Zircaloy cladding thickness of nuclear fuel rods in the advanced CANDU fuel is very thinner than existing ones. Therefore, the soundness of a brazement becomes more important. If the metallic glass alloys are used filler metals, it is possible to get defectless brazement, because they have better advantages than crystalline filler metals and Metallic Be those are using in the nuclear industry.
As the Zr-Be and Ti-Be system alloys are composed with the elements having low thermal neutron absorption cross section, it is suitable for joining of nuclear materials. And the Zr-Be and Ti-Be binary alloy has eutectic reaction below 1000℃, it is possible to form of metallic glass near the eutectic composition by melt spinning. In order to develop the fabrication technique of metallic glass by melt spinning, the composition range where metallic glass can be formed was investigated. The thermal stability and crystallization behavior of metallic glasses were also investigated because they transform to crystalline phase by processing at high temperature prior to brazing process. The Zr-Be and Ti-Be binary amorphous alloys and Zr-X-Be (X=Ti, Nb, V, Mo, etc) ternary amorphous alloys were applicable to the joining the nuclear fuel rods. The microstructures of the brazement with the amorphous alloys were also examined metallographically.
Various component, (spacer pads, bearing pads and buttons), were brazed on the cladding surface by using amorphous filler metals
1. The binary amorphous alloys, $Zr_{1-x}Be_x$ and $Ti_{1-y}Be_y$, were produced in the composition range of 0.3≤x≤0.5 and 0.37≤y≤0.41 respectively by melt-spinning method.
2. A part of the metallic glass of $Zr_{1-x}Be_x$ (0.3≤x≤0.35) crystallized to the α-Zr as increasing the temperature and the rest of it transformed to $ZrBe_2$ at more higher temperature. The metallic glass of $Zr_{1-x}Be_x$ which contains above 40 at% of Be crystallized to α-Zr and $ZrBe_2$ simultaneously. The crystallization of the $Ti_{1-y}Be_y$ binary amorphic alloys proceeds by two steps in the composition range of this experiment. The first step of the crystallization of them is the formation of α-Ti at relatively lower temperature and the second step is that of TiBe at higher temperature.
3. From the result of DSC, the crystallization temperature and activation energy of $Ti_{1-y}Be_y$ binary metallic glass are higher and larger than that of $Zr_{1-x}Be_x$ system respectively. Therefore, it is believed that the $Ti_{1-y}Be_y$ amorphous system is much thermally stable than $Zr_{1-x}Be_x$ system
4. The width of brazement using the amorphous filler metals is thinner than that of physical vapor deposited metallic Be.
5. Among the $Zr_{1-x}Be_x$ amorphous binary alloys, $Zr_{0.7}Be_{0.3}$ binary alloy is the most appropriate for joining of nuclear fuel rod because its brazement is smooth and thin due to prevention of Be diffusion to cladding. The microstructures of brazement using $Ti_{1-y}Be_y$ binary metallic glass are similar to that using $Zr_{1-x}Be_x$ system. In the case of using the $Ti_{1-y}Be_y$ alloy, however, a solid-solution layer composed with Zr and Ti is formed toward the Zr cladding sheath.
6. The $Zr_{(0.7-z)}Ti_zBe_{0.3}$, $Zr_{(0.7-z)}Nb_zBe_{0.3}$, $Zr_{(0.7-z)}V_zBe_{0.3}$, $Zr_{(0.7-z)}Mo_zBe_{0.3}$ (z=0.01, 0.03, 0.05, 0.07, 0.1) ternary amorphous alloys show the similar crystallization behavior with that of the $Zr_{0.7}Be_{0.3}$ binary amorphous alloy. The $Zr_{0.7}Be_{0.3-z}Si_z$ ternary amorphous alloys also show the two step crystallization behavior.
7. In the $Zr_{(0.7-z)}Ti_zBe_{0.3}$, $Zr_{(0.7-z)}Nb_zBe_{0.3}$, $Zr_{(0.7-z)}V_zBe_{0.3}$, $Zr_{(0.7-z)}Mo_zBe_{0.3}$, and $Zr_{0.7}Be_{0.3-z}Si_z$ ternary amorphous alloys, the crystallization temperature and activation energy increase as the substitutional content(z) increases.
8. In the aspect of thermal stability, the ternary amorphous alloys are superior than Zr-Be binary amorphous alloys. The Zr-V-Be, Zr-Ti-Be, Zr-Mo-Be, Zr-Nb-Be amorphous alloy shows sequentially superior thermal stability.
9. The thickness of the brazement with the ternary amorphous alloys are not affected largely by the substitutional content(z). Especially, using the Zr-Nb-Be and Zr-Mo-Be ternary amorphous alloys, 'Nb-precipitated Zr phase' and 'Mo intermetallic compound precipitated Zr phase' were formed at the both sides of the brazement, respectively.
1. $Zr_{1-x}Be_x$ 이원계 합금과 $Ti_{1-y}Be_y$ 이원계 합금을 각각 (0.7≤x≤0.5) 와 (0.37≤y≤0.41) 조성범위에서 급냉응고법에 의하여 비정질합금을 제조할 수 있었다.
2. $Zr_{1-x}Be_x$ 비정질 합금계는 Be 함량이 0.3 과 0.35 at% 인 경우 결정화는 우선 α-Zr 이 형성된 후 온도가 증가하면 $ZrBe_2$가 형성된다. 반면 Be 함량이 0.4 at% 이상인 경우 결정화는 α-Zr 과 $ZrBe_2$ 가 동시에 형성된다. $Ti_{1-y}Be_y$ 비정질 합금계는 본 실험 전체 조성영역에서 결정화는 두단계로 이루어졌으며 저온에서는 α-Ti 이 고온에서는 TiBe 이 형성됐다.
3. DSC 결과로부터 $Ti_{1-y}Be_y$ 이원계 합금의 결정화온도가 $Zr_{1-x}Be_x$ 이원계 합금의 결정화온도보다 높고 또한 $Ti_{1-y}Be_y$ 합금의 결정화 활성화에너지가 $Zr_{1-x}Be_x$ 합금보다 크다. 따라서 $Ti_{1-y}Be_y$ 이원계 합금이 $Zr_{1-x}Be_x$ 이원계 합금보다 열적으로 더 안정 하였다.
4. 비정질 ribbon 을 용가제로 사용하는 경우가 물리증착법에 의한 Be 용가제를 사용하는 것보다 Be의 확산이 적어 접합층의 두께가 얇은 우수한 특성을 보였다.
5. $Zr_{1-x}Be_x$ 이원계 비정질 합금 중 $Zr_{0.7}Be_{0.3}$ 이원계 합금은 용가제로 사용하여 brazing 한 경우 접합층 계면이 매끈하며 Be 확산이 가장 적기 때문에 핵연료봉 접합에 가장 적합한 용가제이다. 비정질 $Ti_{1-y}Be_y$ 이원계 합금을 용가제로 사용하여 brazing 한 경우 접합층의 구조는 $Zr_{1-x}Be_x$ 이원계 비정질 합금을 사용한 경우와 유사하지만 Zr cladding sheath 쪽으로 Zr-Ti 고용층이 형성되었다. 또한 접합층내에서도 다량의 Zr이 검출되었다.
6. z=0.01, 0.03, 0.05, 0.07, 0.1의 조성을 갖는 $Zr_{(0.7-z)}Ti_zBe_{0.3}$, $Zr_{(0.7-z)}Nb_zBe_{0.3}$, $Zr_{(0.7-z)}V_zBe_{0.3}$, $Zr_{(0.7-z)}Mo_zBe_{0.3}$ 삼원계 비정질 합금의 경우 $Zr_{0.7}Be_{0.3}$ 이원계 비정질 합금의 경우와 마찬가지로 α-Zr 의 생성과 $ZrBe_2$ 의 생성의 2단계로 결정화가 진행된다. Be의 일부를 Si으로 치환한 $Zr_{0.7}Be_{0.3-z}Si_z$ 삼원계 비정질 합금의 경우도 2단계로 결정화가 진행된다.
7. $Zr_{(0.7-z)}M_zBe_{0.3}$ (M = Ti, Nb, V, Mo) 삼원계 비정질 합금과 $Zr_{0.7}Be_{0.3-z}Si_z$ 삼원계 비정질 합금의 경우 치환량(z)이 증가함에 따라 결정화 온도와 활성화 에너지가 증가한다.
8. 열적 안정성 측면에서 $Zr_{1-x}Be_x$ 이원계 비정질 합금보다 삼원계 비정질 합금이 더 우수한 특성을 나타내었으며 Zr-V-Be, Zr-Ti-Be, Zr-Mo-Be, Zr-Nb-Be 삼원계 비정질 합금의 순서로 더 우수하였다.
9. $Zr_{(0.7-z)}M_zBe_{0.3}$ (M = Ti, Nb, V, Mo) 삼원계 비정질 합금을 용가제로 사용하여 brazing한 경우 접합부의 두께증가 측면에서는 이원계 비정질 합금의 경우에 비하여 큰 변화가 없었다. 그러나 치환량(z)이 너무 많게 되면 접합부의 두께가 약간 증가하는 경향을 나타낸다. 특히 Nb, Mo을 치환한 경우에는 접합부 주변에 새로운 상이 형성되었다.
10. 이상과 같은 결과로부터 원자로 재료 접합의 용가제로 적합한 최적의 다원계 비정질 합금은 $Zr_{0.65}V_{0.05}Be_{0.29)Si_{0.01}$이라고 사료된다.
