In a nuclear  facility the problem of surface contamination of materials, structures and equipment is mainly  generated  in the nuclear reactor by the neutron reaction with the materials which are permanently or temporarily inside the  reactor core.  The  radioisotopes  occurred in the materials which are temporarily hold, specially  coolants  and moderators, are transferred to the components in the reactor outside  systems; via leakage, seepage or processes such fluids reach the outside of the systems  and contaminate other systems as well as the reactor building and the service building.

The primary contamination sources in a CANDU type Nuclear Power Plant  are represented by the reactor core and the main  process systems which circulate the primary coolant i.e. the moderator.  Considering that in CANDU type NPPs the primary coolant and the moderator  is the heavy water (D₂O), in point of radioprotection , tritium represents one of the problems related to such NPPs.  The radioactive releases are due to the fission products, i.e. the activation products, including tritium and radioactive carbon.

In this  stage II, the project aimed the determination of the work areas affected by tritium and radioactive carbon contamination  related to the material , structure and equipment surfaces in the nuclear systems.

The control of the radioactive contamination in CANDU type NPPs is mainly conducted by:

the minimizing of the heavy water losses from the process systems;

• the heavy water vapour recovery from certain rooms;
• heavy water leakage collection;
• segregation of the rooms containing process systems which circulate the heavy water;
• controlled ventilation of the rooms in the plant;
• tritium monitoring by  stationary , half-portable(temporary located) and portable instrumentation.

The  above  actions will lead to the reduction of the gamma radiation level in the NPP –NSP rooms.

On the other hand, the presence of tritium and radioactive carbon ( very mobile elements)  is generating the spread of the contamination level  by beta active radionuclides to larger  areas , if compared with other types of reactors.

The main radiation sources located in various systems in Cernavoda NPP are :  the reactor core, the primary cooling  circuit, the moderator circuit, the fuelling machine, the spent fuels bays, the active circuit purification systems, radioactive waste handling and storing systems, etc ( see Fig. 4).

The estimation of the room surfaces , of the equipment weights and volumes, of the  radioactive contaminated materials and structures  developed in this stage of the study, is representing input data for the radioactive waste management system to be implemented at Cernavoda NPP  at decommissioning ; implicitly the data  will also have an impact on  sizing the surface  repository aimed to dispose the low and intermediate active radwastes and the deep  geologic repository aimed to safely dispose the long-lived radioactive wastes and spent nuclear fuel for long time- periods.

Tritium and the radioactive carbon are produced by nuclear chemical reactions which occur naturally in the environment, during the nuclear facility operation and following to nuclear weapon testing. In case of  such  releases to the environment, the radionuclides are globally distributed due to the long half-time and the time of remanence in atmosphere and hydrosphere.  Since both tritium and C-14 are mobile in the environment, it is important to control their release from the nuclear facilities and from the radioactive waste management sites by the most operation  protection  procedures, strategies and practices employed in the management of radioactive wastes.

The  tritium and C-14 release impact release assessment shall consider the consequences of one person exposure in the proximity of the exclusion zone exterior and  of the population. It is necessary to have the proper programs for the effluent and environment monitoring ( e.g.: for tritium and C-14) for each nuclear facility individually, in order to provide the population  and environment protection against  radioactive releases.

The discharge/release limits for radioisotopes  are set-up in most countries according to  ICRP recommendations and by also considering other factors in a certain country or requirements typical for that site.  Such limits are vary from one site to another , in function of the assumptions on the nature of the effluent and the environment the discharge/release is  carried-out.

The radioactive wastes  which contain low level  tritium and C-14  contents and which satisfy the waste acceptance requirements  (WAR) may be  disposed in the existing  waste repositories ( e.g. surface disposals on site).

Special attention  is being given to the tritium waste disposal because of  tritium mobility.  It is recommended that the disposal of such a waste be in leak-proof and  stable containers that  can prevent the tritium dispersion to water or an aqueous system. For discharge and dilution, tritium gas is recommended to be used rather than HTO.

In case of wastes  with high tritium and C-14 contents, such as ion exchange resins from PHWRs, which exceed WAR associated to surface disposal, for the moment there is no disposal facility available.

For a CANDU 600 type unit , similar with the ones in Cernavoda NPP, in this document the surfaces of the structures ( buildings- rooms) likely to be high or low  contaminated have been estimated.  Also, a calculation of the  total weight of the main equipment and contaminated materials has been done ( by braking them down in 3 categories: high contaminated (H), low contaminated (L) and low- probability contaminated (N) – See Table 1).  Note that these estimations need to be considered as a guide because the exact calculation of the contaminated structure surface and the weights of the radioactive contaminated materials and equipment cannot be performed but after shutting off each nuclear unit and the radiologic characterization  employing accurate radioactive measurement procedures.  The activity is to be developed  before starting any decommissioning operation upon the Integrated Decontamination, Decommissioning and Radioactive Waste Management Plan which is to be implemented at Cernavoda NPP.  The radiological characterization of the  nuclear facility is mandatory after the final shut-off of the nuclear unit  and it is possible to develop such a characterization only at that moment because one should consider the operation history of the facility, possible  incidents or major accidents occurred throughout the facility operation  period, events that  could  generate a radioactive contamination  also to other  structures or systems which normally would not have  been  contaminated.

Table 1 – The estimation of the contamination degree for the process components in  radiologic area I and II ( Reactor Building and Service Building).

Where:

m_H = total weight of process components estimated with contamination degree H ,expressed in tons;

m_L = total weight of process components estimated with contamination degree L, expressed in tons;

m_N  = total  weight of process components estimated with contamination degree N, expressed in tons;

 %  = percentage of the process component contamination degree.

The activity induced by the neutrons  shall be determined for the calculation of the estimated radiation doses ( see Fig 5).

The contamination degree of the primary  systems shall be determined on basis of the computer models regarding the  build-up of activity and the increase of the radiation field in CANDU type NPPs. ( see Fig. 6).

By decontamination, some materials and components  can be recovered and thus the volume of the radioactive wastes is reduced but also new wastes which are to be  processed, are resulting.  This aspect need to be considered when selecting the decontamination process for each case individually.  The decision on selecting a decontamination method shall follow a technical-economic comprehensive analysis which need to consider the degree of fulfilling the objectives and the selection criteria as well as the influencing factors for each analyzed procedure/method.

The radioactive wastes generated from decommissioning are often different from the  wastes generated during the normal operation or routine maintenance of the nuclear power plant systems. The differences occur due to the diversity of the chemical , physical  and radiological characteristics, the different physical conditions or quantities and volumes generated in the two types of activities.

Due to these  specific characteristics, certain wastes are considered “problem making”, foe example, the wastes foe which the application of the routine handling, treatment and conditioning are not the proper ones and thus, special further consideration should be given to the selection of the specific management procedures.  For such materials and wastes resulted from the decommissioning  process, the correct planning and selection of the waste or material management options have a special importance  in point of economic aspects, nuclear safety and organizing.

The  extent of the Integrated Plan for Decontamination, Decommissioning and Radioactive Waste Management, its contents and the required degree of detailing  depend on the complexity and potential risks of the nuclear facility and the national regulations in force at the date of its implementation.

Within the stages of this Project, one should consider the evolution of the legislation in the domain of NPP decommissioning and  the management of the  radioactive wastes generated by the NPP decommissioning.

When a NPP is decommissioned, in order to optimize the  integrated concept of decommissioning, the Decontamination Plans and the Radioactive Waste Management Plans  will be part of the Decommissioning Plan.   The Decontamination Plan considered in  the Decommissioning Plan, will lead (function of the degree of contamination and  type of resulted waste) to the determination and classification of the radioactive waste volumes required to be processed and disposed off.  The Integrated Decontamination, Decommissioning and Radioactive Waste Management Plan ensures that the decommissioning requirements until the unrestricted release of the site from  the licensing conditions and  final disposal of all radioactive wastes in nuclear safety and radiological  conditions,  are satisfied

An important aspect in the decommissioning of nuclear facilities is the progressive elimination of risks via some sequential decontamination and decommissioning actions  which need to be implemented according to a  Integrated Plan for Decontamination, Decommissioning and Radioactive Waste Management.

The  spatial distribution of radioactivity in all the reactor materials , is employing the use of codes for the calculation of the activity induced by neutrons in the system components.  One of the most commonly used codes  for developing the activity  calculations  is the ORIGIN Code  which is extremely useful because it  can cover many radioisotopes and their decay time.

As a conclusion, the  process of radiological and radiochemical characterization on the shallow and in-depth contamination   of the material surfaces, structures and equipment in case of nuclear  facilities  , is representing an essential step in the  process of defining a nuclear reactor decommissioning plan which in its turn,  is having an essential role in setting-up the radioactive waste classification criteria so to allow the estimation of the costs related to the radioactive  waste management.

The information, data and  findings of the elaborated stage, will fill-in the elements required for the subsequent stage of the in-progress research project.  In the next stage the activities stated to be developed for the scientific and technical substantiation of the decontamination methods  shall be put into practice in order to perform the nuclear  facility  decontamination and decommissioning.  By its contents, the work will be a support for the elaboration of the research studies  which are to define and assess the decontamination methods and their efficiency in case of nuclear facility decommissioning, also considering  the consequencies of such activities on  the technical and human effort.  All these aspects are to be approached in Stage III of the Project , “ The Development of the Decontamination Methodologies for Nuclear Facility Decommissioning”, as per the following activities:

Activity III.1 –  RAAN-SITON Project Coordinator elaboration : “ Study on the scientific and technical basis of the decontamination methodologies for nuclear facility decommissioning. Cost-Benefit Assessment.”

Activity III.2 – RAAN-SCN Pitesti partner elaboration : “ Decontamination Methods Applicable to Tritium and radioactive carbon contaminated surfaces; Measurements and analyses of  various contaminated surfaces.”

Activity III.3. – SC EDATA SRL partner elaboration : “ Assessment on the surface decontamination method efficiency in the case of shallow and in-depth decontamination”.

Finalization deadline :  February , 28^th, 2009.