Study On The Establishment Of Thorium Fuel Cycle By Using Enriched Uranium And Plutonium In Molten Salt Reactor

As one of the six candidates for advanced Generation IV reactors,thorium molten salt reactor?TMSR?nuclear energy system,has remarkable advantages in safety,economics,fuel utilization efficiency and proliferation resistance.It is important to produce available ²³³U for startup of a molten salt reactor?MSR?before it can be developed and deployed to achieve large-scale utilization of thorium fuel.By employing the currently available starter fuels,i.e.,enriched uranium and plutonium,the transition scenarios from uranium/plutonium fuel cycle to thorium fuel cycle in MSR are established in this work with the purpose of providing several feasible solutions to the problem of ²³³U shortage.Effects of the starter fuel composition,feeding modes and reprocessing options on the transition behaviors are discussed in detail.The method for a single-fluid,two-zone MSR burnup calculation is developed based on SCALE/TRITON code in this work.SCALE/TRITON is an integrated module used for criticality calculation,cross-sections processing and depletion calculation.Currently,TRITON only supports multigroup depletion calculations.Duplicate definitions of the fuel salt are required in SCALE/TRITON to specify the unit cells in zone 1 and zone 2 when performing the burnup calculation of the MSR with a two-zone core and consequently,two separate one-group cross-section libraries are generated for each zone.Then Origen-s reads the separate cross sections and performs burnup calculation respectively for zone 1 and zone 2,thus leading a heterogeneous distributed system and obviously,it is unsuitable for the MSR whose fuel is well-distributed in the core.In order to solve this problem and further expand the functions of the SCALE/TRITON,three methods,i.e.,the homogeneous mixing method,equivalent volume method and average cross section method,are developed by using external program.Comparison of the three methods is conducted and the results indicate that the average cross section method is advantageous over the other two methods in computational efficiency and accuracy;The homogeneous mixing method has low computation efficiency although its results are almost consistent with those obtained from the average cross section.However,it is expected to be useful in the MSRs with multi-zone and complex flow behaviors.Moreover,the result also shows that the equivalent volume method is not sufficiently accurate to describe the difference of spectra in zone 1 and zone 2.The study on the MSR startup and achieving the fuel transition to thorium fuel cycle by using enriched uranium is conducted.Two scenarios,the breeding and burning?B&B?scenario and the pre-breeding and burning?PB&B?scenario are both put forward and discussed.In the former scenario,Four uranium enrichment levels,20%,40%,60% and 93%,are chosen as the starter fuels.During the operation,²³³U produced from the decay of the extracted 233 Pa is fed into the reactor and the excess ²³³U is stored outside the core.The results of the B&B scenario indicate that the fuel transition(²³⁵U^enri/²³²Th?²³³U/²³²Th)can potentially be accomplished with 20% enriched uranium but takes a long double time of about 79 years and meanwhile,the fuel transition can be realized smoothly and relatively quickly if the startup fuel with ²³⁵U enrichment greater than 40%.In latter case,the double time is within 40 years.Additionally,the PB&B scenario in which the criticality of the core is maintained by on-line adding of enriched uranium rather than ²³³U produced by itself is analyzed and one can find that the PB&B scenario can produce ²³³U effectively.Finally,a comparison of these two scenarios is conducted,which indicates that the B&B scenario can significantly enhance resource utilization efficiency owing to the depletion of the bred ²³³U.The transition from startup with Pu to thorium fuel cycle in MSR has also been studied in this work.Similarly,the B&B scenario and PB&B scenario based on Pu are employed.The effect of reprocessing rate is analyzed in both scenarios.It can be concluded from the B&B scenario that the transition(Pu/²³²Th?²³³U/²³²Th)can be accomplished by employing a relatively fast fuel reprocessing with a cycle time less than 60 days.At the equilibrium state,the reactor can achieve a conversion ratio of about 1.047 for the 10-day reprocessing period?RP?case and about 0.996 for the 60-day RP case.The results also show that the above fuel transition can not be accomplished with limited reprocessing?RP =180 days?and in such a case,external Pu would have to be fed periodically into the core to maintain criticality of the core and the reactor operates as a converter.Similarly to the enriched uranium,the PB&B scenario based on Pu is efficient to produce ²³³U.This thesis has developed a coupling module for two-zone MSR burnup calculation based on SCALE/TRITON code and also established some possible transition scenarios based on enriched uranium and plutonium in order to give the solutions to MSR startup and the fuel transition to thorium fuel cycle.Evolutions of the inventories of main nuclides,the performances of the core and the on-line reprocessing options during the transition are all analysed systematically.The work that has been carried out can provide some essential reference for developing the thorium fuel cycle strategy in MSRs.