Assessment of the Impact of Modification of Calcium Sorbents and the Possibility of Their Use in Desulfurization for Oxy-Fuel Combustion Process

The paper describes the possibilities of simple and effective modification of calcium sorbents used for flue gas desulfurization with a size between of 125–250 µm. The additives to the sorbents in the amount of 0.5% and 1.0% were inorganic sodium and lithium compounds. The research on the reactivity of sorbents was analyzed in the process of simultaneous calcination and sulfation at the temperature of 850 °C. The type of Na+ or Li+ cations and the inorganic salt anions have an influence on the modification of calcium sorbents in order to improve the efficiency of the calcination and sulfation process. Modification of calcium sorbents by adding inorganic sodium and lithium compounds, regardless of the amount, changes the reactivity coefficient RI [mol/mol] and the absolute sorption coefficient CI [g S/kg sorbent]. In the case of inorganic sodium salt (Additive 1), regardless of the amount of modifier added, there was a visible improvement in the reactivity of the sorbent: 1.0% of the additive caused an increase in the RI coefficient in relation to the raw sorbent by over 14%, and in the case of the CI coefficient by over 24%. Additional research was the analysis of the limestone behavior mechanism during the simultaneous calcination and sulfation (SCS) process under conditions of elevated temperature and with variable CO2 and O2 contents in the flue gas. The behavior of sorbents with a size distribution of 125–250 µm was assessed on the basis of the change in mass of the samples by determining the reactivity coefficient RI, [mol/mol] and the absolute sorption coefficient CI, [g S/kg sorbent]. Using the mercury porosimetry technique, the change in sorbent porosity in the subsequent stages of the simultaneous calcination and sulfation process was investigated. The process was carried out in the temperature range corresponding to the oxy-combustion (i.e., from 850 °C to 1000 °C).

Improved FAST Device for Inherent Safety of Oxide-Fueled Sodium-Cooled Fast Reactors

The floating absorber for safety at transient (FAST) was proposed as a solution for the positive coolant temperature coefficient in sodium-cooled fast reactors (SFRs). It is designed to insert negative reactivity in the case of coolant temperature rise or coolant voiding in an inherently passive way. The use of the original FAST design showed effectiveness in protecting the reactor core during some anticipated transients without scram (ATWS) events. However, oscillation behaviors of power due to refloating of the absorber module in FAST were observed during other ATWS events. In this paper, we propose an improved FAST device (iFAST), in which a constraint is imposed on the sinking (insertion) limit of the absorber module in FAST. This provides a simple and effective solution to the power oscillation problem. Here, we focus on an oxide fuel-loaded SFR that is characterized by a more negative Doppler reactivity coefficient and higher operating temperature than the metallic-loaded SFR cores. The study is carried out for the 1000 MWth advanced burner reactor with an oxide fuel-loaded core during postulated ATWS events that are unprotected transient over power, unprotected loss of flow, and unprotected loss of the heat sink. It was found that the iFAST device has promising potentials for protecting the oxide SFR core during the various studied ATWS events.

Analysis of Doppler reactivity of SMART reactor core for hybrid fuel configurations of UO2, MOX and (Th/U)O2 using OpenMC

In this study, the Doppler reactivity coefficient has been investigated for UO2, MOX, and (Th/U)O2 fuel types. The calculation has been carried out using the Monte Carlo method ( OpenMC). The effective multiplication factor keff has been evaluated for three materials with four different configurations without Integral Fuel Burnable Absorber (IFBA) rods and soluble boron. The results of MOX fuel, homogenous and heterogeneous thorium fuel configuration has been compared with the core of the reference fuel assembly (UO2). The calculation showed that the effective multiplication factor at 1 000 K was 1.26052, 1.14254, 1.22018 and 1.23771 for reference core, MOX, homogenous and heterogeneous configurations respectively. The results shows that reactivity has decreased with increasing temperature while the doppler reactivity coefficient remained negative. Moreover, the use of (Th/U)O2 homogenous and heterogeneous configuration had shown an improved response compared to the reference core at 600 K and 1 000 K. The doppler reactivity coefficient has been found as –8.98E-3 pcm/K, -0.8 655 pcmK for the homogenous and –8.854 pcm/K, -1.2253 pcm/K for the heterogeneous configuration. However, the pattern remained the same as for the reference core at other temperature points. MOX fuel has shown less response compared to the other fuel configuration because of the high resonance absorption coefficient of Plutonium. This study showed that the SMART reactor could be operated safely with investigated fuel and models.

A simplified analysis of the Chernobyl accident

We show with simplified numerical models, that for the kind of RBMK operated in Chernobyl: The core was unstable due to its large size and to its weak power counter-reaction coefficient, so that the power of the reactor was not easy to control even with an automatic system. Xenon oscillations could easily be activated. When there was xenon poisoning in the upper half of the core, the safety rods were designed in such a way that, at least initially, they were increasing (and not decreasing) the core reactivity. This reactivity increase has been sufficient to lead to a very high pressure increase in a significant amount of liquid water in the fuel channels thus inducing a strong propagating shock wave leading to a failure of half the pressure tubes at their junction with the drum separators. The depressurization phase (flash evaporation) following this failure has produced, after one second, a significant decrease of the water density in half the pressure tubes and then a strong reactivity accident due to the positive void effect reactivity coefficient. We evaluate the fission energy released by the accident

A SEMI-ANALYTIC EIGENVALUE EXTENSION TO THE DOPPLER SLAB ANALYTIC BENCHMARK1

Advancement in multiphysics simulation has motivated interest in availability of analytic and semi-analytic benchmark solutions. These solutions are sought because they can be used to assess the accuracy of complicated numerical schemes necessary to simulate coupled physics systems. While there exist analytic solutions for fixed-source problems, benchmark-quality eigenvalue solutions are of interest because eigenvalue problems more closely align with analyses undertaken with coupled solvers. This paper extends a fixed-source benchmark, the Doppler Slab benchmark, to the eigenvalue case. A novel solution for this benchmark is derived. Numerical implementation of the benchmark is demonstrated through verification of numerical computation of the power reactivity coefficient.

Investigation of the Safety Features of Advanced PWR Assembly Using SiC, Zr, FeCrAl and SS-310 As Cladding Materials

In this work, SiC (Silicon carbide), FeCrAl (ferritic), SS-310 (stainless steel 310) and Zirconium are simulated by MCNPX code as cladding materials in advanced PWR assembly. A number of reactor safety parameters are evaluated for the candidate cladding materials as reactivity, cycle length, radial power distribution of fuel pellet, reactivity coefficients, spectral hardening, peaking factor, thermal neutron fraction and delayed neutron fraction. The neutron economy presented by Zr and SiC models is analyzed through the burnup calculations on the unit cell and assembly levels. The study also provided the geometric conditions of all cladding materials under consideration in terms of the relation between fuel enrichment and cladding thickness from the viewpoint to achieve the same discharge burnup as the Zircaloy cladding. It was found that the SiC model participated in extending the life cycle by 2.23% compared to Zr. The materials other than SiC largely decreased discharge burnup in comparison with Zircaloy. Furthermore, the claddings with lower capture cross-sections (SiC and Zr) exhibit higher relative fission power at the pellet periphery. The simulation also showed that using SiC with a thickness of 571.15 µm and 4.83% U-235 can satisfy the EOL irradiation value as Zr. For reactivity coefficient, the higher absorbing materials (SS-310 and FeCrAl) exhibit more negative FTCs, MTCs and VRCs at the BOL But, at the intermediate stages of burnup Zr and SiC have a strong trend of negative reactivity coefficients. Finally, the delayed neutron fraction of SiC and Zr models is the highest among all the four models.

Oxygen migration through a cover with capillary barrier effects colonized by roots

Covers with capillary barrier effects (CCBEs) are multi-layered oxygen barrier covers used in humid climates to reclaim reactive mine tailings and limit the generation of acid mine drainage. Once constructed, CCBEs are colonized by surrounding plants. Roots modify water storage and respire oxygen. The performance of CCBEs could evolve over time due to root colonization. Twenty-five plots with varying vegetation were investigated at a 17-year-old CCBE in the mixed forest of Quebec, Canada. Geotechnical parameters and root colonization of the moisture-retaining layer (MRL) of the CCBE were characterized. The performance of the MRL to control oxygen migration was assessed using oxygen consumption tests and numerical modeling. Despite root colonization at the surface of the MRL, oxygen fluxes generally complied with the CCBE’s design criteria. Root presence created oxygen consumption in the MRL, which could be expressed with a reactivity coefficient (Kr). A positive correlation (R2 = 0.65) was found between root length density and Kr. Oxygen consumption by root respiration helped to lower oxygen fluxes by 0.5 to 76 g/m2/year, with a mean of 13 g/m2/year. These results will help to better understand the influence of roots on CCBEs’ performance to control oxygen migration.

Use of erbium as a burnable absorber for the VVER reactor core life extension

The paper presents the results of a computational and theoretical analysis concerned with the use of erbium as a burnable absorber in VVER-type reactors. Partial refueling options for the reactor life extension to 18 and 24 months is considered, the refueling ratio being equal to three for the 18-month life and to two for the 24-month life. Erbium is expected to be present in all fuel elements in the FA with the same weight content. The influence of the erbium weight content on such neutronic characteristics of the reactor and fuel as burn-up, reactivity coefficients, residual volume of “liquid" control, and amounts of the liquid radioactive waste (LRW) formed was assessed. The calculations were performed using a simplified model of refueling without FA reshuffling. An infinite array of polycells consisting of FAs with different in-core times was considered. The escape of neutrons from the core was taken into account by selecting the critical value K∞ at the end of life. Erbium does not burn up in full for the lifetime which affects the fuel burn-up as compared with the liquid excessive reactivity compensation system. The reduction is 0.7% per 0.1% of the erbium weight load in the fuel elements. This, however, also reduces the maximum content of the boron absorber in the coolant and the LRW accumulation in the ratio of 5% per 0.1% of the erbium weight load. Erbium influences the spectral component of the coolant temperature reactivity coefficient which turns out to be negative even with its minor weight fraction in fuel elements, and a reduction in the boron absorber fraction leads to a positive value of the density reactivity coefficient. As a result, the overall coolant temperature reactivity coefficient has a negative value throughout the lifetime.