• Energy Research

Abstract This paper presents the verification and validation of the radiation source term calculation capability implemented in the reactor analysis code STREAM for pressurized water reactor (PWR) spent nuclear fuel (SNF) analysis. Activity, decay heat, neutron and gamma source spectra of irradiated PWR fuel assemblies are calculated with STREAM and ORIGEN and the results obtained are compared. The verification is performed with Westinghouse 15 × 15, 17 × 17 and Combustion Engineering 16 × 16 designs with enrichment of 3–5 wt.% 235U, burnup of 30–60 GWd/t and cooling times of 0.3–1000 years. The radiation source term comparison between STREAM and ORIGEN indicates that the implementation of the source term equations is correct. The validation is done by comparing STREAM calculation results against 91 decay heat calorimetric measurements for 52 PWR fuel assemblies performed at the Swedish central storage facility for spent nuclear fuel, CLAB, and United States Hanford Engineering Development Laboratory (HEDL) and General Electric Morris Operations. The measured fuel assemblies have enrichment of 2–4 wt.% 235U, burnup of 28–51 GWd/t and cooling times of 2–27 years. The comparison between calculated and measured decay heat shows good agreement. For most of the measurements analyzed, the calculated decay heat is within the range of the measurement uncertainty. The average, overall C/E over 71 CLAB decay heat measurements is 1.000 ± 0.017, and 0.997 ± 0.013 for 20 US measurements. Overall, this work demonstrates that STREAM can be reliably used to predict PWR SNF radiation source terms.

Abstract A new Monte Carlo (MC) neutron/photon transport code, called MCS, has been developed at Ulsan National Institute of Science and Technology (UNIST) with the aim of performing the high-fidelity multi-physics simulation of large-scale power reactors, especially pressurized water reactors (PWR). The high-fidelity multi-physics analysis of large-scale PWR is a challenging problem due to two aspects, the first being the difficulty of implementing various state of the art techniques into a single code system, and the other making it feasible to run such simulations on practical computing machines within reasonable amount of memory usage and computing time. In this paper, features implemented into MCS for large-scale PWR simulations are described including but not limited to depletion, thermal/hydraulics coupling, fuel performance coupling, equilibrium xenon, on-the-fly neutron cross-section Doppler broadening, and critical boron search. The efficient memory usage for burnup simulation and the high performance of MCS through various algorithms and optimizations (parallel fission bank, hash indexing) are illustrated on Monte Carlo performance benchmarks. Finally, the large-scale PWR analysis capability is fully demonstrated with BEAVRS Cycles 1 & 2 calculations.

Abstract A photon transport capability has been implemented and verified in the Monte Carlo code MCS of Ulsan National Institute of Science and Technology for the purpose of radiation shielding studies. The MCS photon fixed-source mode simulates the transport of photons between 1 keV and 20 MeV for all elements from hydrogen to fermium. The specific physics for the main four photo-atomic reactions (Rayleigh scattering, Compton scattering, photoelectric effect and pair production) and three secondary processes of photon production (positron–electron annihilation, atomic relaxation and electron/positron bremsstrahlung) are reviewed. Verification results against Monte Carlo codes MCNP6.1 and SERPENT2.1.29 are presented. The verification cases include the comparison of energy distributions of photon flux in an infinite medium, of spatial distributions of photon flux in a cylinder, of the spatial distribution of photon body-equivalent dose in a spent nuclear fuel transport cask, and of photon KERMA (Kinetic Energy Released per MAss) in photon detector calibration geometries. Good calculation/calculation agreement is observed overall, with some marked differences in the detailed photon flux comparison at given energy ranges traced back to differences in photon physics implementation. MCS can from now on be applied for the purpose of advanced photon studies and corresponding validation against experimental shielding benchmarks will follow in the future.

A review of the documentation and an interpretation of the NEA-1517/74 and NEA-1517/80 shielding benchmarks (measurements of photon leakage flux from a hollow sphere with a central 14 MeV neutron source) from the SINBAD database with the Monte Carlo code MCS and the most up-to-date ENDF/B-VIII.0 neutron data library are conducted. The two analyzed benchmarks describe satisfactorily the energy resolution of the photon detector and the geometry of the spherical samples with inner beam tube, tritium target and cooling water circuit, but lack information regarding the detector geometry and the distances of shields and collimators relatively to the neutron source and the detector. Calculations are therefore conducted for a sphere model only. A preliminary verification of MCS neutron-photon calculations against MCNP6.2 is first conducted, then the impact of modelling the inner beam tube, tritium target and cooling water circuit is assessed. Finally, a comparison of calculated results with the libraries ENDF/B-VII.1 and ENDF/B-VIII.0 against the measurements is conducted and shows reasonable agreement. The MCS and MCNP inputs used for the interpretation are available as supplementary material of this article.

Abstract This study presents the development of a new integral-type rack design, characterized by the use of gadolinium (Gd)-containing structure materials that enhances the capacity of spent fuel (SF) pool storage by exploiting the high neutron absorption capability of Gd. Appropriate types and contents of Gd-based neutron-absorbing materials are selected for the new design through parametric studies. For high-reactivity fuels (region I of SF storage pools), neutron-absorbing material composed of Gd 0.7 at% with Eu 2.73 at% is found to be an optimal neutron absorber whereas for low-reactivity fuels (region II of SF storage pools), a composition of Gd 0.7 at% with Eu 8.38 at% is found to be optimal. A criticality safety analysis shows that the newly designed racks are more subcritical than conventional racks for both regions I and II. The additional reactivity margin yielded by the new integral-type design can be used to reduce the pitch of the rack while maintaining equivalent subcriticality compared to conventional rack design. This study demonstrates the potential of Gd-based neutron absorbers in structure materials for increasing the total amount of fuel assemblies that can be stored in a SF storage pool.

Abstract This paper presents the verification of the DeCART2D/CAPP code system for the Very High Temperature Gas-Cooled Reactor (VHTR) analysis with the Prismatic Modular Reactor 200 (PMR-200) benchmark. The McCARD Monte Carlo (MC) code is used to obtain the reference solution. The verification has been performed for the effective multiplication factor (keff) and reactivity coefficients at the levels of fuel compact, fuel block, and full core. Furthermore, the verification of the depletion calculation has been conducted for the fuel block and the verification for the power distribution has been performed at the levels of fuel block, two-dimensional (2D) and three-dimensional (3D) full core. The verification results of DeCART2D, CAPP, and DeCART2D/CAPP are compared systematically against the reference McCARD solutions to demonstrate the VHTR modeling capability and accuracy of the codes. It was successfully shown that the keff errors of the DeCART2D/CAPP code system are smaller than ∼510 pcm, the isothermal temperature coefficient (ITC) errors are smaller than ∼0.66 pcm/K, and the power distribution errors are smaller than 2.80%. It was also shown that the maximum keff errors of DeCART2D fuel block depletion calculations are smaller than ∼460 pcm.

Abstract This paper presents the validation of the continuous-energy Monte Carlo neutron-transport code MCS with the ENDF/B-VII.0 neutron cross-section library for the criticality safety analysis of PWR spent fuel pools and storage casks. The MCS code is developed by the COmputational Reactor physics and Experiment laboratory (CORE) of Ulsan National Institute of Science and Technology (UNIST) for the analysis of Pressurized Water Reactors (PWRs) with high fidelity and high performance. The validation is conducted with 279 selected critical benchmarks from the International Criticality Safety Benchmark Evaluation Project (ICSBEP). The 279 validation cases are representative of PWR spent fuel pools and storage casks with 235U enrichment ranging from 2.35 wt% to 4.74 wt%, pin pitch ranging from 1.075 cm to 2.540 cm, moderator to fuel ratio (H/U) ranging from 0.4683 and 11.5398, Energy of the Average Lethargy causing Fission (EALF) ranging from 0.0109 eV to 1.0600 eV, without soluble boron and with soluble boron concentration ranging from 0.015 g/L to 5.030 g/L. The calculation of the effective neutron multiplication factor by MCS is validated by the comparison between experiment and calculation for the selected critical benchmarks. The Upper Safety Limit (USL) of the MCS code is established in accordance to the NUREG/CR-6698 guideline recommended by the NRC (US National Regulatory Commission). The full validation process and determination of USL based on the selected critical benchmarks was also repeated with the MCNP6.1 and SERPENT2.1.27 codes in order to compare the performances of MCS with other reactor analysis codes. This paper demonstrates the capability of the MCS code for the criticality safety analysis of PWR spent fuel pools and storage casks.

The criticality analysis of VVER-1000 mock-up benchmark experiments from the LR-0 research reactor operated by the Research Center Rez in the Czech Republic has been conducted with the MCS Monte Carlo code developed at the Computational Reactor Physics and Experiment laboratory of the Ulsan National Institute of Science and Technology. The main purpose of this work is to evaluate the newest ENDF/B-VIII.0 nuclear data library against the VVER-1000 mock-up integral experiments and to validate the criticality analysis capability of MCS for light water reactors with hexagonal fuel lattices. A preliminary code/code comparison between MCS and MCNP6 is first conducted to verify the suitability of MCS for the benchmark interpretation, then the validation against experimental data is performed with both ENDF/B-VII.1 and ENDF/B-VIII.0 libraries. The investigated experimental data comprises six experimental critical configurations and four experimental pin-by-pin power maps. The MCS and MCNP6 inputs used for the criticality analysis of the VVER-1000 mock-up are available as supplementary material of this article.