This study presents the development of a new integral type neutron absorber design, characterized by structural materials containing gadolinium (Gd) and boron (B) that enhance critical safety and performance for a spent fuel transport and storage system. Gd and B stainless steel could act both as a structural member and as a neutron absorber. Sensitivity studies were performed to optimize the design parameters of fuel assembly spacing, internal cell width, and cell wall thickness in the spent fuel transport and storage system.
The new neutron absorber, Gd and B stainless steel with optimized dimensions, was applied to the spent fuel transport and storage vessel and the spent fuel storage rack. For the flux trap-type transportation and storage system, the spent fuel pool of Shin-kori units 3 and 4 was selected for the spent fuel storage pool, and KORAD-21 was selected for the spent fuel storage vessel. In conclusion, the Gd and B stainless steel with optimized size could improve the criticality control.
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Introduction
Commercially, boron (B) based neutron absorber is commonly used in spent fuel storage systems such as BORAL®, METAMICTM and borated stainless steel. The flux trap is located in the center of the reactor core and is surrounded by an annular region of fuel. The best position of the flux trap is in the center, but there is a limit to the reactor size.
The flux trap in the core is used for fuel/material testing or radioisotope production [18], [19]. The storage of spent fuel of the flux trap type improves the thermalization of neutrons and reduces the reactivity of the system. Therefore, in this paper, the analysis of neutron motion and the effect of flux trap were studied with neutron absorbing material.
![Table 1 Spent fuel storage status in Korea (data until 2019 3 rd quarter) [1]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10494388.0/18.892.119.782.150.968/table-spent-fuel-storage-status-korea-until-quarter.webp)
Analysis Methodology
Analysis code
Analysis model
- Fuel assembly
- Spent nuclear fuel transportation and storage system
The spent fuel transport and storage system is divided into two types of flux trap design and non-flux trap design as described in Figure 1. On the contrary, the non-flux trap type consists of a neutron absorber between fuel assemblies. The basic component of the spent fuel transport and storage system is a cell wall, which is a square structure of stainless steel that surrounds the fuel assembly, neutron absorbers that absorb neutrons for criticality control and attached to the cell wall, and cladding that covers the neutron absorbers to protect them.
The dimensions of the SNF transport and storage system vary depending on the type of fuel assembly.
Application to conventional cask
- KSC-1
- KSC-7
- KORAD-21
Study of new neutron absorber for spent fuel transportation and storage system
Conventional-type neutron absorber
New integral-type neutron absorber
- Effect of new neutron absorbing materials
Neutron absorbers of Gd, B, Sm, Er and Eu are inserted independently into stainless steel. In the calculations, the density of stainless steel with neutron absorbing materials is calculated by the mixing law in Eq. Within a 2 wt% range of absorber content, Gd shows the best performance among Gd, B, Sm, Er and Eu as depicted in Figure 10 .
The keff of other absorbers such as B, Sm and Eu decreases exponentially with increasing absorber content. When only Gd is used, the Gd content must be at least 2 wt%. For use in spent fuel systems, the mixture content of Gd and B is calculated in Figure 12.
Sensitivity analysis in spent fuel transportation and storage system
Neutron movement in spent fuel transportation and storage system
- Neutron flux variation of fuel assembly case under water
- Neutron flux variation of fuel assembly with spent fuel transportation and storage case
In Figure 16, the total neutron flux becomes higher as the distance gets further from the fuel assembly. At the epithermal energy region, the neutron flux becomes lower when the position comes far from the fuel composition. In the fast energy region, the neutron flux variation has a similar configuration with the epithermal energy region.
As the distance away from the fuel composition, the neutron flux decreases the amount of cm. Both the epithermal and fast energy regions, the neutron flux decreases as far from the fuel composition. This is similar between neutron flux at the edge of the fuel assembly and that at the center region of the flux trap.
The thermal neutron flux shows a similar configuration when only fuel composition exists in the water in Figure 19 . The neutron flux increases as the position is far from the fuel composition from the overall reach point of view. In Figure 20, the neutron flux of epithermal and fast energy decreases as the position is far from the fuel assembly.
The total neutron flux of Gd 1 wt% is much lower than that of stainless steel inside the cell wall as shown in Figure 18. The depression of the neutron flux in the cell wall can be more clearly observed for the Gd 1 wt% case than the stainless steel case . The 1 wt% Gd case has lower thermal neutron flux than the stainless steel case for the entire region.
The thermal neutron flux of stainless steel is 10 times higher than that of Gd 1 wt%.
Sensitivity analysis of flux trap type for criticality control
- Inner width
- Thickness
From the cell wall to the center of the flux trap, the neutron flux gradually increases as the distance from the fuel assembly increases. When the cell wall is attached to fuel composition, the neutron flux increases continuously as a sinusoid (0 to pi/2). The neutron flux decreases as the distance from the fuel assembly to the cell wall increases.
For Gd 1 wt% case, with neutron absorber, the neutron flux distribution at the cell wall is not affected by inner width as described in Figure 36. As the flux trap decreases, the absorption rate of water and neutron flux at the flux trap decreases. However, the amount of water that comes close to fuel composition increases, therefore the neutron flux inside the cell wall increases.
Although the rate of cell wall absorption increases, the rate of water absorption in the flux trap decreases. Also, the neutron flux inside the cell wall increases, so the neutron multiplication factor increases as the inner width increases. The neutron flux decreases as the cell wall thickness increases over a distance from the fuel assembly.
The neutron flux is highest at the central position of the flux trap for all cases. As the thickness of the cell wall increases, the amount of water in the flux trap decreases. Neutron flux spectra by cell wall thickness have been studied in an advanced manner.
The outer region of the cell wall shows that the neutron flux decreases with increasing thickness. However, the inner region of the cell wall shows that the neutron flux increases with increasing thickness for thicknesses of 0.2 to 0.9 cm. At the position of the flux trap in Figure 48 , the neutron flux distribution for the Gd 1 wt% case appears similar to the stainless steel case.
Application to spent fuel transportation and storage system for criticality control
Spent fuel storage pool
- Region I
- Region II
As explained in Chapter 4, keff shows a minimum value at the lowest internal width as summarized in Table 17. A clearance between the rack walls and the fuel assemblies is required, which takes into account the envelope of the fuel assemblies and manufacturing tolerances. With an optimal internal width of 21.2 cm and an optimal thickness of 0.45 cm, the pitch could be reduced to 26.0 cm, which has similar performance to the Boral, as shown in Figure 51.
With enriched B-10, reactivity can be reduced and pitch can be reduced more than using natural B. With the optimal internal width of 21.2 cm and the optimal thickness of 0.50 cm, the pitch can be reduced to 24.9 cm as depicted in Figure 52. As shown in Figure 53, the total spent nuclear fuel stored in region I is five modules; four for 8 9 and one for 7 9 .
In this study, the 75% value for the neutron absorber was used. Therefore, the concentration of B-10 used in the calculations is 0.0220 g B-10/cm for area II. The fresh fuel condition of a 5 wt% enriched UO2 fuel assembly was analyzed for optimization study in region II. Therefore, with optimized dimensions and Gd and B stainless steel, the pitch could be reduced from 22.5 cm to 22.05 cm, as described in Figure 54.
Therefore, the burn-up credit must be taken into account to store an enrichment of more than 1.5 wt.% for region II. Nuclide sets selected from NUREG/CR-7109 (actinide only, actinide with 16 fission products and all nuclides). Among all nuclide sets, the load curves show consistency with Boral and Gd 0.9 wt% + B 1.0 wt% (enriched B-10) stainless steel with optimized dimensions.
As described in Figure 53, there are thirteen modules of the rack in region II; four modules of 9x10, six modules of 10x10, two modules of 9x8 and one module of 9x9.
Spent fuel transportation and storage cask
The surface area to store the same amount of fuel assemblies is reduced by the circle calculating from Eq.
Accident analysis for spent nuclear fuel transportation and storage system
Spent fuel storage pool
- Region I
- Region II
Spent fuel transportation and storage cask
Conclusions
This is due to the fact that the total amount of stainless steel increases with increasing thickness. Although stainless steel has a low neutron absorption cross section, it still has the ability to absorb neutrons. To reduce the reactivity, the pitch should be maximum and the internal width should be minimum if there is a neutron absorber.
Through optimization studies, the inner width was optimized to 21.2 cm, and the thickness was optimized to 0.45 cm for natural B and 0.5 cm for enriched B. By combining Gd and B stainless steel and optimized dimensions, the pitch could be reduced from 27, 0 cm to 26.0 cm (natural B) and 24.9 cm (enriched B). Through optimization studies, an internal width of 22.0 cm was optimized, and thicknesses were optimized to be 0.55 cm (natural B) and 0.5 cm (enriched B).
With the combination of Gd and B stainless steel and optimized dimensions, the pitch can be reduced from 27.7 cm to 27.3. Kim et al., “A design and performance evaluation for a new neutron absorber based on an artificial rare earth compound,” Ann. Kim, "A Feasibility Study on Criticality Control Method Using Vitrified Radioactive Forms for Spent Fuel Storage," Nucl.
Lim, “Development of Gd containing duplex stainless steel for spent nuclear fuel applications,” Sungkyunkwan University, 2016. Kim, “Effect of stainless steel plate position on neutron multiplication factor in spent fuel storage racks,” Nucl. Lambert, "Handbook of Neutron Absorber Materials for Spent Nuclear Fuel Transport and Storage Applications: 2009 Edition," Palo Alto, CA.
29] ASTM International, “Standard Specification for Welded Stainless Steel Sheet, Plate and Strip for Core Application,” ASTM A.
![Table 13 Natural atomic abundances (in at%) of Gd, B, Sm, Er and Eu [34]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10494388.0/47.892.112.786.150.416/table-natural-atomic-abundances-gd-sm-er-eu.webp)
