Dong-Hak Kook

A STUDY ON THE INITIAL CHARACTERISTICS OF DOMESTIC SPENT NUCLEAR FUELS FOR LONG TERM DRY STORAGE

During the last three decades, South Korean nuclear power plants have discharged about 5,950 tons of spent fuel and the maximum burn-up reached 55 GWd/MTU in 2002. This study was performed to support the development of Korean dry spent fuel storage alternatives. First, we chose V5H-17 17 and KSFA-16 16 as representative domestic spent fuels, considering current accumulation and the future generation of the spent fuels. Examination reveals that their average burn-ups have already increased from 33 to 51 GWd/MTU and from 34.8 to 48.5 GWd/MTU, respectively. Evaluation of the fuel characteristics shows that at the average burn-up of 42 GWd/MTU, the oxide thickness, hydrogen content, and hoop stress ranged from 30 ~ 60 μm, 250 ~ 500 ppm, and 50 ~ 75 MPa, respectively. But when burn-up exceeds 55 GWd/MTU, those characteristics can increase up to 100 μm, 800 ppm, and 120 MPa, respectively, depending on the power history. These results demonstrate that most Korean spent nuclear fuels are expected to remain within safe bounds during long-term dry storage, however, the excessive hoop stress and hydrogen concentration may trigger the degradation of the spent fuel integrity early during the long-term dry storage in the case of high burn-up spent fuels exceeding 45 GWd/MTU.

REVIEW OF SPENT FUEL INTEGRITY EVALUATION FOR DRY STORAGE

Among the several options to solve PWR spent fuel accumulation problem in Korea, the dry storage method could be the most realistic and applicable solution in the near future. As the basic objectives of dry storage are to prevent a gross rupture of spent fuel during operation and to keep its retrievability until transportation, at the same time the importance of a spent fuel integrity evaluation that can estimate its condition at the final stage of dry storage is very high. According to the national need and technology progress, two representative nations of spent fuel dry storage, the USA and Japan, have established different system temperature criteria, which is the only controllable factor in a dry storage system. However, there are no technical criteria for this evaluation in Korea yet, it is necessary to review the previously well-organized methodologies of advanced countries and to set up our own domestic evaluation direction due to the nations need for dry storage. To satisfy this necessity, building a domestic spent fuel test database should be the first step. Based on those data, it is highly recommended to compare domestic data range with foreign results, to build our own criteria, and to expand on evaluation work into recently issued integrity problems by using a comprehensive integrity evaluation code.

DESIGN AND CONSTRUCTION OF AN ADVANCED SPENT FUEL CONDITIONING PROCESS FACILITY (ACPF)

KAERI has worked on the development of an advanced spent fuel conditioning process (ACP) since 1997. A hot cell facility, termed the ACPF, has also been developed. The ACPF consists of two air-sealed hot cells. The results of a safety analysis as part of the license procurement process stipulated by the Korean Government showed that the facility was designed safely. After its construction, an integrated performance test was performed. The results of this test confirmed that the facility satisfies the design requirements.

Stress and temperature-dependent hydride reorientation of Zircaloy-4 cladding and its effect on the ductility degradation

A hydride reorientation can deteriorate the mechanical ductility of spent fuel cladding and make it more susceptible to failure. Therefore, an evaluation of the reorientation under dry storage conditions and their effects on the cladding ductility are critical issues in terms of the regulation criteria. In this work, biaxial stress was applied to Zircaloy-4 cladding by pressurizing Ar gas. The study showed that the hydride reorientation can occur at around 60 and 80 MPa at 400 and 300 °C, respectively. The ring compression test at room temperature showed that the ductility decreases with an increase in radial hydride quantity: Fl(45) and radial hydride continuity factor. In addition, a significant hydride reorientation can occur at high temperature conditions even if the hoop stress is equal to or less than 90 MPa which can bring a significant ductility degradation.

WASTE CLASSIFICATION OF 17 x 17 KOFA SPENT FUEL ASSEMBLY HARDWARE

Metal waste generated from the pyroprocessing of 10 MtU of spent fuel was classified by comparing the specific activity of a relevant radionuclide with the limit value of the specific activity specified in the Korean acceptance criteria for a lowand intermediate-level waste repository. A Korean Optimized Fuel Assembly design with a 17 x 17 array, an initial enrichment of 4.5 weight-percent, discharge burn-up of 55 GWD/MtU, and a 10-year cooling time was considered. Initially, the mass and volume of each structural component of the assembly were calculated in detail, and a source term analysis was subsequently performed using ORIGEN-S for these components. An activation cross-section library generated by the KENO-VI/ORIGENS module was utilized for top-end and bottom-end pieces. As a result, an Inconel grid plate, a SUS plenum spring, a SUS guide tube subpart, SUS top-end and bottom-end pieces, and an Inconel top-end leaf spring were determined to be unacceptable for the Gyeongju low- and intermediate-level waste repository, as these waste products exceeded the acceptance criteria. In contrast, a Zircaloy grid plate and guide tube can be placed in the Gyeongju repository. Non-contaminated Zircaloy cladding occupying 76% of the metal waste was found to have a lower level of specific activity than the limit value. However, Zircaloy cladding contaminated by fission products and actinides during the decladding process of pyroprocessing was revealed to have 52 and 2 times higher specific activity levels than the limit values for alpha and 90Sr, respectively. Finally, it was found that 88.7% of the metal waste from the 17 x 17 Korean Optimized Fuel Assembly design should be disposed of in a deep geological repository. Therefore, it can be summarized that separation technology with a higher decontamination factor for transuranics and strontium should be developed for the efficient management of metal waste resulting from pyroprocessing.

PYROPROCESS WASTE DISPOSAL SYSTEM DESIGN AND DOSE CALCULATION

PWR spent fuels produced in the Republic of Korea are expected to be recycled by pyroprocess in the long term future. Even though pyroprocess waste amounts can be smaller than that of PWR spent fuel assembly in case of direct disposal, this process essentially will produce various and unique radioactive wastes. The goals of this article are to characterize these wastes, calculate the amount of wastes, design disposal systems for each waste and evaluate the radiation safety of each system by dose assessment. The absorbed dose results of the metal and ceramic waste for the engineering barrier system (EBS) showed 2.21 x 10 -2 Gy/h and 1.15 x 10 -2 Gy/h, which are lower than the recommended value of 1 Gy/h. These results confirmed that the newly proposed disposal systems have a safety margin for the radiation produced from each waste.

Mechanical Property Degradation of Unirradiated Zircaloy-4 Cladding After Creep Deformation

Abstract One of the limiting mechanisms of pressurized water reactor spent fuel cladding is creep owing to high temperature and rod internal pressure. Based on extensive studies, many countries have tentatively concluded that creep rupture is hard to occur under dry storage conditions and cannot severely degrade the integrity of the cladding if it meets the 400°C limitation owing to a self-limiting property. However, the changes in mechanical properties after creep deformation are not well understood due to the limited amount of relevant tests and analyses. In this regard, mechanical property degradation of unirradiated Zircaloy-4 cladding by creep deformation was investigated using a ring compression test and microscopic observation. In addition, the implication regarding spent fuel cladding integrity based on the test results is described.

Characteristics of the Spent Fuel Generated in Korea

Nuclear power has satisfied the national electric power demand for three decades, and there are only two reactor types in Korea. The nuclear fuel species, however, have a large variety of fuel types, dimensions, initial enrichment, and fuel supply vendors. A spent fuel accumulation problem has arisen like any other country that uses nuclear power. The spent fuel wet storage capacity in the reactor pool is getting close to its limit, and so hence, short & long-term solutions are being actively proposed. First the general status for the nuclear industries and spent fuels will be introduced, then spent fuel characteristics will follow, and last the future anticipation of spent fuel management will close this article.Copyright