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Vol.7, No.4, NT74
 
Dry Ice Blast Decontamination to In-service equipment in Japanese PWR Plant
 
Mitsubishi Heavy Industries, LTD.
 
KEYWORDS:
Dry Ice Blast, Decontamination, Dose rate reduction, Secondary waste reduction
 
1. Technical summary

Classification
7 - A (Decontamination)

In primary system of PWR plants, it is important to reduce dose rate in terms of radiation exposure during maintenance work and inspection. Therefore chemical decontamination is used to be applied to primary system equipment before maintenance work and inspection. Chemical decontamination is beneficial when it is applied to equipment whose surface is large. However much secondary waste, such as high dose resin, is produced in chemical decontamination process and disposal of high dose rate resin is difficult.

On the other hand, MHI had developed several mechanical decontamination methods. Mechanical decontamination is beneficial when it is applied to equipment whose surface is narrow. Especially in terms of secondary waste reduction, MHI started the study of application of Dry Ice Blast Decontamination to actual PWR plant.

This paper provides an introduction to Dry Ice Blast Decontamination principle, its system and actual application result to PWR plant.

2. Development Phase

Phase 2 : Industrial Confirmation Phase

3. Scope
  1. (1) Components:
    Material surface
  1. (2) Location:
    Nuclear power plants
  1. (3) Materials:
    Deposition on the material surface
  1. (4) Condition:
    In the air



4. Features
  1. (1)Principle of Dry Ice Blast Decontamination
  2. Solid CO2 is used as its blast media in Dry Ice Blast Decontamination method. Dry ice particles are accelerated to high velocity by a stream of compressed air and the blast stream is exposed to physical object. Three reactions on the surface of physical object are mainly occurred by dry ice particles shock as described in Fig.1.
    1. a)First reaction; Blast stream collides against radioactive scale on the surface of physical object. It develops small crack in scale.
    1. b)Second reaction; Extreme small dry ice particles in blast stream thrusts into crack in scale.
    1. c)Third reaction; Temperature difference between dry ice particles(-75°C) and scale varies dry ice particles to vapor CO2 (that is sublimation reaction). The volume of vapor CO2 becomes about 800 times as much as that of dry ice particles. Expansion of dry ice volume destroys scale on the surface of equipment.



    EJAM7-4NT74_Fig.1 Principle of Dry Ice Blast Decontamination

    Fig.1 Principle of Dry Ice Blast Decontamination



  3. Unique features of Dry Ice Blast Decontamination are shown in Table.1. It produces no secondary waste, abrasive, toxic electrically conductive as the media sublimates upon impact unlike other decontamination methods. One of other features of Dry Ice Blast Decontamination is that it does not need to take many facilities and places.



    Table 1 Comparison with mechanical decontamination method
      Secondary
    Waste
    Abrasive Toxic Electrically
    conductive
    Dry ice NO NO NO NO
    Solvents YES NO YES CAN BE
    Sand YES YES NO YES
    Water or Steam YES NO NO YES
    Ultrasonic bath YES NO CAN BE NO


  1. (2)System configuration
  2. The System configuration of Dry Ice Blast Decontamination is shown in Fig.2. Dry ice particles are exposed by a stream of compressed air from station air compressor. They impact and remove the scale on the surface of physical object. Removed corrosion products (scale) in the atmosphere are collected by dust collector and filter box. Dust collector has extraction rate over 98% and filter box has extraction rate over 99.97% by AFI duct test.


    EJAM7-4NT74_Fig.2 The system of Dry Ice Blast Decontamination

    Fig.2 The system of Dry Ice Blast Decontamination



5. Examples of Application

MHI applied Dry Ice Blast Decontamination to evaporator in waste disposal system at actual PWR plant. This application to in-serviced equipment was first trial in Japanese PWR plant. The structure of the evaporator is shown in Fig.3. Targets of decontamination are the surface of heater exchanger tube and inside bottom of evaporator.



EJAM7-4NT74_Fig.3 The structure of evaporator in waste disposal system

Fig.3 The structure of evaporator in waste disposal system



MHI performed decontamination through 3 steps as shown in Fig.4.
At first step, MHI performed decontamination of upper surface of heater exchanger tube bundle using tool in order to reduce dose exposure. Tool could be accessed through Man hole of the evaporator.
At second step, MHI performed decontamination all the way to center of heater exchange tube bundle after pulling out tube bundle from evaporator. As shown in Fig.5, MHI set shield box and additional shield for reducing radiation exposure. At third step, MHI performed decontamination of inside bottom of evaporator.
As shown in Fig.6, MHI confirmed the detail of work procedure through training in Kobe.



EJAM7-4NT74_Fig.4 Work procedure of decontamination to evaporator in waste disposal system

Fig.4 Work procedure of decontamination to evaporator in waste disposal system




EJAM7-4NT74_Fig.5 Countermeasure for reducing radiation exposure

Fig.5 Countermeasure for reducing radiation exposure




EJAM7-4NT74_Fig.6 Picture of training at Kobe shop

Fig.6 Picture of training at Kobe shop




The pictures and results of decontamination effect in site implementation are shown in Fig.7. Scale on inner surface of the evaporator was almost removed by Dry Ice Blast Decontamination. Therefore drastic dose rate reduction inside of evaporator was achieved in this site implementation (Decontamination Factor (DF): about 20). And any crack on the evaporator was not recognized after Dry Ice Blast Decontamination. The work period was approximately 1 week.



EJAM7-4NT74_Fig.7 Pictures of decontamination effect

Fig.7 Pictures of decontamination effect




6. Conclusion

  1. (1)Summary of application
  2. MHI had successfully applied Dry Ice Blast Decontamination to in-serviced equipment in Japanese PWR plant. MHI could show the effectiveness of Dry Ice Blast Decontamination and comprehensive benefit in terms of radioactive waste reduction and reduction of radiation exposure during inspection as shown in Fig.8.


    EJAM7-4NT74_Fig.8 Pictures of decontamination effect

    Fig.8 Pictures of decontamination effect




  3. (2)Future
  4. It is indispensable to reduce dose rate in terms of radiation exposure during maintenance work and inspection. Therefore continuous study of beneficial chemical and mechanical decontamination method is valuable. Based on this experience, MHI intends to improve decontamination method continuously. Also, MHI keeps making contribution to reduction of dose rate in primary system of PWR plants.

7. Reference

  1. 1.https://vimeo.com/coldjet/videos
  2. 2.http://www.coldjet.com/en/industries/power-generation.php
8. Contact

Japan Society of Maintenology (ejam@jsm.or.jp)