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Research on laser composite decontamination technology of radioactive contaminated metal parts in nuclear power plant
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摘要
探究放射性污染金属部件的清洁解控或重新利用新技术，可用于减少核电站废金属产生量。本研究针对锈蚀的4140钢，提出激光复合去污技术，并与单一的干冰去污和激光去污进行对比试验。通过对基体试样的微观形貌、成分分布、金相组织、三维形貌进行测试，分析其表面粗糙度、显微硬度的变化趋势，以确定激光复合去污效果，最后在核电站中实现工程验证。测试和验证结果表明，激光复合去污技术可将4140钢表面的锈蚀层完全去除，获得最佳的去污效果，且不影响基体材料的成分、组织及力学性能。沾污电机叶轮去污后表面沾污水平均＜0.4 Bq/cm2，表面接触剂量率均＜40 nSv/h，达到核电站固体废物清洁解控标准。因此，激光复合去污技术可为核电站放射性污染金属部件的去污提供新方法，实现废物最小化。
Abstract
In order to reduce the amount of waste metal produced by nuclear power plants, a new technology for clean control or reuse of radioactively contaminated metal components is been studied. In this study, laser composite decontamination technology was proposed for corroded 4140 steel, and compared with dry ice decontamination and laser decontamination alone. By testing the microstructure, composition distribution, metallographic structure, and three-dimensional morphology of the matrix sample, the variation trend of surface roughness and microhardness was analyzed to determine the composite decontamination effect. Finally, the engineering verification was realized in the nuclear power plant. The test and verification results show that the laser composite decontamination technology can completely remove the rust layer on the surface of 4140 steel and obtain the best decontamination effect. Furthermore, it does not affect the composition, microstructure, and mechanical properties of the matrix material. The average amount of sewage on the surface of the contaminated motor impeller after decontamination is less than 0.4 Bq/cm2, and the surface contact dose rate is less than 40 nSv/h, which meets the clearance level of solid waste in nuclear power plants. Thus, laser composite decontamination technology can provide a new method for decontamination of radioactively contaminated metal components in nuclear power plants and minimize waste.
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Key words:
- laser composite decontamination /
- surface morphology /
- roughness /
- damage /
- clean decontrol
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Overview
Overview: In order to reduce the pressure on nuclear power plants to store and dispose of radioactive solid waste, waste minimization has become the mainstream development concept. As a new decontamination technology in the field of nuclear energy, laser decontamination and dry ice decontamination solve the problems that some traditional decontamination methods are difficult to deal with. However, based on the diversity of radioactive solid waste types and pollutants in nuclear power plants, a single decontamination scheme is difficult to meet the demand. In this work, laser composite decontamination was carried out on the surface of corroded 4140 steel, and the decontamination effect of a single laser and dry ice was compared to explore its decontamination mechanism. By testing the microstructure, composition distribution, metallographic structure, and three-dimensional morphology of the matrix sample, the variation trend of surface roughness and microhardness was analyzed to determine the composite decontamination effect. Finally, the engineering verification was realized in the nuclear power plant. The main results show that: 1) Laser composite decontamination can completely remove the rust layer and oxide on the surface of 4140 steel and expose the metal surface of the matrix. Only a few point objects exist. The oxygen content is reduced from 32.5 wt% of the matrix to 1.7 wt%, and the decontamination effect is better than that of single dry ice decontamination and laser decontamination technology; 2) After laser composite decontamination, the surface roughness of the corroded substrate reached 4.49 μm, which was 59.4% lower than that of the substrate ( 11.06 μm ). The metallographic structure of the matrix cross-section did not change, and it was still tempered sorbite. The composition distribution and hardness value of the cross-section are similar to the standard value of the matrix before decontamination. It shows that the laser composite decontamination has no damaging effect on the substrate; 3) The laser composite decontamination technology parameters determined by the test were used to decontaminate the contaminated motor impeller of the nuclear power plant. After decontamination, the average contaminated sewage of each part of the motor impeller was less than 0.4 Bq/cm2, and the surface contact dose rate was less than 40 nSv/h, which reached the solid waste cleaning and control standard of the nuclear power plant. Laser composite decontamination technology can become a new technology to minimize waste in nuclear power plants and will be widely used in the future.
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图 5 试样去污前后表面微观形貌。(a)基体；(b)干冰去污；(c)激光去污；(d)激光复合去污
Figure 5. Surface micro-morphology of the sample before and after decontamination. (a) Substrate; (b) Dry ice decontamination; (c) Laser decontamination; (d) Laser composite decontamination
图 6 试样去污前后表面成分分布。(a)基体；(b)干冰去污；(c)激光去污；(d)激光复合去污
Figure 6. Surface composition distribution of the sample before and after decontamination. (a) Substrate; (b) Dry ice decontamination; (c) Laser decontamination; (d) Laser composite decontamination
图 7 试样去污前后截面金相组织。(a)基体；(b)干冰去污；(c)激光去污；(d)激光复合去污
Figure 7. Cross-section metallographic structure of the sample before and after decontamination. (a) Substrate; (b) Dry ice decontamination; (c) Laser decontamination; (d) Laser composite decontamination
图 8 试样去污前后截面成分分布。(a)基体；(b)干冰去污；(c)激光去污；(d)激光复合去污
Figure 8. Cross-section composition distribution of the sample before and after decontamination. (a) Substrate; (b) Dry ice decontamination; (c) Laser decontamination; (d) Laser composite decontamination
图 9 试样去污前后表面三维形貌及轮廓曲线。(a)基体；(b)干冰去污；(c)激光去污；(d)激光复合去污
Figure 9. 3D surface morphology and contour curve of the sample before and after decontamination. (a) Substrate; (b) Dry ice decontamination; (c) Laser decontamination; (d) Laser composite decontamination
图 10 试样去污前后表面粗糙度
Figure 10. Surface roughness of the sample before and after decontamination
表 1 干冰清洗机参数
Table 1. Parameters of the dry ice decontamination equipment
参数 数值 干冰流量/(kg/h) 0~50 气体压力/(105 Pa) 2~10 干冰料斗容量/kg 25 干冰尺寸/mm Φ3×3-6，圆柱形 
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表 2 激光去污设备参数
Table 2. Parameters of the laser decontamination equipment
参数 数值 激光器类型 脉冲 输出中心波长/nm 1064 单脉冲能量/mJ 1.8 脉冲宽度/ns 10~500 功率调节范围/W 0~200 重复频率/kHz 1~4000
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表 3 去污前后污染水平及表面剂量率测量结果
Table 3. Measurement results of pollution level and surface dose rate before and after decontamination
电机叶轮 去污前 干冰去污后 激光去污后 去污因子 去污效率 沾污水平
/(Bq/cm2)表面剂量率
/(nSv/h)沾污水平
/(Bq/cm2)表面剂量率
/(nSv/h)沾污水平
(/Bq/cm2)表面剂量率
(nSv/h)根部 6.30 4790 0.92 1510 0.32 29.9 19.7 94.9% 叶片 5.78 3760 0.8 1270 0.20 10.0 28.9 96.5% 轴心 5.13 3420 0.83 1390 0.26 31.7 19.7 94.9%
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