Investigation On Thorium Used In Pebble-Bed Fluoride Salt-cooled High Temperature Reactor

Nuclear energy provides important means for solving climatic change,environmental pollution and energy crisis. However, nuclear reactor itself has to facethe problems of sustainable development. Accordding the present consumption rate ofuranium, nuclear reactor can only last for100years. Thorium is a kind of potentialnuclear fuel and is three to four times more abundant in the earth’s crust than uranium.The cost of thorium mining is cheap. If thorium can be used in nuclear reactor on alarge scale, it will promote the sustainable development of nuclear reactor.Lots of thorium investigations have been done in PWRs, HWRs, SCWRs,HTGRs and MSRs, but thorium is still not utilized in commercial reactor becausehigh burnup of thorium (>80MWd/kgHM) is required to improve the fuel utilizationrate while materials used in conventional PWRs and HWRs cannot meet therequirement. SCWRs, HTGRs and MSRs belong to the G-IV reactors and are in astate of research and development. The discharge burnup of thorium-uraniumbreeding in SCWRs is about40MWd/kgHM, the economy of thorium-uraniumclosed cycle in SCWRs is yet to be assessed. MSRs is the best candidate reactor forthorium-uranium breeding, but many technical issues such as reprocessing techniqueneed to be settled. HTGRs display high burnup ability, and are suitable to utilizethorium fuel, however, further work need to be done.PB-FHR (Pebble Bed Fluoride molten salt-cooled High temperature Reactor) isan advanced and innovative reactor based on the H9G7s’ technology of coated fuelparticles (high temperature resistance and high burnup), and molten salt coolant (lowpressure, high melting point, high boiling point and high heat capacity). Many provedtechnologies, such as, passive residual heat removal system in liquid metal fast reactor,and brayton cycle in advanced coal-fired power plant can be applied in PB-FHR,which shows high commercial feasibility. There are some design and safetycharacteristics in PH-FHR, such as,(1) flexible reactor scale. Both50MWe smallmodular reactor and GWe level large reactor are allowable;(2) high outlettemperature, high power density (20-30MW/m3) and high thermal efficiency;(3) lowoperating pressure reduces the release probability of radioactive substances;(4) highsafety level with controllable accident. PB-FHR could provide high neutron utilization and high burnup ability and has potential advantage of thorium fuelutilization.In this paper, we introduce the thorium fuel utilization in PB-FHR. The contentinvolves:(1) development of burnup code. In view of the flow behavior of fuel inPB-FHR, we develop a general burnup code MOBAT and an equilibrium burnup codeof pebble bed reactor (PBRE), which coupled with MCNP5and ORIGEN2;(2)thorium utilization in PB-FHR with open cycle to save uranium fuel;(3)thorium-uranium breeding in PB-FHR using dispersion fuel particles with closedcycle to find breeding region and breeding ability and neutron properties of Flibe.In the development of burnup code part, we introduce:(1) the coupled principle,coupled mode and accuracy analysis of a general burnup code MOBAT. Results showthat MOBAT could achieve the same accuracy with SCALE and MCNPX, and issuitable for burnup calculation of PB-FHR;(2) the coupled principle, coupled mode,convergence method and accuracy analysis of PBRE. PBRE uses one-loop-positive-correlation method to save calculation time, and the accuracy of PBRE has beenverified in a HTR-10model. Besides, PBRE has the function to searchthorium-uranium sustainability state.In the part of thorium utilization with open cycle, we introduce:(1) neutronicdesign in unit cell model, including thorium-uranium homogeneity fuel unit cellmodel, thorium-uranium heterogeneity unit cell model and thorium-pebble-uranium-pebble unit cell model. Results show that more uranium fuel is needed inthorium-uranium homogeneity fuel unit cell model,6.8%uranium fuel could be savedin thorium-uranium heterogeneity unit cell model and20%uranium fuel could besaved in thorium-pebble-uranium-pebble unit cell model.(2) Neutronic design in coremodel, including thorium-region-uranium-region core model, thorium-pebble-uranium-pebble-mixing core model and only-uranium-pebble core model.20%uranium fuel can be saved in thorium-region-uranium-region core model but theradial power peak factor is about1.8;10%uranium fuel can be saved inthorium-pebble-uranium-pebble-mixing core model and the radial power peak factoris about1.48; only-uranium-pebble core model is used for reference, the burnup ofuranium is about220MWd/kgU.(3) thermal-hydraulic analysis in thorium-region-uranium-region core model. For the limit of1300°C fuel temperature, if thediameter of pebble is3cm, the core average power density can reach22.5MW/m3, if the diameter of pebble is6cm, the core average power density is only about6MW/m3.For the limit of2.E-4TRISO failure probability, the kernel radius in uranium pebbleshould be lower than250μm to obtain the burnup of180MWd/kgU, the burnup ofthorium pebble is recommended as100MWd/kgHM.In the part of thorium-uranium breeding with closed cycle, we introduce:(1) theoption of fuel element. The TRISO-based PB-FHR is not suitable forthorium-uranium breeding due to the low fuel loading (not beyond5vol%) and strongneuron slowing down power. While dispersion fuel has a50vol%fuel loading abilityand the SiC matix or cladding in dispersion fuel element has a weak neutron slowingdown power.(2) thorium-uranium sustainability and neutronic properties of Flibe.Results show that it can breed and keep negative void coefficient during20%-60%fuel volume ratio in pebble. The maximum discharge burnup is more than200MWd/kgHM. Power density has an important effect on the discharge burnup ofdispersion-based PB-FHR, and30MWd/kgHM drop could happen if the powerdensity increases from10to30MW/m3. Besides, we found that fluoride molten salthas different neutronic properties between fast spectrum and thermal spectrum. In fastspectrum, the neutron absorption rate of Li-6and Li-7is low,99.95%enriched Li-7isallowed, and in the equilibrium state, Li-6can reach1000pcm. In fast spectrum, theproduction rate of H-3is only30-40g/GW/year, far lower than that in thermalspectrum.(3) thermal-hydraulic analysis. For6cm pebble, the core power density islower than20MW/m3; for3cm pebble, the core power density can be60MW/m3. Thebig challenge for dispersion-based PB-FHR may be the long irradiation time,therefore, power density should increase and discharge burnup will decrease. Besides,further investigation need to be performed in fuel manufacture and post-processing.