• Energy Research

Abstract Sustainability of a nuclear fuel cycle can be strongly increased by fuel recycling. Not all reactor types have sufficient neutron economy to enable this recycling. Nevertheless, even if the neutron economy is not sufficient, simulation of repetitive recycling, with constant imposed power and fuel cycle parameters, results in converged fuel composition. The final equilibrium state represents an eigenvalue of the respective Bateman equations and strongly differs between the reactor types. Equilibrium reactivity, as a product of the neutron spectrum and fuel composition, determines inherent neutron economy of the reactor and thus its potential for closed fuel cycle and legacy waste burning. In this study the performance of sixteen selected reactor types, eight thermal and eight fast, was evaluated in both U-Pu and Th-U equilibrium fuel cycles. The reactor types were selected so that all major designs and spectra are covered. Even though the equilibrium composition and spectrum mutually influence each other, the general spectrum shape is determined by the relative strength of coolant and structural materials scattering properties. EQL0D v2 MATLAB procedure coupled to the SERPENT 2 code was used for the simulation. Several simplifying assumptions have been applied to enumerate the eigenvector of Bateman matrix; the reprocessing losses were zero, the FPs were neglected and instantaneously replaced by either 238U or 232Th feed, the reactors were represented only by an infinite lattice, and the generated power was fixed at nominal value independently from the criticality or subcriticality level and fissile fuel share. In the obtained equilibrium closed cycle every core acts, per definition, as an iso-breeder. Burned and produced masses are the same for each isotope. As expected, some of the reactors are subcritical and thus cannot be operated with such a fuel cycle in reality. Still the obtained eigenvalues well characterize each reactor performance. To provide additional insight into the equilibrium behavior unique reactivity break-down method was applied. Furthermore, two options for equilibrium reactivity increase were discussed and two major safety related parameters evaluated. The general conclusion of this study is that the U-Pu cycle profits more from the spectrum hardening and has better neutron economy, where more neutrons are produced but also parasitically captured. On the other hand, the Th-U cycle is less sensitive to the spectrum hardening and has better neutron efficiency, where the lower neutron production is compensated by the lower parasitic captures.

Abstract The Sodium-cooled Fast Reactor (SFR) is one of the most promising Generation IV systems with many performance advantages, but has one dominating neutronics drawback – a positive sodium void reactivity. The starting point for the present study is an SFR core design considered in the Collaborative Project on the European Sodium-cooled Fast Reactor (CP-ESFR). The aim is to analyze, for this reference core, four safety and performance parameters from the viewpoint of four different optimization options, and to propose possible optimized core designs. In doing so, the study focuses not only on the beginning-of-life state of the core, but also on the beginning of equilibrium closed fuel cycle. The four studied optimization options are: (a) introducing an upper sodium plenum and boron layer, (b) varying the core height-to-diameter (H/D) ratio, (c) introducing moderator pins into the fuel assembly, and (d) modifying the initial plutonium content. The sensitivity of the void reactivity, Doppler constant, nominal reactivity and breeding gain has been evaluated. In particular, the void reactivity, which is the most crucial safety parameter for the SFR, has been decomposed into its reaction-wise, isotope-wise and energy-group-wise components using a methodology based on the neutron balance equation. Extended voiding in the upper sodium plenum region – in conjunction with the effect of a boron layer introduced above the plenum – is found to be particularly effective in the void effect reduction while, at the same time, having almost no impact on the other considered parameters. A lower H/D ratio can also reduce void reactivity, but normally corresponds to worse neutron economy and, as such, leads to a less positive breeding gain and a smaller nominal reactivity margin. The Doppler constant, which is very sensitive to the neutron spectrum, can be significantly improved by the spectrum softening in the moderator pins case. Initial plutonium content effectively influences the nominal reactivity and breeding gain at the beginning-of-life, without significant changes of the safety related parameters. Finally, two different “synthesis core” concepts are proposed for further study, the common boundary condition set being to have a positive nominal reactivity margin at the end of equilibrium closed fuel cycle. Their considered safety and performance parameters at different fuel cycle states are presented in the end.

The Sodium-cooled Fast Reactor (SFR) is one of the most promising Generation IV systems with many advantages, but has one dominating neutronic drawback - a positive sodium void reactivity. The aim of this study is to develop and apply a methodology, which should help better understand the causes and consequences of the sodium void effect. It focuses not only on the beginning-of-life (BOL) state of the core, but also on the beginning of open and closed equilibrium (BOC and BEC, respectively) fuel cycle conditions. The deeper understanding of the principal phenomena involved may subsequently lead to appropriate optimization studies.

Abstract This paper reports about the DYN1D-MSR code development and dynamics studies of the molten salt reactors (MSR) – one of the ‘Generation IV International Forum’ concepts. In this forum the graphite-moderated channel type MSR based on the previous Oak Ridge National Laboratory research is considered. The liquid molten salt serves as a fuel and coolant, simultaneously and causes two physical peculiarities: the fission energy is released predominantly directly into the coolant and the delayed neutrons precursors are drifted by the fuel flow. The drift causes the spread of delayed neutrons distribution to the non-core parts of primary circuit and it can lead to a reactivity loss or gain in the case of fuel flow acceleration or deceleration, respectively. Therefore, specific 3D tool based on in house code DYN3D was developed in FZR. The code DYN3D-MSR is based on the solution of two-group neutron diffusion equation by the help of a nodal expansion method and it includes models of delayed neutrons drift and specific MSR heat release distribution. In this paper the development and verification of 1D version DYN1D-MSR of the code is described. The code has been validated with the experimental data gained from the molten salt reactor experiment performed in the Oak Ridge and after the validation it was applied to several typical transients (overcooling of fuel at the core inlet, reactivity insertion, and the fuel pump trip).

Abstract In the pebble-bed high-temperature reactor under construction in China, called the HTR-PM, the spherical fuel elements continuously flow downward in the cylindrical core. The burnup of each pebble is checked at the core outlet and, according to the achieved burnup level, the pebble might be disposed or reinserted into the upper section of the core. Upon reinsertion, each pebble is radially distributed in a random manner and, according to its downward path, faces different burnup conditions. Hence, the number of passes necessary to achieve the average discharge burnup of 90 MWd/kgU may vary. Discrete element method (DEM) simulations have been carried out to achieve a clear understanding of the movement of the 420000 fuel pebbles in the HTR-PM core. At the same time, neutronics properties have been investigated for a single pebble and for the full core with the Serpent 2 Monte Carlo code. As a result, one-group microscopic cross sections (XS) have been parametrized at the core level. The pebble movement has been loosely coupled with the depletion of a single pebble in a dedicated burnup script called moving pebble burnup (MPB), developed in matlab. 3000 single pebble burnup histories were simulated to obtain sufficient statistics and an insight into the HTR-PM burnup process. The decrease of the average burnup gained per single pass implies that a miss-handling of recirculated fuel elements is unlikely to lead to an excess of the maximum allowed burnup of 100 MWd/kgU. The core demonstrates a self-compensation effect of burnup, meaning that it always compensates burnup under- or over-runs in the successive passes. In addition, gamma detection of 137Cs has been studied as a practical method for monitoring the burnup of the discharged pebbles, turning out to be an applicable measurement technique. Finally, it is possible to conclude that the fuel cycle of the HTR-PM, as it has been laid out, is well designed and feasible.

The operation of a reactor on an open but self-sustainable cycle without actinide separation is known as breed-and-burn. It has mostly been envisioned for use in solid-fueled fast-spectrum reactors such as sodium-cooled fast reactors. In this paper the applicability of breed-and-burn to molten salt reactors is investigated first on a cell level using a modified neutron excess method. Several candidate fuel salts are selected and their performance in a conceptual three-dimensional reactor is investigated. Chloride-fueled single-fluid breed-and-burn molten salt reactors using enriched chlorine are shown to be feasible from a neutronics and fuel cycle point of view at the cost of large fuel inventories.

The Generation IV initiative was launched with the goal of developing nuclear reactors which surpass current designs in safety, sustainability, economics and non-proliferation. From the six most promising concepts the Gas Cooled Fast Reactor (GFR) represents a challenging and innovative idea that is prominent in the sustainability aspect with the ability to have a closed fuel cycle and the potential to burn minor actinides (MAs). The European FP7 GoFastR project was one of the latest steps in the development and further optimization of GFRs. This paper presents a comprehensive overview of the neutronic performance of GFR2400 which was considered as a conceptual design for a large scale GFR within the collaboration. This reactor is the newest on the evolutionary path of fully ceramic GFRs featuring ceramic fuel and structural materials allowing high temperatures and efficiency using helium coolant. An important innovation of the current design is the application of refractory metallic liners to enhance the fission product retention of the cladding, resulting in a significant neutronic penalty during normal operation, at the same time being advantageous under transient conditions involving spectrum softening. Using the ERANOS and SCALE code systems several parameters were determined for beginning of life (BOL) conditions, including excess reactivity, various reactivity effects such as depressurization, Doppler or thermal expansion effects, as well as kinetic parameters. An extensive sensitivity and uncertainty analysis of these parameters was also done with the 15 group BOLNA and 44 group SCALE covariance libraries. Open and closed fuel cycle operations were investigated and the transmutational capabilities were studied with the GFR connected to traditional light water reactors in a symbiotic system. The presented analysis shows that the GFR2400 design is a major improvement compared to previous concepts. All preliminary constraints are respected resulting in a manageable initial Pu inventory of 10 t/GWel at 45% plant efficiency, a low MA mass fraction of 1% by self-recycling and a near zero breeding gain without the use of fertile blankets. At the same time the reactor has acceptable safety features precluding super-prompt-criticality in depressurized conditions at BOL and in open cycle equilibrium. Either of the two planned control devices is sufficient to shut down the reactor independently of the other and the refractory liners introduce significant negative reactivity in case of water ingress. However the occurrence of hot spots when all control rods are inserted needs further analysis. The design also shows promising closed fuel cycle and transmutational performance. However as is the case in other fast reactors the fuel cycle closure causes safety related parameters to degrade, most importantly the depressurization reactivity effect to exceed the effective delayed neutron fraction in the current design. To assess the acceptability of this deterioration further analysis is needed. Finally, it can be concluded that current commercial codes are satisfactory for such analysis; however there is a need for better covariance data. Several parameters exceed their target uncertainty value, most notably the k-effective by a factor of 6, the main source of the uncertainty being the inelastic scattering of U-238. (C) 2014 Elsevier Ltd. All rights reserved.

Abstract Advanced fast reactors of the fourth generation should be capable to breed their own fuel from 238 U feed and to recycle the actinides from their own spent fuel. This recycling or virtually the closure of fuel cycle can converge to an equilibrium fuel cycle and has impact on the safety-related parameters. The goals of this study are: (i) to apply an equilibrium cycle procedure EQL3D to the Gas cooled Fast Reactor (GFR), (ii) to simulate and confirm the GFR neutronics capability for closed fuel cycle, and (iii) to evaluate the safety-related parameters of the equilibrium cycle. Equilibrium cycle method for considering the homogeneous recycling of actinides is a known approach. However, in EQL3D the equilibrium method is newly applied for hexagonal-z 3D core geometry and 33 energy-groups neutron-flux calculation. This geometry enables to characterize the equilibrium cycle for complex reloading patterns within a multi-batch cycle. Two GFR geometries were studied, the first based on an international neutronics benchmark with a simple set-up and the second based on more advanced core design. For the advanced design, three reloading patterns within a multi-batch cycle with four different feeds were compared. The GFR neutronics capability for closed cycle was proved. The negative impact of the fuel cycle closure on safety-related parameters was confirmed and quantified. The GFR core with closed fuel cycle could serve after prospective optimization as a sustainable and clean energy source.

Abstract The adoption of Th fuel in fast reactors is being reconsidered due to the potential favorable impact on actinide waste management and resource availability. A closed Th cycle leads to an actinide inventory with lower radiotoxicity and heat load for the first several thousands of years. Due to the typically low TRansUranic (TRU) Conversion Ratio (CR), Th can also be advantageous to expedite the consumption of legacy TRU. One of the main obstacles to the implementation of Th is the highly radioactive recycled fuel which requires remote handling under heavy shielding, inevitably penalizing economics and challenging conventional pin-based fuel manufacturing. From this perspective, the development of liquid-fuelled reactors, with Molten Salt Reactors regarded as the most promising, appears particularly attractive as fuel handling would be greatly simplified. The present paper investigates the fuel cycle performances of the reference GEN-IV Molten Salt Fast Reactor (MSFR) in terms of isotope evolution, radiotoxicity generation and safety-related parameters. Similarly to most MSR concepts proposed in the past, the MSFR is based on the fluoride molten salt technology, but it features the novelty of a fast neutron spectrum. Calculations are performed using state-of-the-art equilibrium-cycle methodologies, i.e., the ERANOS-based EQL3D procedure developed at the Paul Scherrer Institut and extended to the simulation of the MSFR. Selected results have been benchmarked with the Monte Carlo code PSG2/SERPENT. These results have also been used for the assessment of a diffusion module based on the COMSOL multi-physics toolkit, which is the subject of current studies aimed at efficiently simulating the peculiar MSFR transient behavior.