Reactor Vessel Head Penetration Research Articles

Numerical analysis of sodium-argon two-phase flow in narrow channels under severe accident conditions in pool type SFR

A two-phase flow model for sodium-argon mixture flow through the reactor vessel head penetrations under the energetic core disruptive accident in a sodium fast reactor (SFR) is presented. The model evaluates the total sodium mass in sodium-argon mixture released to the reactor containment building (RCB) through penetrations. The driving pressure at the penetration entrance is given as input. The two-phase frictional and local pressure drops, inertial and piezo-metric heads are accounted for in the model. Depending on the argon volume fraction, either a homogenous or separated flow approach is adopted to evaluate the mixture flow. Reactor-scale analysis show that the presence of argon in vessel head penetrations can significantly mitigate the total sodium release mass during the slug impact phase. Also, the two-phase frictional pressure drop plays a mitigating role and needs to be included in the model formulation to avoid overestimation of the sodium release mass. Fluid-structure interaction effects such as vessel head bending and hold-down bolt elongation have negligible influence on sodium release mass as long as the deformations are within the elastic limit. The model is conceived to be part of an integrated code system to evaluate the sodium fire and the maximum RCB pressure under severe accident conditions in a pool-type SFR.

Identification of nonregular indication according to change of grain size/surface geometry in nuclear power plant (NPP) reactor vessel (RV)-upper head alloy 690 penetration

Abstract During the fabrication process of reactor vessel head penetration (RVHP), the grain size of the tube material can be changed by hot or cold work and the inner side of the tube can also be shrunk due to welding outside of the tube. Several nonregular time-of-flight diffraction (TOFD) signals were found because of deformed grains. In this paper, an investigation of nonregular TOFD indications acquired from RVHP tubes using experiments and computer simulation was performed in order to identify and distinguish TOFD signals by coarse grains from those by Primary Water Stress Corrosion Crack (PWSCC). For proper understanding of the nonregular TOFD indications, microstructural analysis of the RVHP tubes and prediction of signals scattered from the grains using Finite Element Method (FEM) simulation were performed. Prediction of ultrasonic signals from the various sizes of side drilled holes to find equivalent flaws, determination of the size of the nonregular TOFD indications from the coarse grains, and experimental investigation of TOFD signals from coarse grain and shrinkage geometry to identify PWSCC signals were performed. From the computer simulation and experimental investigation results, it was possible to obtain the nonregular TOFD indications from the coarse grains in the alloy 690 penetration tube of RVHP; these nonregular indications may be classified as PWSCC. By comparing the computer simulation and experimental results, we were able to confirm a clear difference between the coarse grain signal and the PWSCC signal.

원자로 상부 헤드 관통관 TOFD 신호 시뮬레이션

비파괴검사 분야에 대한 시뮬레이션은 다양한 결함에 대한 신호의 예측과 검사 절차 개발에 사용되어진다. 특히 비파괴검사 전용 시뮬레이션 툴인 CIVA는 정확도가 높고 빠른 계산이 가능하며, 비파괴평가 기술과 동일한 형태의 화면 표시와 시각적으로 개선된 3차원 그래픽 유저 인터페이스를 제공한다. CIVA 소프트웨어 개발자가 내부적으로 타당성 검증을 시행하겠지만, 사용 이전에 소프트웨어의 정확도를 평가하는 독립적인 유효성 검증 연구가 필요하다. 이러한 목적으로 이번 연구에서는 CIVA를 이용하여 원자로 상부 헤드 관통관 검사에 사용되는 보정시험편에 대하여 TOFD 신호를 시뮬레이션하고, 실제 검사 신호와 비교하여 시뮬레이션 신호의 정확도와 적용 범위에 대하여 검증하였다. 종합적으로, A-scan 신호, B-scan 이미지, 깊이 측정 측면에서 CIVA 시뮬레이션 결과와 실험 결과 간에 전반적으로 일치를 보였다. The simulation of nondestructive testing has been used in the prediction of the signal characteristics of various defects and in the development of the procedures. CIVA, a simulation tool dedicated to nondestructive testing, has good accuracy and speed, and provides a three-dimensional graphical user interface for improved visualization and familiar data displays consistent with an NDE technique. Even though internal validations have been performed by the CIVA software development specialists, an independent validation study is necessary for the assessment of the accuracy of the software prior to practical use. In this study, time of flight diffraction signals of ultrasonic inspection of a calibration block for reactor vessel head penetrations were simulated using CIVA. The results were compared to the experimentally inspected signals. The accuracy of the simulated signals and the possible range for simulation were verified. It was found that, there is a good agreement between the CIVA simulated and experimental results in the A-scan signal, B-scan image, and measurement of depth.

Discussion on In-Service Inspection after the Application of Embedded Flaw Repair Technique

Embedded Flaw Repair technique has been developed in JSME S NA-1. The technique is applied to prevent propagation of Stress Corrosion Crack (SCC). The welding metal with high corrosion resistance is embedded on the surface of component, and could protect cracking part from the aggressive primary water. In this method, the crack is not fully removed from the component. After discussion in JSME, inspection rules after application of Embedded Flaw Repair were established for penetrations of Class 1 vessel's penetration (for example, reactor vessel head penetration and bottom mounted instrument in PWR) and reactor internal components. The focus of discussion is placed on whether the flaw should be monitored by nondestructive inspection. This document describes the technical aspects that JSME code committee studied, such as, flaw evaluation, tolerance of components with defects, inspectability of defects in seal-welded components and the operating experiences of repaired components at U.S. plants. Finally, in-service inspection items for a component repaired by Embedded flaw technique are proposed.