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Difference in the neutron spectrum caused by the insertion of moderating material calculated with the 190 group library of HELIOS 1.9 and compared to the radiative capture cross-section of uranium-238.
Source publication
The use of fine distributed moderating material to enhance the feedback effects and to reduce the sodium void effect in sodium-cooled fast reactor cores is described. The influence of the moderating material on the fuel assembly geometry, the neutron spectrum, the feedback effects, the power and burnup distribution, and the transmutation performanc...
Citations
... Many researchers have tried to overcome the sodium void effect, which can increase the reactor thermal power. Merk [4] reduced the sodium void effect by using moderating material, such as zirconium hydride, for the cladding layer to enhance the safety characteristics of SFR without changing the reactor core design. Kim et al. [5] attempted to maintain the sodium void worth low with a pan-shape core design that has two different heights of fueled region. ...
The Floating Absorber for Safety at Transient (FAST) is a safety device used in the innovative Sodium-cooled Fast Reactor (iSFR). The FAST insert negative reactivity under transient or accident conditions. However, behavior of the FAST is still unclear under transient conditions. Therefore, the existing Floating Absorber for Safety at Transient Analysis Code (FASTAC) is improved to analyze the FAST movement by considering the reactivity and temperature distribution within the reactor core. The current FAST system is simulated under a single control rod withdrawal accident condition. In this investigation, the reactor thermal power does not return to its initial thermal power even if the FAST inserts negative reactivity. Only a 9 K of coolant temperature margin, in the hottest fuel assembly at EOL, can lead to unnecessary insertion of the negative reactivity. On the other hand, the FASTs cannot contribute to controlling the reactivity when normalized radial power is less than 0.889 at BOL and 0.972 at EOL. These simulation results suggest that the current FAST design needs to be optimized depending on its installed location. Meanwhile, the FAST system keeps the fuel, cladding and coolant temperatures below their limit temperatures with given conditions.
... Several ideas and concepts have been proposed to maintain CVR near zero or slightly negative such as making the core more heterogeneous by placement of internal blanket (Sciora et al., 2011), inserting moderating materials to slightly soften the neutron spectrum (Merk, 2013;Wakabayashi, 2013;Won et al., 2014), or increasing the neutron leakage (Kim et al., 2000). These ideas may possibly reduce the CVR, but they also deteriorate the neutron economy, and thus they cannot be used in the current iSFR core requiring a high level of neutron economy. ...
This paper presents the neutronics conceptual design of a long-life 392.6 MWth innovative sodium-cooled fast reactor (iSFR). The iSFR, which was derived based on Korean Prototype Generation-IV sodium cooled fast reactor (PGSFR), was intended as an alternative fast reactor option while waiting for the availability and readiness of the fuel recycling technology in the Republic of Korea. The iSFR core was designed for a lifetime of over 20 EFPYs (effective full power years) and an average core discharged burnup greater than 100 GWd/MTHM. It was loaded with annular metallic low enriched uranium (LEU) fuels, which were arranged in eight rings of hexagonal assemblies and surrounded by three rings of PbO radial reflectors. The core neutronic performance and characteristics such as core lifetime, power profiles and thermal–hydraulics analysis, kinetics parameters and reactivity feedback coefficients, and cladding fast neutron fluence were analyzed by using McCARD Monte Carlo code and MATRA-LMR thermal–hydraulics sub-channel code. A balance of reactivity (BOR) analysis was also investigated to preliminary assess the safety capability of the iSFR against several unprotected transient scenarios. In addition, two new unique passive safety devices named FAST (Floating Absorber for Safety at Transient) and SAFE (Static Absorber Feedback Equipment) were also introduced and discussed in this paper.
... The second factor that affects the CVR is reduced capture by the coolant when the coolant voiding occurs [1]. To improve the CVR, many ideas and concepts have been proposed, which include introduction of an internal blanket [2], spectrum softening [3,4,5], or increasing the neutron leakage [6]. These ideas may reduce the CVR, but they deteriorate the neutron economy. ...
... Fuel breeding is not the major task anymore; thus, wellbalanced systems are designed with optimized feedback effects, which results in excellent safety behavior [21]. Additionally, the recently investigated use of fine distributed moderating material offers the possibility of designable feedback effects, since the most important inherent feedback effects, the Doppler and the coolant effect, can be influenced significantly without negative implications to the other safety and operationally relevant system parameters [26,27,28,29]. Unfortunately, the amount of TRUs in the core of a SFR is strictly limited due to the negative influence of some TRU isotopes on the inherent safety effects of the reactor core even if the effects can be reduced by the insertion of moderating material [30,31]. ...
In the view of transmutation of transuranium (TRU) elements, molten salt fast reactors (MSFRs) offer certain advantages compared to solid fuelled reactor types like sodium cooled fast reactors (SFRs). In the first part these advantages are discussed in comparison with the SFR technology, and the research challenges are analyzed. In the second part cycle studies for the MSFR are given for different configurations - a core with U-238 fertile, a fertile free core, and a core with Th-232 as fertile material. For all cases, the transmutation potential is determined and efficient transmutation performance for the case with thorium as a fertile material as well as for the fertile free case is demonstrated and the individual advantages are discussed. The time evolution of different important isotopes is analyzed. In the third part a strategy for the optimization of the transmutation efficiency is developed. The final aim is dictated by the phase out decision of the German government, which requests to put the focus on the determination of the maximal transmutation efficiency and on an as much as possible reduced leftover of transuranium elements at the end of the reactor life. This minimal leftover is achieved by a two step procedure of a first transmuter operation phase followed by a second deep burning phase. There the U-233, which is bred in the blanket of the core consisting of thorium containing salt, is used as feed. It is demonstrated, that transmutation rates up to more than 90% can be achieved for all transuranium isotopes, while the production of undesired high elements like californium is very limited. Additionally, the adaptations needed for the simulation of a MSFR, and the used tool HELIOS 1.10 is described.
Negative reactivity feedback is one of the most important inherent safety characteristics of reactors for suppressing changes in thermal power and component temperatures during accidents. The floating absorber for safety at transient (FAST) has been suggested to improve the coolant temperature coefficient (CTC) in a sodium-cooled fast reactor (SFR) for a better self-stabilization of the reactor system. Previous studies on the FAST application showed that the FAST causes power excursion and temperature oscillations under anticipated transient without scram (ATWS) conditions. This study explains the variables affecting the FAST behavior and oscillations with mathematical expressions. In addition, a damping FAST is proposed to passively remove oscillations without any mechanical system. The damping FAST increases the damping ratio by reducing the pin diameter at a height where the difference between the FAST bottom elevation and the equilibrium position is maximum. The performance evaluation for the damping FAST is conducted in an innovative sodium-cooled fast reactor (iSFR) under ATWS conditions. The 7-mm damping FAST unilaterally inserts the negative reactivity in unprotected transient overpower event, dramatically lowering the core power without oscillations. While the previous FAST shows oscillatory behavior repeatedly approaching the coolant boiling temperature, the damping FAST stably keeps the coolant temperature below 800 °C. Moreover, the damping FAST lowers the maximum coolant temperatures for other ATWS events. These findings show the potential of the FAST system to significantly improve the inherent safety of SFRs.
The sensitivity of operational and safety parameters on different strategies for the improvement of in core breeding on fuel assembly level are investigated using the HELIOS 2.1 code. The operational characteristics is analyzed regarding criticality, breeding, pin power and burnup distribution. As additional key parameters, the conservation of the safety related feedback effects of the assembly are examined. It is demonstrated that the insertion of 1/3 of fertile fuel rods into the fuel assembly, while the overall Pu content of the assembly is kept constant, can improve the breeding of fresh plutonium. A second proposal is the reduction of the initial Pu content of the assembly which is compensated by eliminating one ring of the fertile blanket around the core. This method proofs to be very efficient to improve the in-core breeding. The consequences on the fuel assembly multiplication factor, the fissile material content, and the pin wise power as well as burnup distribution is analyzed. Additionally, the effect of fine distributed material on breeding as well as on the safety related feedback effects is investigated for both proposals. A clear enhancement of the feedback effects is proven.
