Experimental Study On Spray Cooling Of Spent Fuel Rods

Spent fuel is nuclear fuel that has been removed from the core after reaching certain depth of burnup.The spent fuel contains radionuclides which decay and release heat known as decay heat.These heat must be removed in time,or due to their own decay heat,the temperature of spent fuel rod will gradually increase,even cause that the cladding damage and radioactive material leakage.After the Fukushima nuclear accident,the safety of spent fuel pool has drawn much attention.Especially in extreme accident,the spent fuel pool is damaged and the cooling water empty accidentally.The traditional water-cooling method is difficult to work,in which the spray cooling method become an effective cooling means.The sprinkler cooling system was designed inside the spent fuel plant of the AP1000 and CAP 1400.The water is sprayed directly onto the spent fuel assembly to take decay heat away.However,limited by the storage capacity of the cooling water in the sprinkler system,it is very crucial to obtain the best spray flow density,which can ensure the safety of spent fuel and save the water.In this paper,the experimental study was carried out on the optimum spray flow density and the spray cooling effect of spent fuel pool.The spray cooling experimental device with 5×5 fuel rods and water distribution test device with 17×17 bundle were constructed.The spray cooling experiment not only studied the optimum spray flow density under different decay power and safety limit,but also analyzed the influence of nozzle sensitivity(nozzle position,droplet diameter)on the cooling effect and the influence of the surrounding rod on the temperature of center rod.An expansive coefficient of the spray flow density was obtained by the water distribution test,and analyzed the flow distribution and temperature distribution on the 17 x 17 bundle Experimental data and research results provide technical support and reference for the design of spent fuel sprinkler system.The conclusion include:(1)The optimum spray flow density is obtained,which enables the temperature of the fuel bundle to be stable,and the maximum temperature is stabilized near the safety limit.(2)Under the same spray flow rate,the change of nozzle position has little effect on the cooling effect of the fuel bundle,which is negligible.(3)The diameter of the droplet with the best cooling effect is 850-950p.m.(4)The power of the first and second fuel rods around the target rod plays a major role to influence the temperature of target rod,and the outermost fuel rod has no effect on it.(5)The expansive coefficient of spray flow density is obtained and applied to 17× 17 fuel assembly,which shows that the maximum temperature can be stabilized near or below the temperature limit.Most fuel rods get more adequate cooling and their maximum temperature below the safety limit.