Irradiation tests of materials at HANARO have usually been conducted using a standard capsule at temperatures of about 300°C for irradiation of materials used at PWR. Thus, the standard capsule uses aluminum as the specimen holder, which acts to dissipate the thermal energy. Future nuclear systems such as a VHTR and SFR require the irradiation tests at a relatively high temperature. As an alternative to aluminum which has been used as the thermal media in a standard material capsule, the characteristics of liquid metals such as NaK and LBE are reviewed. The temperatures of the capsule are affected by the variation of parameters such as the gap and wall thickness of the container. In particular, the external gap is most important in determining the temperature of the specimen. LBE raises the temperature of the specimen higher than NaK at the same configuration of the capsule. Thus, LBE can lessen the gap of the parts and reduce the vibration for a stable long-term test in reactor.
Nuclear systems have evolved in accordance with concerns over energy resource availability, climate change, and energy security. Among them, Gen-IV nuclear systems are in the spotlight as future energy sources. Sustainability, safety, cost-effectiveness, and proliferation risk reduction for future commercial development are pursued [
To meet these requirements, a new type of capsule is being developed at HANARO. The irradiation tests of the materials at HANARO up to the present have been performed usually at temperatures below 300°C under which the RPV (Reactor Pressure Vessel) materials of a PWR are being operated [
The standard material capsule has a simple shape containing the specimen holders in a sealed tube. The external tube is made of stainless steel to have enough strength to withstand an external impact when it is installed in the irradiation holes in the reactor, but the specimen holders are usually made of aluminum to process the specimen holes and instrument grooves easily on the surface. The specimen holder plays a role of thermal media to dissipate heat generated in the reactor well. The specimen holder of a standard capsule usually has 4 specimen holes that are arranged at a 90-degree angle at the same distance from the center. However, the aluminum specimen holder cannot be used at the high-temperature irradiation capsule owing to the low melting point, and thus another material is being sought out. Liquid metal has been accepted as an alternative way for use in an application of high-temperature irradiation for Gen IV reactor material development. And thus, liquid metal such as NaK (Sodium Potassium) and LBE (Lead Bismuth Eutectic) is reviewed for use as a material to replace aluminum in capsule [
The geometrical shape of the HANARO standard capsule is shown in Figure
Geometrical data of the standard capsule.
Descriptions | Dimension (mm) |
---|---|
Outer diameter of the external tube | 56 |
Inner diameter of the external tube | 52 |
Center hole diameter of the holder | 10 |
Specimen hole size (width × height × length) | 10 × 10 × 114 |
Distance between the center hole and specimen hole | 15 |
Instrumented capsule for the material irradiation test.
The specimen holder is a cylinder with four rectangular specimen holes and one circular center hole of 10 mm in diameter. It has a length of 114 mm and is used for fixing the test specimens. The five holders in the main-body are arranged in the axial direction, and insulators made of alumina between the holders are placed to prevent heat from transferring between the stages and to control the temperature of each stage independently. Figure
Schematic view of the standard capsule.
Since the irradiation tests for the future nuclear systems in an SFR and VHTR will be conducted at a relatively high temperature of 550 to 950°C [
LBE is a eutectic alloy of lead and bismuth [
Cross sections of capsules with solid and liquid thermal media.
The capsule is 56 mm in diameter and 870 mm long and consists of a specimen container, a liquid metal container, and an external tube. The 4 columns of the specimen holders are placed at 90 degrees to maximize the space. Helium gas is filled into the gap between the container and the external tube and between the specimen and the specimen holder. There are walls and gaps that block the heat transfer out from the specimen to the outside cooling water. They are the wall of the external tube, the specimen container (thickness T1), and the liquid metal container (thickness T2) and the gap (thickness G1) between the specimen and the specimen container and the gap (thickness G2) between the liquid metal container and the external tube.
A temperature analysis for capsules with three kinds of thermal media, that is, aluminum, NaK, and LBE, was performed. The temperature calculation is performed using a finite element analysis program, ANSYS. The two-dimensional model of a quarter-section has two specimens shown in Figure
Calculation model ((a) solid thermal media/(b) liquid thermal media).
The temperature of cooling water in the reactor in-core is about 40°C, and the heat transfer coefficient at the outer surface of the external tube is 30.3 × 103 W/m2°C, which is experimentally determined [
Heating rate at OR5.
Material | Density (g/cm3) |
|
---|---|---|
LBE | 9.96 | 1.04 |
NaK | 0.86 | 1.80 |
Aluminum | 2.7 | 1.67 |
Stainless steel | 7.8 | 2.08 |
The temperature distribution in the capsule depends on the detailed configuration and the thermal environments in which it is placed at the reactor. To evaluate the effect of the design variables such as G1, G2, T1, and T2 relatively, one parameter is varied, while, on the other hand, holding other variables constant. To obtain the thermal characteristics of the liquid thermal media capsule, the gap size of G1 varies from 0.1 to 1.0 mm and the G2 varies from 0.1 to 1.5 mm; on the other hand, thicknesses T1 and T2 of the specimen container and the liquid metal container are considered at the same range from 0.5 to 1.2 mm. The temperature data for a circular cylinder with multiple specimens are obtained by a finite element analysis. The gamma heating rates at the OR5 hole are obtained for a reactor power of 30 MW. The boundary conditions in the analysis are symmetric for the
An analysis was conducted to study the effect of variation in the design parameters on the peak temperature of the specimens, the temperature difference within the specimen, and the surface temperature.
The results of thermal analyses for capsules with solid and liquid thermal media, in which the solid thermal media is aluminum and the liquids are NaK and LBE, are shown in Figure
Temperature distribution of the capsules with three kinds of thermal media.
When gap G2 in the capsule using LBE thermal media (LBE capsule) is 1.575 mm, the specimen reaches the target temperature of 950°C, where the values of other variables such as G1, T1, and T2 are fixed. On the other hand, the specimen temperature reaches 950°C when gap G2 in the capsule using NaK thermal media (NaK capsule) becomes 3.48 mm, and the specimen reaches 950°C when gap G2 in the capsule using aluminum thermal media (Al capsule) is 3.175 mm.
In the aluminum capsule, when the specimen temperature reaches 950°C, aluminum thermal media would reach a similar temperature. This temperature exceeds the melting point of aluminum, and therefore aluminum cannot be used as the thermal media in a high-temperature irradiation capsule. In the case of a NaK capsule, to raise the temperature of the specimen to 950°C, gap G2 of the NaK capsule should be 3.48 mm, which is greater than the 1.575 mm gap in the LBE capsule. The capsule will be loaded into the irradiation hole of the reactor for testing. In HANARO, the capsule will be placed in the cooling water flowing upward with high pressure and high speed, and thus it will tremble with vibration. The vibration becomes larger as the gap between the parts in the capsule grows more. Because the gap between the parts in the NaK capsule is greater than that in the LBE capsule, the vibration increases more. It will be difficult to hold the parts in the capsule at a constant position and keep the temperature constant during the irradiation test.
In the helium gap between the inner container and the external tube, temperature falls sharply. The surface temperatures of the inner containers, NaK, LBE, and aluminum, reach as in Table
Temperature changes of the external tube by radiation heat.
Thermal media | Temperature of inner container | Radiation heat to external tube (W/g) | Temperature of external tube by radiation heat (°C) | |
---|---|---|---|---|
Before absorbing radiation heat | After absorbing radiation heat | |||
NaK | 867 | 5.61 | 53 | 59 |
LBE | 837 | 5.00 | 65 | 71 |
Aluminum | 906 | 6.38 | 55 | 62 |
Figure
Effects of gaps G1 and G2 and container thicknesses T1 and T2 on the specimen temperature (from the left; capsule with NaK/LBE/Al thermal media).
The effect of the thickness of the liquid container on the specimen’s temperature is also analyzed and shown in the figure. The thickness of the liquid container, T1 and T2, changes from 0.5 to 1.5 mm in both cases. The temperature changes from 438 to 514°C in the NaK capsule and from 745 to 769°C in the LBE capsule. The variation rate in the NaK capsule is much bigger than in the LBE capsule. It indicates that the LBE is more stable than NaK on the specimen temperature according to the variation of the container thickness. This analysis showed that G2 plays an important role in determining the temperature of the capsule.
Figure
Effects of gaps G1 and G2 and container thicknesses T1 and T2 on the surface temperature (from left: NaK/LBE/Al thermal media).
Figure
Effects of gaps G1 and G2 and container’s thicknesses T1 and T2 on the temperature difference within the specimen (from left: NaK/LBE/aluminum thermal media).
The optimal specimen size depends on the design configuration of a capsule as well as the nuclear characteristics of the test hole. The effect of the gap is larger than that of the container thickness. The temperature differences decrease according to the increase of the gap. G1 is much more sensitive to the difference in the specimen’s temperature than G2, and both affect it adversely. On the other hand, the variations of T1 and T2 have little effect. From the analysis results, the larger gap size of G1 can cause a smaller temperature difference within the specimen. Since the increase in the gap raises the temperature of the specimen, although it lessens the temperature differences within the specimen, it is important to find the optimum gap sizes. The temperature differences within the specimen of an Al capsule are within the range of ±10°C, as shown in the right side of Figure
For irradiation of high-temperature materials to be used in a future nuclear system like a VHTR and SFR, a new type of capsule using liquid metal was reviewed for application to high-temperature irradiation tests. As an alternative to aluminum which has been used as the thermal media in a standard material capsule, the characteristics of liquid metals such as NaK and LBE are reviewed. The temperatures of the capsule are affected by the variation of parameters such as the gap and wall thickness of the container. In particular, the external gap G2 has the greatest influence on the temperature of the specimen, and thus G2 is the most important in determining the target temperature. The surface temperatures are almost constant regardless of any change in the gap and container thickness in all kinds of capsules. LBE raises the temperature of the specimen higher than NaK at the same configuration of the capsule. The LBE capsule can lessen the gap of the parts to reduce the vibration for a long-term stable test in the reactor. In addition, LBE has a higher boiling point and is convenient to treat in comparison with NaK.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2013M2A8A1035822).