New in-core instrumentation for Atucha I Nuclear Power Plant

In-Core Neutron flux Monitoring Assemblies were developed and constructed for Atucha I Nuclear Power Plant (CNA I) in the Argentine National Atomic Energy Commission (CNEA) to replace the German original ones that had to be removed for maintenance reasons. Each assembly contains seven Self-Powered Neutron Detectors (SPND) in different measurement positions, protected with an external Zy-4 tube. Detector's mineral insulated wires pass the reactor pressure vessel (RPV) cover head through seven holes made in a removable steal-sealing plug. A braze welding is made between the plug and the wires to seal such holes. The manufacture of this component was divided into two big processes. One of them was the fabrication and testing of the detectors and the other was the construction and checking of the assembly. a) The SPND were totally developed and constructed in CNEA. A composite bar is assembled at the beginning of the process. Such bar is composed by an Inconel 600 external tube, an Inconel 600 central bar, a small tube of vanadium (neutron sensitive material) placed around the middle of the central bar and tubular OMg pellets, between the external tube and the central bar, as isolation. A sketch of the composite bar is shown. The composite bar is deformed by cold working with intermediate thermal treatments. The external diameter is reduced in several steps with the consequent increase in length. When the final diameter is obtained, the bar is cut in the middle of the vanadium (two detectors are obtained) and the ends are sealed. The fabrication method is patented and allows obtaining integral cable-detector devices. The electric contact between the vanadium and the central conductor is obtained by plastic deformation and the external sheath is continuous with a conic reduction from the detector up to the cable. In this way the internal welding, that is a cause of failure, and the welding between the sheaths are avoid. The main advantage of these detectors is to have greater reliability than the welded ones. The small disadvantage is to have lower neutron sensitivity due to the fact that the neutron sensitive material, in this case vanadium, is a tube with an Inconel core and not a solid bar. In the practice, a first step was carried out to verify the construction feasibility and to validate the fabrication process. In order to do this, a Prototype SPND was constructed for electrical, radiographic and neutron tests and a Simile SPND without vanadium was constructed to evaluate the evolution of dimensions and metallurgical properties during the plastic deformation sequence. In the second step the definitive detectors were fabricated according to the validated sequence. These sensors were electrically (continuity and isolation) and radiographic evaluated and a final neutron calibration was done. b) The main technological problem during the manufacture of the assembly was the braze welding between the detector cables and the sealing plug of the RPV cover head perforation. Several trials were made with different conditions in the significant variables to reach satisfactory and repetitive results. Qualification specimens were welded and evaluated. Leakage and metallographic tests were made. Finally the assembly welding was made and checked for helium leakage and high temperature hydrostatic test. The criteria to finish the rest of the assembly were to reproduce as much as possible the original design. The detectors passed the continuity and isolation electrical tests after hydrostatic immersion. The neutron response was adequate. Obtained sensitivities were compatible with the original design with a ±6% dispersion. The first assembly installed in March of 2001 is working with no failures up to now. The seven detectors present good responses and this is an indication that there is electrical contact between the vanadium and the central conductor and that no humidity entered into the sensors. No primary coolant leakage was detected trough the sealing plug of the RPV cover head perforation. This is a consequence of the cables-plug welding watertighness and the metallic contact watertighness between the plug and the RPV cover head. The montage of the assembly in the Nuclear Power Plant was made with no problems and this is an indication of a good compatibility with the original design. As a conclusion it can be affirmed that, as a consequence of the developed work and the successful instrumentation provision, the Argentine National Atomic Energy Commission has the knowledge and the technological capability to construct and provide this kind of components or similar ones, according to a specific requirement. All this under strict safety controls and a Quality System matching ISO 9001. For the future, CNEA will continue the international diffusion, among nuclear reactor operators (PWR, BWR or HWR), of its capability to solve this aspect of NPP life management. (author)