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.

As if the wet storage of Spent Nuclear Fuel (SNF) becomes saturated, a transition from wet storage to dry storage could be required. The first process for dry storage is to move SNF from the wet storage into a canister for dry storage, and secondly perform a drying process to remove the moisture in the canister to prevent a potential impact such as deterioration of cladding or corrosion of the interior material. Nuclear Regulatory Commission (NRC) accepts the conditions describing the adequate dryness state that remain below the pressure of 3 Torr for 30 minutes in the drying process. That is, the most pressure of water vapor that may exist inside the canister is 3 Torr. If it is maintained below 3 Torr, it can be determined that the dryness criterion is satisfied. Based on this, relative humidity and dew point trends can be identified. Relative Humidity (RH) is calculated by dividing the vapor pressure by the saturated vapor pressure. Here, if the vapor pressure is fixed at 3 Torr, which is the dryness criterion value, the relative humidity has a value according to the saturated vapor pressure. Saturated vapor pressure is a value that varies with temperature, so relative humidity varies with temperature. On the other hand, the dew point temperature has a value according to the water vapor pressure. Therefore, when the internal temperature of the canister is 120°C and the water vapor pressure is 3 Torr, the relative humidity is 0.2% and the dew point temperature is -4.4°C. We will confirm the suitability of the dryness criterion through the drying tests, and secure a technology that can measure and evaluate the amount of moisture remaining inside the canister.

To dry storage of spent nuclear fuel withdrawn the wet storage, all moisture inside the dry storage container must be removed to ensure the long-term integrity and retrievability. Substantial amounts of residual water in dry storage container may have potential impacts on the fuel, cladding, and other components in the dry storage system, such as fuel degradation and cladding corrosion, embrittlement, and breaching. The drying could perform as a vacuum drying process or a forced helium dehydration process. In NUREG-1536, the evacuation of most water contained within the canister is recommended a pressure of 0.4 kPa (3 torr) to be held in the canister for at least 30 minutes while isolated from active vacuum pumping as a measure of sufficient dryness in the canister. Monitoring the moisture content in gas removed from the canister is considered as a means of evaluating adequate dryness. Dew point monitoring and special techniques could be used to evaluate this adequacy. Various studies are continuing for quantitative evaluation of residual moisture inside the dry storage system. Andrawes proposed a methodology for determining trace water contents in gaseous mixtures, utilizing gas chromatography together with a helium ionization source. A microwave plasma source and emission spectrometry were utilized to determine trace amounts of bound water in solid samples using peak areas of atomic oxygen (O) and hydrogen (H) emissions. Bryans measured the gas samples taken from the High Burn-Up Demonstration Cask at three intervals: 5 hours, 5 days, and 12 days after the completion of drying and backfilling in the North Anna power Station. To measure water content, a Vaisala humidity probe was used. Final results indicated that the cask gas water content built up over 12 days to a value of 17,400 ppmv ±10%, equivalent to approximately 100 g of water within the entire cask gas phase. Tahiyats also proposed a methodology that involves a direct current (dc) driven plasma discharge and optical emission spectroscopy for detecting and quantifying water vapor in a flowing gas stream under both trace and high water vapor loading conditions. For detecting water vapor concentration, the emission from H at 656.2 nm was employed. The H emission is the red visible spectral line generated by a hydrogen atom when an electron falls from the third lowest to the second lowest energy level, this suggests that the normalized H intensity can be used as a marker for water vapor detection and quantification. Several of the attempts are continuing to quantify water contents in dry storage system. Lessons learned by Case studies would be provided insights into how to improve future measurements.