The in situ breeding and burning reactor (ISBBR), which makes use of the outstanding breeding capability of metallic pellet and the excellent irradiation-resistant performance of SiCf/SiC ceramic composites cladding, can approach the design purpose of ultralong cycle and ultrahigh burnup and maintain stable radial power distribution during the cycle life without refueling and shuffling. Since the characteristics of the fuel pellet and cladding are different from the traditional fuel rod of ceramic pellet and metallic cladding, the multiphysics behaviors in ISBBR are also quite different. A computer code, named TANG, to model the specific multiphysics behaviors in ISBBR has been developed. The primary calculation results provided by TANG demonstrate that ISBBR has an excellent comprehensive performance of GEN-IV and a great development potential.
After 60 years of development and deployment, nuclear energy has become one of the three main energy sources supporting human society. Although it happened to three major nuclear accidents during past decades and projected a big shadow on the nuclear energy development, especially the Fukushima Nuclear Event, occurred in March, 2011, significantly twisted people’s understanding to nuclear safety, but, people still could not stop 1 the prospects on nuclear energy, as the fossil energy on earth is gradually drying up and increasingly expensive, and the other renewable energy is not enough stable and reliable. By now, except for a few European countries, who claim to terminate their nuclear power projects, the main energy consumers of the world, including China and the United States, have clearly declared their positive position to their established nuclear power route, and several new projects just have been approved during the past recent years.
However, it should be pointed out that the existing nuclear power technology could not support the sustainable nuclear energy development for long-term prospects. The reason is that the existing nuclear power technology is mainly based on water-cooled reactor. It is well known that the fuel utilization of water-cooled reactor is less than 1%, which means that the existing water-cooled reactors are quickly consuming the limited natural uranium resource and producing the huge volume of depleted uranium in front end and the high radioactive spent fuel in back end. According to the 2009 edition of the IAEA red book [
In fact, the international society has long known the limitation of the existing nuclear energy technology and has been actively looking for effective ways to pursue the sustainability for nuclear development. Among them, a general consensus is that fuel breeding and closed fuel cycle are the crucial options to realize sustainable nuclear energy development. As is well known, fast reactor or harden spectrum is the necessary condition to achieve fuel breeding for current uranium-plutonium fuel cycle, whereas it seems a little optimistic if we think that the existing fast reactor technology is just the solution to the sustainable issue. Taking the existing sodium-cooled fast reactor (SFR) adopting oxide fuel as an example, the one year or so of the refueling cycle brings not only heavy burden of reprocessing and a significant increase in the fuel cycle cost, but also the 20 to 25 years of doubling time seems to give people a choking sense. In recent years, Terra Power LLC, which is founded by Bill Gates, proposed an innovative concept of traveling wave reactor (TWR), which is based on the platform of pool-type sodium-cooled fast reactor by using metallic fuel pellet and HT9 cladding; TWR can approach 30~40 years of ultralong cycle life and around 30 at% of ultrahigh burnup without refueling, but with periodical fuel shuffling during the cycle life. Theoretically, the TWR can well satisfy the need of sustainable nuclear power development, whereas, because the TWR core is composed of igniting region of medium-enriched uranium and blanket region of depleted uranium, the core radial power distribution shall become severely heterogeneous and have significant variation during the cycle life, and also the ultrahigh burnup shall pose a big challenge to the dose limitation of the HT9 cladding. Therefore, there are still a series of questions for TWR’s engineering implementation.
Based on the above understanding, this paper proposed an innovative concept of in situ breeding and burning reactor (ISBBR), which is based on the platform of traditional sodium-cooled fast reactor (SFR), and can approach ultralong cycle and ultrahigh burnup and maintain stable radial power distribution during the cycle life without refueling and shuffling. A computer code TANG modeling main multiphysics phenomenon in ISBBR has been developed for the core design balance. The primary calculation results provided by TANG demonstrate that ISBBR has an excellent comprehensive performance of GEN-IV and a great development potential.
Obviously, the prerequisites to approach in situ breeding and burning are that the fuel should have great breeding capability and the fuel reactivity should change very slowly during the cycle life. Figures
Reactivity versus burnup for typical fuel materials.
Normalized 239Pu versus burnup for typical fuel materials.
In addition, in order to ensure the integrity of the fuel rod under the conditions of ultralong cycle and ultrahigh burnup, the ISBBR’s structure material, especially the fuel cladding material, should have outstanding radiation resistant performance besides the better heat conduction capability and mechanical performance. The cladding material of the traditional sodium-cooled fast reactor (SFR) is Austenite or Martensite Stainless Steel, whose radiation-resistant performance is not enough promising. Some new structure materials under developing, such as ODS and HT9, are predicted to have good radiation-resistant capability; however, it is difficult to have a revolutionary solution because these materials are still iron based. Recently, an innovative ceramic composite material SiCf/SiC has been causing more and more attention from the nuclear field because of its comprehensive performance of heat conduction, mechanical properties, and radiation resistant. It is said that SiCf/SiC is the most promising for the first wall material in fusion reactor [
DPA cross section for stainless steel and SiCf/SiC.
DPA dose for stainless steel and SiCf/SiC.
In the view of the above analysis, our proposed in situ breeding and burning reactor (ISBBR) shall select the ternary alloy of Uranium-Plutonium-Zirconium as fuel pellet, SiCf/SiC as cladding material, and liquid sodium as coolant. Table
General parameters for reference core of ISBBR.
Thermal power, MWt | 800 |
Electricity power, MWe | 300 |
Coolant flow rate, kg/sec | 5000 |
Inlet temperature, °C | 350 |
System pressure, MPa | 0.1 |
Number of fuel assembly | 222 |
Number of control assembly | 31 |
Number of shielding assembly | 270 |
Flat to flat distance of FA, cm | ~12.5 |
Number of fuel pin in a FA | 60 |
Fuel rod pitch, cm | 1.50 |
Fuel rod diameter, cm | 1.40 |
Active fuel height, cm | 200 |
Plenum height, cm | 200 |
Cladding material | SiCf/SiC |
Pellet material | UPuZr |
Coolant material | Sodium |
Control absorber material | B4C |
Detail size | Reference Figure |
Heavy metal inventory in core, ton | 40.4 |
DU (0.3 w/o 235U), ton | 35.8 |
Reactor grade Pu, ton | 4.6 |
Composition of Pu: | |
238Pu (%) | 3.54 |
239Pu (%) | 50.94 |
240Pu (%) | 22.99 |
241Pu (%) | 15.15 |
242Pu (%) | 7.38 |
Schematic for the reference core of in situ breeding and burning reactor.
The reference core is a small modular reactor. The rated thermal power is 800 MWt, the rated mass flow rate is 5000 kg/Sec, and the inlet coolant temperature is 350°C. The core is composed of 222 fuel assemblies, 30 control assemblies, 270 shielding assemblies, and a barrel. The equivalent diameter of active core is about 252 cm and the outer diameter of barrel is about 350 cm. The active core is divided into inner zone (108 assemblies, identified with 1 in Figure
Grouping and Layout of Control Assembly.
The fuel assembly has an overall length of 460 cm and contains 90 fuel pins arranged in a triangular pitch array within a duct, see Figure
The control assemblies consist of an absorber bundle contained within a duct. The absorber bundle is a closely packed array of tubes containing compacted boron carbide pellets. The natural boron whose B-10 enrichment is 19.9 a/o is used. Thirty control assemblies are grouped into A, B, and C banks, where bank A is the primary manipulated bank, bank B is the secondary control bank, and bank C is the shutdown bank. The grouping of the control assemblies and its layout are given in Figure
Traditionally, nuclear design, thermal-hydraulic analysis, and fuel performance analysis for a reactor core are performed independently. Actually, the neutronics behavior, thermal-hydraulic behavior, fuel thermodynamics behavior, and fuel irradiation behavior in a reactor core are tightly coupled with each other. In PWR core design, thermal-hydraulic feedback has been considered widely in core analysis code due to the significant spectrum effect of coolant density and Doppler effect of fuel temperature. As for ISBBR, besides the thermal-hydraulic feedback, the reactivity effect of thermal expansion and irradiation swelling also have significant influence on the core reactivity and the cycle life.
Figure
Multiphysics behaviors for the core with metallic fuel.
Based on the understanding to the above multiphysics phenomena in the core using metallic fuel and the specific ISBBR fuel design of metallic pellet and ceramic cladding, we developed the specific multiphysics model and the core simulation code TANG for ISBBR. The following sections briefly describe the technical characteristics of the multiphysics model and computer code TANG.
Neutronics model is the kernel of multiphysics model and also the driving force for heat conduction, heat transfer, and deformation of fuel rod. Neutronics model involves depletion model, parameterized cross section model, and multidimension/multigroup neutron diffusion model.
The depletion model solves nonlinear depletion chains by using Matrix Exponential Algorithm and tracks the evolution of nodal-wise number density for major actinides based on the 3D neutron flux and reaction rate during full cycle life. Figure
Depletion chain for actinides tracked in TANG code.
Parameterized cross section model captures the instant effect and historical effect of various factors on homogenized assembly cross sections. The instant effect is caused by the deviation of instant local conditions from the reference conditions, such as local burnup (bu), local coolant temperature/density (
The integration of the above methodologies and technologies endows TANG code abundant calculation functions and flexible simulation ability.
The thermal expansion coefficient of metallic fuel is large (approximately 2 times of ceramic fuel), and the irradiation swelling effect of metallic fuel is also significant (around 30% at high burnup). As a result, the geometrical change of metallic fuel pellet during heating process and/or irradiation process will not only give penalty on the extra reactivity and cycle life of the core, but also will directly affect the transient behavior of the core, as the negative feedback effect of axial growth can effectively restrain the increase of the reactor core power and automatically bring the reactor to a safe lower power level. The experiments on EBR-II had demonstrated this inherent safety characteristic of the core with metallic fuel [
Relative to the metallic pellet, the SiCf/SiC cladding has very good thermal stability, irradiation stability, and mechanics stability. Firstly, the thermal expansion coefficient of SiCf/SiC is only about half of the zircaloy; secondly, the existing irradiation experiment (about 43 DPA) shows that the swelling and creeping phenomena are very weak [
Properties of several typical fuel materials.
Thermal expansion coefficient ppm/°C | Brinell hardness kg/mm2 | Young’s modular ( |
Poisson's ratio ( | |
---|---|---|---|---|
UO2 (ceramic) | ~9 | ~2000 | ~96 | ~0.3 |
UPuZr (alloy) | ~18 | ~260 | ~85 | ~0.3 |
| ||||
Zircaloy | ~10 | ~120 | ~100 | ~0.3 |
Stainless steel | ~17 | ~100 | ~196 | ~0.3 |
SiCf/SiC (ceramic) [ |
~4 | ~2800 | ~300 | ~0.14 |
According to the above analysis on the metallic pellet and SiCf/SiC ceramic cladding, we proposed a “rigid cladding model” to describe the fuel rod deformation behavior in ISBBR, which assumes the following. The deformation of SiCf/SiC ceramic cladding is only due to thermal expansion, irradiation swelling, creeping, and stress/strain are ignored. The deformation of metallic pellet may be caused by thermal expansion, irradiation swelling, and creeping. After the metallic pellet contacts with ceramic cladding, the contact stress shall not cause any strain to ceramic cladding. After the metallic pellet contacts with ceramic cladding, the metallic pellet shall become yielded consequently due to the contact stress, and according to the Prandtl-Reuss flow rule, the expansion shall develop to the inner hole of the annular pellet; once the inner hole is closed, the expansion shall switch to axial direction.
Thermal expansion is recoverable and the irradiation deformation (swelling and creeping) is irrecoverable. Therefore, with the assumption of “rigid cladding model,” the cladding deformation is recoverable, and the pellet deformation involves the recoverable and irrecoverable compositions. The recoverable deformation of metallic pellet can affect the core dynamic parameters and the transient behavior of the core, and the irrecoverable deformation of the metallic pellet has direct penalty on the core extra reactivity and the cycle life.
Metallic fuel had significant irrecoverable deformation (swelling and creeping). Figures
Radial swelling versus burnup for metallic fuel.
Axial swelling versus burnup for metallic fuel.
Each fuel assembly in ISBBR is contained within a duct, which directs the coolant flow to fuel rods of the fuel assembly, and there is no exchange of coolant mass and momentum among the assemblies. Therefore, it is reasonable for TANG code to adopt the “single channel model” to simulate the heat conduction within fuel rod and heat transfer between rod surface and coolant; TANG code has a “single channel model” for each fuel assembly modeled in the core to calculate the averaged effect of coolant density/temperature, cladding temperature, and fuel temperature in each elevation of the fuel assembly, and then, the 3D nodal-wise coolant density/temperature, cladding temperature, and fuel temperature are passed to 3D neutronics model, rod deformation model, and other models, so that all models are tightly coupled into a multiphysics model.
The “single channel model” in TANG code uses finite difference method to solve time-dependent heat conduction equation in cylindrical R-Z geometry for steady-state and transient solution, which shall be coupled with steady-state or transient 3D neutronics model. The heat conduction along
The discrete of heat conduction equation is based on the nominal rod sizes so as to maintain the stability during equation solution, but the deformed rod sizes are used for the gap conductance calculation.
The fuel rod in ISBBR might endure extra high internal pressure and even endanger the fuel rod integrity due to the fission gas release and accumulation under ultralong cycle and ultrahigh burnup. Therefore, the fuel rod internal pressure in ISBBR is an important design constraint. Based on the “single channel model” and 3D burnup distribution of the core, TANG code tracks the fission gas release fraction for 3D nodes and calculates assembly-averaged fuel rod internal pressure at each burnup step.
The main components of fission gas are Xe and Kr, and the total fission yield of Xe and Kr is about 0.25 (totally 2.0). Fission gas is gathered in the grain boundary in earlier stage; with the burnup accumulation, the fission gas gradually gathers into bubble; bubbles grow along with the increased inner pressure and connected mutually; finally, it forms a coherent tunnel. Eventually, fission gas is driven by the temperature and the pressure and release to the gap between pellet and cladding and then the plenum of the rod. The coherent tunnel in metallic fuel is formed at around 2~3 at% of burnup, the release fraction of fission gas increases quickly prior to 3 at% of burnup; thereafter, the release fraction maintains at around 70%. Figure
Fission gas releases fraction for metallic fuel.
The fuel rods internal pressure is calculated by using the free gas state equation
The structure material, especially the cladding material, in ISBBR might endure severe radiation damage due to ultralong time exposure of fast neutron spectrum. The radiation damage to material usually is measured by displacement per atom (DPA), which means the accumulative displacement number of each medium atom. TANG code equips a fuel rod exposure dose model, so that the maximum cladding dose can be monitored during core design process.
The multigroup DPA cross sections (
The reference ISBBR core was analyzed with TANG code, and the calculation results are introduced in following sections.
Figure
Burnup versus cycle length for reference core.
Figure
Critical rod position and power peaking factor versus cycle lifetime.
Assembly-wise power distribution at 0 EFPY/15 EFPY/25 EFPY.
Figure
Assembly-wise maximum cladding dose, rod internal pressure, and average axial deformation factor at EOL.
Influence of axial expansion on cycle lifetime.
Figure
Rod peaking temperatures versus cycle lifetime.
The positive Void Reactivity Coefficient and smaller Fuel Doppler Temperature Coefficient are the main characteristics of the sodium-cooled fast reactor and also the main concerns of people to the safety of the sodium-cooled fast reactor. Some studies in [
Influence of SiCf/SiC on void reactivity.
Influence of SiCf/SiC on Doppler effect.
The early demonstration experiments on EBR-II had proved that the SFR using metallic fuel could safely approach lower power level during the anticipated transient without scram (ATWS) [
TANG code is used to simulate the transient response for ULOFA and ULOHSA of reference ISBBR core. The calculated results are given in Figures
Core key parameters response during ULOFA.
Core key parameters response during ULOHSA.
ISBBR makes use of the outstanding breeding capability of metallic fuel, produces more fissile material as consuming existing fissile material, and finally achieves ultralong cycle life and ultrahigh burnup.
Figure
Core heavy metal inventory and breeding ratio versus cycle lifetime.
The motivation to propose ISBBR is not only to pursue safe and economical energy, but also for the following strategic prospects: continuously consume the huge volume of the depleted uranium and spent fuel accumulated by the development and deployment of water-cooled reactor, and finally achieve the minimization of the waste volume; Support sustainable development and deployment of fission energy and provide abundant and reliable energy for the peace and development of human society.
The precondition to achieve the above strategic goals is that ISBBR should implement closed fuel cycle. Fortunately, the low molten point (around 1200°C) property of metallic fuel has provided a very favorable condition for economical reprocessing of spent fuel. And then, the prerequisite to achieve sustainable closed fuel cycle for ISBBR is the quantity and quality of the fission material in the spent fuel which has no degradation compared with the initial inventory. Figure
Core plutonium inventory and fissile plutonium fraction versus cycle lifetime.
ISBBR does not pursue the accumulation or doubling of the extra plutonium, but the synchronized breeding and burning of fissile isotopes within an ultralong cycle life; therefore, it naturally satisfies the requirement of nonproliferation.
And also, only relying on the huge amount of depleted uranium and spent fuel accumulated by water-cooled reactor, ISBBR can achieve sustainable fission energy supply for long term and finally achieve the minimization of the waste volume. Let us assume that all the natural Uranium resource on the earth shall be utilized by PWR, it means that there will be about 6 million tons of depleted uranium and about 0.6 million tons of spent fuel accumulated finally; usually, the content of Reactor Grade Plutonium in the PWR spent fuel is nearly 1%, so the accumulated Reactor Grade Plutonium shall be 6000 tons; taking the reference ISBBR core as an example, where the initial plutonium inventory is 4.6 tons, then, the accumulated Reactor Grade Plutonium is enough to equip
Based on the platform of traditional sodium-cooled fast reactor, making use of the innovative fuel design and core design, ISBBR can achieve ultralong cycle and ultrahigh burnup and maintain stable radial power distribution during the cycle life without refueling and shuffling.
Primary calculation results, provided by specifically developed computer code TANG, demonstrate that the ISBBR core has enough thermal margins during steady-state operation and inherent passive safety during anticipated transients.
The fuel cycle analysis indicates that the fuel utilization rate of ISBBR can approach 18% and have significant fuel cycle economy compared with the current water-cooled reactors, where fuel utilization is about 0.5~0.6%.
In addition, the features of ultralong cycle, no extra plutonium accumulation, and sustainable closed fuel cycle demonstrate that ISBBR can realize the minimization of waste volume, nonproliferation, and sustainable nuclear power supply.
In conclusion, ISBBR can well satisfy the requirements of Gen-IV nuclear energy system, such as sustainability, economy, safety, and nonproliferation and has a great development potential.