The saturation of wet storage facilities constructed and operated within nuclear power plant sites has magnified the significance of research concerning the dry storage of spent nuclear fuel. Not only do wet storage facilities incur higher operational and maintenance costs compared to dry storage facilities, but long-term storage of metal-clad fuel assemblies submerged in aqueous tanks is deemed unsuitable. Consequently, dry storage is anticipated to gain prominence in the future. Nevertheless, it is widely acknowledged that quantitatively assessing the residual water content remains elusive even when employing the apparatus and procedures utilized in the existing dry storage processes. The presence of residual water can only be inferred from damage or structural alterations to the spent nuclear fuel during its dry storage, making precise prediction of this element crucial, as it can be a significant contributor to potential deformations and deterioration. The aforementioned challenges compound the issue of retrievability, as substantial complexities emerge when attempting to retrieve spent nuclear fuel for permanent disposal in the future. Consequently, our research team has established a laboratory-scale vacuum drying facility to investigate the sensitivity of various parameters, including canister volume, pump capacity, water surface area, and water temperature, which can exert thermohydraulic influences on residual water content. Moreover, we have conducted dimensional analysis to quantify the thermohydraulic effects of these parameters and express them as dimensionless numbers. These analytical approaches will subsequently be integrated into predictive models for residual water content, which will be further developed and validated at pilot or full-scale levels. Furthermore, our research team is actively engaged in experimental investigations aimed at fine-tuning the duration of the pressure-holding phase while optimizing the evaporation process under conditions designed to avert the formation of ice caused by abrupt temperature fluctuations. Given that the canister is constructed from acrylic material, we are able to identify, from a phenomenological perspective, the specific juncture at which the boiling phenomenon becomes manifest during the vacuum drying process.

Measuring the amount of water remaining in the canister after drying is critical to ensuring the integrity of Dry Storage. There are many ways to measure residual moisture, but dew point sensors are typically used to measure residual moisture after drying the canister. Because the dew point temperature inside the canister depends on the water vapor partial pressure, the water vapor partial pressure present in the canister can be determined using the dew point temperature. The British Standard (BS1336) proposes a formula for converting dew point temperature into vapor partial pressure. It is possible to validate changes in residual water concentration throughout drying and at the end of drying. It has around 500 ppmv when the dew point temperature hits -73°C at 3 torr. Nuclear Regulatory Commission (US NRC) presented at 3 torr for 30 minutes as a criterion for the suitability of spent nuclear fuel drying. When the canister’s internal pressure is around 1,000 torr and the dryness criteria are met, the moisture concentration for this value is around 3,000 ppmv. We conducted a vacuum drying test of a 57 liter test vessel. It is filled with helium after vacuum drying was completed, and the concentration of residual water is measured by AquaVolt Moisture Analyzer (AMA) connected by a sample flow line. After the vacuum pressure of 1.5 torr was reached, the test vessel was filled to a pressure of 1,140 torr of helium after 30 minutes. The average temperature inside the basket inside the test vessel is 50°C, the dew point temperature is below -70°C, the pressure of test vessel is around 1,000 torr, and the measurement results of the AMA connected to the sample line showed less than 200 ppmv. From these results, we can evaluate that the residual moisture in the test vessel is about 0.01 gram.

When storing spent fuel in a dry condition, it becomes essential to ensure that any remaining moisture bound to the canister and spent fuel is effectively removed and stored within an inert gas environment. This is crucial for preserving the integrity of the spent fuel. According to the NRC- 02-07-C-006 report, it is advised to reduce pressure gradually or in incremental stages to prevent the formation of ice. In the context of vacuum drying, it is desirable to perform testing using a prototype model; however, utilizing a prototype model can be difficult due to budget constraints. To address this limitation, we designed and constructed a laboratory-scale vacuum drying apparatus. Our aim was to assess the impact of vacuum pump capacity on the drying process, as well as to evaluate the influence of canister volume on drying efficiency. The vacuum drying tests were carried out until the surface temperature of the water inside reached 0.1°C. In the tests focusing on vacuum pump capacity, vacuum pumps with capacities of 100, 200, 400, and 600 liters were employed. The outcomes of these tests indicated that smaller vacuum pump capacities resulted in increased evaporation rates but also prolonged drying times. In the case of drying tests based on canister volume, canisters with volumes of approximately 100 and 200 liters were utilized. The results revealed that larger canister volumes led to longer drying times and lower rates of evaporation. Consequently, if we were to employ an actual dry storage cask for vacuum drying the interior of the canister, it is anticipated that the process would require a substantial amount of time due to the considerably larger volume involved.

There is a need to develop a quantitative residual water measurement method to reduce the measurement uncertainty of the amount of residual water inside the canister after the end of vacuum drying. Therefore, a lab-scale vacuum drying apparatus was fabricated and its characteristics were evaluated by performing vacuum drying experiments based on the amount of residual water, vacuum drying experiments based on the surface area of residual water, and vacuum drying experiments based on the energy of residual water using the lab-scale vacuum drying apparatus. As a result of the vacuum drying experiments, if the surface area of water is the same, the greater the amount of water, the greater the energy of the water, so more energy is transferred to the surface of the water. Therefore, more water evaporated, and the average temperature of the remaining water was higher. The larger the surface area of the water, the more energy it takes to vaporize it, so the faster it dries and the faster the drying time. Before ice formed, energy was actively transferred by conduction heat transfer from the top, center, and bottom of the water to provide the energy needed for the water to evaporate from the surface. However, no energy was transferred from the water just before it turned into ice. When vacuum drying water, you can dry more water if you dry it slowly over a longer period of time. Therefore, by using a vacuum pump with a low flow rate, the pressure can be lowered slowly to prevent ice from freezing, thereby improving the drying quantity. It was evaluated that there was a good agreement between the energy used when water evaporated and the energy absorbed from the surroundings to within about 4%. Therefore, if the energy absorbed from the surroundings is known, it is possible to evaluate the amount of water evaporated in vacuum drying.

For the decommissioning or continuous long-term power generation of nuclear power plants, it is necessary to transfer the spent nuclear fuel from the wet storage pool to the dry storage. Spent nuclear fuel should go through the drying process, which is the first step of dry storage. The most important part in the drying process is the removal of the residual water. The spent fuel might be stored in a dry storage system for a long time. The integrity of internal components and spent fuel cladding should be maintained during the storage period. If residual water is present, problems such as aging of metal materials, oxidation of cladding, and the hydride-reorientation could occur. The presence or absence of residual water after vacuum drying is evaluated by pressure. If there is residual water in the vacuum drying process, it evaporates easily at low pressure to form water vapor pressure and the internal pressure rises. In the recent EPRI High burn up demonstration test, the gas inside the canister that satisfied the dryness criteria was extracted and analyzed. It showed that the water content was higher than the expected value. We are conducting verification studies on the pressure evaluation method, which is an indirect evaluation method of vacuum drying. The vacuum drying test was performed on small specimens at Sandia National Laboratory, and quantitative residual water evaluation was also performed. The report did not mention a detailed method for the assessment of residual water. Based on the test results of SNL, direct residual water evaluation was performed using energy balance. If the dryness criteria were satisfied, the quantitative amount of residual water was also evaluated. As a result, almost the same result as the evaluation result of SNL was derived, and it was confirmed that most of the water was removed when the dryness criteria was satisfied.

The synthesis of porous W by freeze-casting and vacuum drying is investigated. Ball-milled WO3 powders and tert-butyl alcohol were used as the starting materials. The tert-butyl alcohol slurry is frozen at –25oC and dried under vacuum at –25 and –10oC. The dried bodies are hydrogen-reduced at 800oC and sintered at 1000oC. The XRD analysis shows that WO3 is completely reduced to W without any reaction phases. SEM observations reveal that the struts and pores aligned in the tert-butyl alcohol growth direction, and the change in the powder content and drying temperature affects the pore structure. Furthermore, the struts of the porous body fabricated under vacuum are thinner than those fabricated under atmospheric pressure. This behavior is explained by the growth mechanism of tert-butyl alcohol and rearrangement of the powders during solidification. These results suggest that the pore structure of a porous body can be controlled by the powder content, drying temperature, and pressure.