Radiological characterization of non-activated parts

Knowledge of the radioactive inventory is a key prerequisite for the decommissioning process, being needed for the initial planning, control of radioactive waste flow during dismantling and clearance of the site at the completion of the decommissioning programme. There is considerable similarity of estimation methods in use at various facilities, although the codes used at each step in the calculation tend to be chosen on the basis of local history and preference.

Contamination modelling is considerably less developed than that of activation modelling.

The route to improved modelling will depend on whether the most cost effective way of achieving the required level of knowledge is perceived as direct measurement or development of a model supported by direct measurements.

Characterization of WWER contamination comprises the following two main areas:

• estimation of NPP contamination levels (contamination map)

• measuring dose rates (dose rate inventory).

7.3.1. Contamination levels

The main purposes for determination of contamination levels are:

• estimation of total activity content for licensing purposes

• identification of personnel risks during decommissioning activities

• quantification of possible releases to the environment

• detailed classification of materials for final processing (disposal, decontamination, melting, clearance).

A majority of the material in contact with primary circuit water (containing boric acid 0–

12 g/L, pH maintained mainly by KOH) is austenitic stainless steel stabilized with titanium, and the fuel cladding made of zirconium alloy with 1% niobium. The surfaces of this material react with the primary coolant and become contaminated with activated corrosion products or fission products. The contact surface for a typical WWER-440 is about 16 500 m² (not including the surface of fuel elements) [35]. The volume of the primary circuit together with the volume of pressurizer (volume control system) is about 240 m³.

Data on surface contamination available at operational NPPs cover only a limited part of the plant systems. Operational records and expert estimation have been used in preparing decommissioning studies for the WWER-440 Plants at Paks, Dukovany, and Jaslovske Bohunice. Data resulting from a conservative approach used for classification of contaminated technological systems at NPP Paks [18] are shown in Appendix 4.

Comparison of data collected at operational WWER-440 NPPs [36, 37] indicates that the contamination levels of the main parts of the primary circuit (piping, main circulation pump housing, main isolation valve, steam generator collectors) vary in the range of 10⁴– 10⁵ Bq/cm². Similar results were obtained during radiological investigations of the Armenian NPP Unit I [38]. Some results of this investigation are shown in Table XII. The level of total surface activity of different primary loop pipes changed only slightly, with a major portion of the surface activity resulting from ⁵⁴Mn, ⁶⁰Co, ^110mAg and ¹³⁷Cs. The contribution of ⁶⁰Co to

total surface activity varied from 20 to 80%. Significant differences can be found in ⁶⁰Co and

58Co distribution at some operational NPPs (Table XIII [36, 37, 39]). This is due to the variation of Co content in construction materials produced by different manufacturers.

Table XII. Averaged surface activity in inner surface of primary loop pipelines at different sections (Armenian NPP Unit 1 after 12 years of operation -1992) [38]

Loop section (kBq/cm²)

Reactor–main isolation valve 6.1 × 10¹

Main isolation valve–steam generator 4.1 × 10¹

Steam generator–main circulation pump 3.8 × 10¹

Main circulation pump–main isolation valve 5.3 × 10¹

Main isolation valve-reactor 2.8 × 10¹

Table XIII. Surface contamination levels at various WWER-440s [36–39]

Composition of surface contamination (kBq/cm²)

51Cr ⁵⁴Mn ⁵⁸Co ⁵⁹Fe ⁶⁰Co ^110mAg ¹²⁴Sb NPP

Main circulation pump

-housing 0.52 1.1 0.2 5.6 0.3 Paks

Main circulation piping

-cold leg 1.6 1.9 13.7 1.6 3.6 0.4 Dukovany

Main circulation piping

-hot leg 4.32 27.8 1.1 6.1 0.4 1.2 Dukovany

Main isolation valve 18.2 17.8 3 52 1.9 Paks

Main isolation valve 2.1 16.9 2.4 18.4 0.7 Paks

Steam generator cold

collector 6.4 10.4 0.8 4.9 5.7 7 Dukovany

Steam generator hot

collector 11.4 39.8 1.8 12.9 0.4 4.8 Dukovany

Distribution of radionuclides in systems where a thermal gradient occurs (heat exchangers, steam generator) may differ from other parts of the same circuit. Levels of contamination in auxiliary systems such as drainage systems, valves or filters also need special attention because of the presence of hot spots. A high level of ^110mAg is a specific feature of systems connected with the spent fuel storage pools [40].

An extensive radiological survey performed in the framework of the NPP Greifswald decommissioning project has provided a valuable set of data covering all areas of the NPP including the controlled zone, monitored area, secondary circuit systems, concrete structures and activated components. A complete radiochemical analysis including all relevant nuclides requires considerable radiochemical analytical effort that cost time and money. These data were used for the definition of the nuclide vector. A nuclide vector is a representative mathematical model for estimation of all relevant nuclides based on the measurement of a limited number of key nuclides. This approach can only be used if the radionuclide content of the various systems has been properly analyzed. This approach is very useful during

easily measurable nuclides (gamma emitters) such as ⁶⁰Co and ¹³⁷Cs (Ba). The nuclide vectors determined for the Greifswald Unit 5 (controlled area), Units 1–4 (controlled area) and turbine halls Units 1–4 are shown in Table XIV [41].

7.3.2. Dose rates

Dose rates measured on major components of the WWER reactor coolant system (Table XV [42–44]) are up to an order of magnitude lower than for most western PWRs. This is due to different composition of construction materials (lower nickel and cobalt content) in the WWERs (see Appendix 5). The dose rate from major components in the primary loop at the NPP Greifswald was comparatively low although some hot spots up to 10 mSv/h were found [34]. Detailed radiological characterization of the NPP Kozloduy Unit 3 (Table XV) was performed at specific points of primary system equipment before the full system decontamination in 1994 [42]. Lower reported occupational exposures in operating WWERs as compared to Western PWRs (Table XVI [45]) are generally due to lower dose rates and may entail somehow lower decommissioning doses, although this assumption is not proven.

Table XIV. Nuclide vectors (NPP Greifswald 1995) [41]

area ⁶⁰Co

Note: Unit 5: only 70 days of operation in 1989

Table XV. Equivalent gamma-dose rates of some primary system equipment

Measuring point Equivalent

gamma-dose rate (mSv/h)

NPP Outer surfaces

Steam generator outer surface 0.003–0.08 Greifswald

Main circulation pump outer surface 0.15–0.45 Greifswald

Main isolation valve outer surface 0.05–0.24 Greifswald

Pressurizer outer surface 0.02–0.05 (max.7) Greifswald

Heat exchanger 0.5–10.2 Greifswald

Inner surfaces

Steam generator — collector inner surface 12–45 Kozloduy

Steam generator — tube bundle outer surface 3–11 Kozloduy

Pressurizer inner surface 0.6–8.9 Kozloduy

Primary coolant blow down heat exchanger 10–20 Kozloduy

Primary circuit pipeline up to 7 Kozloduy

PWR data [44]

Steam generator - channel head 100 (typical value) Westinghouse and KWU NPPs, 1992

Primary circuit pipeline 1–5 KWU NPPs, 1992

Table XVI. Occupational exposures of WWERs, elaboration from [45]

Country Annual collective dose, average per reactor (man Sv)

1986–1989 1990–1996

Czech Republic 0.31 0.37

Finland 0.93 1.14

Hungary 0.56 0.60

Slovakia 0.62 0.58

World, PWR 2.51 1.70