Kamil Tucek

Definition and application of proton source efficiency in accelerator-driven systems

Abstract In order to study the beam power amplification of an accelerator-driven system (ADS), a new parameter, the proton source efficiency ψ* is introduced. ψ* represents the average importance of the external proton source, relative to the average importance of the eigenmode production, and is closely related to the neutron source efficiency φ*, which is frequently used in the ADS field. φ* is commonly used in the physics of subcritical systems driven by any external source (spallation source, (d,d), (d,t), 252Cf spontaneous fissions, etc.). On the contrary, ψ* has been defined in this paper exclusively for ADS studies where the system is driven by a spallation source. The main advantage with using ψ* instead of φ* for ADS is that the way of defining the external source is unique and that it is proportional to the core power divided by the proton beam power, independent of the neutron source distribution. Numerical simulations have been performed with the Monte Carlo code MCNPX in order to study ψ* as a function of different design parameters. It was found that, in order to maximize ψ* and therefore minimize the proton current needs, a target radius as small as possible should be chosen. For target radii smaller than ~30 cm, lead-bismuth is a better choice of coolant material than sodium, regarding the proton source efficiency, while for larger target radii the two materials are equally good. The optimal axial proton beam impact was found to be located ~20 cm above the core center. Varying the proton energy, ψ*/Ep was found to have a maximum for proton energies between 1200 and 1400 MeV. Increasing the americium content in the fuel decreases ψ* considerably, in particular when the target radius is large.

Application of Burnable Absorbers in an Accelerator-Driven System

Abstract The application of burnable absorbers (BAs) to minimize power peaking, reactivity loss, and capture-to-fission probabilities in an accelerator-driven waste transmutation system has been investigated. Boron-10-enriched B4C absorber rods were introduced into a lead-bismuth-cooled core fueled with transuranic (TRU) discharges from light water reactors to achieve the smallest possible power peakings at beginning-of-life (BOL) subcriticality level of 0.97. Detailed Monte Carlo simulations show that a radial power peaking equal to 1.2 at BOL is attainable using a four-zone differentiation in BA content. Using a newly written Monte Carlo burnup code, reactivity losses were calculated to be 640 pcm per percent TRU burnup for unrecycled TRU discharges. Comparing to corresponding values in BA-free cores, BA introduction diminishes reactivity losses in TRU-fueled subcritical cores by ~20%. Radial power peaking after 300 days of operation at 1200-MW thermal power was <1.75 at a subcriticality level of ~0.92, which appears to be acceptable, with respect to limitations in cladding and fuel temperatures. In addition, the use of BAs yields significantly higher fission-to-capture probabilities in even-neutron-number nuclides. Fission-to-absorption probability ratio for 241Am equal to 0.33 was achieved in the configuration studied. Hence, production of the strong alpha-emitter 242Cm is reduced, leading to smaller fuel-swelling rates and pin pressurization. Disadvantages following BA introduction, such as increase of void worth and decrease of Doppler feedback in conjunction with small values of βeff, need to be addressed by detailed studies of subcritical core dynamics.

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.

FUEL CYCLE INVESTIGATION FOR WALLPAPER-TYPE HTR FUEL

As early as the 1970s, attempts have been made to reduce the peak fuel temperature in pebble bed-type high-temperature reactors (HTRs) by means of so-called “wallpaper fuel,” in which the fuel is arranged in a spherical shell within a pebble. By raising the particle packing fraction, fuel kernels are condensed to the outer diameter of the fuel zone, leaving a central part of the pebble free of fuel. This modification prevents power generation in this central fuel-free zone and decreases the temperature gradient across the pebble. Besides the reduction of maximum and average particle temperature, the wallpaper concept also enhances neutronic performance through improved neutron economy, resulting in reduced fissile material and/or enrichment needs or providing the potential to achieve higher burnup. To assess such improvements, calculations were performed using the PANTHERMIX code. Among other tests, investigations of fuel cycle under steady-state conditions and loss-of-coolant-accident calculations were conducted. Based on PANTHERMIX steady-state conditions, both particle failure fraction [with the CRYSTAL code (Code foR analYsis of STress in coAted particLes)] and fissile material cost can be determined. It is demonstrated that the wallpaper fuel type positively impacts the fuel cycle, reduces the production of minor actinides (MAs), and improves the safety-relevant parameters of the reactor. A comparison of these characteristics with those of Pebble Bed Modular Reactor (Pty) Limited (PBMR) type of fuel is presented: In comparison with PBMR fuel, the wallpaper design results in an increase of the effective neutron multiplication coefficient (by [approximately]925 pcm). This reactivity increase can lead to a burnup extension (from 96.4 to 101.3 MWd/kg), therefore improving the burnup of HTRs, or to an enrichment reduction (from 9.6 to 9.277 wt%). Both options decrease MA production [as defined in g/TW(thermal)·h, between 5.9% and 34.5%], making fuel reprocessing easier and reducing fuel cost (by 4.6% for the high-burnup option and by 3.7% for the low-enrichment option). Safety is also improved, with particle temperature being reduced during steady-state operations (by >55 K for the most exposed particles and by almost 10 K on average). This positively impacts particle failure fraction as calculated by the fuel performance code CRYSTAL, leading to a reduction of up to 85% of the particle failure fraction over its in-core lifetime. This reduces the in-core fission product release. While an increase of the graphite density in the central fuel-free zone increases thermal inertia, initiates a faster reactor shutdown, and delays recriticality, it also disturbs the thermal flux that raises pebble powers in the inner part of the core. This increases the highest kernel temperature during a depressurized loss-of-coolant accident from 1872 K for the PBMR case to 1876, 1917, and 1895 K, respectively, for the three wallpaper designs proposed. The fuel changes suggested in this paper offer more versatility to the HTR concept. The conversion ratio can be decreased, leading to lower MA buildup and fuel reprocessing cost, or raised, leading to lower fuel consumption and fuel cost.

Revisiting the Concept of HTR Wallpaper Fuel

Good safety characteristics are an outstanding feature of High Temperature Reactors (HTR): • The high graphite inventory in the core provides significant thermal inertia. Graphite also has a high thermal conductivity, which facilitates the transfer of heat to the reflector, and it can withstand high temperatures; • The strongly negative Doppler coefficient gives a negative feedback, such that the reactor shuts down by itself in overpower accidental conditions; • The high quality of fuel elements — tri-isotropic (TRISO) coated particles — minimizes operational and accidental fission gas release. The materials selected have resistance to high temperatures; • The low power density enables stabilization of core temperature significantly below the maximum allowable, even in case of severe accidents (such as loss-of-coolant accident). Together, these aspects significantly reduce the risk of massive fission product release, which is one of the attractive features of HTRs. The fuel that is currently used in pebble bed reactors such as AVR, HTR-10 and soon PBMR is based on a homogeneous distribution of coated particles within a fuel pebble. This homogenizes power density in the pebble, but creates a radial temperature gradient across the fuel sphere. Fuel particles placed at its centre has the highest temperature. Reducing the average temperature of particles would help preserve their integrity and maintain the resistance of the first barrier against fission product release. As early as the 1970s, attempts were made to reduce the peak fuel temperature by means of so-called “wallpaper fuel”, in which the fuel is arranged in a spherical shell within a pebble. At that time, the production process was not sufficiently mature and had caused unacceptable damage to the (less performing) BISO particles, which is why this fundamentally promising concept was abandoned. In this paper, proposals will be put forward to improve the production process. This paper further exploits the wallpaper concept, not only from the point of view of temperature reduction, but also for enhanced neutronic performance through improved neutron economy, resulting in reduced fissile material and/or enrichment needs or providing the potential to achieve higher burn-up. Parameters modified were the density of the central fuel-free graphite zone and the packing fraction of the fuel zone. It is demonstrated that this fuel type impacts positively on the fuel cycle, reduces production of minor actinides (MA) and improves the safety-relevant parameters of the reactor. A comparison of these characteristics with PBMR-type fuel is presented. The calculations were performed using Monte Carlo neutron transport and depletion codes MCNP/MCB and the deterministic code WIMS. By comparison with PBMR fuel, the “wallpaper design” of the fuel pebble results in an effective neutron multiplication coefficient increase (by about 2%), which is combined with a decrease of between 3 and 15% in MA production. An improved neutron economy of the heterogeneous design enables enrichment of the “wallpaper type” of fuel to be reduced by more than 6%.Copyright

Investigations of Alternative Steam Generator Location and Flatter Core Geometry for Lead-Cooled Fast Reactors

This paper concerns two independent safety investigations on critical and sub-critical heavy liquid metal cooled fast reactors using simple flow paths. The first investigation applies to locating the steam generators in the risers instead of the down-comers of a simple flow path designed sub-critical reactor of 600 MWth power. This was compared to a similar design, but with the steam generators located in the downcomers. The transients investigated were Total-Loss-of-Power and unprotected Loss-Of-Flow. It is shown that this reactor peaks at 1041 K after 29 hours during a Total-Loss-Of-Power accident. The difference between locating the steam generators in the risers and the downcomers is insignificant for this accident type. During an unprotected Loss-Of-Flow accident at full power, the core outlet temperature stabilizes at 1010 K, which is 337 K above nominal outlet temperature. The second investigation concerns a 1426 MWth critical reactor where the influence of the core height versus the core outlet temperature is studied during an unprotected Loss-Of-Flow and Total-Loss-Of-Power accident. A pancake type core geometry of 1.0 m height and 5.8 m diameter, is compared to a compact core of 2 m height and 4.5 m diameter. Moderators, like BeO and hydrides, and their influence on safety coefficients and burnup swings are also presented. Both cores incinerate transuranics from spent LWR fuel with minor actinde fraction of 5%. We show that LFRs can be designed both to breed and burn transuranics from LWRs. It is shown that the hydrides lead to the most favorable reactivity feedbacks, but the poorest reactivity swing. The computational fluid dynamics code STAR-CD was used for all thermal hydraulic calculations, and the MCNP and MCB for neutronics, and burn-up calculations.Copyright