Study on the effect of various burnable poisons on pebble bed reactor with OTTO fuelling schemes using Serpent 2 Monte Carlo code

Pebble bed reactor with a once-through-then-out fuelling scheme has the advantage of simplifying the refueling system. However, the core upper-level power density is relatively higher than the bottom, producing an asymmetric core axial power distribution. Several burnable poison (BP) configurations are used to flatten the peak power density and improve power distribution while suppressing the excess core reactivity at the beginning of the burnup cycle. This study uses HTR-PM, China’s pebble bed reactor core, to simulate several burnable poison (BP) configurations. Serpent 2 coupled with Octave and a discrete element method simulation is used to model and simulate the pebble bed reactor core. It is found that erbium needs a large volumetric fraction in either QUADRISO or distributed BP to perform well. On the other hand, gadolinium and boron need a smaller volumetric fraction but perform worse in radial power distribution criteria in the fuel sphere. This study aims to verify the effect of BP added fuel pebbles on an OTTO refueling scheme HTR-PM core axial power distribution and excess reactivity.

Study on transmutation efficiency of the VVER-1000 fuel assembly with different minor actinide compositions

The feasibility of transmutation of minor actinides recycled from the spent nuclear fuel in the VVER-1000 LEU (low enriched uranium) fuel assembly as burnable poison was examined in our previous study. However, only the minor actinide vector of the VVER-440 spent fuel was considered. In this paper, various vectors of minor actinides recycled from the spent fuel of VVER-440, PWR-1000, and VVER-1000 reactors were therefore employed in the analysis in order to investigate the minor actinide transmutation efficiency of the VVER-1000 fuel assembly with different minor actinide compositions. The comparative analysis was conducted for the two models of minor actinide loading in the LEU fuel assembly: homogeneous mixing in the UGD (Uranium-Gadolinium) pins and coating a thin layer to the UGD pins. The parameters to be analysed and compared include the reactivity of the LEU fuel assembly versus burnup and the transmutation of minor actinide nuclides when loading different minor actinide vectors into the LEU fuel assembly.

Burnable Poison Selection and Neutronics Analysis of Plate Fuel Assemblies

Burnable poisons play a critical role in long-life pressurized water reactors. Plate fuel elements have good application prospects in long-life pressurized water reactors. In long-life pressurized water reactors with large initial residual reactivity in the core, a reasonable selection of burnable poisons can suppress the large residual reactivity at beginning of lifetime and can achieve a long burnup depth at end of lifetime. Therefore, the selection of burnable poisons is a crucial factor to be considered in the design of long-life pressurized water reactors. In this study, the selection of burnable poisons and neutronics characteristics of long-life PWR plate fuel assembly were studied. The transport-burnup calculations of different burnable poison fuel assemblies were carried out. Some candidate BPs are selected to realize the effective control of reactivity. The results show that when the enriched isotopes 157Gd, 167Er and B4C are used as burnable poisons, there is almost no reactivity penalty; when PACS-J and 231Pa are used as burnable poisons, due to their own characteristics, not only does not cause reactivity penalty at end of lifetime, but also the fuel assembly life is extended, the fuel utilization rate is improved. The combination of PACS-J and the slow-burnup burnable poisons can obtain a better reactivity curve. The results of this article show that the plate fuel assemblies can be selected with enriched isotope 157Gd, enriched isotope 167Er, B4C, 231Pa and PACS-J as burnable poisons, and the combinations of burnable poisons can be selected with two combination schemes, PACS-Er and PACS-Pa.

The Ring RPT Method for DH Systems Containing Dispersed Particle-Type of Fuel and Burnable Poisons

Because dispersed particle-type fuel and burnable poisons both have double heterogeneity (DH), using the traditional volumetric homogenization method (VHM) to treat DH systems will bring about large reactivity calculation deviation. The improved reactivity-equivalent physical transformation (IRPT) method can be applied to DH systems which have both dispersed particle-type fuel and burnable poisons because of the features of simplicity and high calculation accuracy. In this article, the calculations show that the IRPT method becomes invalid for some DH systems when the volume fraction of dispersed particle-type burnable poisons is relatively high or the absorb cross section of burnable poison particles is relatively large. Then the two-step ring reactivity-equivalent physical transformation (TRRPT) method is proposed to be applied to the DH systems with both dispersed particle-type fuel and burnable poisons. Results of reactivity at zero burnup and depletion calculations for different types of dispersed particle-type fuel and burnable poisons and the comparison with Monte Carlo results of grain models prove the validity of the TRRPT method, and it has been proven that the TRRPT method has higher accuracy in reactivity calculation and a wider scope of transformation than the IRPT method.

Hybrid RPT Method On Double-Heterogeneous System Containing Dispersed Fuel Particles and Burnable Poison Particles

In this paper it was found that for DH systems containing both particle-dispersed fuel and particle-dispersed burnable poison, the TRRPT method may be invalid in some situation. In his paper a new method named Hybrid RPT (HRPT) method has been proposed for DH systems containing both particle-dispersed fuel and particle-dispersed burnable poisons. And then the HRPT are analyzed and studied on the equivalent transformation range of DH systems comparing with the TRRPT method and the IRPT method. It was found that the HRPT method not only has a wider equivalent transformation range of DH systems than the TRRPT method, but also has higher calculation accuracy than the IRPT method. Results of depletion calculations for different types, different volume fractions and different particle sizes of burnable poisons particles dispersed the DH systems with dispersed particle-type fuel and the comparison with Monte Carlo results of grain models have proved the effectiveness and applicability of HRPT method.

AN IMPROVED DOUBLE HETEROGENEITY MODEL FOR PEBBLE-BED REACTORS IN DRAGON-5

In pebble-bed reactors, the fuel is contained in small grains, which are included in a graphite matrix. Some burnable poison particles may also be present. In this work, an additional ’double heterogeneity’ model is introduced in the DRAGON5 lattice code. The model is based on the legacy work of She (INET) and has been improved to overcome intrinsic limitations. It is based on a simplified physical model whereas the two already existing models were based on the collision probability analysis or on renewal theory. The advantage of this new model is its simplicity to implement. The theory shows that the correction suggested in the original model should not be arbitrary, but a constant equivalent particle fraction of 0.63. Numerical comparison between the models is generally good. However it does not support the theory of a constant equivalent fraction. Additional work is needed to reduce the discrepancy between the models in some cases.

QUALIFICATION OF ANC9 GENERIC INSERT METHODOLOGY (GIM)

Discrete burnable poisons like the Wet Annular Burnable Absorber and Pyrex have been used in PWR core to improve core power distribution and provide more negative moderator temperature coefficient. The burnable absorber in the burnable poison rods burns fast and is completely gone after one cycle exposure in the core. Traditionally, the burnable poison rods are designed to be part of the fuel assembly. They are burned together with the fuel in the first loading cycle and discharged after one cycle. In recent years, different insertion scenarios of the burnable poison rods have been introduced in the PWR plants operation to improve the fuel performance, for instance, fresh burnable poison rods are inserted into a burned assembly; burned burnable poison rods stay in the original assembly or are replaced in a different assembly. A Generic Insert Methodology [1] was developed in Westinghouse and implemented in NEXUS/ANC9 code system. With this new methodology, ANC9 is able to follow the assembly history and model all types of absorber insert components for all kinds of insertion scenarios. An extensive methods validation and qualification effort has been completed by modeling different insertion cases. This paper provides details of the qualification cases along with the analysis of the results.

A SIMPLIFIED SMAHTR BENCHMARK PROBLEM SET

The Small, Modular Advanced High Temperature Reactor (SmAHTR) is a preconceptual design for a fluoride salt-cooled small modular reactor (SMR) [1]. In this paper, a stylized 2D benchmark problem set has been created based on SmAHTR. Certain gaps and considerations in burnable poison and control rod content were unspecified/undetermined in the preconceptual design, but those gaps were filled for the stylized problem set. With those features, this problem set could then be used for benchmarking neutron transport methods as well as its low order methods in 2D single assembly and full core configurations. For this benchmark set, continuous energy Monte Carlo calculations were performed. Those calculations provided keff values of 0.9459 (± 11 pcm) and 1.1436 (± 12 pcm) in the full core configuration with all the control rods fully inserted and withdrawn, respectively. The single assembly calculations yielded an eigenvalue, kinf, of 0.9987 (± 15 pcm) and 1.2117 (± 15 pcm) with all of the control rods either inserted or removed, respectively. In the full core configuration, the worth of all the control rods and burnable poison particles were determined to be 197.6 (± 0.16) mk and 311.6 (± 0.23) mk, respectively. The corresponding results in the single assembly configurations are 213 (± 0.21) mk and 337.4 (± 0.20) mk, respectively. A near-critical configuration was also determined for the reactor by inserting control rods in some assemblies, thus providing a case with a keff value of 0.9909 (± 12 pcm).