Hydrogen is considered a promising future energy solution, as it is clean, abundant, and storable, and has highenergy density. One of the major challenges in establishing a hydrogen economy is how to produce massive quantities of hydrogen in a clean, safe, and economical way. Among the various hydrogen production methods, nuclear hydrogen production has garnering attention worldwide since it can produce hydrogen, a promising energy carrier, without environmental burden. Research demonstrating the massive production of hydrogen using a VHTR (Very High Temperature Reactor) designed for operation at up to 950˚C has been actively carried out worldwide, including in the USA, Japan, France, and the Republic of Korea (ROK) [1-3].

A Sulfur-Iodine (SI) process that requires high temperature energy is well known as a feasible technology to produce hydrogen from water [4]. The SI process for hydrogen production requires sufficient heat that can be supplied by a nuclear reactor. A VHTR should be used as a high temperature energy source. The nuclear hydrogen program in the ROK has been strongly considering for producing hydrogen by employing an SI water-splitting hydrogen production process as shown in Figure 1 [5,6]. An intermediate loop that transports the nuclear heat to the hydrogen production process is necessitated for a nuclear hydrogen program. In the intermediate loop, unlike the hot gas duct, which provides a route of hightemperature gas from the nuclear reactor to the intermediate heat exchanger, the PHE (Process Heat Exchanger) is a component that utilizes the nuclear heat from the nuclear reactor to provide for hydrogen production. PHE is used in several processes such as nuclear steam reforming, nuclear methanol, nuclear steel, nuclear oil refineries, and nuclear steam [1]. The PHE of the SO₃ decomposer, which generates process gases such as H₂O, O₂, SO₂, and SO₃ at a very high temperature, is a key component in the nuclear hydrogen program in the ROK.

Recently, KAERI (Korea Atomic Energy Research Institute) established a small-scale nitrogen gas loop for a performance test of VHTR components, and manufactured a 10 kW class lab-scale PHE prototype made of Hastelloy-X. The 10 kW class lab-scale PHE prototype is composed of two kinds of flow plates; grooves 1.0 mm in

diameter machined into the flow plate for the primary coolant, and waved channels bent into the flow plate for the secondary coolant. Inside the 10 kW class lab-scale PHE prototype, twenty flow plates for the primary and secondary coolants are stacked in turn. A performance test of the 10 kW class lab-scale PHE prototype is underway in the small-scale gas loop at KAERI, as shown in Figure 2. In reality, the flow plates inside the 10 kW class lab-scale PHE prototype were not metallurgically joined with each other. During the performance test of the 10 kW class lab-scale PHE prototype, the flow plates might not make contact with each other.

In this study, to understand the macroscopic structural behavior of the PHE prototype under a steady-state operating condition of the gas loop, high-temperature structural analyses on the 10 kW class lab-scale PHE prototype were performed for two extreme cases: in the event of the flow plates making contacting with each other, and cases in which they do not make contact. The analysis results for the extreme cases were also compared.

A schematic view of the inside of the 10 kW class labscale PHE prototype is illustrated in Figure 3. The PHE prototype is designed as a hybrid concept to meet the design pressure requirements between a nuclear system and hydrogen production system [7]. That is to say, the hot nitrogen gas channel has a compact semicircular shape, similar to a printed circuit heat exchanger, and is designed to withstand the high pressure difference between loops, while the sulfuric acid gas channel has a plate fin shape with sufficient space to install and replace the catalysts for sulfur trioxide decomposition as shown in Figure 4.

All parts of the 10 kW class lab-scale PHE prototype

are made of Hastelloy-X of high-temperature alloy. Grooves 1.0 mm in diameter are machined into the flow plate for the primary coolant (nitrogen gas). Waved channels are bent into the flow plate for the secondary coolant (SO₃ gas). Twenty flow plates for the primary and secondary coolants are stacked in turn, and are bonded along the edge of the flow plate using a solid-state diffusion bonding method. After stacking and bonding the flow plates, the outside of the PHE is covered with a Hastelloy-X plate 3.0 mm thick, and is welded along its edges using TIG welding.

Figure 5 shows the overall dimensions and each part of the 10 kW class lab-scale PHE prototype from a 3-D CAD modeling. Figure 6 shows the set-up of the PHE prototype in a small-scale nitrogen gas loop. Based on Figure 6, FE modeling using commercial code I-DEAS was carried out. For the sake of simplicity and an understanding of the overall structural behavior of the PHE prototype, the FE model was formulated with linear solid elements including brick elements, wedge elements, and tetrahedron elements. The structural FE model of the PHE prototype was formulated using 870,696 brick elements.

For the FE model, the inflow/outflows of the primary and secondary coolants are shown in Figures 7 and 8 [8]. The inflow of the primary coolant into the PHE prototype and the outflow of the primary coolant from the PHE

prototype, after the heat is transferred to the secondary coolant, are shown in Figure 7. The inflow of the secon-

dary coolant into the PHE prototype and the outflow of the secondary coolant from the PHE prototype, after the heat is received from the primary coolant, are shown in Figure 8.