Low Reactivity Swing Research Articles

Burnable Absorber Design Study for a Passively-Cooled Molten Salt Fast Reactor

The Passively-Cooled Molten Salt Fast Reactor (PMFR) is one of the advanced design concepts of the Molten Salt Fast Reactor (MSFR) which utilizes a natural circulation for the primary loop and aims to attain a long-life operation without any means of fuel reprocessing. For an extended operation period, it is necessary to have enough fissile material, i.e., high excess reactivity, at the onset of operation. Since the PMFR is based on a fast neutron spectrum, direct implementation of a burnable absorber concept for the control of excess reactivity would be ineffective. Therefore, a localized moderator concept that encircles the active core has been envisioned for the PMFR which enables the effective utilization of a burnable absorber to achieve low reactivity swing and long-life operation. The modified PMFR design that incorporates a moderator and burnable absorber is presented, where depletion calculation is performed to estimate the reactor lifetime and reactivity swing to assess the feasibility of the proposed design. All the presented neutronic analysis has been conducted based on the Monte Carlo Serpent2 code with ENDF/B-VII.1 library.

Neutronics calculation of the conceptual TRISO duplex fuel rod design

• The neutronic properties of TRISO duplex fuel rod for a small block-type thorium HTR was investigated. • TRISO duplex fuel rod consists of an inner region filled with UO 2 particles and an outer layer filled with ThO 2 particles. • TRISO duplex fuel increases the discharged burnup and has a lower reactivity swing. • TRISO duplex fuel capable of maintaining a softer neutron spectrum throughout its burnup period. The duplex fuel pellet includes UO 2 components as seed and ThO 2 blanket, originally designed to increase the breeding rate of fissile materials in light water breeder reactors. However, the seed portion alone concentrates most of the energy released from the fission reaction, which can make the seed temperature too high. The duplex fuel potential for the high-temperature reactor (HTR) using TRISO fuel has never been researched. MCNPX simulation was conducted to compare the neutronics performance of normal TRISO fuel with that of TRISO duplex fuel. Two types of HTR fuel rod, the cylinder and the annular rod, were simulated and compared to see whether the duplex neutron characteristics would be better in the cylinder or annular fuel compact design. The TRISO duplex rods produced higher burnup than normal rods up to 5% higher for cylinder and 6% for annular rods. The TRISO duplex rod is also capable of maintaining longer criticality, producing smaller k inf changes and reducing the potential for plutonium build-up. The proposed TRISO duplex conceptual fuel design has an interesting neutronic properties, which offers possibilities for future studies.

Fuel options for nuclear ship reactors featuring reactivity swing below one dollar

Environmental protection against air pollution and climate change has drawn great and increasing public attention in recent decades. Nuclear power involving no pollutant and CO2 emissions provides a more sustainable way for commercial shipping activities. Nine fuel options are simulated to find the combinations of key parameters that make the core to have a lifetime reactivity swing below one dollar. Such a low reactivity swing, when combined with a negative feedback reactivity coefficient, may facilitate passive load follow and passive safety, which means the reactivity can be controlled through temperature or thermal expansion of the reactor core. These fuel options are compared regarding reactor physics and thermal hydraulics, under certain constraints associated with ship reactors. WIMS code is used for burnup, perturbation and parameter calculating. MONK and SERPENT codes are used to verify the WIMS model. It is found that ship-based lead-cooled fast reactor can be designed to have superior core safety and good economics. Different fuel options show diverse characteristics in terms of power output limits, refuelling intervals, natural circulation cooling capabilities and other safety related parameters.

Neutronic investigation of alternative & composite burnable poisons for the soluble-boron-free and long life civil marine\xa0small modular reactor cores

Concerns about the effects of global warming provide a strong case to consider how best nuclear power could be applied to marine propulsion. Currently, there are persistent efforts worldwide to combat global warming, and that also includes the commercial freight shipping sector. In an effort to decarbonize the marine sector, there are growing interests in replacing the contemporary, traditional propulsion systems with nuclear propulsion systems. The latter system allows freight ships to have longer intervals before refueling; subsequently, lower fuel costs, and minimal carbon emissions. Nonetheless, nuclear propulsion systems have remained largely confined to military vessels. It is highly desirable that a civil marine core not use soluble boron for reactivity control, but it is then a challenge to achieve an adequate shutdown margin throughout the core life while maintaining reactivity control and acceptable power distributions in the core. High-thickness ZrB2 150 μm Integral Fuel Burnable Absorber (IFBA) is an excellent burnable poison (BP) candidate for long life soluble-boron-free core. However, in this study, we want to minimize the use of 150 μm IFBA since B-10 undergoes an (n, α) capture reaction, and the resulting helium raises the pressure within the plenum and in the cladding. Therefore, we have considered several alternative and novel burnable BP design strategies to minimize the use of IFBA for reactivity control in this study: (Case 1) a composite BP: gadolinia (Gd2O3) or erbia (Er2O3) with 150 μm thickness ZrB2 IFBA; (Case 2) Pu-240 or Am-241 mixed homogeneously with the fuel; and (Case 3) another composite BP: Pu-240 or Am-241 with 150 μm thickness ZrB2 IFBA. The results are compared against those for a high-thickness 150 μm 25 IFBA pins design from a previous study. The high-thickness 150 μm 25 IFBA pins design is termed the “IFBA-only” BP design throughout this study. We arrive at a design using 15% U-235 fuel loaded into 13 × 13 assemblies with Case 3 BPs (IFBA+Pu-240 or IFBA+Am-241) for reactivity control while reducing 20% IFBA use. This design exhibits lower assembly reactivity swing and minimal burnup penalty due to the self-shielding effect. Case 3 provides ~10% more initial (beginning-of-life) reactivity suppression with ~70% less reactivity swing compared to the IFBA-only design for UO2 fuel while achieving almost the same core lifetime. Finally, optimized Case 3 assemblies were loaded in 3D nodal diffusion and reactor model code. The results obtained from the 3D reactor model confirmed that the designed core with the proposed Case 3 BPs can achieve the target lifetime of 15 years while contributing to ~10% higher BOL reactivity suppression, ~70% lower reactivity swings, ~30% lower radial form factor and ~28% lower total peaking factor compared to the IFBA-only core.

Small modular reactor core design for civil marine propulsion using micro-heterogeneous duplex fuel. Part II: whole-core analysis

Civil marine reactors face a unique set of design challenges. These include requirements for a small core size and long core lifetime, a 20% cap on fissile loading, and limitations on using soluble neutron absorbers. In this reactor physics study, we seek to design a core that meets these requirements over a 15 effective full-power-years (EFPY) life at 333 MWth using homogeneously mixed all-UO2 and micro-heterogeneous ThO2-UO2 duplex fuels. In a companion (Part I) paper, we found assembly designs using 15% and 18% 235U for UO2 and duplex fuels, respectively, loaded into 13 × 13 pin arrays. High thickness (150 μm) ZrB2 integral fuel burnable absorber (IFBA) pins and boron carbide (B4C) control rods are used for reactivity control. Taking advantage of self-shielding effects, these designs maintain low and stable assembly reactivity with little burnup penalty.In this paper (Part II), whole-core design analyses are performed for small modular reactor (SMR) to determine whether the core remains critical for at least 15 EFPY with a reactivity swing of less than 4000 pcm, subject to appropriate constraints. The main challenge is to keep the radial form factor below its limit (1.50). Burnable poison radial-zoning is examined in the quest for a suitable arrangement to control power peaking. Optimized assemblies are loaded into a 3D reactor model in PANTHER. The PANTHER results confirm that the fissile loadings of both fuels are well-designed for the target lifetime: at the end of the ∼15-year cycle, the cores are on the border of criticality. The duplex fuel core can achieve ∼4% longer core life, has a ∼3% lower initial reactivity and ∼30% lower reactivity swing over life than the final UO2 core design. The duplex core is therefore the more successful design, giving a core life of ∼16 years and a reactivity swing of less than 2500 pcm, while satisfying all the neutronic safety parameters. In particular, one of the major objectives of this study is to offer/explore a thorium-based candidate alternative fuel platform for the proposed marine core. It is proven by literature reviews that the ability of the duplex fuel was never explored in the context of a single-batch, LEU, SBF, long-life SMR core. In this regard, the motivation of this paper is to understand the underlying physics of the duplex fuel and ‘open the option’ of designing the functional cores with both the duplex and UO2 fuel cores.

Small modular reactor core design for civil marine propulsion using micro-heterogeneous duplex fuel. Part I: Assembly-level analysis

In an effort to de-carbonise commercial freight shipping, there is growing interest in the possibility of using nuclear propulsion systems. In this reactor physics study, we seek to design a soluble-boron-free (SBF) and low-enriched uranium (LEU) (<20% 235U enrichment) civil nuclear marine propulsion small modular reactor (SMR) core that provides at least 15 effective full-power-years (EFPY) life at 333 MWth using 18% 235U enriched micro-heterogeneous ThO2-UO2 duplex fuel and 15% 235U enriched homogeneously mixed all-UO2 fuel. We use WIMS to develop subassembly designs and PANTHER to examine whole-core arrangements.The assembly-level behaviours of candidate burnable poison (BP) materials and control rods are investigated. We examine gadolinia (Gd2O3), erbia (Er2O3) and ZrB2 integral fuel burnable absorber (IFBA) as BPs. We arrive at a design with the candidate fuels loaded into 13 × 13 assemblies using IFBA pins for reactivity control. Taking advantage of self-shielding effects, this design maintains low and stable assembly reactivity with relatively little burnup penalty. Thorium-based duplex fuel offers better performance than all-UO2 fuel with all BP options considered. Duplex fuel has ∼20% lower reactivity swing and, in consequence, lower initial reactivity than all-UO2 fuel. The lower initial reactivity and smaller reactivity swing make the task of reactivity control through BP design easier in the thorium-rich duplex core. For control rod design, we examine boron carbide (B4C), hafnium, and Ag-In-Cd alloy. All the candidate materials exhibit greater rod worth for the duplex design. For both fuels, B4C has the highest rod worth. In particular, one of the major objectives of this study is to offer/explore a thorium-based candidate alternative fuel platform for the proposed marine core. It is proven by literature reviews that the ability of the duplex fuel was never explored in the context of a single-batch, LEU, SBF, long-life SMR core. In this regard, the motivation of this paper is to observe the neutronic performance of the proposed duplex fuel with respect to the UO2 fuel and ‘open the option’ of designing the functional cores with both the duplex and UO2 fuel cores.A companion paper will examine key physics and core safety analysis parameters in the whole-core environment.

Design of self-sustainable small modular reactor with multiple fissile and fertile layers

Summary In order to achieve the challenging design goal of a future-advanced small modular reactor, this paper suggests newly three concepts for small modular reactor such as a fissile and fertile multiple arrangement, a molten-fueled salt coolant, and a self-adjustable pitch size. The multiple arrangements of fissile and fertile layers makes it possible to enhance breeding fissile isotopes through the optimal placement of the thinner fissile layer between the thicker fertile regions in the core. The second idea of a creative use of molten-fueled salt as a coolant decreases the fuel loading as well as promotes safety significantly even if a severe core accident happens such as the loss of coolant accident. In that case, the reactor maintains the subcritical state owing to loss of molten-fueled salt coolant that contributes loss of neutron source. When the fuel is melt down, it is smeared into coolant that incorporates the molten-fueled salt coolant and both fuel and coolant are circulated simultaneously like the typical molten salt breeder reactor. The third idea of the new small modular reactor is an automatic size change of fuel lattice pitch, which plays an important role on keeping reactivity swing within a specified limit. This self-adjustment of fuel rod and assembly pitch sizes can be realized by using of the shape memory material such as Ti-Ni alloy in the fuel assembly bottom grid. Simulation results of the Monte Carlo depletion calculations indicate that the proposed brand-new small modular reactor system is sufficiently achievable by providing an ultra-long fuel cycle length and a sufficiently low reactivity swing through the optimization of design parameters. Copyright © 2017 John Wiley & Sons, Ltd.

Thorium utilization in a small long-life HTR. Part II: Seed-and-blanket fuel blocks

In order to utilize thorium in high temperature gas-cooled reactors (HTRs), the concept of seed-and-blanket (S&B) fuel block is introduced into the U-Battery, which is a long-life block-type HTR with a thermal power of 20MWth. A S&B fuel block consists of a seed region with uranium in the center, and a blanket region with thorium. The neutronic performance, such as the multiplication factor, conversion ratio and reactivity swing, of a typical S&B fuel block was investigated by SCALE 6.0 by parametric analysis of the composition parameters and geometric parameters of the fuel block for the U-Battery application. Since the purpose of U-235 in the S&B fuel block is to ignite the fission reactions in the fuel block, 20% enriched uranium is recommended for the S&B fuel block. When the ratio of the number of carbon to heavy metal atoms changes with the geometric parameters of the fuel block in the range of 200–250, the reactivity swing reaches very small values. Furthermore, for a reactivity swing of 0.1 Δk during 10 effective full power years, three configurations with 36, 54 and 78 UO2 fuel rods are recommended for the application of the U-Battery. The comparison analysis of the S&B fuel block with the Th/U MOX fuel block shows that the former has a longer lifetime and a lower reactivity swing.

Thorium utilisation in a small long-life HTR. Part III: Composite-rod fuel blocks

The U-Battery is a small long-life high temperature gas-cooled reactor (HTR) with power of 20MWth. In order to increase its lifetime and diminish its reactivity swing, the concept of composite-rod fuel blocks with uranium and thorium was investigated. Composite-rod fuel blocks feature a specific axial separation between UO2 and ThO2 compacts in fuel rods. The design parameters, investigated by SCALE 6, include the number and spatial distribution of fuel compacts within the rods, the enrichment of uranium, the radii of fuel kernels and fuel compacts, and the packing fractions of uranium and thorium TRISO particles. The analysis shows that a lower moderation ratio and a larger inventory of heavy metals results in a lower reactivity swing. The optimal atomic carbon-to-heavy metal ratio depends on the mass fraction of U-235 and is commonly in the 160–200 range. The spatial distribution of the fuel compacts within the fuel rods has a large influence on the energy spectrum in each fuel compact and thus on the beginning-of-life reactivity and the reactivity swing. At end-of-life, the differences caused by the spatial distribution of the fuel compacts are smaller due to the fissions of U-233 in the ThO2 fuel compacts. This phenomenon enables to design fuel blocks with a very low reactivity swing, down to less than 4% in a 10-year lifetime. Among three types of thorium fuelled U-Battery blocks, the composite-rod fuel block achieves the highest end-of-life reactivity and the lowest reactivity swing.

Netronic Design of Small Long-Life PWR Using Thorium Cycle

A small long-life core loaded with thorium fuel and 231Pa as burnable poison material has been performed in Pressurized Water Reactor (PWR). Thorium cycle fuel has higher conversion ratio in the thermal spectrum domain and lower reactivity swing than the Uranium-Plutonium cycle fuel. 231Pa have very large capture cross section that can pressed reactivity in the beginning of life. The neutronic analysis result of infinite cell calculation shows that mixed nitride is better than oxide and carbide in thorium fuel system. In the present study we consider thorium nitride system with 3 ~ 8 % 233U percentage and 0.2~ 7% 231Pa as fuel for small PWR and can be burn up for the long time. The purpose of the study is to optimize the design of 350MWt PWR which can be operated without refueling in 10 years The core was designed by cylindrical two-dimension R-Z (radial and axial). The multigroup diffusion and Burn-up analysis was performed by SRAC-CITATION code using libraries based on JENDL 3.2. By using this concept, small PWR can be designed for long time operation with reduced excess reactivity until under 1 % and flatted power distribution during its operation.

Solitary breeding/burning waves in a supercritical water cooled fast reactor

This paper demonstrates a possible application of the so-called CANDLE concept, namely solitary breeding/burning wave concept, in a supercritical water cooled fast reactor (SCWFR). The high enthalpy increment and the large density variation of the supercritical water through the core are favourable to this concept. However, based on an SCWFR design, it is found that only the Th–U conversion cycle offers the possibility of a solitary breeding/burning wave without using a burnable poison. A one-group diffusion equation coupled with simplified burn-up equations is investigated for the burn-up drift wave problem, where the coupling with the thermal hydraulics of the supercritical water, especially the dependence of microscopic cross sections on the water density, and consequently on the neutron fluence, is also taken into account. A specific example of the solitary breeding/burning wave in the one-dimensional case is obtained numerically and presented in this paper. The asymptotic burn-up is about 33 at.%. Besides usual advantages of the CANDLE concept, e.g. low reactivity swing and high fuel utilization, the negative coolant feedback is an inherent feature of this kind of reactor concept.

Safety and design concepts of the 400 MW th-class EFIT accelerator driven transmuter and considerations for further developments

European R&D for ADS design and fuel development is driven in the 6th FP of the EU by the EUROTRANS Programme. In EUROTRANS, the longer-term EFIT development, the european facility for industrial transmutation, aims at a generic conceptual design of a full transmuter. A CERCER U-free fuel core with an MgO matrix and a CERMET core with a Mo-92 matrix have been designed. Both the CERCER and the CERMET EFIT concept were optimized towards: a high transmutation efficiency, high burn-up, low reactivity swing, low power peaking, adequate subcriticality, reasonable beam requirements and a high level of safety. Protected and unprotected transients which are initiated by a mismatch of power-to-flow or resulting from a beam disturbance or overpower situation were analyzed. Potentials which can lead to the introduction of positive reactivity into the core were identified, as e.g. the steam generator tube rupture (SGTR) accident or the pin failure with a gas release from the fission gas plena. Both for the CERCER and the CERMET fuelled core the design and safety analyses are close to completion in EUROTRANS and thus a first preliminary résumé can be drawn on the achieved design and safety goals. In addition, scenario studies in the framework of the PATEROS CA project of the 6th FP highlighted the specific needs on transmutation machines serving in countries with different nuclear options. Based on these studies in the 6th FP first reflections can be performed on needs or options for further optimizing an EFIT type ADS.

Accelerator driven systems for transmutation: Fuel development, design and safety

European R&D for ADS design and fuel development is driven in the 6th FP of the EU by the EUROTRANS Programme. In EUROTRANS two ADS design routes are followed, the XT-ADS and the EFIT. The XT-ADS is designed to provide the experimental demonstration of transmutation. The EFIT, the European Facility for Industrial Transmutation, aims at a conceptual design of a full transmuter. A key R&D issue is the choice of an adequate fuel. Various fuel forms have been assessed and CERCER and CERMET fuels, specifically the matrices MgO and Mo, have finally been selected. Within EUROTRANS, the domain ‘AFTRA’ is responsible to more deeply assess the behavior of these dedicated fuels and to provide the fuel database for the EFIT. The EFIT is optimized towards: a good transmutation efficiency, high burnup, low reactivity swing, low power peaking, adequate subcriticality, reasonable beam requirements and a high safety level. In the current paper the fuels under investigation are described, including their production route and their safety limits. First core designs of CERCER and CERMET fuelled 400 MWth EFITs have been developed within AFTRA. The trends found in the design studies and first safety analyses are presented.

On TiN-particle fuel based helium-cooled transmutation systems

We have designed a sub-critical helium-cooled core with TiN-coated particle fuel, dedicated to the transmutation of minor actinides. The excellent neutronic properties of helium allows for a low plutonium fraction in the fuel, which yields a low reactivity swing, Δ k = 2600 pcm, for a burnup of 31.2%. Further the neutron spectrum is hard, limiting the buildup of Cm and Cf. The high burnup combined with a minor actinide burning rate of 355 kg/GWth year makes the present design an attractive transmutation system.

Safety aspect of long-life small safe power reactors

Safety aspects of several design options of long-life small safe fast power reactors using nitride fuel and lead—bismuth as coolant are discussed. In the present study hypothetical accidents are simulated for these reactors, i.e., unprotected simultaneous ULOF (total loss of primary pumping system) and UTOP (rod run out transient over power) accidents, caused by the simultaneous withdrawal of all control rods. The proposed designs have some important safety characteristics such as low reactivity swing (only 0.2–0.25$), and negative coolant void coefficient over whole burnup period. Effectively negative value of all components of reactivity during an accident is observed. The safety performances of the balance, pancake, and tall slender type of core, each of them satisfy reactivity and negative coolant void coefficient constraint, against the above accident are compared. The simulation results show that all of the design options can survive the above accidents without the help of reactor scram and without the need of operator actions. The margin to coolant boiling and fuel melting is large. From the parametric survey results it is found that primary pump coastdown half time and relative position of primary side outlet of SG to the inlet of the core can effectively influence the maximum coolant, cladding, and pellet temperature during accidents.