Fuel Storage Racks Research Articles

Computational method for the added mass of fluid imposed on spent fuel storage racks considering three-dimensional flow

Spent fuel in nuclear power plants is usually stored temporarily in spent fuel pools in the form of racks. Seismic analysis of the racks is required for the structural safety of the spent fuel. Since the racks are arranged in water, the seismic analysis of the racks should consider the fluid–structure interaction effect (i.e. the added mass of fluid imposed on spent fuel storage racks). The traditional computational method for the added mass is to simplify the racks to a concentric square column with a two-dimensional flow. This simplification improves computational efficiency but results in excessive margins. In addition, since the rack of CAP1400 (a Chinese AP-type PWR) uses a porous structure, which is different from the conventional rack, the traditional simplified method cannot be used to calculate the added mass of the rack of CAP1400. Therefore, this paper proposes a computational method for the added mass, in which the three-dimensional flow, as well as the internal pores of the rack, are considered so that the excessive margin can be reduced. Using this new method, a formula with four factors was given to calculate the added mass coefficient. Among four factors, the dimensionless gaps in X and Y directions represent two-dimensional flow effect, while the dimensionless height of the rack represent three-dimensional flow effect. The new method is verified by a scaled down experiment for rack of CAP1400.

Methodology, applications and performance of the CAD-based geometry type in the serpent 2 Monte Carlo code

The Serpent 2 Monte Carlo code features a CAD-based geometry option for the modeling of complicated and irregular systems. The methodology is based on the STL data format, which is commonly used for computer graphics and 3D printing, and supported by a wide range of software tools. The methodology has been available for several years, but not described in complete detail. This paper presents the geometry routine with its advantages, limitations and known flaws. A brief overview on practical applications and a workflow example involving neutron and photon transport calculations for a spent nuclear fuel storage rack are provided for discussion. It is concluded that the CAD-based geometry type is a convenient option for various neutron and radiation transport problems, and does not suffer from significant deterioration in computational efficiency compared to conventional CSG models. It is also noted, however, that taking full advantage of the methodology requires some level of understanding on the software tools, the STL data format and the Serpent geometry routine to avoid the most common pitfalls.

International case survey and literature review of maintenance strategy for decommissioning nuclear power plant during transition period

The Chinshan nuclear power plant in Taiwan has currently shutdown when the operating license expired in 2018. However, because the operation license of the independent spent nuclear fuel dry storage facility has not been approved yet, and the fuel storage racks of Spent Fuel Pool (SFP) has almost saturated, there is not enough space to accommodate all irradiated fuels from the reactor core during the transition period. Therefore, part of the mechanical systems, plant structures and electrical equipment of the plant still have to meet the functional requirements for maintaining the safety under the pre-decommissioning activities.In this article, we introduce the international cases of permanent shutdown but the fuel still in the reactor core, and also describe the actions of the Taiwan Power Company in response to this situation. For the providing regulatory advice, we have also proposed a framework of the maintenance strategy associated with important issues for the transition period since 2018, including a revised Aging Management Program (AMP) for keeping the safety of spent nuclear fuel. From the knowledge level and important issues related to the topic, the Phenomena Identification and Ranking Table (PIRT) is quoted to complete the understanding level of requirements during the transition period.

Fluid-structure coupled vibration characteristics of liquid immersed square channels within confinement

In research reactors, liquid immersed square section geometries like plate-type fuel assemblies or spent fuel storage racks are arranged such that the gap between them is small (<10 mm). These liquid-filled gaps affect and alter the dynamics of these structures under vibratory loads. The present work investigates vibration characteristics of liquid immersed square sectional geometries like square channels. A correlation for added mass coefficient (Cm) is developed for the immersed square channel in terms of gap ratio (η) and eccentricity ratio (ε), and the influence of the NAVMI factor is studied. It is found that Cm increases with decreasing η and increasing ε. Subsequently, the insights obtained from these investigations are utilized to determine the vibration characteristics of typical plate-type fuel assemblies separately and for a fluid coupled 3 × 3 array.

The determination of added mass matrix of CAP1400 spent fuel storage racks by porous media method

The fluid-structure interaction (FSI) effect is crucial to the seismic response of spent fuel storage racks in an earthquake. Previous studies show that the tiny gaps between rack’s cells (end-tube construction) strongly influence the rack’s added mass. However, accurate added mass for such racks requires huge computational resources. To simplify the determination of added mass, porous media is used. The porous media coefficients determined by oscillating flow are used as input for rack-wall model with an experimental benchmark. The fluid forces on rack/wall by the porous media model agree with the experiment and detail CFD model. After proving that oscillating frequency has limited influence on added mass, the added mass matrix is obtained by multiple transient simulations on whole-pool racks. The porous media method is successfully used to simplify the determination of CAP1400 racks’ added mass matrix with desirable accuracy.

The fluid-structure interaction effect on seismic response of double CAP1400 spent fuel storage racks

The spent fuel storage racks (racks) are immersed in the spent fuel pool to storage the spent fuels temporarily. The safe and reliable operation of racks is crucial to the safety of nuclear power plants. The free-standing rack's design gradually replaces the traditional bottom-fixed design because the free-standing design uses friction and water to decrease the seismic load transfer. The fluid–structure interaction (FSI) effect, which is the key phenomenon for racks under earthquake, can be simplified as an added mass matrix among the racks and the spent fuel pool. The added mass matrix is strongly associated with the rack’s distribution and the rack’s geometry. CAP1400 (a newly designed reactor type designed by China) racks adopt end-tube-connection construction and abandon outer plates, significantly changing the FSI effect. The end-tube-connection construction’s influence on double-rack seismic response hasn’t be studied numerically. Moreover, the large uncertainties in the previous experimental response bring some difficulties in model validation. To solve the above two problems, we built two CFD models to study the influence of the rack’s outer plates on the added mass matrix. Then double-rack seismic models with /without the FSI effect are validated by the experiments of sine waves and seismic waves input. The validated model is then used to study the influence of the friction coefficient and the outer plates on the rack’s seismic response. Results show the previous experimental large uncertainties in racks’ displacement response are mainly caused by the extremely low-frequency components. Moreover, the seismic displacement response of racks without outer plates is much larger compared to racks with outer plates, which means racks’ seismic performance may be improved if an outer plate is added to CAP1400 racks.

Development of Underwater Lifting Tool for Spent Fuel Storage Rack in Nuclear Power Plants

In response to the requirement for the underwater lifting of spent fuel storage racks in a second-generation nuclear power plant, according to the current situation of the narrow underwater layout of spent fuel storage racks and the analysis of the safety and convenience of underwater lifting interfaces, a grab-and-release rack was developed. Bottom-bottom lattice lifting tool with underwater self-locking lattice is realized by running water holes on the bottom of the rack, and the structure, operation and working principle of this underwater lifting tool are introduced in detail. Finally, the lattice is used in a second-generation nuclear power plant for a lifting operation of 20 rack frames in the factory. The field application shows that the lifting tool is easy to operate and reliable in structure, which meets the requirements for the underwater lifting of the lattice frame.

Shaking table experiments on 3/10 reduced-scale CAP1400 spent fuel storage rack considering the effect of fluid-structure interaction

The seismic study of the spent fuel storage rack (SFSR) is of great significance for ensuring the safety of spent fuel. But it is a challenging topic for the complex nonlinear behaviors such as fluid–structure interaction (FSI), friction and impact. At present, owing to the lack of the understanding of these nonlinear behaviors, in engineering design process, the seismic analysis of SFSRs is usually conducted with a conservative method. To reduce the excessive margin in the seismic design of CAP1400 (a new reactor type developed by China) rack, we have performed a 1/10 scale shaking table experimental study in the previous work. As an extension of the previous study, a series of 3/10 reduced-scale seismic experiments considering the effect of FSI were carried out to study the scale effects in this paper. The experiments obtained friction coefficient between the rack and the tank (spent fuel pool model), the maximum sliding displacement of the rack and the fluid pressure distribution of the tank wall. Furthermore, the seismic responses of the rack under different experimental variables such as the number of racks, experimental scales and vertical excitation were derived. The results showed that the relative uncertainty of the sliding displacement response is very large. Moreover, the large sliding displacement differences between single-rack case and double-rack case indicated that fluid–structure interaction has a strong influence on sliding of rack. We adopted a generally used similarity criterion for seismic experiment of rack, and the experimental results showed the scaling analysis of fluid pressure satisfies the similarity relationship, however, the displacement results were influenced greatly by scale effect. These results can be the benchmarks for the seismic analysis of CAP1400 spent fuel storage rack under dynamic seismic loading.

Study on added mass model for fluid-structure interaction of spent fuel storage rack

The spent fuel storage racks are placed freely inside a pool that is filled with water. Under seismic condition, the fluid in the gap between the racks will be forced to flow due to the movement of the racks. Inversely, the fluid may impose an added mass to the rack and affect the movement of the racks. The added mass imposed to the rack in this kind of fluid-structure interaction phenomenon (FSI) is very important for the dynamic analysis of racks. Previous researches on rack mainly focus on theoretical analysis and numerical simulation. Due to the complexity of the structure, application of the theoretical analysis is limited, while the effectiveness of numerical method need to be validated by experiments. In order to develop an added mass model for rack with end-tube-connection construction like CAP1400 spent fuel storage rack, in this paper, a two-dimensional model of the CAP1400 rack is established and added mass of rack were obtained by the numerical simulation. The numerical results are in good agreement with the experimental ones. Therefore, the effective of numerical method for the calculation of the added mass is validated. In order to study added mass of the multi-rack, the hydraulic mass matrix of double-rack is calculated by CFD. The results can provide added mass input for the design and analysis of the CAP1400 series rack.

Criticality safety analysis of spent fuel pool for TRIGA Puspati Reactor using MCNP5

Spent fuel pool uses water to temporarily cool spent nuclear fuels before final disposal. The Malaysian Nuclear Agency is recently working on its spent fuel pool facility to store irradiated TRIGA fuel rods. These irradiated fuels still contain significant amount of fissile material which are capable to induce criticality in the fuel storage facility. This study is therefore performed to investigate the safety criticality analysis for the proposed fuel storage rack design of 5x5 arrays. MCNP5 code was used to model the 5x5 array of fuel storage rack in order to determine the optimal fuel pitch distance. The analysis was based on the standard UZrH1.6 TRIGA fuel cladded with stainless steel used in PUSPATI TRIGA Reactor, which is 19.9% enriched 235U and 20% weight of Uranium. By varying the pitch distance between 4 cm to 14 cm with no neutron absorber included, the optimum fuel rack design was determined where sub-criticality condition can be maintained.

Two-scale CFD analysis of a spent fuel pool involving partially uncovered fuel storage racks

Abstract The thermohydraulic conditions in a storage pool for spent nuclear fuel are studied for a loss of cooling accident leading to partially uncovered fuel racks. While under normal operating conditions, the fuel is cooled by means of single-phase natural convection in the water of the pool, the heat transfer rate into the gaseous pool atmosphere for the above scenario involves several mechanisms to a similar amount which are interacting in a complicated manner, such as heat convection, thermal radiation and heat conduction. These mechanisms are analyzed with the aid of Computational Fluid Dynamics using two complementary models that address different length scales, one for the large scales of the pool and the containment geometry, another one for the small scales of the fuel rods. The models emulate the conditions at reactor unit 4 of the former Fukushima Daiichi nuclear power plant at the time of the accident, because this is a case example for a loss of cooling scenario and sufficient material for adequate modeling is available in the literature. The pool scale model serves to analyze the developing flow patterns. It includes the fuel represented as porous medium as well as the pool and reactor building atmosphere. The results show that a characteristic flow field forms in the pool atmosphere that prevails for all studied water levels and decay heat rate distributions. The temperature level, on the other hand, can be reduced significantly by means of a checkerboard storage. However, the boundary conditions in the head region of a fuel assembly are clearly a function of its storage location in the pool. Representative conditions are extracted and applied to the second model that represents a single geometry-resolved fuel assembly with a portion of the atmosphere above it. The corresponding simulations show that the heat exchange rate between the fuel assembly and the atmosphere above is higher for the conditions near the pool wall compared to the conditions in the center of the pool. The study provides important information on how to best arrange the fuel assemblies in a spent fuel pool for optimal heat exchange in case of an accident involving loss of cooling water. Checkerboarding should be combined with a strategy to place fuel with higher decay heat rate near the pool walls.

Numerical investigation on the fluid added mass of spent fuel storage rack

Abstract Spent fuel storage rack is the most important equipment for storing spent fuel after refueling in nuclear power station. Free standing storage rack has an advantage to protect the structural integrity of the rack, because the fluid-structure interaction of the free standing rack can dissipate the earthquake energy and reduce the movement of rack, when earthquake occurs. The fluid added mass is a main parameter for studying the fluid-structure interaction effect, which provides an important parameter for the transient structural analysis of the storage rack. However, the calculation methods of fluid added mass, which in engineering, are only for the simple geometric structure such as circular cylinder or square cylinder. It cannot apply to the spent fuel storage rack, which the geometric structure is extraordinarily complex. In this paper, in order to investigate a more accurate fluid added mass, a 2-dimensional dynamic model was constructed. Furthermore, the transient analysis of the 2-dimensional model was conducted by Computational Fluid Dynamics (CFD) software, Fluent. Then, the added mass of the model was obtained according to CFD transient fluid force. Moreover, a model which corresponding to the previous vibration experiment of rack was conducted to verifies the correctness of the model. From numerical results, some important features for the design of rack were found. This study can offer the assistance for the design and manufacture of spent fuel pools and high-density spent fuel storage racks, ultimately.

Study on the fluid added mass of CAP1400 spent fuel storage rack under vibration condition

Abstract The fluid has great influence on the motion and deformation of the structure under vibration condition, which makes the structure produce a strongly nonlinear vibration. The spent fuel storage rack, which submerged in liquid, is an important equipment in nuclear power plant. Thus, it is extremely important to study fluid-structure interaction problem of the spent fuel storage rack among the equipment in nuclear power plant. Moreover, the fluid-structure interaction between the water around the spent fuel storage rack and the structure may compromise the integrity of the storage rack. Therefore, it is highly necessary to research the characteristic of the fluid-structure interaction among the racks. In this paper, the free vibration experiments, based on the 3:10 model of the spent fuel storage rack in CAP1400 (a new reactor type designed by China), were conducted to study the fluid-structure interaction effect of adjacent racks. The parameters of fluid-structure interaction among the racks was measured under different clearances through free vibration experiment. Ultimately, the formula for calculating the added mass of the rack under different gap sizes was put forward in this paper. This study can provide fluid-structure interaction parameters for structural dynamic analysis of the spent fuel storage rack, and offer the assistance for the design and manufacture of spent fuel pools and high-density spent fuel storage racks, ultimately.

Performance evaluation of METAMIC neutron absorber in spent fuel storage rack

Abstract High-density spent fuel (SF) storage racks have been installed to increase SF pool capacity. In these SF racks, neutron absorber materials were placed between fuel assemblies allowing the storage of fuel assemblies in close proximity to one another. The purpose of the neutron absorber materials is to preclude neutronic coupling between adjacent fuel assemblies and to maintain the fuel in a subcritical storage condition. METAMIC neutron absorber has been used in high-density storage racks. But, neutron absorber materials can be subject to severe conditions including long-term exposure to gamma radiation and neutron radiation. Recently, some of them have experienced degradation, such as white spots on the surface. Under these conditions, the material must continue to serve its intended function of absorbing neutrons. For the first time in Korea, this article uses a neutron attenuation test to examine the performance of METAMIC surveillance coupons. Also, scanning electron microscope analysis was carried out to verify the white spots that were detected on the surface of METAMIC. In the neutron attenuation test, there was no significant sign of boron loss in most of the METAMIC coupons, but the coupon with white spots had relatively less B-10 content than the others. In the scanning electron microscope analysis, corrosion material was detected in all METAMIC coupons. Especially, it was confirmed that the coupon with white spots contains much more corrosion material than the others.

A new design concept and seismic margin assessment for a spent fuel storage system

The design and structural performance of a spent fuel storage system, which plays a role in storing irradiated and damaged fuel assemblies generated during the operation of a reactor, have been regarded as very important issues in relation to nuclear safety. The recent accident at the Fukushima Daiichi Nuclear Power Plant once again highlights the importance of the safe design of such a system. Thus, this paper proposes a new design concept of a spent fuel storage system composed of spent fuel storage racks (SFSR) and a support frame. The design of each SFSR is enhanced by increasing the natural cooling capacity of the pool water. The support frame is newly devised such that it physically prevents the overturning and sliding of racks and collisions among them during an earthquake event.In addition, to verify the structural integrity of the proposed design and evaluate its seismic margin, static analyses, response spectrum analyses and nonlinear time history analyses are conducted. The response spectrum analyses provide the maximum response of the structures during and after seismic events. The nonlinear time history analyses were carried out to predict contact sliding, rocking, twisting, and turning between the floor of the pool bottom and the support frame. The numerical results were analyzed based on the allowable code limits to assess the structural integrity. The possibility of a collision between the support frame and the adjacent pool wall is investigated. The seismic margin of the proposed design is studied within the seismic fragility analysis framework.The analysis results show that the maximum stress values of these SFSRs and their support frame under seismic loads are within the specified code limits. An impact between the support frame and the adjacent pool wall will not occur because the sliding distance calculated from the nonlinear time history analysis is less than the gap between the two elements. These results confirm that they cannot be overturned but will instead slide under an SSE event. Finally, the seismic fragility analysis results demonstrate that the designed spent fuel storage system has a sufficient seismic margin which exceeds the targeted seismic design level.

The shaking table experiments on sliding and overturning of CAP1400 spent fuel storage rack with the effect of FSI

Under seismic condition, for CAP1400 (a new nuclear reactor type designed by China) spent fuel storage rack; water can flow through the channels between the rack storage cells. Unfortunately, the fluid structure interaction (FSI) effect for such perforate structure has not been studied in depth. In engineering approach of spent fuel rack’s structural analysis, the perforations are ignored, leading to an over-conservative added mass. To study the friction coefficient and FSI effect for CAP1400 rack, a series of shaking table experiments had been carried out using a reduced-scale spent fuel rack model in a water tank (spent fuel pool model). To derive the friction coefficient between rack and tank wall, sweep frequency tests were carried out in shallow-water-level condition and deep-water-level condition. Large uncertainty was found in the measurements of friction coefficient; consequently, the data were organized by statistical methods. Moreover, the relationship between gap size and fluid pressure were studied. Finally, the response to seismic loading was studied in different gap size conditions. The research can provide the basic input parameter for the dynamic analysis for spent fuel storage rack; moreover, the experimental seismic response can be the benchmark for dynamic analysis of CAP1400 spent fuel rack.

Comparison of the welding behavior of P/M borated and I/M borated stainless steel

Borated stainless steels are being used to control neutron criticality in reactors as control rods, shielding material, spent fuel storage racks and transportation casks. Borated stainless steels fabricated via powder metallurgy (P/M) route (grade A) exhibit superior neutron absorption, enhanced toughness and ductility compared to cast ingot metallurgy (I/M) steels (grade B), due to fine and uniform dispersion of borides. In this study, welds were made using both 304B grade A and grade B materials by gas tungsten arc welding and activated gas tungsten arc welding processes. The effect of the activated flux on the bead geometry, microstructure and mechanical properties was investigated and compared. The heat input required for P/M steel material to produce full penetration welds has been found to be significantly higher compared to I/M steel, while all welds exhibited high joint efficiencies. Metallographic examinations revealed that no porosities and shrinkage cracks have been observed either in the fusion zone or in partially melted zone of the weldments. The fusion zone structure of 304 P/M type consists of a higher amount of eutectics with less number of austenite dendrites compared to I/M type. Both P/M and I/M steel welds exhibited partially melted zones adjacent to fusion zone. P/M steel welds were found to fracture in the partially melted zone, where a significant loss in hardness was noticed due to irregular boride eutectics, while in case of I/M steel welds, failure occurred in base material. It has been concluded that the P/M steel 304B grade A exhibits good GTA weldability compared to grade B material.

Effect of neighboring wall on the fluid added mass of CAP1400 spent fuel storage rack

Under seismic conditions, the motion of spent fuel storage racks will be influenced greatly by fluid-structure interaction (FSI) between water, racks and walls. In the design configurations of CAP1400 (a new reactor type designed by China), known as end tube connection construction, the water between the rack storage cells can flow out of the rack, forming a perforate structure. However, the FSI effect for such structure has not been studied in depth. In engineering, the perforations are ignored, leading to an over-conservative added mass. The FSI effect of racks can be categorized into rack-wall FSI (also known as the FSI effect of neighboring wall) and rack-rack FSI. Free vibration experiments for a rack model were carried out to study the FSI effect of neighboring wall of CAP1400 rack. By dimension analysis, it is determined that KC number, oscillatory Re number, and nondimensional gap size are key factors for added mass. The experiments obtained the fluid added mass with different combinations of key parameters. An added mass expression for CAP1400 rack near a wall was obtained and the error was evaluated. The results can be utilized as a reference for the FSI input of the dynamic analysis of CAP1400 spent fuel rack.

Benchmarking criticality analysis of TRIGA fuel storage racks

A criticality analysis was benchmarked to sub-criticality measurements of the hexagonal fuel storage racks at the United States Geological Survey TRIGA MARK I reactor in Denver. These racks, which hold up to 19 fuel elements each, are arranged at 0.61m (2 feet) spacings around the outer edge of the reactor. A 3-dimensional model was created of the racks using MCNP5, and the model was verified experimentally by comparison to measured subcritical multiplication data collected in an approach to critical loading of two of the racks. The validated model was then used to show that in the extreme condition where the entire circumference of the pool was lined with racks loaded with used fuel the storage array is subcritical with a k value of about 0.71; well below the regulatory limit of 0.8. A model was also constructed of the rectangular 2×10 fuel storage array used in many other TRIGA reactors to validate the technique against the original TRIGA licensing sub-critical analysis performed in 1966. The fuel used in this study was standard 20% enriched (LEU) aluminum or stainless steel clad TRIGA fuel.

The Sliding and Overturning Analysis of Spent Fuel Storage Rack Based on Dynamic Analysis Model

Spent fuel rack is the key equipment for the storage of spent fuel after refueling. In order to investigate the performance of the spent fuel rack under the earthquake, the phenomena including sliding, collision, and overturning of the spent fuel rack were studied. An FEM model of spent fuel rack is built to simulate the transient response under seismic loading regarding fluid-structure interaction by ANSYS. Based on D’Alambert’s principle, the equilibriums of force and momentum were established to obtain the critical sliding and overturning accelerations. Then 5 characteristic transient loadings which were designed based on the critical sliding and overturning accelerations were applied to the rack FEM model. Finally, the transient displacement and impact force response of rack with different gap sizes and the supporting leg friction coefficients were analyzed. The result proves the FEM model is applicable for seismic response of spent fuel rack. This paper can guide the design of the future’s fluid-structure interaction experiment for spent fuel rack.