Study On Transition To Thorium-Uranium Cycle And Safety Characteristics In A Small Heavy Water Moderated Molten Salt Reactor

Molten salt reactor is one of the suitable reactors for efficient utilization of thorium resources,which can realize online removal of fission products and online extraction of ²³³Pa because of the liquid fuel form.The heavy water moderated molten salt reactor(HWMSR)is a novel concept for a molten salt reactor proposed recently,which uses liquid fuel and heavy water as moderator,combining the advantages of excellent neutron economy of heavy water reactor and of online reprocessing and atmospheric pressure operation of traditional molten salt reactor.Meanwhile,it avoids a series of nuclear waste management problems caused by regular replacement of graphite in graphite moderated molten salt reactor due to neutron irradiation.With the advantages of low construction cost and high deployment flexibility,small modular reactor is one of the important reactor types in the future nuclear energy development.In view of this,a small HWMSR(S-HWMSR)with a power of 500MWth is proposed in this work.The core design,transition to thorium-uranium fuel cycle and safety characteristics are studied in detail.In order to obtain better breeding ratio(BR)and negative temperature reactivity coefficient(TRC)to ensure the safety of reactor operation,the design objectives and criteria of S-HWMSR are firstly summarized in this paper,and the initial critical search calculation code(CSCC)is developed as the calculation tool.By changing the lattice pitch(P)?(5?24 cm)and the molten salt volume fraction(VF)?(4?28%),the target parameters such as the initial fissile nuclide loading mass,initial conversion ratio(CR)and temperature reactivity coefficient(TRC)are optimized.Considering configuration of breeding and transition,three kinds of starting fuel ²³³U-Th,LEU-Th and TRU-Th(LEU,17.95 wt%²³⁵U/U)are selected,for comparison to obtain a better initial thorium-uranium fuel cycle performance.The results show that the TRC of ²³³U-Th,LEU-Th fuels are negative in the selected range of Ps and VFs.Meanwhile,when P and VF are20 cm and 20%respectively,better initial loading mass and CR and can be obtained.For TRU fuel,a smaller P(?5cm)and a larger VF(?24%)core is recommended to obtain negative TRC.In addition,the thickness of Si C and heat insulation should be less than 2 mm and 7 mm,respectively,in order to obtain a better breeding performance.The ex-core transition to Th-U fuel cycle requires existing fissile nuclides as the starting fuel,and meanwhile ²³³Pa is extracted online outside the reactor and then decays into ²³³U(t_1/2=27 day)until enough ²³³U is accumulated to start the new reactor.The ex-core transition is an effective way to address the issue of the lacking of the ²³³U in nature since only a small amount of ²³³U is consumed in the core during operation.However,in the existing ex-core transition scheme,using the same fuel as the starting fuel and feeding fuel(such as LEU)would inevitably lead to the increase of heavy nuclide concentration,which limits the core operating time(to avoid excessive increase in heavy metal nuclide concentration),since the increase of heavy nuclide concentration is detrimental to the chemical stability of fuel salt and subsequently endangers the safety of core operation.In this work,an improved ex-core transition scheme is proposed to maintain heavy metal concentration unchanged in the core,which includes two stages.In the first stage,the transuranuric(TRU)nuclides extracted from the spent fuel of light water reactors(LWRs)are added online to maintain core criticality with LEU as the starting fuel,and ²³³Pa is extracted online at the same time.To ensure a negative TRC and a certain safety margin,the first stage operation time is set as 300 days.In the second stage,the ²³³U obtained from the first stage is mixed with TRU as feeding fuel.The results show that the mixed fuel can maintain the core criticality and safe operation with negative TRC,and can realize transition to Th-U fuel cycle and breed ²³³U.The TRC value can be further improved by increasing ²³³U fraction in the mixed fuel.When the fraction of ²³³U in the mixed fuel is 15 mol%,it will take only three years to accumulate enough ²³³U to start a new reactor.After 60 years of operation,about 11,512kg TRU can be added,3,830 kg TRU remains in the reactor core,and 7,680 kg TRU is consumed.The radiotoxicity of the added TRU fuel can be reduced by 11%.The steady-state and transient characteristics of reactor core are the important basis for evaluating the safety of the core operation.Based on the unique core structure of S-HWMSR,a neutronic-thermohydraulic coupling program is developed firstly.Then the steady-state characteristics of the core are analyzed in terms of insulation thickness,heavy water flow velocity,neutron flux distribution,molten salt temperature distribution and heavy water temperature distribution.The analysis results show that when the insulation thickness is reduced from 3 mm to 1 mm,the temperature of heavy water increases significantly.Considering the effect of the insulation thickness on the breeding performance,it is recommended to choose 3 mm as the insulation thickness.Under this thickness,even when the velocity of heavy water drops from 0.6 m/s to 0.02m/s,the temperature of heavy water rises from 61.5?to 90?which is still below the boiling point and is still therefore within the safe range.At the same time,the maximum outlet temperature of fuel salt is 667?,lower than the design upper limit of 700?.Meanwhile,several typical transient accident conditions,such as temperature driven by molten salt inlet,speed driven by heavy water and molten salt,are analyzed.In the transient driven by fuel salt inlet temperature and flow velocity,the heavy water temperature,fuel salt temperature and power are all within the safe range.For the transient driven by heavy water velocity,the change of heavy water temperature is mainly caused by the change of heavy water velocity due to the small change of reactor core power,but the temperature of heavy water is within the safe range.The above analysis results show that the designed core meets the safety design requirements.