【Technical Introduction】
The experience obtained from decommission of nuclear facilities in NARI has been accumulated including those complex tasks in dismantling the Taiwan Research Reactor (TRR) such as the disassembling of the reactor and underwater cutting of reactor assemblies and the packing of radioactive waste. These are major challenges in terms of technology, program management and radiation protection. The underwater cutting operation methods are mainly classified into thermal and mechanical cuttings. According to those foreign nuclear facility decommission experiences, mechanical cutting is the mainstream to avoid the problem of secondary waste disposal. Those components of TRR reactor vessel has been planned to be cut underwater after considering personnel dosage absorption and TRR structure conditions, that include Axial Thermal Shield-Upper(ATSU), a Zircaloy Calandria, Radial Thermal Shields and Axial Thermal Shield-Lower(ATSL).
 
After prudent evaluation based on the size of reactor vessel components, packing container and cutting workshop, some underwater cutting devices were developed to cut TRR reactor vessel components, such as cutting operation platforms, band saws, and disk saws. Underwater band saw was successfully applied to cut ATSU and ATSL, and underwater band saw was used for Zircaloy Calandria and Radial Thermal Shields disassembling.


Figure 1.Main components of TRR


Figure 2.Cutting device assessment for TRR internal component

【Project Planning/Technology Application】
The main functions of the underwater cutting operation platform were to adjust relative position between the reactor vessel component and the cutting machine, and to control the cutting parameter of the machine. The underwater cutting operation platform was composed of a motion mechanism, a control unit, and a hydraulic unit. As the reactor vessel component was lifted out and moved to the cutting workshop, the ATSU could be locked and rotated to a reserved location by setting a motion mechanism. The platform could be integrated with different cutting machines, such as disk saw or band saw. On the other hand, the parameters of the cutting process, such as initial position, final position, feeding rate, etc., could be controlled by the control unit.
After assessment, those reactor vessel components with less than 0.18 m cutting section thickness, such as Radical Thermal Shields and Zircaloy Calandria, were suitable to be cut by disk saw. Based on 3D model and cutting process assessment, NARI developed a disk saw suitable for use under water and finished cutting cast iron samples with 0.15 m thickness. In order to reduce underwater maintenance work during the period of cutting process, a quick-release mechanism between the cutting machine and the underwater cutting operation platform was designed to lift cutting device for daily maintenance.
An underwater band saw was designed to cut reactor vessel components, whose cutting section thickness is less than 0.6 m.  The maximum cutting section thickness of Axial Thermal Shield-Upper and Axial Thermal Shield-Lower was 0.43 m.  In this case, underwater band saw would be used to accomplish the cutting process.


Figure 3.Underwater cutting operation platform


Figure 4.Underwater disk saw device testing


Figure 5.Underwater band saw cutting

【Future Plan】
Underwater cutting technology and methods play an important role to help achieve the success of nuclear facilities decommission. In order to meet required deadline of the TRR reactor vessel dismantling plan, NARI had developed wet-cut workshop, underwater cutting operation platform. Meanwhile, NARI will try its best to improve the design of the cutting machine suitable to be used under water environment continuously. In the future, it is expected that the cumulative experience of TRR reactor vessel components cutting can be extended to the radioactive component dismantling of domestic nuclear facilities and help strengthen the foundation of nuclear dismantling industry.
 
 
【Contact Information】
Name：Jhih-Jhong Huang
Tel：03-4711400 Ext：3632
E-mail：jjhuang@nari.org.tw