In this stage the following activities were performed:

• Research report regarding the scientific and technical bases of the  decontamination methodologies associated to the nuclear installation decommissioning; costs-benefit evaluations
• Research report regarding the decontamination methods applicable to tritium and radiocarbon contaminated surfaces; measurements and analyses of various contaminated surfaces.
• Research report regarding the assessment the efficiency of the decontamination  methods employed for  shallow and deep contaminated surface areas.

 This phase revealed the necessity to know a methodology for selecting the decontamination techniques related to nuclear installations decommissioning, a selection methodology which should govern the selection of the appropriate decontamination  technique for the  decontamination of metal and concrete structure surfaces in such installations.
The scientifically and technically  established decontamination methodology  considers that the selection of a particular decontamination method should meet  the requirements related to safety, efficiency, cost - benefit and waste minimization to the maximum extent possible.
The scientific bases of the decontamination methodology are stating the fundamental chemical and physical decontamination processes by the application of specific selection criteria (reducing the occupational radiation exposure and population exposure , minimizing the potential risk of contamination spread during the decommissioning process, reducing the residual contamination down to allowable radiation levels for components or metal or concrete structures ).
The technical bases of the decontamination  methodology establish the  applicability of the  decontamination techniques by the use of identifiable scientific bases.

The methodology for selecting the appropriate decontamination techniques, outlined in the relevant decontamination method  will determine the importance of each criterion for the  selection according to:

• The necessary prerequisites
• The characteristics of the analyzed method.

 
To facilitate the selection process, each selection criterion is been assigned, in descending order of preference, different levels of performance.
The object of decontamination may be the surface of an equipment , a subsystem or an entire process  system or in case of concrete structures  the wall or floor area, etc. .. For components whose contamination can be easily removed from the system, the most usual option is to use physical or electrochemical decontamination, unless chemical decontamination is the only feasible method.
If pursuant to the evaluation, several techniques are considered acceptable, their characteristics are reviewed to determine which type of technology  meets best the constraints set on postulated charge  and which offers may be available for analysis.
The net cost of the decontamination process has a role in selecting the methodology because it includes the cost of materials, equipment and activities for the  implementation of the method and the cost of processing and storage of the resulted  radioactive waste. The lower the value of the cost-benefit ration the better, because it signifies the cost per unit of radiation dose reduction. for a certain method.
The final choice of the most appropriate decontamination technique is made by comparing the costs with other similar techniques and the method with the most  competitive cost is selected.
In case of nuclear installation decommissioning, decontamination methods relevant for the decontamination of steel structures and equipment, i.e concrete surfaces and buildings are chemically and physically  decontaminated.
As a basis for selecting the decontamination methods , a technology must be able to remove radioactive contaminants. It might be useful to remove  non-radioactive contaminants  such as organic materials, metals or inorganic materials. Second, the technology should be commercially available by one or more suppliers. Third, the technology must be feasible as demonstrated earlier in the process of contaminants removal in similar installations.
Considering the selection methodology , the cost efficiency significantly influences the appropriate technology implementation.
The chemical decontamination methods are based on the chemical properties of contaminants and their host matrix in order to obtain the removal of the contaminant.
The following chemical decontamination techniques were developed:

- Chelation and organic acids;
- Strong mineral acids and related substances;
- Chemical foams and gels;
- Oxidizing and reducing agents;
- TechXtract

Physical decontamination methods are based on some form of physical or mechanical abrasion of the surface contaminants or host material to get the effect of contaminant removal.

The following  physical decontamination techniques were developed:

- Strippable coatings;
- Centrifugal Shot Blasting;
- Concrete grinder;
- Concrete shaver;
- Concrete spaller;
- Dry vacuum cleaning;
- Electro-Hydraulic scabbling;
- En-vac robotic wall scabbler;
- Grit Blasting;
- High Pressure Water;
- Soft Media Blast Cleaning;
- Steam Vacuum Cleaning;
- Piston Scabbler.
 

Each decontamination technology was highlighted by the following aspects

- Description of the technology;
- Target contaminants;
- Applicable media and features of the area;
- Waste flow and issues of waste management;
- Stage performance.
 

For example, see the en-vac robotic wall scabbler technical.

In the nuclear installation decommissioning  process the decontamination of metal structures and concrete surfaces and buildings is considered.
Decontamination of metallic structures involves a decontamination before decommissioning and a decontamination after decommissioning.
Decontamination before decommissioning is aimed  to reduce the occupational exposure and the decontamination can take place on closed systems, such as process pipeline systems, and on open systems such as equipment, ponds, tanks, etc. ..
For closed systems the  chemical or physical methods are recommendable.
In case of open systems the  physical methods are recommendable  out of which the following techniques are considered relevant for decontamination: water jet,  blasting, strippable coatings.
Decontamination after decommissioning is aimed  to  recycle the contaminated metal and reduce the quantities of secondary wastes. Decontamination can develop either in closed systems or open systems, including pipes, components or equipment.
Generally, the decontamination after decommissioning  employ  physical methods, chemical or sometimes derivative, such as the electrochemical method (by electrolytic polishing technique) or by ultrasound method.
Among the most applied decontamination techniques  associated to the chemical method  it is worth mentioning the chemical immersion methods such as the ones which use  strong mineral acids and related substances or oxidants and reducing agents.

Decontamination of concrete surfaces and buildings  aims  to the free release of buildings and reduction of the concrete radioactive contamination.. Decontamination of concrete buildings involves a decontamination of concrete surfaces (e.g. walls, floors, etc..) and the subsequent demolishion of the concrete and disposal as non-radioactive wastes.

For the decontamination of concrete surfaces  relevant physical methods of decontamination techniques are mainly used  as follows:

- shaving
- en-vac robotic wall scabbler
- strippable coatings
- grit blasting

Decontamination of concrete surfaces may be achieved using derived thermal methods such as :microwave irradiation or flame scarifying.
Decontaminated concrete buildings decided to be destroyed, are demolished by explosions or mechanically, by hammering or drilling and crushing.

Figure 1 shows the diagram of the decontamination methods applicable to  nuclear installation decommissioning, in accordance with those described above.

The capacity of each decontamination technology characterizing a decontamination technique , function of  the type of contaminated surface (metal, concrete) is shown in table 1 and table 2, structured on methods of chemical and physical decontamination.
Starting with the scope of applicability, i.e. imposed limitations, and evidencing the special considerations for each decontamination technique  the efficiency of each decontamination technique was determined on basis of the professional decision and available information in the literature.
Based on the specification of the general application media for each technique, the dominant medium of application for which the applicability characteristics were obvious, has been determined.

 The  decontamination techniques that are best for certain areas of the CANDU 600 NPP were determined and  for each selected technique, the estimated concrete or metal surfaces which are to be decontaminated by that technique, were established.
The selection of the  techniques started from the recommendations in the scientific literature and has resulted in the establishment of techniques to be applied on certain contaminated areas.
On the other hand , on basis of the detailed  studies in the previous stages, the  total areas  requiring decontamination were estimated , namely: 85000 m2 metal surfaces  and 65000m²  concrete surfaces.
It was considered that all surfaces identified in the contaminated areas must be decontaminated either for  the license release or  for to pass to an inferior contamination category.
The following  decontamination techniques  were considered relevant and  selected for  the decontamination of the nuclear power plant in Cernavoda:

- Out of the chemical methods: TechXtract

- Out of the physical methods:


- Strippable coatings;
- Centrifugal Shot Blasting;
- Concrete shaver;
- En-vac robotic wall scabbler;
- Grit Blasting;
- Soft media blast cleaning;
- Steam vacuum cleaning;
- Piston scabbler.
 

A unit cost for each decontamination technique was determined by relating the total cost to the average surface to be decontaminated ( i.e. 500 m3), corresponding to one month work period.

In order to  develop a comparative study on the decontamination techniques, an estimation of the operational parameters corresponding to the work procedures associated to the analyzed decontamination techniques,  has been conducted by means of  mathematical modeling ( Monte Carlo simulation  method).

The steps of the simulation ( modeling), according to the simulation methodology adopted by the manufacturer of ProVision V.6.1.1. device, imvolved the following  actions:

• the definition of the work procedures specific to the decontamination techniques;
• the parametrization of the work procedures in point of necessary time ;
• the parametrization  of the work procedures in point of costs associated to the component activities;
• the simulation of the work procedures by ProVision 6.1.1. by  Monte Carlo mathematical method;
• the generation of post-simulation reports  on which basis the efficiency of each decontamination technique is  assessed.

The  definition of the work procedure specific to the decontamination techniques and  the parametrization of the work procedures in point of time-consumption and costs related to the component activities were made on basis of the information in the decontamination technique descriptions for to demonstrate their performance. The parameters represented the input data for the soft simulation of the work procedures employing Monte Carlo mathematical modeling method.

Information contained in table 1 and table 2 lead to the following conclusions:

• The relevant techniques associated to  the chemical decontamination methods that show to be  appropriate and have a good efficiency and unit costs which are not  exceeding 40 Euro / m² , are the chemical foams & gels and TechXtract.
• The relevant techniques associated to the physical decontamination methods  showing  to have a good efficacy and the  unit costs are not exceeding 40 Euro/m²  are : strippable coatings, centrifugal shot blasting, concrete shaver, en-vac robotic wall scabbler, grit blasting, soft media blast cleaning,
• The techniques associated to the physical decontamination methods  are :  dry vacuum cleaning and piston scabbler  whi though efficient, because of the high costs requiring larger funds, have a smaller applicability in CANDU type NPPs. 
• The techniques associated to  the chemical decontamination methods  are :  chemical foams and gels & TechXtract and the techniques related to the  physical decontamination methods  are :  strippable coatings, centrifugal shot blasting, concrete shaver, en-vac robotic wall scabbler, grit blasting, soft media blast cleaning, and they are  the most recommended methods to be employed in the  decontamination of Cernavoda NPP units.

 
Note that for some situations, decontamination techniques are considered appropriate, even if the degree of use is low, because of considering the  geometry of the contaminated surface. In other circumstances , for the decontamination of surfaces , more decontamination techniques, successively applied ,need to be employed..
As a  conclusion, it is important to know and improve  the decontamination techniques used in the nuclear industry for to ease the  operation, inspection, maintenance and modification of nuclear installations and to meet their needs for decommissioning.

The technical procedures are needed to make possible the recovery of equipment, the  reuse of buildings and the proper waste management.
It is necessary to point out that  there is no universally applicable  decontamination method and every situation tends to become a special case. Moreover, from the practical experience in the decontamination of a nuclear reactor or a reprocessing facility  it results that the technologies used should be further improved. It is necessary to continue the  efforts to find new techniques and make improvements to the existing techniques in order to develop and apply the experimentally tested techniq