Reactivity Swing Research Articles

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.

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.

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.

Core design options of an ultra-long-cycle sodium cooled reactor with effective use of PWR spent fuel for sustainable energy supply

In this work, 350MWe ultra-long-cycle sodium-cooled reactor cores are designed to supply electric energy over ~60 Effective Full Power Years (EFPYs) without refueling and with an effective use of Transuranics (TRU) and uranium from large pressurized water reactor (PWR) spent fuel stocks. The core employs the axial blanket-driver-blanket (ABDB) burning strategy, which was recently proposed by the authors to achieve an ultra-long-cycle length with self-controllability under unprotected accidents. In particular, a thorium–uranium fuel cycle is considered to remove the heterogeneity of the fuel assemblies for design simplification and to improve the core performance parameters by selectively adding thorium into both blanket and driver fuels. The results show that the use of TRU nuclides from PWR spent fuel leads to significant extension of the fuel cycle length, but considerable increase of burnup reactivity swing. In addition, these results also indicate that the uranium–thorium mixed fuels both in the lower blanket and driver considerably improve the inherent safety of the ultra-long-cycle core by reducing burnup reactivity and sodium void worth; this makes it possible to simplify the previous heterogeneous fuel assembly design with improved core performances. Copyright © 2016 John Wiley & Sons, Ltd.

Design study of long-life small modular sodium-cooled fast reactor

Summary This paper presents a new design for a small modular sodium-cooled fast reactor core with an optimized lifetime and reactivity swing through the analysis of various breed-and-burn strategies and its neutronic analyses in terms of active core movements, isotopic mass balance, kinetic parameters, and inherent safety. The new core design aims at a power level of 260 MW with a long lifetime of 30 years without refueling and a reactivity swing smaller than 1000 pcm. Starting from five initial candidate cores with various breed-and-burn strategies, an optimum core was selected from a combination of the two candidates that shows a proper breeding behavior with the optimized uranium enrichment in the low-enriched uranium region and the optimized size of the blanket region. The depletion analysis of the new core provides various reactor design parameters such as the core multiplication factor, breeding ratio, heavy metal mass change, power distribution, and summary of neutron balance. In addition, the perturbation analysis provides the reactor kinetic parameters and reactivity feedback coefficients for the inherent safety analysis of the core. The integral reactivity parameters of the quasi-static reactivity balance analysis demonstrate that the new core is inherently safe in cases of unprotected loss of flow, unprotected loss of heat sink, and unprotected transient over power. Copyright © 2016 John Wiley & Sons, Ltd.

Boron-free small modular pressurized water reactor design with new burnable absorber

Summary This paper presents a preliminary design of a Small Modular Pressurized Water Reactor (SMPWR) aimed at boron-free operation which can reduce the size of a nuclear power plant (NPP) and the amount of liquid radioactive waste, and also reduce the corrosion issues caused by boric acid in the coolant. The design parameter limits, such as reactivity swing, axial offset (AO), 3D pin peaking factor (Fq), and the required shutdown margin, have been established for the boron-free SMPWR. Furthermore, a new ring type of burnable absorber (R-BA) with Zr–167Er is adopted as a burnable absorber (BA) and HfB2 is adopted as a control rod material to satisfy those design limits without liquid boron. Optimal fuel assemblies (FAs) and loading patterns have been searched through a sensitivity study, and the excess reactivity control capacity and design requirements have been demonstrated to be satisfactory. Copyright © 2016 John Wiley & Sons, Ltd.

Advanced CANDU reactors axial xenon oscillation controllability validation

Advanced CANDU reactor fuel channel assembly MCNP model with top, bottom, and four lattice sides reflecting boundaries is used to generate a conservative flat axial neutron flux distribution for the 135Xe oscillation controllability analyses. Burnup-dependent neutron Fission Tally and 135Xe axial profiles are calculated using the Monte Carlo burnup script MCOS. The controllability of the 135Xe oscillations is validated by the MCOS-calculated xenon reactivity worth swings with respect to the mean. The validated upper bound of the xenon reactivity swing band of ±3mk is well within the zone controller reactivity bank of ±7mk.

Neutronic Design Features of a Transportable Fluoride-Salt-Cooled High-Temperature Reactor

The fluoride-salt-cooled high-temperature reactor (FHR) is a new reactor concept, which combines low-pressure liquid salt coolant and high-temperature tristructural isotropic (TRISO) particle fuel. The refractory TRISO particle coating system and the dispersion in graphite matrix enhance safeguards (nuclear proliferation resistance) and security. Compared to the conventional high-temperature reactor (HTR) cooled by helium gas, the liquid salt system features significantly lower pressure, larger volumetric heat capacity, and higher thermal conductivity. The salt coolant enables coupling to a nuclear air-Brayton combined cycle (NACC) that provides base-load and peak-power capabilities. Added peak power is produced using jet fuel or locally produced hydrogen. The FHR is, therefore, considered as an ideal candidate for the transportable reactor concept to provide power to remote sites. In this context, a 20-MW (thermal power) compact core aiming at an 18-month once-through fuel cycle is currently under design at Massachusetts Institute of Technology (MIT). One of the key challenges of the core design is to minimize the reactivity swing induced by fuel depletion, since excessive reactivity will increase the complexity in control rod design and also result in criticality risk during the transportation process. In this study, burnable poison particles (BPPs) made of B4C with natural boron (i.e., 20% B10 content) are adopted as the key measure for fuel cycle optimization. It was found that the overall inventory and the individual size of BPPs are the two most important parameters that determine the evolution path of the multiplication factor over time. The packing fraction (PF) in the fuel compact and the height of active zone are of secondary importance. The neutronic effect of Li6 depletion was also quantified. The 18-month once-through fuel cycle is optimized, and the depletion reactivity swing is reduced to 1 beta. The reactivity control system, which consists of six control rods and 12 safety rods, has been implemented in the proposed FHR core configuration. It fully satisfies the design goal of limiting the maximum reactivity worth for single control rod ejection within 0.8 beta and ensuring shutdown margin with the most valuable safety rod fully withdrawn. The core power distribution including the control rod’s effect is also demonstrated in this paper.

Power flattening study of ultra-long cycle fast reactor using thorium fuel

This paper presents a design study of power shape flattening for an optimized ultra-long cycle fast reactor with a power rate of 1000 MWe in order to mitigate the power peaking issue and improve the safety with a lower maximum neutron flux and reactivity swing. There are variations in the core designs by loading thorium fuel or zoning fuels in the blanket region and the bottom driver region of ultra-long cycle fast reactor with a power rate of 1000 MWe. While it has lower breeding performance in a fast breeder reactor, thorium fuel is one of the promising fuel options for future reactors because of its abundance and its safety characteristics. It has been confirmed that the thorium fuels, when loaded into the center region of a reactor core, lower the power peaking factor from 1.64 to 1.25 after 20 years and achieves a more flattened radial power distribution. This consequently reduces the maximum neutron flux and the speed of the active core moving from 3.0 cm/year to 2.5 cm/year on the average over the 60-year reactor operation. It has been successfully demonstrated that the three-zone core is the most optimized core, has the most flattened radial power shape, and is without any compromise in the nature of long cycle core, from the neutronics point of view, in terms of average discharge burnup and breeding ratio. Copyright © 2016 John Wiley & Sons, Ltd.

A Core Physics Study of Advanced Sodium-Cooled TRU Burners with Thorium- and Uranium-Based Metallic Fuels

In this work, 400-MW(electric) sodium-cooled fast reactor cores using thorium- and uranium-based metallic fuels for high burning rates of light water reactor spent-fuel transuranics (TRUs) are neutronically designed and analyzed based on equilibrium cycles with a focus on consistent comparative analysis of the differences in performance between thorium- and uranium-based fueled cores. Axial uranium and thorium blankets are introduced in thorium- and uranium-based driver fueled burner cores to improve TRU burning rates without considerable increases of burnup reactivity swing. For this core configuration, it was shown that cores using thorium and depleted uranium blankets can be designed to have a high TRU burning rate, a low sodium void reactivity (SVR) worth, and a low burnup reactivity swing. In particular, the use of uranium or thorium blankets without recycling in the thorium-based driver fueled cores led to significant reductions of burnup reactivity swing with considerable increases of the TRU burning rate and small increases of SVR. In addition, the core configuration having central nonfuel regions was considered to show the effects of the thorium-based driver metallic fuel versus the uranium-based metallic fuel coupled with moderator rods. The core configuration with thorium-based fuel led to a negative SVR without moderator rods, and the use of moderator rods further improved the Doppler coefficient and reduced SVR. Also, a decomposition analysis of SVR was performed to better understand the differences in the contributing factors between the uranium- and thorium-based fueled cores, and a quasi-static reactivity balance analysis was performed to show the inherent safety of the cores in terms of self-controllability.

A sodium-cooled ultra-long-life reactor core having improved inherent safety with new driver–blanket burning strategy

In this paper, two sodium-cooled ultra-long-life reactor cores are designed by using new axial blanket-driver burnup strategy such that they have ultra-long-life longer than 40 EFPYs (Effective Full Power Years), high average fuel burnup of ∼180 MWD/kg, small burnup reactivity swing, and improved inherent safety in terms of self-controllability under unprotected accidents. An exploration on the arrangement of driver and blanket fuels revealed that axially blanket-driver-blanket (B-D-B) configuration with different thicknesses in inner and outer core regions is effective to achieve the design goals. The final core was determined through the reduction of sodium void worth with an upper sodium plenum above the core and with partial use of dummy rods in the lower blanket of the inner core, and through the reduction of burnup reactivity swing with the adjustment of driver and blanket heights and with partial use of dummy rods in outer driver region. The neutronic analysis shows that the final core has desirable features such as long life of 53 EFPYs, high average burnup of 182 MWD/kg, small burnup reactivity swing of 773 pcm, and satisfaction of all the conditions for self-controllability under unprotected accidents.

Thorium utilization in existing and advanced reactor types

A comparative study of the capability of some of the existing power reactor designs to produce fissile material that is vital for sustenance of fission nuclear power is made. The fissile conversion rate is less in reactors using enriched fuel compared to the natural uranium fueled ones. Even in fast reactors the net fissile breeding is feasible only due to blanket regions. A case is made for reconfiguring the existing reactor designs to accommodate internal blankets or exclusive fissile breeding zones. For maximizing the net fissile accumulation it is suggested to use suitably large volume/mass of fertile breeding zones and extend the fuel cycle duration to as long as 2 or 3 years. The burnup reactivity swing of such modified core designs can be as little as ±5 mk over 2 to 3 full power years, thus obviating the need for large excess reactivity control through either burnable absorbers like boron, Gd or control absorbers. The safety characteristics of the new core designs improve with minimal reactivity changes due to temperature, Xe, Sm, burnup and voiding.

Burning high-level TRU waste in fusion fission reactors

Recently, the concept of actinide burning instead of a once-through fuel cycle for disposing spent nuclear fuel seems to get much more attention. A new method of burning high-level transuranic (TRU) waste combined with Thorium–Uranium (Th–U) fuel in the subcritical reactors driven by external fusion neutron sources is proposed in this paper. The thorium-based TRU fuel burns all of the long-lived actinides via a hard neutron spectrum while outputting power. A one-dimensional model of the reactor concept was built by means of the ONESN_BURN code with new data libraries. The numerical results included actinide radioactivity, biological hazard potential, and much higher burnup rate of high-level transuranic waste. The comparison of the fusion–fission reactor with the thermal reactor shows that the harder neutron spectrum is more efficient than the soft. The Th–U cycle produces less TRU, less radiotoxicity and fewer long-lived actinides. The Th–U cycle provides breeding of [Formula: see text]U with a long operation time (>20 years), hence significantly reducing the reactivity swing while improving safety and burnup.

3D in-core fuel management optimization for breed-and-burn reactors

Breed-and-burn (B&B) reactors are a special class of fast reactors that are designed to utilize low grade fuel such as depleted uranium without fuel reprocessing. One of the most challenging practical design feasibility issues faced by B&B reactors is the high level of radiation damage their fuel cladding has to withstand in order to sustain the B&B mode of operation – more than twice the maximum radiation damage cladding materials were exposed to so far in fast reactors. This study explores the possibility of reducing the minimum required peak radiation damage by employment of 3-dimensional (3D) fuel shuffling that enables a significant reduction in the peak-to-average axial burnup, that is, more uniform fuel utilization. A new conceptual design of a B&B core made of axially segmented fuel assemblies was adopted to facilitate the 3D shuffling. Also developed is a Simulated Annealing (SA) algorithm to automate the search for the optimal 3D shuffling pattern (SP). The primary objective of the SA optimization is to minimize the peak radiation damage while its secondary objective is to minimize the burnup reactivity swing, radial power peaking factor and maximum change of fuel assembly power over the cycle. Also studied is the sensitivity of the 3D shuffled core performance to the number of axially stacked sub-assemblies, core height and power level.It was found that compared with the optimal 2-dimensional (2D) shuffled core, the optimal 3D shuffled B&B core made of four 70 cm long axially stacked sub-assemblies and 12 radial shuffling batches offers a 1/3 reduction of the peak radiation damage level – from 534 down to 351 displacements per atom (dpa), along with a 45% increase in the average fuel discharge burnup, and hence, the depleted uranium utilization, while satisfying all major neutronics and thermal-hydraulics design constraints. For the same peak dpa level, the 3D shuffling offers more than double the uranium utilization and the cycle length relative to 2D shuffling. The minimum peak radiation damage is increased to 360 or to 403 dpa if the core is made of, respectively, three – 70 cm or two – 140 cm long axially stacked subassemblies. Reducing the length of the subassemblies of B&B cores made of three-segment assemblies from 70 cm to 60 or 50 cm results in an increase in the peak radiation damage from 360 dpa to, respectively, 368 and 397 dpa.

Preliminary studies of a new accelerator-driven minor actinide burner in industrial scale

Pursuing high transmutation rate of minor actinide (MA), a preliminary conceptual design of a lead-bismuth (LBE) cooled accelerator-driven system (ADS) is proposed in this study. Parametric studies are performed to optimize the neutronics and thermal–hydraulics performances. The proton energy and axial position of the proton beam impact is investigated to obtain high neutron source efficiency and spallation neutron yield. The influences of MA/Pu mixing ratio and the ratio of pin pitch to diameter (P/D) are also optimized to control the burnup reactivity swing and the minimum coolant velocity for adequate cooling. To reduce the power peak, three kinds of power flattening techniques are adopted and compared. The results show that the inert matrix ratio zone loading method seems more versatile. Based on the analyses, an optimized three zone loading pattern is proposed for the 800MWth subcritical core. The total transmutation rate of MA is 328.8kg per effective full power year. Preliminary safety analyses based on the balance of power method (BOP) are performed and the results show that in the reactivity insertion and beam overpower transients, the core shows inherent safety, but the scram is necessary by cutting off the beam current to protect the core from possible damages caused by the loss of flow.

Breed-and-burn strategy in a fast reactor with optimized starter fuel

The in situ breed-and-burn (B&B) strategy in fast reactor is capable of offering very high fuel utilization efficiency. CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy) or TWR (Traveling Wave Reactor) is a possible scheme to implement the B&B strategy. The approach in realizing the equilibrium state of CANDLE is based on neutron source supply from ignition zone with enriched uranium or plutonium. And burnable poison is usually employed to compensate the extremely high excess reactivity during the startup. In the present work, the optimization of the starter fuel with only plutonium and depleted uranium is performed to reduce the initial positive excess reactivity swing and to flatten the power distribution. The optimization is done with refer to a nominal steady state CANDLE core. In this regard the swing of the reactivity is limited during the startup. And the initial power shape is flattened significantly compared with that without optimization. It is proved that constructing the ignition zone with refer to the equilibrium state of a CANDLE is effective and feasible to improve the operation performance during the startup.

Particle-type Burnable Poisons for Thorium-based Fuel in HTGR

Abstract Thorium-based fuels have been proposed for use as alternative nuclear fuels in a 100-MW t prismatic high temperature gas-cooled reactor (HTGR) to overcome the limitation of natural uranium resources. Safety design features have been addressed by using particle-type burnable poison to prevent a prompt supercritical accident in the reactor. The present study compared the proposed system with the use of 235 U- 238 U conventional nuclear fuel in an HTGR. The use of thorium-based fuel showed usefulness in terms of neutron economics, but from the safety standpoint, the thorium fuel reversed the inherent safety characteristics of a negative temperature coefficient. Introducing the appropriate composition of particle-type burnable poison can be useful for reactivity control in long-term operation, but it cannot guarantee the reactor will be free from the prompt supercritical accidents. The use of particle-type burnable poisons did sufficiently minimize the reactivity and flatten the reactivity swing to decrease the burden on the control rods mechanism, and reduce the positive temperature coefficient or even make it negative. These results did improve the safety features of the reactor.