability but it is not regarded as cost-effective.