Burnup Reactivity Swing Research Articles

Multi-batch core design study for innovative small modular reactor based on centrally-shielded burnable absorber

Various core designs with multi-batch fuel management (FM) are proposed and optimized for an innovative small modular reactor (iSMR), focusing on enhancing the inherent safety and neutronic performance. To achieve soluble-boron-free (SBF) operation, cylindrical centrally-shielded burnable absorbers (CSBAs) are utilized, reducing the burnup reactivity swing in both two- and three-batch FMs. All 69 fuel assemblies (FAs) are loaded with 2-cylindrical CSBA. Furthermore, the neutron economy is improved by deploying a truly-optimized PWR (TOP) lattice with a smaller fuel radius, optimized for neutron moderation under the SBF condition. The fuel shuffling and CSBA loading patterns are proposed for both 2- and 3-batch FM with the aim to lower the core leakage and achieve favorable power profiles. Numerical results show that both FM configurations achieve a small reactivity swing of about 1000 pcm and the power distributions are within the design criteria. The average discharge burnup in the two-batch core is comparable to three-batch commercial PWR like APR-1400. The proposed checker-board CR pattern with extended fingers effectively assures cold shutdown in the two-batch FM scenario, while in the three-batch FM, three N-1 scenarios are failed. The whole evaluation process is conducted using Monte Carlo Serpent 2 code in conjunction with ENDF/B-VII.1 nuclear library.

Neutronics design optimization of a small modular fast reactor based on response surface methodology

RSM (response surface methodology) and multi-objective optimization are employed to optimize the preliminary design of a Small Modular Fast Reactor. The basic idea of RSM is to use mathematical models to represent and describe the complicated relationship between inputs and outputs for multi-variable and multi-objective problems. In this study, by means of RSM, a preliminary core design that meets the requirements of both small excess reactivity and small burnup reactivity swing is proposed without empirical preliminary calculation and time-consuming search. The core design is then fine-tuned according to the common engineering requirements. Finally, a 50MWt core design with a burnup reactivity swing<1$ in 20 years has been proposed. In addition, two independent control systems are designed and performance of the core has been studied. Results of the basic core design using RSM show that RSM is a candidate method feasible for preliminary core design.

Conceptual core design of the molten metal fueled microreactor with self-regulating capability

The microreactor concept has received significant attention in the United States for its lower capital investment, siting flexibility and high mobility. A microreactor aims to provide reliable electricity in remote communities, mining areas, or isolated islands, which will save the high fuel transportation cost. Thus, minimizing the staff level for operating and maintaining a microreactor is desirable. The objective of the study is to design a self-regulated Molten Metal fueled microReactor core (MMR) and to demonstrate the self-regulating capability of the MMR core. The control of the reactor is solely relying on reactivity feedbacks from fuel temperature perturbation. Molten UMn (liquid form at the temperature over 720 °C) was utilized as the fuel material in MMR due to the larger thermal expansion coefficient compared to the conventional solid fuel. The MMR core has a fast neutron spectrum, which corresponds to a small burnup reactivity swing. The core design parameters were selected based on DAKOTA, REBUS, and ANLHTP calculations. The MMR is designed to run for 10 years at a power level of 15 MWth. No refueling is needed during the reactor lifetime. The system dynamic analyses were performed, and it was found that the reactor can be self-regulating within a temperature range of 800 ± 80 °C for a loss of heat sink transient.

An ultra-long-life small safe fast reactor core concept having heterogeneous driver-blanket fuel assemblies

New 80-MW (electric) ultra-long-life sodium cooled fast reactor core having inherent safety characteristics is designed with heterogeneous fuel assemblies comprised of driver and blanket fuel rods. Several options using upper sodium plenum and SSFZ (Special Sodium Flowing Zone) for reducing sodium void reactivity are neutronically analyzed in this core concept in order to improve the inherent safety of the core. The SSFZ allowing the coolant flow from the peripheral fuel assemblies increases the neutron leakage under coolant expansion or voiding. The Monte Carlo calculations were used to design the cores and analyze their physics characteristics with heterogeneous models. The results of the design and analyses show that the final core design option has a small burnup reactivity swing of 618 pcm over ~54 EFPYs cycle length and a very small sodium void worth of ~35pcm at EOC (End of Cycle), which leads to the satisfaction of all the conditions for inherent safety with large margin based on the quasi-static reactivity balance analysis under ATWS (Anticipated Transient Without Scram).

MicroURANUS : Core design for long‐cycle lead‐bismuth‐cooled fast reactor for marine applications

A core design of MicroURANUS, a long-cycle lead-bismuth-cooled fast nuclear reactor for marine applications, is presented. It aims to generate a power of 60MWth, which can be regulated during operation. MicroURANUS was designed to achieve a small burnup reactivity swing for 30 effective full-power years of a lifetime without refueling. To attain these goals, a unit cell study with uranium oxide fuel was initiated to lay a solid foundation for core design, owing to size constraints. A reflector optimization was also performed to minimize neutron leakage. MicroURANUS adopts onion zoning constructed by two enrichment zones for flattening the power and lengthening the core lifetime. The coolant is driven by electromagnetic pumps to achieve inlet and outlet temperatures of 250°C and 350°C, respectively. MicroURANUS analyses were performed using the Argonne Reactor Computation suite and the Monte Carlo code MCS. Core performance features were analyzed for criticality, power profiles, fuel isotope mass inventory, reflector coefficients, reactivity feedback coefficients, and shutdown margin. Additional thermal-hydraulic calculations were performed to confirm that the fuel and cladding temperatures were within the acceptable range. Furthermore, a load follow analysis using the quasi-static reactivity balance method confirmed the feasibility of regulating power by adjusting the inlet coolant temperature.

HIGH-PERFORMANCE MULTI-BATCH FUEL MANAGEMENTS FOR THE ADVANCED SOLUBLE-BORON-FREE ATOM CORE

The autonomous transportable on-demand reactor module (ATOM), a 450 MWth PWR-type small modular reactor (SMR), is under development at Korea Advanced Institute of Science and Technology (KAIST). The ATOM core is designed for soluble-boron-free and passive autonomous load-following operations by utilizing successfully an advanced reactivity control technology, centrally-shielded burnable absorber (CSBA). To enhance the ATOM core safety, CrAl-coated Zircaloy-4 is adopted as an accident-tolerant-fuel cladding. For a long operational cycle, the reference ATOM core has primarily accomplished with a single-batch fuel management (FM). In this paper, for more flexible operation and enhanced fuel utilization, various multi-batch FMs are investigated while the core performance is maintained in terms of both neutronic and safety aspects. These aspects are refueling pattern, cycle length, burnup reactivity swing, discharge burnup, axial and radial power peaking factor (PPF), total PPF, and temperature coefficients. Several refueling types are examined: In-out (low leakage), out-in (flattened power), and randomly scattered schemes. In addition, new heavy reflector designs, ZrO2 and PbO, are introduced instead of stainless steel reflector for an improved core performance. Moreover, a new CSBA loading pattern is also proposed for an effective reactivity control of multi-batch FM strategy. Numerical results show that with a two-batch FM the cycle length can achieve above 2 years with an average discharge burnup of 40 GWd/tU, while the burnup reactivity swing remains less than 1,200 pcm. On top of that, the coolant and fuel temperature coefficients are highly negative at the beginning of cycle and power profile is comparable to that with the single-batch FM. All calculations in these multi-physics assessments of the ATOM core are performed using a Monte Carlo-diffusion hybrid code system based on Monte Carlo Serpent 2 and nodal diffusion COREDAX codes.

Validation of SARAX code system using the SFR operational tests

This paper introduces extended validation works on SARAX code based on the hot state data of two SFRs, the JOYO and Phénix reactors. Compared with the previous validation based on the zero-power physical tests, these validation cases challenge the code because the calculation should consider more under the operational condition, including the fuel depletion, temperature effect and geometry expansion. Besides, more computational assumptions are discussed in this paper, which contains approximation of constant microscopic cross-sections, sensitivity to different depletion chain and so on. The isothermal temperature effect and burnup reactivity swing calculation were validated using the JOYO reactor and the control rod worth were validated based on the withdrawal tests in the Phénix final tests. The results show that SARAX code has good accuracy in simulating the SFR in the hot states. The neutronic parameters such as reactivity, control rod worth, and power distribution agreed well with the experimental data.

Comparisons of Reduced Moderation Small Modular Reactors With Heavy Water Coolant

This paper presents the comparison of two reduced moderation small modular reactor concepts with heavy water coolant. Two reduced moderation small modular reactors, RMSMR-Th and RMSMR-MOX, are proposed for the sustainable utilization of nuclear resources. The design concepts are established on modifications of the well-experienced pressurized water reactor technology. Tightly-packed lattice and heavy water coolant are employed to yield a hard neutron spectrum, which proved advantageous for increasing the conversion ratio as well as lowering the burnup reactivity swing between beginning of cycle and end of cycle. Thorium-uranium dioxide fuel and MOX fuel are compared using the same core arrangement, and the small modular reactor concept is adopted to reduce void coefficients. Radial blanket region and axial blanket region are adopted to enhance the fissile breeding and a three-zone fuel configuration is adopted to flatten the power distribution. Core burnup calculations were carried out to investigate the available cycle length, the conversion ratio, the power peaking factors, reactivity coefficients, etc. Light-water based thermal-hydraulic models were employed to examine the safety features of the concepts. Numerical simulations indicate that both RMSMR-Th and RMSMR-MOX can sustain the power generation of 100MWe by 7 years without refueling. Compared with thorium-uranium dioxide fuel, MOX fuel is helpful in reducing the burnup reactivity swing, increasing the conversion ratio and increasing the mDNBR value. However, the positive void coefficient becomes a serious problem making RMSMR-MOX less attractive than the RMSMR-Th concept.

Physical study of an ultra-long-life small modular fast reactor loaded with U-Pu-Zr fuel

The physical characteristics of a small modular fast reactor loaded with U-Pu-Zr fuel were studied in this paper. The reactor can be operated for more than 20 years without refueling. The small burnup reactivity swing was achieved thanks to the good balance between the fuel breeding characteristics and decay of 241Pu with a half-life of 14.239 years, which avoids increasing the initial excess reactivity of the core. Nuclide effect analyses in this paper show that the decay of 241Pu plays a non-negligible role in burnup reactivity swing for the ultra-long-life low power reactors loaded with U-Pu-Zr fuel. The reactor neutronic parameters such as burnup behavior, displacement per atom (DPA) of the fuel rod cladding, spent fuel composition, and reactivity feedback coefficients were analyzed. The operating modes of the reactor were also analyzed.

Conceptual design of an innovative reduced moderation thorium‐fueled small modular reactor with heavy‐water coolant

This paper presents an innovative conceptual design for small modular reactors, the reduced-moderation small modular reactor (RMSMR), for the sustainable use of nuclear resources. The concept is established by a modification of the well-understood pressurized water reactor technology. A reduced-moderation lattice and heavy-water coolant are used to yield an epithermal-to-fast neutron spectrum, which is beneficial for attaining a large conversion ratio and reducing the burnup reactivity swing throughout the core lifetime. Two-dimensional pin cell and three-dimensional core burnup calculations are performed to systematically analyze the neutronics influences of important parameters, such as the coolant type, moderator-to-fuel ratio, and fuel type. The RMSMR adopts a three-zone uranium-thorium dioxide fuel configuration to flatten the power distribution and ensure a negative void coefficient. The radial and axial blanket regions are found to enhance the breeding effect. The proposed RMSMR can sustain power generation of 100 MWe for 7 years without refueling and achieve a conversion ratio of 0.85 at the end of the cycle. Numerical simulations indicate that the proposed concept has satisfactory shutdown margins and reactivity coefficients and conservative thermal-hydraulic safety. The RMSMR may be a promising candidate to fill the gap between light-water reactors and fast breeder reactors.

Minor Actinide Transmutation in Supercritical-CO2-Cooled and Sodium-Cooled Fast Reactors with Low Burnup Reactivity Swings

This paper presents the investigation of minor actinide (MA) transmutation in supercritical CO2-cooled and sodium-cooled fast reactors (S-CO2-FR and SFR) with the thermal output of 600 MW(thermal) for simultaneously attaining low burnup reactivity swings and reducing long-life radioactive waste. Minor actinides are loaded uniformly in the fuel of the cores, and the MA contents are determined to minimize the burnup reactivity swings. In the S-CO2-FR, the burnup reactivity swing is minimized to 0.11% ∆k/kk’ when the MA content is 6.0 wt%. In the SFR, the MA content was determined to reduce the burnup reactivity swing while maintaining sodium void reactivity under a design limitation of 5 $. The burnup reactivity swing of the SFR is reduced to 1.94% ∆k/kk’, whereas sodium void reactivity is about 4.7 $ when 10.0 wt% MAs are loaded. The low burnup reactivity swing enables minimization of control rod operation during fuel burnup. The number of control rods in the two reactors is reduced to ten, which is half of a typical sodium-cooled mixed-oxide fuel MONJU reactor without MA loading. The MA transmutation rates in the S-CO2-FR and SFR are 42.2 and 52.2 kg/year, respectively, which are equivalent to the production rates in seven and nine light water reactors of the same electrical output.

An advanced core design for a soluble-boron-free small modular reactor ATOM with centrally-shielded burnable absorber

Abstract A complete solution for a soluble-boron-free (SBF) small modular reactor (SMR) is pursued with a new burnable absorber concept, namely centrally-shielded burnable absorber (CSBA). Neutronic flexibility of the CSBA design has been discussed with fuel assembly (FA) analyses. Major design parameters and goals of the SBF SMR are discussed in view of the reactor core design and three CSBA designs are introduced to achieve both a very low burnup reactivity swing (BRS) and minimal residual reactivity of the CSBA. It is demonstrated that the core achieves a long cycle length (∼37 months) and high burnup (∼30 GWd/tU), while the BRS is only about 1100 pcm and the radial power distribution is rather flat. This research also introduces a supplementary reactivity control mechanism using stainless steel as mechanical shim (MS) rod to obtain the criticality during normal operation. A further analysis is performed to investigate the local power peaking of the CSBA-loaded FA at MS-rodded condition. Moreover, a simple B4C-based control rod arrangement is proposed to assure a sufficient shutdown margin even at the cold-zero-power condition. All calculations in this neutronic-thermal hydraulic coupled investigation of the 3D SBF SMR core are completed by a two-step Monte Carlo-diffusion hybrid methodology.

Research on the application of burnable poison ZrB2 and Gd2O3 in a small modular lead-based thermal reactor

A lead-based reactor with thermal or epithermal spectra is considered to be a good choice for small modular reactors design. Beside the safe and better thermal property of lead coolant it also takes the advantage of the smaller critical mass characteristics of thermal reactor. However, it needs to control the substantial amounts of excess reactivity at the beginning of core life which is essential to get a long-life cycle. Therefore, choosing the appropriate burnable poison material and optimizing its arrangement is a big concern during the design of a small modular lead-based thermal reactor.In this article, the analysis on use of burnable poison in a small modular lead-based thermal reactor has been carried out to achieve effective control of reactivity. The burnup characteristics of burnable poison ZrB2 and Gd2O3 were investigated and compared. The results show that ZrB2 is better than Gd2O3 for flatting the keff curve, while the depletion rate of Gd2O3 is much faster than ZrB2. Reducing the poison content in poison rods and increasing the number of poison assemblies can speed up the poison depletion. Based on the results, an optimized core (with the use of burnable poison) which can give a more flat keff curve with the burnup reactivity swing less than 3303 pcm was given. And the influences of the burnable poison on the main parameters of the optimized core scheme such as power distribution and reactivity coefficient were analyzed. The results show that with the use of burnable poison, the radial power distribution becomes more flat and the coolant void effect and fuel temperature reactivity become more negative.

Characteristics of a gas-cooled fast reactor with minor actinide loading

This paper presents the neutronics characteristics of a prototype gas-cooled (supercritical CO2-cooled) fast reactor (GCFR) with minor actinide (MA) loading in the fuel. The GCFR core is designed with a thermal output of 600 MWt as a part of a direct supercritical CO2 (S-CO2) gas turbine cycle. Transmutation of MAs in the GCFR has been investigated for attaining low burnup reactivity swing and reducing long-life radioactive waste. Minor actinides are loaded uniformly in the fuel regions of the core. The burnup reactivity swing is minimized to 0.11% ∆k/kk’ over the cycle length of 10 years when the MA content is 6.0 wt%. The low burnup reactivity swing enables minimization of control rod operation during burnup. The MA transmutation rate is 42.2 kg/yr, which is equivalent to the production rates in 7 LWRs of the same electrical output.

An Advanced Sodium-Cooled Fast Reactor Core Concept Using Uranium-Free Metallic Fuels for Maximizing TRU Burning Rate

In this paper, we designed and analyzed advanced sodium-cooled fast reactor cores using uranium-free metallic fuels for maximizing burning rate of transuranics (TRU) nuclides from PWR spent fuels. It is well known that the removal of fertile nuclides such as 238U from fuels in liquid metal cooled fast reactor leads to the degradation of important safety parameters such as the Doppler coefficient, coolant void worth, and delayed neutron fraction. To resolve the degradation of the Doppler coefficient, we considered adding resonant nuclides to the uranium-free metallic fuels. The analysis results showed that the cores using uranium-free fuels loaded with tungsten instead of uranium have a significantly lower burnup reactivity swing and more negative Doppler coefficients than the core using uranium-free fuels without resonant nuclides. In addition, we considered the use of axially central B4C absorber region and moderator rods to further improve safety parameters such as sodium void worth, burnup reactivity swing, and the Doppler coefficient. The results of the analysis showed that the final design core can consume ~353 kg per cycle and satisfies self-controllability under unprotected accidents. The fuel cycle analysis showed that the PWR–SFR coupling fuel cycle option drastically reduces the amount of waste going to repository and the SFR burner can consume the amount of TRUs discharged from 3.72 PWRs generating the same electricity.

The excess reactivity management in small pressurized water reactor utilizing fully ceramic microencapsulated fuel

After Fukushima Daiichi nuclear disaster, different techniques to overcome the weaknesses of pressurized water reactor (PWR) were developed, such as a change of the cladding and fuel materials. The Fully Ceramic Microencapsulated (FCM) fuel is one of those potential materials to replace traditional UO2 pellets fuel to make PWR safer.In this paper, a neutronics study has been performed to apply burnable poison (BP) for an excess reactivity management in a 200 MWt small PWR adopted soluble boron free (SBF) concept. In this core design, fuel enrichment is set as 19.9 w/o to get long enough cycle length due to the low packing faction of FCM fuels. As a result, the excess reactivity control of the core would be huge at the beginning of core life. In order to suppress excess reactivity efficiently, burnable poison such as Pu-238, Integral Fuel Burnable Absorber (IFBA) and Wet Annular Burnable Absorber (WABA) were evaluated for a cycle in terms of the major core performance parameters such as the suppressing excess reactivity, shutdown margin, and core cycle length. MCNP4C and ORIGEN were used as a tool to perform the results. It has been demonstrated that the burnup reactivity swing over a long cycle period can be reduced from 50,000 pcm to about 20,000 pcm due to utilizing different type burnable poison. It is shown that combinations of burnable poison with control rods make it possible to reduce excess reactivity and get enough shutdown margin during the core life for soluble boron free operation.

Advanced Burner Reactors with Breed-and-Burn Thorium Blankets for Improved Economics and Resource Utilization

This paper assesses the feasibility of designing seed-and-blanket (S&B) sodium-cooled fast reactor (SFR) cores to generate a significant fraction of the core power from radial thorium-fueled blankets that operate in the breed-and-burn (B&B) mode. The radiation damage on the cladding material in both seed and blanket does not exceed the presently acceptable constraint of 200 displacements per atom (dpa). The S&B core is designed to have an elongated seed (or driver) to maximize the fraction of neutrons that radially leak into the subcritical B&B blanket and reduce the neutron loss via axial leakage. A specific objective of this study is to maximize the fraction of core power generated by the B&B blanket that is proportional to the neutron leakage rate from the seed to the blanket. Since the blanket feed fuel is very inexpensive and requires no reprocessing and remote fuel fabrication, a larger fraction of power from the blanket will result in a lower fuel cycle cost per unit of electricity generated by the SFR core. It is found possible to design the seed of the S&B core to have a lower transuranics (TRU) conversion ratio (CR) than a conventional advanced burner reactor (ABR) core without deteriorating core safety. This is due to the unique synergism between a low CR seed and the B&B thorium blanket. The benefits of the synergism are maximized when using an annular seed surrounded by inner and outer thorium blankets. Two high-performance S&B cores are designed to benefit from the annular seed concept: (1) an ultra-long-cycle core having a CR = 0.5 seed and a cycle length of ~7 effective full-power years (EFPYs) and (2) a high-transmutation core having a TRU CR of 0.0. The TRU transmutation rate of the latter core is comparable to that of the reference ABR with a CR of 0.5, and the thorium blanket can generate close to 60% of the core power. Because of the high blanket power fraction along with the high discharge burnup of the CR = 0 seed, the reprocessing capacity per unit of core power required by this S&B core is only approximately 1/6th that of the reference ABR core with a TRU CR of 0.5. Although the seed fuel CR is nearly zero, the burnup reactivity swing is low enough to enable a cycle length of more than 4 EFPYs. This is attributed to a combination of reactivity gain in the thorium blankets over the cycle and the relatively high heavy metal inventory. Moreover, despite the very low leakage, the S&B cores feature a less positive coolant reactivity coefficient and large enough negative Doppler coefficient even when using nonfertile fuel for the seed, because of the unique physics properties of the 233U and Th in the thorium blankets. With the long cycles, the S&B SFR is expected to have a higher capacity factor, and therefore a lower cost of electricity, than conventional ABRs. The discharge burnup of the thorium blanket fuel is typically 70 MWd/kg such that the thorium fuel utilization is approximately 12 times that of natural uranium in light water reactors. A sensitivity study is subsequently undertaken to quantify the trade-off between the core performances and several design variables: amount of zirconium in the inert matrix seed fuel, active core height, coolant pressure drop, and radiation damage constraint. The effect of the criterion used for quantifying acceptable radiation damage is evaluated as well. It is concluded that a viable S&B core can be designed without significant deviation from typical SFR core design practices.

An innovative core design for a soluble-boron-free small pressurized water reactor

This research investigates the neutronic feasibility of a high-performance soluble-boron-free (SBF) small modular reactor (SMR) core based on a new burnable absorber concept called the “Burnable Absorber-Integrated Guide Thimble” (BigT). Three unique BigT designs were loaded into the core; each BigT design was judiciously ascertained from the core radial power profile to tailor the required reactivity depletion patterns for an SBF operation. The approach is demonstrated to work well as the SBF SMR design exceeds the targeted cycle length while successfully controlling its burnup reactivity swing between 634 and 800 pcm throughout most of its operation. This paper also describes the use of hafnium-doped stainless steel as mechanical shim (MS) rods to attain the core criticality. Because the worth of the MS rods is relatively small, insertion and withdrawal of the rods during operation hardly alter the core radial power distributions. The resulting axial power profile, meanwhile, displays a more refined bottom-skewed pattern during the early portion of the irradiation cycle due to partial top-half insertion of the MS rods. This investigation further deliberates on a modified checker board control rod pattern to assure safe cold shutdown of the core. All calculations in this multiphysics assessment of the 3D SBF SMR core were completed by using a 2-step Monte Carlo deterministic hybrid procedure based on the Monte Carlo Serpent and COREDAX diffusion codes with the ENDF/B-7.1 nuclear data library.

Robust PID control of power in lead cooled fast reactors: A direct synthesis framework

Output core power control for an innovative design of lead cooled fast reactors (BREST) is investigated applying a PID design approach. The reactor core model is developed through a lumped parameter thermo-neutronic coupled layout of the plant. Control design is carried out within a direct synthesis framework whereby desired closed loop characteristics are incorporated. The PID coefficients are calculated employing a Laurent expansion of the intended controller and the resultant control scheme is employed on the uncertain nonlinear reactor plant. A worst case robust stability analysis is thereby conducted for the extreme hypothetical working point of the reactor subject to a near prompt criticality reactivity insertion. Results confirm that the control practice proposed may quite satisfactorily be implemented on the original plant as reasonable robustness characteristics are preserved for this ultimate operational point of the system. Nyquist stability criterion has been resorted for robust stability analysis with cycle burnup reactivity swing taken as the source of uncertainty and further recast into a multiplicative layout. The overall control method for desired power trajectories provides a safe reactivity insertion rate and a feasible rod speed.

Design Study of Thorium Cycle Based Long Life Modular Boiling Water Reactors

Design study of long life (cycle) Boiling Water Reactor, which can be operated for 20-30 years of operation time without the necessity of refuelling during that period, has been performed. In shown in the previous study Th232-U233 cycle based fuel has potential for longer operation time in the thermal reactor domain. To increase reactor operation time and minimize excess-reactivity burnable poison may be used. Protactinium (Pa-231), Np-238, or gadolinium has good properties for such purpose. Here based on the previous study we select Protactinium and Gadolinium poisons to be used in the present study. Optimizations the content of 231Pa in the core enables the BWR core to sustain enough reactivity for long period of time with reasonable burn-up reactivity swing. Based on the optimization of fuel element composition (Th and Pa) in various moderation compositions can be achieved reactor core with longer operation time, 20 ~ 30 years operation without fuel shuffling or refuelling. Similarly Gadolinium has been successfully used to extend refuelling time and reduce excess reactivity during burnup period for both oxide fuel and nitride fuel.