Study On Molten Salt Infiltration And Mechanical Properties Of Nuclear Graphite Based On The Molten Salt Reactor

Molten salt reactors?MSRs?are a class of nuclear fission reactors where the primary coolant,even the fuel itself,is a molten salt mixture.MSRs have some unique characteristics that offering a safer,good neutron economy,and high thermal efficiency.In the MSR,nuclear graphite is used as reflector and moderator material generally,which is directly contact with molten salt.Given the porous nature of graphite materials,molten salts can readily penetrate into the microstructure in MSR environments,which may change the properties of the graphite,such as the mechanic and thermal properties and increasing the rate of damage.Hence,the permeability of graphite materials has a significant impact on operating characteristics and service life of MSR.Currently,the research and development of nuclear graphite materials is mainly used for high-temperature gas-cooled reactor,and there is no commercial graphite materials used for MSR.Thus the research and development of nuclear graphite and screening out a qualified one for MSR is an indispensable research topic.In 2001,Chinese Academy of Science launched the“Thorium Molten Salt Reactor”?TMSR?project,aiming at developing solid and liquid fueled MSR,which strive for realizing effective Thorium energy utilization and hydrogen production within 20³0 years.In the TMSR,nuclear graphite is used as the reflector and moderator materials,molten 2LiF-BeF₂?FLi Be?salts as primary coolant.Three grades of candidate graphite have been developed for the TMSR:fine-grained graphite NG-CT-10,ultrafine-grained graphite NG-CT-50 and T220.To verify whether the candidate nuclear graphite will be penetrated by molten FLi Be salt,and the amount of the infiltration in TMSR environments is vital importance to the safe operation of the reactor.Therefore,one research focus of this paper is the compatibility of candidate nuclear graphite that used for TMSR with molten FLiBe salt,and the effect of molten salt infiltration on the microstructure of graphite.Including the following three research aspects:1.The pore size distribution and pore structure characteristics of five grades?IG-110,NBG-18,NG-CT-10,NG-CT-50,and T220?of nuclear graphite were investigated by mercury intrusion porosimetry and optical microscopy?OM?.The results showed that there existed concentrated distribution of pore diameter for graphite IG-110,NG-CT-10,NG-CT-50,and T220,and apparent critical infiltration pressure.Nuclear graphite NBG-18 had a wide pore size distribution and low open porosity.Infiltration laws of molten FLiBe salt into graphite under virous pressures were measured using a self-developed molten salt test assembly.The threshold pressure for FLiBe salt infiltration for both two grades of ultrafine-grained graphite NG-CT-50 and T220 was between 600⁷00 kPa,indicating that the two grades may be able to resist salt infiltration in TMSR.The infiltration law of molten FLiBe salt/graphite and mercury/graphite are similar,which means the infiltration pressure of two systems is proportional according to Washburn formula,the coefficient k approximately equal to 2.4.Hence,mercury intrusion porosimetry is an effective alternative method to predict molten salt amount of infiltration.In addition,according to Washburn formula,we can calculate the contact angle of molten FLiBe salt/graphite system,was 135.8°,which was consistent with the experimental data value obtained by Oak Ridge Natinal Laboratory?ORNL?.2.In order to study molten salt infiltration properties in MSR environment,we conducted effects of pre-gas-filled sample vs vacuum sample,temperature,infiltration time duration,and sample sizes on molten FLi Be salt infiltration behaviors of nuclear graphite.The infiltration depth of salt in pre-gas-filled graphite sample was basically the same and shorter than degassed sample under the same differential infiltration pressure,which means that the inner gas of graphite sample can prevent molten salt from infiltration to a certain extent.A simple model was established to calculate the depth of infiltration,the result showed the depth of molten salt infiltration according to model was about 80%the value that obtained by infiltration test.The real pore shape of graphite is quite different from the ideal cylindrical pore shape can lead the major differences between the analysis and reality.The infiltration of five grades of graphite after infiltration tests in different temperatures?600 vs 700?C?and durations?20²000 h?showed no significant difference,which indicated 20 h was adequate to balance the infiltration process.Within the sample size of D60*60 mm,there was no much difference to the weight gain ratios of graphite after infiltration.3.The distribution and morphology of FLiBe salt that penetrated into graphite samples were observed by Scan Electron Microscope?SEM?,Electron Probe Micro Analyzer?EPMA?,and Time of Flight Secondary Ion Mass Spectrometry?TOF-SIMS?.EPMA and TOF-SIMS characterization showed that for the vacuum samples the FLiBe salt was distributed relatively uniformly,indicating the presence of cross-linked networks of open pores throughout the samples.SEM characterization showed that the FLiBe salt existed in graphite sample with a bright white,forming flake-like crystals,which mainly occupied the open pores of graphite materials.X-ray diffraction?XRD?patterns of the as-received and salt-penetrated graphite materials were conducted to analysis the microstructure changes.The shift of the?002?diffraction peak positions to higher 2?angles revealed that d?002?of graphite decreased after the molten salt infiltration.Additionally,a decrease in the full width at half maximum?FWHM?of?002?diffraction peak of the salt-penetrated graphite indicated a decrease of microcrystalline size Lc?002?.In the MSR,nuclear graphite is applied as both neutron-moderator and reflector material,but also as structural material,as well as used as structure material to ensure the structure integrity of reactor internal components.The mechanical properties of the graphite material are the main factors affecting the structure integrity,such as force between graphite components,stress concentration caused by the asymmetric structure and discontinuity,mechanical properties change due to uneven distribution of neutron fluence rate and temperature in reactor.In addition,microstructure of graphite materials will be changed if molten salt infiltration into the internal pores of graphite,which,in turn,definitely affects the stress field distribution of graphite,and likely to affect the macromechanical properties of graphite.Therefore,effect of molten salt infiltration on mechanical properties of graphite materials another significant composition in this paper.A self-developed high temperature vacuum test device with versatile functions is introduced to study effect of molten salt infiltration on high-temperature strength property of graphite material.The result indicated that the high-temperature compressive strength of salt-penetrated graphite samples was decreased when compared to as-received graphite samples.The more the amount of salt infiltration,the weaker the high-temperature compressive strength of the graphite material.The main reasons,on one hand,there exists internal stress caused by asymmetric structure of graphite material itself.On the other hand,internal stress distribution affected as internal pore structure changed by interaction between graphite material and molten salt.The infiltration of molten salt into graphite may change pore structure which to some extent hindered internal stress release of graphite.In the macro,high-temperature compressive strength decreased for salt-penetrated graphite when compared to as-received graphite.