Burnup Reactivity Swing Research Articles

Studies of an Accelerator-Driven Transuranium Burner with Hafnium-Based Inert Matrix Fuel

Neutronic and burnup characteristics of an accelerator-driven transuranium burner in a startup mode were studied. Different inert and absorbing matrices as well as lattice configurations were assessed in order to identify suitable fuel and core design configurations. Monte Carlo transport and burnup codes were used in the analyses. The lattice pin pitch was varied to optimize the source efficiency and coolant void worth while respecting key thermal and material-related design constraints posed by fuel and cladding. A HfN matrix appeared to provide a good combination of neutronic, burnup, and safety characteristics: maintaining a hard neutron spectrum, yielding acceptable coolant void reactivity and source efficiency, and alleviating the burnup reactivity swing. A conceptual design of a (TRU,Hf)N fueled, lead-bismuth eutectic-cooled accelerator-driven system was developed. Twice higher neutron fission-to-absorption probabilities in Am isotopes were achieved compared to reactor designs relying on ZrN or YN inert matrix fuel. The production of higher actinides in the fuel cycle is hence limited, with a Cm fraction in the equilibrium fuel being ~40% lower than for cores with ZrN matrix-based fuel. The burnup reactivity swing and associated power peaking in the core are managed by an appropriate choice of cycle length (100 days) and by core enrichment zoning. A safety analysis shows that the system is protected from instant damage during unprotected beam overpower transient.

Evaluation of Long-Life Transuranics Breakeven and Burner Cores for Waste Minimization in a PB-GCFR Fuel Cycle

Transuranics (TRU) breakeven and burner core designs have been studied for the Pebble-Bed Gas-Cooled Fast Reactor (PB-GCFR), which was developed under a 2-yr U.S. Department of Energy Nuclear Energy Research Initiative project. The issues of minimizing waste production, fuel cost, and burnup reactivity swing, and maximizing TRU burning have been investigated primarily from a neutronics viewpoint. For TRU breakeven cores, it was found that for the given core power [300 MW(thermal)] and power density (50 MW/m3), the lowest amount of radiotoxic TRU to be processed is obtained for a long-life (single-batch) core of 30-yr duration. Minimizing the TRU processed results in a minimization of the TRU losses that ultimately will have to be entombed in a geologic repository.The results show that the single-batch, long-life PB-GCFR could be designed to operate over a wide range of cycle lengths and fuel loadings. By modifying the TRU feed to have a higher minor actinide (MA) fraction than contained in light water reactor spent fuel, the burnup reactivity swing for the long-life core can be reduced significantly. With this approach, it is also possible to configure the long-life PB-GCFR core as a TRU burner using nonuranium fuel. A nonuranium fuel PB-GCFR with 24% plutonium and 76% MAs can operate for 17 full-power years and achieve 25% burnup with a reactivity swing of 3%Δk.

Fuel Design and Core Layout for a Gas-Cooled Fast Reactor

The gas-cooled fast reactor (GCFR) is regarded as the primary candidate for a future sustainable nuclear power system. In this paper a general core layout is presented for a 2400-MW(thermal) GCFR. Two fuel elements are discussed: a TRISO-based coated particle and the innovative hollow sphere concept. Sustainability calls for recycling of all minor actinides (MAs) in the core and a breeding gain close to unity. A fuel cycle is designed allowing operation over a long period, requiring refueling with 238U only. The evolution of nuclides in the GCFR core is calculated using the SCALE system (one-dimensional and three-dimensional). Calculations were done over multiple irradiation cycles including reprocessing. The result is that it is possible to design a fuel and GCFR core with a breeding gain around unity, with recycling of all MAs from cycle to cycle. The burnup reactivity swing is small, improving safety. After several fuel batches an equilibrium core is reached. MA loading in the core remains limited, and the fuel temperature coefficient is always negative.

A study on improvement of sodium cooled TRU burner design using burnable poison

A new design approach to improve safety characteristics of sodium cooled core for transuranic element transmutation is discussed. In the new option, some amount of fertile material is removed for reduction of sodium void reactivity. Simultaneously, a burnable absorber material is loaded in replacement of fertile material to compensate for reactivity drop during the fuel depletion. Two methods of burnable absorber loading are considered such as the homogeneous and the heterogeneous. In the results, it is found that the homogeneous loading cannot reduce the sodium void reactivity but makes the reactivity more positive. On the other hand, the heterogeneous loading can reduce the sodium void reactivity successfully. It is also found that the increment in burnup reactivity swing is negligible when the burnable poison is heterogeneously loaded. It is concluded that if the burnable poison material is loaded appropriately, the sodium void reactivity can be reduced without any significant penalty of increase in burnup reactivity swing.

Molten salt cooled Encapsulated Nuclear Heat Source (ENHS)-like reactors

The feasibility of designing molten-salt cooled ENHS (Encapsulated Nuclear Heat Source)-like reactor cores with Pu 15N-U 15N nitride fuel for high temperature applications is assessed. The cores considered have uniform fuel composition and no blanket elements and solid reflectors. They are to operate for at least 20 effective full power years without refueling, without fuel shuffling and with burnup reactivity swing less than 0.52%. Three molten-fluoride-salts: NaF(57)-BeF 2(43), 7LiF(66)-BeF 2(34), and LiF(46.5)-NaF(11.5)-KF(42) are considered as the coolant and six materials: SS304, Hastelloy-N, HT-9, Mn-316SS, PCA, and SiC, are considered for the structures. It is found that, neutronically, ENHS-like cores can be designed for all combinations of molten-salt coolants and structural materials considered. Relative to the reference ENHS core, the molten-salt cooled cores require significantly tighter lattice, have softer neutron spectra, significantly more negative Doppler reactivity effect, much more positive coolant temperature and void reactivity effect and smaller reactivity worth of the control elements. Of the molten salts considered, LiF-NaF-KF offers the largest p/d ratio and is most suitable for natural circulation cooling.

The Encapsulated Nuclear Heat Source (ENHS) Reactor Core Design

A once-for-life, uniform composition, blanket-free and fuel-shuffling-free reference core has been designed for the Encapsulated Nuclear Heat Source (ENHS) to provide the design goals of a nearly zero burnup reactivity swing throughout ~20 yr of full-power operation up to the peak discharge burnup of more than 100 GWd/t HM. What limits the core life is radiation damage to the HT-9 structural material. The temperature coefficients of reactivity are all negative, except for that of the coolant expansion. However, the negative reactivity coefficient associated with the radial expansion of the core structure can compensate for the coolant thermal expansion. The void coefficient is positive but of no safety concern because the boiling temperature of lead or lead-bismuth is so high that there is no conceivable mechanism for the introduction of significant void fraction into the core. The core reactivity coefficients, reactivity worth, and power distributions are almost constant throughout the core life.It was found possible to design such once-for-life cores using different qualities of Pu and transuranics as long as U is used as the primary fertile material. It is also feasible to design ENHS cores using nitride rather than metallic fuel. Relative to the reference metallic fuel core, nitride fuel cores offer up to ~25% higher discharge burnup and longer life, up to ~38% more energy per core, a significantly more negative Doppler reactivity coefficient, and less positive coolant expansion and coolant void reactivity coefficient but a somewhat smaller negative fuel expansion reactivity coefficient. The pitch-to-diameter ratio (1.45 of the nitride fuel cores using enriched N) is larger than that (1.36) for the reference metallic fuel core, implying a reduction of the coolant friction loss, thus enabling an increase in the power level that can be removed from the core by natural circulation cooling.It is also possible to design Pu-U(10Zr) fueled ENHS-type cores using Na as the primary coolant with either Na or Pb-Bi secondary coolants. The Na-cooled cores feature a tighter lattice and are therefore more compact but have spikier power distribution, more positive coolant temperature reactivity coefficients, and smaller reactivity worth of the control elements.

Neutronics Design for Lead-Bismuth Cooled Accelerator-Driven System for Transmutation of Minor Actinide

Neutronics design study was performed for lead-bismuth cooled accelerator-driven system (ADS) to transmute minor actinides. Early study for ADS indicated two problems: a large burnup reactivity swing and a significant peaking factor. To solve these problems, effect of design parameters on neutronics characteristics were searched. The design parameters were initial plutonium loading, buffer region between spallation target and core, and zone fuel loading. Parametric survey calculations were performed considering fuel cycle consisting of burnup and recycle. The results showed that burnup reactivity swing depends on the plutonium fraction in the initial fuel loading, and the lead-bismuth buffer region and the two-zone loading were effective for solving the problems. Moreover, an optimum value for the effective multiplication factor was also evaluated using reactivity coefficients. From the result, the maximum allowable value of the effective multiplication factor for a practical ADS can be set at 0.97. Consequently, a new core concept combining the buffer region and the two-zone loading was proposed base on the results of the parametric survey.

Neutronics Design for Lead-Bismuth Cooled Accelerator-Driven System for Transmutation of Minor Actinide

Neutronics design study was performed for lead-bismuth cooled accelerator-driven system (ADS) to transmute minor actinides. Early study for ADS indicated two problems: a large burnup reactivity swing and a significant peaking factor. To solve these problems, effect of design parameters on neutronics characteristics were searched. The design parameters were initial plutonium loading, buffer region between spallation target and core, and zone fuel loading. Parametric survey calculations were performed considering fuel cycle consisting of burnup and recycle. The results showed that burnup reactivity swing depends on the plutonium fraction in the initial fuel loading, and the lead-bismuth buffer region and the two-zone loading were effective for solving the problems. Moreover, an optimum value for the effective multiplication factor was also evaluated using reactivity coefficients. From the result, the maximum allowable value of the effective multiplication factor for a practical ADS can be set at 0.97. Consequently, a new core concept combining the buffer region and the two-zone loading was proposed base on the results of the parametric survey.

Neutronics and thermal effects of graphite foams in the performance of nuclear energy systems

Procedures to produce light graphite foam (∼0.5 g/cm 3) that exhibits heat conductivities similar to full density graphite have been developed at ORNL. The consequent substantial reduction in the thermal inertia might have a significant impact in standard designs of graphite system and it could make possible new concepts. We discuss two possible applications: (a) a modular, zero burnup reactivity swing, reactor and (b) the pebble bed accelerator-driven transmutator.

Core burnup calculation and accidents analyses of a pressurized water reactor partially loaded with rock-like oxide fuel

A rock-like oxide (ROX) fuel – light water reactor (LWR) burning system has been studied for efficient plutonium transmutation. For the improvement of small negative reactivity coefficients and severe transient behaviors of ROX fueled LWRs, a partial loading core of ROX fuel assemblies with conventional UO 2 assemblies was considered. As a result, although the reactivity coefficients could be improved, the power peaking tends to be large in this heterogeneous core configuration. The reactivity initiated accident (RIA) and loss of coolant accident (LOCA) behaviors were not sufficiently improved. In order to reduce the power peaking, the fuel composition and the assembly design of the ROX fuel were modified. Firstly, erbium burnable poison was added as Er 2O 3 in the ROX fuel to reduce the burnup reactivity swing. Then pin-by-pin Pu enrichment and Er content distributions within the ROX fuel assembly were considered. In addition, the Er content distribution was also considered in the axial direction of the ROX fuel pin. With these modifications, a power peaking factor even lower than the one in a conventional UO 2 fueled core can be obtained. The RIA and LOCA analyses of the modified core have also shown the comparable transient behaviors of ROX partial loading core to those of the UO 2 core.

A Neutronics and Burnup Analysis of the Accelerator-Driven Transmutation System with Different Cross Section Libraries

The difference of the neutronics and burnup characteristics of the Accelerator-Driven System (ADS) caused by the different nuclear data libraries, JENDL-3.2, ENDF/B-VI and JEF-2.2, was analyzed. It is important to know the current accuracy of the cross section data of the Minor Actinides (Np, Am, Cm) to improve the reliability of the analysis and to guarantee the subcriticality of the fuel region of ADS. The analysis was performed with benchmark problems of the OECD/NEA ADS transmutor. An ATRAS code system and a MVP code were used for the analysis. Group cross section sets were newly created from JENDL-3.2, ENDF/B-VI and JEF-2.2, respectively. To make the difference of the MA cross section data clearly, the group cross sections generated from JENDL-3.2 were used for cooling and structural nuclides of ENDF/B-VI and JEF-2.2. From the results of the analysis, JENDL-3.2 and JEF-2.2 gives close k-effective values and similar trends of burnup reactivity swing. However, ENDF/B-VI gives different k-effective values and burnup changes from other two libraries

Burn-up characteristics of ADS system utilizing the fuel composition from MOX PWRs spent fuel

Burn-up characteristics of accelerator-driven system, ADS has been evaluated utilizing the fuel composition from MOX PWRs spent fuel. The system consists of a high intensity proton beam accelerator, spallation target, and sub-critical reactor core. The liquid lead–bismuth, Pb–Bi, as spallation target, was put in the center of the core region. The general approach was conducted throughout the nitride fuel that allows the utilities to choose the strategy for destroying or minimizing the most dangerous high level wastes in a fast neutron spectrum. The fuel introduced surrounding the target region was the same with the composition of MOX from 33 GWd/t PWRs spent-fuel with 5 year cooling and has been compared with the fuel composition from 45 and 60 GWd/t PWRs spent-fuel with the same cooling time. The basic characteristics of the system such as burn-up reactivity swing, power density, neutron fluxes distribution, and nuclides densities were obtained from the results of the neutronics and burn-up analyses using ATRAS computer code of the Japan Atomic Energy research Institute, JAERI.

Comparison of the Burnup Characteristics and Radiotoxicity Hazards of Rock-like Oxide Fuel with Different Types of Additives

Burnup characteristics of rock-like oxide (PuO2-ZrO2: ROX) fueled LWR have been studied with an additive ThO2, UO2 or Er2O3. A ROX fueled LWR core has reactivity coefficients and burnup reactivity swing problems, which can be mitigated by these additives. From the burnup calculations of the ROX fueled LWR, it was found that the additive ThO2 causes a few percent decrease in Pu transmutation in ROX fuel, while the additive UO2 or Er2O3 decreases it largely. The production of 241 Am, which is a parent of long-lived 237Np, is smaller in ThO2 added fuel than that in UO2 or Er2O3 added fuel. However, the productions of 243Am and 244Cm are more in ThO2 added fuel than those in UO2 or Er2O3 added fuel. The long-lived fission products (LLFPs) production in ThO2 added fuel is nearly the same as that in UO2 added fuel, and smaller than that in Er2 O3 added fuel. As a result of these Pu transmutation and minor actinides (MAs) and LLFPs productions, ingestion radiotoxicity hazard index of ThO2 added ROX spent fuel is lower than that of UO2 or Er2O3 added ROX spent fuel, except at the 105 years cooling. From these calculations, ThO2 is recommended to be used as the additive in ROX fuel.

A pan-shape transuranic burner core with a low sodium void worth

A 1575 MWt transuranic (TRU) burner reactor core with a low sodium void worth has been developed by devising a pan-shaped active core design. The core consists of two types of fule subassemblies that differ in the height of the fueled regions. This strategy has allowed an extreme "pancaking" of the inner core region while the radial dimension increase is limited by placing longer fuels in the outer core region. The fuel cycle analysis has been performed in the equilibrium cycle consisting of external feed fuel with reprocessed typical PWR spent fuel and fissile makeup with recycled TRU elements. The neutronic performance characteristics obtained from the equilibrium cycle analysis show that it would work safely as well as economically as measured in terms of burnup reactivity swing peak power density Doppler coefficient TRU burning and sodium void worth. The core has relatively low double power peaks in both the inner and outer cores without enrichment zoning and this enables to make an active core volume smaller. The developed TRU burner core has been subjected to an extensive parametric study on the reprocessing schemes. Investigations are given for sodium void worth transmutation capability burnup reactivity swing and minor actinide (MA) contents. Through this series of study a variant of the TRU burner core that is aimed at preferentially burning MA has been determined. This MA burner core uses U-235 as well as the homogeneously recycled TRU elements. This MA burner core consumes 140 kg of MA per year without penalizing the sodium void reactivity observed in the TRU burner core. Finally the combined introduction of developed TRU and MA burner cores shows the functional effectiveness of reducing PWR discharged TRU inventory.

A Liquid-Metal Reactor for Burning Minor Actinides of Spent Light Water Reactor Fuel—I: Neutronics Design Study

A liquid-metal reactor was designed for the primary purpose of burning the minor actinide waste from commercial light water reactors (LWRs). The design was constrained to maintain acceptable safety performance as measured by the burnup reactivity swing, the Doppler constant, and the sodium void worth. Sensitivity studies were performed for homogeneous and decoupled core designs, and a minor actinide burner design was determined to maximize actinide consumption and satisfy safety constraints. One of the principal innovations was the use of two core regions, with a fissile plutonium outer core and an inner core consisting only of minor actinides. The physics studies performed here indicate that a 1200-MW(thermal) core is able to consume the annual minor actinide inventory of about 16 LWRs and still exhibit reasonable safety characteristics.

A Liquid-Metal Reactor for Burning Minor Actinides of Spent Light Water Reactor Fuel—II: Nuclear Data Uncertainty Analysis

A comprehensive sensitivity and uncertainty analysis was performed on a 1200-MW(thermal) minor actinide burner designed for a low burnup reactivity swing, negative Doppler constant, and low sodium void worth. Sensitivities of the performance parameters were generated using depletion perturbation methods for the constrained closed fuel cycle of the reactor. The uncertainty analysis was performed using the sensitivity and covariance data taken from ENDF/B-V and other published sources. The uncertainty analysis of a liquid-metal reactor for burning minor actinides has shown that uncertainties in the nuclear data of several key minor actinide isotopes can introduce large uncertainties in the predicted performance of the core. The relative uncertainties in the burnup swing, Doppler constant, and void worth were conservatively estimated to be 220, 120, and 59%, respectively. An analysis was performed to prioritize the minor actinide reactions for reducing the uncertainties.

Radioactive Waste Transmutation and Safety Potentials of the Lead Cooled Fast Reactor in the Equilibrium State.

The studies performed on a two-dimensional reactor model with burnup and fuel management simulation prove that the lead cooled fast reactor concept provides the possibility to develop critical equilibrium reactor confining all of the self-produced transuraniums and long-lived fission products in the core. Such inherent safety features of the reference, non-confining lead cooled reactor design as negative void reactivity effect and 'zero' burnup reactivity swing may also be provided for equilibrium confining design by implementing the design approaches described in this paper.

Uncertainty in the Burnup Reactivity Swing of Liquid-Metal Fast Reactors

The uncertainty in the burnup reactivity swing {sigma}k{sub b} attributable to nuclear data uncertainties is analyzed using depletion-dependent sensitivity coefficients for single- and multicycle equilibrium depletion. Four systems are analyzed with design features that encompass many of the design options considered for current U.S. advanced liquid-metal reactor cores. These systems, while characterized by very different {sigma}k{sub b} values in the range from {minus}0.22 to 3.87% {Delta}k, exhibit much smaller differences in their {sigma}k{sub b} uncertainties, which range from 0.18 to 0.33% {Delta}k. The {sigma}k{sub b} uncertainties depend primarily on the design choices of core size and fissile fuel type, as well as whether the analysis represents multicycle effects. For all reactors analyzed, the burnup swing uncertainty is dominated by the {sup 238}U capture reaction. In this paper the potential for reducing uncertainties by a factor of 3 by use of available integral experiment results is demonstrated.

Status of Axial Heterogeneous Liquid-Metal Fast Breeder Reactor Core Design Studies and Research and Development

The current status of axial heterogeneous core (AHC) design development in Japan, which consists of an AHC core design in a pool-type demonstration fast breeder reactor (DFBR) and research and development activities supporting AHC core design, is presented. The DFBR core design objectives developed by The Japan Atomic Power Company include (a) favorable core seismic response, (b) core compactness, (c) high availability, and (d) lower fuel cycle cost. The AHC concept was selected as a reference pool-type DFBR core because it met these objectives more suitably than the homogeneous core (HOC). The AHC core layouts were optimized emphasizing the reduction of the burnup reactivity swing, peak fast fluence, and power peaking. The key performance parameters resulting from the AHC, such as flat axial power/flux distribution, lower peak fast fluence, lower burnup reactivity swing, etc., were evaluated in comparison with the HOC. The critical experiments at the Japan Atomic Energy Research Institute’s Fast Critical Assembly facility demonstrate the key AHC performance characteristics. The large AHC engineering benchmark experiments using the zero-power plutonium reactor and the AHC fuel pin irradiation test program using the JOYO reactor are also presented.

酸化物燃料を用いた大型高速炉の高燃焼度炉心概念の提案と炉心構成の最適化

With the aim of reducing the fuel cycle cost of LMFBRs, the high burnup and long operating cycle core concept has been studied and optimized for a large LMFBR.The power shape for the high burnup core is found to be more flattened by dividing the core into different k∞ regions with different fuel volume fraction rather than with conventional different fuel enrichment. An axial heterogeneous core, in which a disk-shaped thin internal blanket is introduced at the core midplane in inner core region with small fuel volume fraction, is furthermore preferable to reduce the power swing during burnup.It becomes clear that the elimination of the core duct material is one of the key components to bring the burnup reactivity swing nearly zero for the core with mixed oxide fuels, in addition to increasing the core height and fuel pin diameter.It is concluded that the core with average burnup of 150GWd/t and continuous operating cycle length of 3 years is feasible by the axial heterogeneous core concept with the radial power distribution flattened by varying the fuel volume fraction.