Patent Number: 048062774
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of decontaminating solid surfaces, and particularly to a method of decontamination which is suitable for the purpose of decontaminating solid surfaces as a measure designed to reduce the potential danger of personnel being exposed to radioactive substances in nuclear installations. DESCRIPTION OF THE PRIOR ART A method of decontaminating solid surfaces in which bubbles are produced in a liquid and the impulsive forces produced when the bubbles burst are employed for separating and removing substances adhered to the solid surfaces has certain advantages in that it is also suitable for the decontamination of articles having complicated forms, produces only a small amount of secondary waste solution, and does not necessitate any use of chemicals, and it has thus recently attracted attention. This method includes ultrasonic washing methods such as the one disclosed in Japanese Unexamined Patent Publication No. 104799/1980. This method utilizes pressure vibrations in a liquid caused by ultrasonic waves whereby bubbles are repeatedly generated and allowed to burst in the liquid. This method is thus able to remove so-called soft clads such as substances that have adhered to the outer layer of a solid surface but has not been able to remove so-called hard clads such as oxide films in the depths of the object to be decontaminated. In addition, since the ultrasonic decontamination apparatus used in practice comprises an ultrasonic generator, a piezoelectric transducer, and a cleaning bath and since it utilizes a method in which an object to be decontaminated is decontaminated while being immersed in a liquid in the cleaning bath, it has been impossible to decontaminate piping or instruments in the state in which they are installed. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of decontamination which overcomes the disadvantages of the above-described conventional method of decontaminating solid surfaces by means of ultrasonic waves and which is capable of effectively removing hard clads and other substances even in the state wherein piping and instruments are fixed in position. The present invention is characterized by comprising the steps of immersing an object to be decontaminated in a liquid, producing bubbles in the liquid by blowing vapor therein, and causing the bubbles to burst on a solid surface which is brought into contact with the liquid and which constitutes the object to be decontaminated so that substances adhered to the solid surface are separated and removed therefrom by the impulsive force produced when the bubbles burst. The present invention can produce bubbles which are extremely large in comparison with those formed by an ultrasonic washing method, and the method of the invention employs vapor and thus is capable of obtaining a greater impulsive force, whereby an excellent decontamination effect can be obtained. The function and effect of the decontamination method of the present invention is described in detail below. (1) Effect of decontamination The present invention is similar to the abovedescribed ultrasonic washing method in that both involve the production of bubbles in a liquid and deal with decontamination by utilizing the impulsive force produced when the bubbles burst. In the present invention, however, it is possible to produce bubbles having a larger diameter than those of the ultrasonic washing method by directly injecting vapor in the liquid and the impulsive force produced is proportional to the third power of the initial diameter of the bubbles, whereby an impulsive force which is greater than that obtainable by the ultrasonic washing method can be achieved. Therefore, not only soft clads that are adhered to the surface in the outer layer thereof but also hard clads in the depths can be removed. (2) Soundness after decontamination Since the present invention uses no decontaminating agent or abrasive, no agent remains after decontamination and no adverse effect upon the soundness of piping or instruments is suffered. (3) Reduction of the amount of waste solution In the method of decontaminating solid surfaces in accordance with the present invention, the amount of secondary waste solution produced following the decontamination work is equal to the amount of vapor injected as the source of generation of bubbles, but the volume of the vapor is reduced to about 1/1500 when condensed and the amount of waste solution to be dealt with is thus kept to a small amount. (4) Workability The present invention uses only vapor and thus exhibits a very high level of safety in comparison with conventional methods of decontamination that use specific chemicals and high-pressure water. In addition, the invention generates no dust during the decontamination work and thus allows a working environment to be kept under sanitary conditions. (5) Applicable field Since the present invention employs the impulsive force produced when bubbles burst, it is possible to decontaminate a surface having a complicated form. In addition, since the method of decontamination utilizes only the injection of vapor, it is possible to decontaminate the inside of piping or an instrument while installed in situ. (6) Prevention of the spread of contamination In the present invention, since all the operations of decontamination are conducted in a liquid and the pressure of the vapor to be injected may be low, there is no possibility of the contamination being spread due to splashed water.