Neutronic and Fuel Cycle Consideration: from Single Stream to Two Fluid Th-U Molten Salt System. Olga S. Feinberg

Transcription

1 Neutronic and Fuel Cycle Consideration: from Single Stream to Two Fluid Th-U Molten Salt System. Olga S. Feinberg RRC-Kurchatov Institute, , Moscow, RF

2 The History of the Problem In the 60 s and 70 s the design of a 1000 MWe molten salt breeder reactor (MSBR) with a core graphite moderator, thermal spectrum and thorium-uranium fuel cycle was designed. Two main configurations were developed at ORNL: (1) single stream and (2) two-fluid. From the point of view of fuel cycle characteristics two-fluid fuel circuit configuration where fertile and fissile materials are dissolved in separate streams, has some advantages in comparison with single stream systems where fissile and fertile are components of one molten salt mixture: two-fluid system will have maximal meaning of breeding ratio in the blanket minimal fuel loading in the central core part system can have attractive meanings of temperature reactivity coefficients fuel moves out of the core due heating and at the same time amount of thorium in blanket is constant. Only construction difficulties connected with heterogeneous structure of such kind breeder limited its development. In both configurations (two and one-fluid), salt(s) served as fuel and blanket fluid(s) at temperatures 700 o C. The technical feasibility of such systems now does not raise the doubts.

3 MOlten Salt Actinide Recycler & Transmuter (MOSART) In Russia, the molten salt program was started in the second half of 1970 th. These studies were directed on the improvement of two and one-fluid MSBR type concepts in two main reactor geometries heterogeneous and homogenous. Within recent ISTC#1606 project main focus was placed on experimental and theoretical evaluation of single stream MOlten Salt Actinide Recycler & Transmuter (MOSART) system. It was shown that optimum spectrum for MOSART is intermediate/fast spectrum of homogeneous core without graphite moderator. Promising configuration for 2400 MWt MOSART is the homogeneous cylindrical core (3.6 m high and 3.4 m in diameter) with 0.2 m graphite reflector filled by 100 % of molten 15LiF- 58NaF-27BeF 2 or 73LiF-27BeF 2 salt mixture. ISTC#1606 It is feasible to design critical homogeneous core fuelled only by transuranium elements (TRU) trifluorides from UOX and MOX LWR spent fuel while equilibrium concentration for trifluorides of actinides (about 1 mole% for the rare earth removal cycle 300 epdf) is truly below solubility limit at minimal fuel salt temperature in primary circuit 600 o C. MOSART has maximum capacity and high enough transmutation efficiency. The fraction required of MOSART units in nuclear power system is about 25%.

4 Molten Salt Th-U Breeder (MSFR) Recent molten salt Th-U breeder developments in Europe (CNRS,France) also moved to advanced large power unit without graphite in the core and fast neutron spectrum (MSFR). For MSFR as solvent system for fuel and blanket circuits it is offered to consider molten 78LiF-22ThF 4 (T m =565 o С). This solvent has essentially higher melting temperature and solubility for actinide trifluorides, in comparison with well established 72LiF-16BeF 2-12ThF 4 melt (T m =504 o С). MSFR has all positive features of the homogeneous molten salt reactor without graphite: large negative temperature reactivity coefficients and strongly reduced reprocessing rates. Compared to MSBR basic difficulty of MSFR is that it requires essentially higher starting loadings of fissile materials (near 5 tons of UF 4 or 11 tons of TRUF 3 ) and fuel concentrations for criticality. If MSFR is started with 233 U it can be realized on the base of existing materials. But if it is started with TRU from PWR spent fuel because of limits on solubility in molten 78LiF-16ThF 4-6.5TRUF 3 mixture it can be realized only at the minimum temperature in fuel circuit >700 0 С (maximum core temperature of fuel salt С). That will demand development of a new constructional material for MSFR fuel circuit since developed for MSBR and MOSART Ni-Mo alloys of Hastelloy N type are certified only for temperatures <750 o C.

5 Hybrid MOSART System Unification of MOSART with Th containing molten salt blanket and thus transition to homogeneous two-fluid system can provide effective production of required for MSFR starting uranium loading. In this scheme hybrid MOSART will work in conversion mode TRU- 233 U. From MOSART blanket uranium processed by volatility process can be removed for MSFR loading, but thorium bearing salt is returned to the blanket. From the point of view of structure such symbiosis MFSR + hybrid MOSART can have advantage, since allows to use in a greater degree already available technology at temperatures to С. Symbiosis MFSR + hybrid MOSART will reflect the main feature of molten salt reactors high flexibility of their fuel cycle. It can work with wide range of fuel loadings without design changing and thus can be included in any scenario of nuclear development. Beside these it can be realized on the base of existing technologies within technological margins. Our work was devoted to the calculation investigation of MOSARTtype transmutor abilities as the system for TRU- 233 U conversion.

6 System Description There is, of course, not one possible arrangement of hybrid MOSART. We considered two-fluid homogeneous concept without fertile elements in fuel salt with two basic configurations where core with blanket fill right circular cylinder. Two-region with simple cylinder core Three region with ring core Blanket salt Fuel salt Blanket salt Fuel salt In two-region configuration cores from 2 m up to 3.6 m in diameter and from 2m up to 3.6m height are considered (variants V1-V6). In three-region configuration cores from 1.1 to 0.9 m width with 3.2 m height (V7-V9) are investigated. In main variants width of external blanket was 0.6m. The fuel circuit in its external part was very similar to that of reference MOSART design with 18 m 3 (V1-V3) and 10m 3 (V4-V9) salt volume out of the core. Radial, bottom, and top graphite reflectors of 0.2 m width were attached to the reactor vessel. Ni-Mo alloys of Hastelloy N type were used as construction materials.

7 System Description We compared two basic core salts: 17LiF-58NaF-25BeF 2 (mole percent) with a melting point of 479 C 73LiF-27BeF 2 with a melting point of about 560 C The specific salts were chosen because of their high solubility (up to 3 mole % at 600 C) for actinide and lanthanide trifluorides a requirement for only TRU loading. In our investigations we assumed that solubility limit must not exceed 2.5 mole % taking into account calculation and experimental uncertainties. The fuel salt processing system removes the soluble fission products with an average residence time in the reactor of 300 effective full-power days (efpd). The blanket salt consists of eutectic of LiF and ThF 4 or mixtures of it with additions of beryllium difluoride (e.g. 75LiF-5BeF 2-20ThF 4 ). The melting point of the blanket salt is 565C or lower. The uranium removal time is 15 efpd At nominal conditions, the fuel and blanket salts enter the reactor vessel at 600 C and transport 2400MWt to the secondary salt in the primary heat exchanger, four in parallel for the core and two in parallel for the blanket. In calculations we assumed minimum external volume for the fuel salt 9-11m 3.

8 Calculation Tools Goals of Parametric Study MCNP-4B+ORIGEN2.1 with ENDF/B-5,6 library was chosen as the tool for calculation investigations. This calculation tool was adapted to the specific features of the molten salt transmutor within 1606 MNTC project and here was only added by the option which gives the opportunity to solve the thorium containing multizone problems. In our study we optimized two main parameters of the hybrid system: equilibrium loading of TRU in the central fuel zone rates of 233 U production in the blanket At the same time we tried to realize design accounting for technological constraints, to use only well-known technical solutions and existing equipment (heat exchangers, pumps etc.) and materials with measured characteristics.

9 Critical concentrations of TRU in Li,Na,Be/F and rates of 233 U production in the blanket vs. time for the cores with different configurations Parametric Study Results Critical concentrations of TRU in salt and rates of 233 U production in the blanket vs. time for the cores with different salt solvent V1,V2,V3 - the cores with dimensions of MOSART, V4 reduced cores, V7- ring core. V4,V6 simple cylindrical zones with Li,Na,Be/F and 73LiF-27BeF 2 correspondingly ; V7,V8 ring cores with Li,Na,Be/F and 73LiF-27BeF 2 ;

10 Calculations Results (Simple Cylindrical Zones) In two-zone MOSART core filled with 15LiF-58NaF-27BeF 2 (height 3.6 m, diameter 3.4 m) the equilibrium concentration of TRU is not higher than 1 mol.% (var. V1-V3). In the variants with MOSART core the geometry and material of blanket don t influence the loading of the central part. Production of 233 U can be increased up to 115 kg/year by rising of the blanket width up to optimal cm. The increasing of ThF 4 part in the molten salt (higher than 16-20%) of blanket has no significant influence on the 233 U production rate. In the variant with the decreased dimensions (diameter 2.4m, var.v4) the equilibrium concentrations of actinides and lantanides trifluorides raise from 1mol.% to 2 mol.% due to more hard spectrum in the core. 233 U production rate in the blanket becomes higher up to 180 kg/year. Further decreasing of the core diameter up to 2m (V5) elevates the equilibrium consentrations of the actinides and lantanides trifluorides so that they reach 2.5 mol.%. 233 U production rate in this variant is near 200kg/year. Exchange of 15LiF-58NaF-27BeF 2 (V4) salt solvent on 73LiF-27BeF 2 (V6) in the core of two-zone system permits to reduce TRU loading on 50%, but this leads to some decreasing of 233 U production rate in fertile salt from 180 to 150 kg/year.

11 Calculations Results (Ring Cores) Transition to ring geometry of the core (V7) gives the opportunity to raise 233 U production rate in fertile salt up to 254 kg/year but equilibrium concentration of actinide and lantanide trifluorides reaches it s limiting value for involved salts - 2.5mol.%. The initial and equilibrium loadings of TRUs in the fuel salt move up respectively. Exchange of 15LiF-58NaF-27BeF 2 (V7) on 73LiF-27BeF 2 (V8) molten salt solvent in the ring cores leads to simultaneous relief of equilibrium concentrations of actinides and lantanides trifluorides in the central zone ( from 2.5mol.% to 1.4 mol.%) and 233 U production rate in the blanket (from 254 kg/year to 241kg/year). For the cost of additional fuel salt specific power escalating (decreasing ring width) it is possible to improve 233 U production rate in the blanket of three-zone ring core up to 286kg/year (for the core with the central hole with radius 0.7m). At the same time equilibrium concentrations of TRU are still below 2.1mol.%.

12 Conclusions and Future Work The variants with ring geometry of the core on the base of 73LiF- 27BeF 2 molten salt solvent afford broad capabilities for the rates of new fuel production and for fuel salt specific power choice and correspondingly possible construction materials of the core. The best solution will be the compromise between these factors. For accumulation of necessary loading for MSFR (5200 kg) years of transmutor system work are needed. At the same time there are other possibilities of produced 233 U use inside two-fluid system itself. In this case not only UOX and MOX PWR spent fuel can be used for first loading and feeding but some other more hard types corresponding to different scenarios of nuclear energy development.these possibilities will be discussed in later works. Within MOSART-type reactors line the main advantage of MSR reactors can be demonstrated as systems with very flexible fuel cycle which can work with different kinds of fuels without essential changes in the design and be not only the TRU-burners, but the converters and breeders.