Study On Cable Tray Fires In Nuclear Power Plant And Flame Spread Over Cable Insulation Under Currents

With the development of social economy and the improvement of electrification degree,cables play an important role in our life and industrial development.However,cable fires occur frequently in recent years.Electrical fires account for more than 30% of the total fires every year,most of which are caused by short circuit or overload of cables.In addition,the use of cable is huge in nuclear power plant(NPP).In case of fire,it will seriously threaten the operation safety of NPP and even cause nuclear leakage.Therefore,it is of great significance to study cable fire.The purpose of this paper is to study the flame spread and fire characteristics of cables with different scale according to actual fire scenarios.First of all,the core and insulation of coaxial cables are selected to carry out the small-scale flame spread experiments under large currents.The influence of cable size and electrical current on flame shape,flame spread rate and molten dripping behavior is researched.As the results shown,the cables are preheated because of currents and the flame shape,flame spread rate and molten dripping behavior are changed under different current.With the increase of current,flame height undergoes three phrases of growth,stability and attenuation by fuel melting and dripping competitive effect.In particular,when the current is too large,the dripping rate is obviously enhanced and a flowing fire is formed,which results in the decrease of burning rate and flame height.The flame width changes little under low currents but decreases in flowing fire stage.In addition,the flame shape changes periodically due to the molten dripping behavior.Before molten dripping,the flame height increases gradually and then keeps steady,and the flame width and flame spread rate are unchanged.But the flame height decreases significantly and the flame width and flame spread rate fluctuate greatly after dripping.Then,the full-scale horizontal cable tray fire experiment is carried out in the closed space with typical structural characteristics in NPP.The flame height,mass loss,heat release rate(HRR)and temperature are analyzed.The effects of tight or loose cable arrangement and ignition time on HRR are also discussed.The results show that the flame height increases with the number of ignited cable trays,but as the intermediate cable burns out,the flame height tends to stabilize and then decrease.The upper cable is affected by the thermal feedback of the lower cable,so that the flame spread rate is faster and the spread distance is longer.The three-layer smoke stratification is observed during smoke descent,and the thickness of upper smoke layer increases and the thickness of lower smoke layer decreases gradually.The peak HRR of tight arrangement cable is higher than that of loose arrangement cable,but the HRR of loose arrangement cable increases quickly after ignition.The ignition time has little effect on peak HRR.Finally,a prediction model of fire HRR in ventilated confined space is proposed based on Oxygen Consumption(OC)and Carbon Dioxide Generation(CDG).The three-zone smoke model is applied to this prediction model.The cable tray fire and FMRC fire tests are used to verify the reliability of prediction model.Results show that the cosine similarity between HRR predicted curve and experimental curve is more than 0.95,which means the reliability of prediction model is high.The application of three-zone smoke model improves the prediction accuracy of the HRR model.In addition,the ventilation enhances the smoke mixing and the smoke distribution becomes relatively uniform,which results in the HRR prediction accuracy of CDG is improved.