Production Of Radionuclide And Safety Analysis Of TRISO Coated Particle

The concept of solid fuel molten salt reactor was first proposed by Americanscientists in the beginning of this century, it uses fluoride molten salt as coolant andthe graphite as moderator, the spherical fuel element made of tri-structuraliso-tropic(TRISO) coated particles as fuel. The spherical fuel element contains fueland non-fuel region, the fuel region is formed by TRISO coated fuel particlesuniformly dispersing in the graphite matrix, and the non-fuel region is formed bygraphite matrix material. The TRISO coated particle consists of fuel kernel and fourcoated layers(porous carbon buffer, inner pyrolytic carbon, silicon carbide, outerpyrolytic carbon).Although the four coated layers make the coated particles muchstronger, the particles can be damaged if the operating environment of the reactorbeyond the range of that the coated particles can bear. Pressure shell-damaged causedby the gases accumulation is the main failure mechanism of TRISO coated particles.The study found that the gases generated in the coated fuel particles consist of fissiongases(xenon and krypton), CO and CO2.Experiments show that when the temperatureis at950℃and the burnup is less than15%FIMA,the pressure induced by the fissiongases is about4times of that by CO and CO2,and the lower temperature and burnupgive less production of CO and CO2.Therefore,the investigation of production and theformation of xenon and krypton fission gases is very important for the safe operationof the coated fuel particles.In the first part of the thesis, the ORIGEN2code was applied to study thevariation of the xenon and krypton fission gases with neutron spectrum and flux basedon thermal reactors in TRISO Th/U coated fuel particles. The results show that for theUO2coated particles, the production of xenon is about7times of that of krypton withthe same neutron spectrum and at the flux and running time, and the production ofxenon and krypton increases linearly with the operation time in the thermal neutronspectrum and higher neutron flux, which reason is that the precursor of xenon andkrypton reacting with the neutron. For the ThO2coated particles, the production of xenon is about4.5times of that of krypton at the same condition of UO2coatedparticles.In addition, xenon and krypton tend to be saturated and the saturation valueare concerned with the neutron spectrum and flux, and the fission gases reachsaturation earlier when the neutron spectrum are softer and the flux are higher, and thesaturation values are smaller compared with those in the harder spectrum and lowerflux environment. Taking the fission gases accumulation as the main factor of TRISOcoated particles, the calculation implies that the ThO2coated particles can have alonger lifetime than the UO2coated particles under the same neutron environment.In the other part of the thesis, we use MCNPX code to study and calculate theformation of the important radionuclide and the fission gases generated from the Th/Uspherical fuel element, which were located in different positions along the radial andaxial of the2MW TMSR-SF1(Thorium Molten Salt Reactor-Solid Fuel1)core, andanalyze the utilization efficiency of the thorium in the thorium spherical fuel element.The results show that with the same running time. In the center of2MW TMSR-SF1core, the production of xenon is about6times of that of krypton in the uraniumspherical fuel element, and about3.7times in the thorium spherical fuel element. Thetrend of fission gases production with the time in Th/U spherical fuel element by theMCNPX code is basically the same with that results in the condition of the hardspectrum (PWRU and BWRU) of the ORIGEN2code. Moreover, the calculationsshow that the production and formation of the fission gases of the Th/U spherical fuelelement in the MCNPX model of the reactor are closest to the results of theBWRU.LIB spectrum of the ORIGEN2code. At last, we have found that the Th/Uspherical fuel element located in the top and bottom and the outer ring of the corehave higher safety factor, but have lower utilization efficiency of the thorium in thethorium spherical fuel element at the corresponding positions.