Study On The Behavior Of The Pebble Bed In Pebble Bed Fluoride-salt-cooled High Temperature Reactor

The PB-FHR,which combines the advantages of HTGR(High Temperature Gas Cooled Reactor)and MSR(Molten Salt Reactor),is one of the six candidates for the Generation IV nuclear power plants.Low pressure,liquid coolant and large fuel thermal margins are significant advantages of the technology.The bed structure in molten salt cooled pebble bed reactor may bring uncertainty to the reactor due to the stochastic nature of the pebble bed.So far there is little information on the behavior of pebble bed in process of reactor operation.It is of importance to investigate pebble bed behavior through experimental work and computer simulations.The work completed includes analysis of pebble packing data from scaled experiments based on TMSR-SF1(Thorium-based Molten Salt Reactor with Solid Fuel)and DEM(Discrete Element Method).Model scale experiments are adopted in this study since it is difficult to perform the dynamic experiments by utilizing full-scaled experiments with molten salt and fuel pebbles.Water and polypropylene pebbles were used to simulate the molten salt and fuel pebbles.Similarity analysis is applied according to the experiments.The PRED(Pebble Recirculation Experiment Device)and the PBDE(Pebble Bed Dense Experiment facility),which were mock ups for TMSR-SF1 were constructed.The methods for pebble loading and unloading in fluid have been proposed and proved feasible,that is of great important for the reactor design.Results from experiments under normal operation conditions show that:(1)the average PF(packing factor)of TMSR-SF1(D=21d,H=30d)is supposed to be about 0.5849 in cylindrical zone,while 0.5745±0.0027 in whole,which is much less than 0.60 in dry experiment while the wall effect is same.(2)Void near the wall of the conical reflector is one of the main reasons of small PF.In order to avoid the uncertainty of PF induced by the geometry of cone reflector,the angle of the cone reflector in reactor core is better to be 45° or larger.Beside,low recovery coefficient and less acceleration in buoyancy field also contribut to the small PF.(3)The loading rate of pebbles and the velocity of inlet flow have effect on the bottom shape of the pebble bed.The PF increases with the diameter and the packing height of the reactor core.(4)The structure of the pebble bed remains stable during normal operation.Results from experiments under accident conditions show that:(1)Vortexes caused by increase of flow velocity lead to instability of pebble bed.(2)The rearrangement of pebble bed occurred after liquid level drop and subsequent starting over of pump.(3)The pebble bed will keep stable if the vibration is not strong enough(S?0.03)to break the support between pebbles.But it will change rapidly from randomly loose pack to regular close pack if the vibration is strong(S=0.18).The packing factor will increase 5% in 5min under vibration,which leads about 2500 pcm increasing of reactivity.The vertical vibration which occurs first in earthquake has less impact on pebble bed than horizontal vibration.The pebble bed behavior that investigated in this paper provides the basis phenomenological analysis for reactor design.Moreover,the running of PRED demonstrates the feasibility of the methods for pebble loading and defueling in liquid salted cooled pebble bed reactors.It is worth mentioning that the scaling analysis should be developed,and the molten salt and graphite pebbles should be used in the further study to attain more exact result.In addition,more advanced simulation work on coupled pebble and fluid dynamics is required.