초록 ▼

사고저항성 SiC 복합체 핵연료 피복관 핵심기술 개발을 위해 본 연구의 1단계에서는 길이 50mm 삼중층 SiC 복합체 튜브 제조기술 개발, 경수로 수화학 환경 SiC 부식거동 평가, 봉단마개 접합을 위한 SiC 접합소재 및 접합공정 선별시험 등에 중점을 두어 연구를 수행하였으며 다음과 같은 결과를 얻었음.
• SiC 가스기밀층, 복합체층 SiC 기지상 및 내환경층은 불순물이 포함되어 있지 않은 고순도의 화학정량비를 갖는 β–SiC로 이루어져 있었음.
• 제조된 삼중층 SiC 복합체 피복관은 선발국 대비 우수한 물리·화학

사고저항성 SiC 복합체 핵연료 피복관 핵심기술 개발을 위해 본 연구의 1단계에서는 길이 50mm 삼중층 SiC 복합체 튜브 제조기술 개발, 경수로 수화학 환경 SiC 부식거동 평가, 봉단마개 접합을 위한 SiC 접합소재 및 접합공정 선별시험 등에 중점을 두어 연구를 수행하였으며 다음과 같은 결과를 얻었음.
• SiC 가스기밀층, 복합체층 SiC 기지상 및 내환경층은 불순물이 포함되어 있지 않은 고순도의 화학정량비를 갖는 β–SiC로 이루어져 있었음.
• 제조된 삼중층 SiC 복합체 피복관은 선발국 대비 우수한 물리·화학적 균일성 및 두께 균일성을 나타내었으며 후프강도 또한 250~300 MPa 범위로 외국 대비 우수한 특성을 보였음.
• 경수로 수화학 환경을 모사한 루프시험을 통해 고순도 CVD SiC에 대한 360℃, 2800h 장기 부식특성 자료를 생산하였으며 용존수소가 제어된 경수로 수화학 환경에서 SiC의 부식이 크게 억제됨을 세계 최초로 발견하였음.
• Ti, Mo, Zr, Si 등의 다양한 인터레이어 소재를 이용한 확산접합 공정 및 레이저 접합 공정의 기초실험을 완료하였으며 접합부 미세구조 및 접합강도 평가를 통해 레이저 접합공정의 봉단마개용 접합 활용 가능성을 확인하였음.

Abstract ▼

Recently, there have been large efforts on applying the SiC_f/SiC composites to the pressurized water reactor (PWR) fuel cladding and guide tubes as well as channel boxes for fuel assembly of the boiling water reactor (BWR). Compared to the current Zr alloys, the SiC composite cladding wil

Recently, there have been large efforts on applying the SiC_f/SiC composites to the pressurized water reactor (PWR) fuel cladding and guide tubes as well as channel boxes for fuel assembly of the boiling water reactor (BWR). Compared to the current Zr alloys, the SiC composite cladding will offer a better neutron economy through a reduced neutron absorption cross-section, power uprate and burn-up extension. The high-temperature strength and superior mechanical property of SiC allow the cladding to operate at higher temperatures and mitigate a vibration induced fretting wear. The SiC composite cladding is also expected to significantly increase the safety of PWR system due to it’s high-temperature performance and superior oxidation resistance to high-temperature steam at severe accidents.
In spite of these potential benefits of the composite cladding, there are a lot of multidisciplinary issues that need to be clarified for the PWR application because the previous research on the SiC_f/SiC composite has mostly been focused on high-temperature application. Among the various issues, the corrosion behavior of SiC under PWR condition needs to be clarified. For an exact determination of the performance of SiC composite cladding, the fabrication of composite tubes with well-controlled properties should be preceded. The development of a robust joining technology for the end-plug and the matrix microcracking susceptibility are significant challenges for the application of SiC composite cladding.
In the phase I of this project, we focused on the development of fabrication technology and property evaluation of triplex SiC composite tube, the evaluation and assessment of SiC corrosion behavior under PWR water conditions, and the development of SiC joining technology for end-plug joining.
○ Fabrication and property evaluation of triplex SiC composite tube
Multi-layered SiC composite tubes were fabricated by CVD and CVI methods for the application of PWR nuclear fuel cladding. The deposition rate of SiC linearly increased with a gas flow rate from 800 sccm to 1600 sccm. It also increased at the lower pressure but saturated above 3.3 kPa of total pressure. In the range of 3.3–10 kPa, preferred orientation was changed from (220) and (311) planes to a (111) plane. Hardness and elastic modulus had the maximum values when SiC had (111) preferred orientation, and gradually decreased with changing to (220) and (311) preferred orientation. Isotropic nuclear grade graphite rod with 8 mm in diameter, 8 cm in length was used as a substrate. The deposited SiC was a beta phase with (111) preferred orientation.
The SiC composite cladding tube consisting of three layers, monolithic CVD SiC – SiC_f/SiC composite – another monolithic CVD SiC was fabricated by a chemical vapor method. Various types of the SiC fiber preforms with tubular shapes were fabricated by a filament winding method using three types of Tyranno SA3 grade SiC fibers with 800 filaments/yarn and 1600 filaments/yarn nd Hi-Nicalon Type S. After chemical vapor infiltration of SiC matrix, surface roughness and fiber volume fraction were measured. As filament counts changed from 800 to 1600, the surface roughness increased but the fiber volume fraction decreased. The SiC_f/SiC composite with bamboo-like winding pattern has a smaller surface roughness and a higher fiber volume fraction than zigzag winding pattern.
Pyrolytic carbon (PyC) was employed as an interphase material for a SiC_f/SiC composite layer. The thickness uniformity was measured to be high which was approximately 200 nm in average thickness. The matrix phase of the SiC_f/SiC composite layer was infiltrated by a CVI method, followed by the deposition of a SiC outer layer. High-quality and uniform SiC triplex composite tubes could be fabricated in this study and the SiC phase was highly stoichiometric without any presence of free silicon or free carbon.
Microstructure analysis and measurement of hoop strength were performed for the composite tube fabricated by various filament winding variables such as winding patterns, winding angle, and the type of reinforced SiC fibers. The composite layer was reinforced by nuclear grade SiC fibers such as Tyranno SA3 and Hi-Nicalon Type S which were filament-wound onto a SiC coated graphite mandrel by a helical method with winding angles of ±45 to ±65o. Tyranno SA3-S1I08PX with 800 filaments/yarn exhibited the highest fiber volume fraction when winding pattern was bamboo-like shape which resulted in a higher hoop strength compared to the other types of fiber reinforcements.
○ Evaluation of SiC corrosion behavior
Long-term corrosion tests were performed using chemical vapor deposition (CVD) SiC specimens in 360℃ water and 400℃ steam to evaluate the corrosion behavior of SiC cladding under normal operation conditions. The CVD SiC specimens exhibited weight loss caused by dissolution in both 360℃ water and 400℃ steam. In the corrosion test in 360℃ water, the dissolution rate was much higher in the static autoclave, where the dissolved oxygen content was not controlled, than in the PWR-simulating water loops, where the dissolved oxygen content was maintained at approximately 5 ppb. The grain boundaries of the SiC dissolved preferentially during the early stage of corrosion, and the grains became thinner and detached from the surface, thereby leading to an acceleration of the weight loss.
The formation of pre-oxide on the surface of CVD SiC was not able to protect the SiC from dissolving in water, even though it was effective, to a certain extent, at mitigating the dissolution of the SiC. In the corrosion test in 400℃ steam, the weight loss was significant from the early stage of corrosion. No acceleration in the corrosion rate was observed in 400℃ steam for up to 135 days, and the weight loss at 90 days was similar to that obtained fromthe 360℃ water autoclave test.
We also investigated the corrosion behavior of CVD SiC under PWR-simulating water condition with a control of dissolve hydrogen (DH) content. The dissolution rate of SiC in the DH control environment was extremely low and the weight loss of CVD SiC was only 0.013 mg/cm2 after 30 days, which was at least 15~40 times smaller than the weight loss of SiC in the PWR-simulating loop test without the control of the DH content. This indicates that the dissolved hydrogen significantly affects the corrosion behavior of SiC under PWR water condition, which is first observed in this study and will have a great impact on the SiC composite community for the application to the PWR fuel cladding.
○ Development of joining technology
SiC plates were diffusion bonded using a hot press method and laser beam scanning method. Metallic interlayers of Ti, Mo, and Zr foils were utilized for the joining. The interfacial microstructures along with their atomic compositions of the SiC/SiC joints were analyzed.
For the Ti interlayer, a Ti₃SiC₂ phase was formed owing to the diffusion of silicon and carbon from the SiC part. At the middle of the Ti interlayer, a TiSi₂ phase also existed, forming a dual phase region. For the Mo interlayer, the diffusion of silicon into Mo induced the formation of the Mo₅Si₃C phase at the vicinity of the SiC/Mo interface. The compositional change was dependent on the diffusion depth of the silicon, which resulted in an unreacted metallicphase in the middle of the Mo insert. For the Zr interlayer, diffused Si formed ZrSi₂, Zr₅Si₃C_x, and Zr₄Si phases. Carbide transformation was also observed (Zr to ZrC_x) near the Zr/SiC interface. No crystallographic orientation relationship was found in most of the grains.