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.

Phase 2 : Industrial Confirmation Phase

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

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.
3. 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.
   
   
   
   

   

   Fig.1 Principle of Dry Ice Blast Decontamination

   
   
   
4. 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.
   
   
   

   

   Fig.2 The system of Dry Ice Blast Decontamination

   
   
   

Fig.3 The structure of evaporator in waste disposal system

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

Fig.5 Countermeasure for reducing radiation exposure

Fig.6 Picture of training at Kobe shop

Fig.7 Pictures of decontamination effect

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.
   
   
   

   

   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.

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

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