Chemical decontamination has significantly reduced radioactive contamination in the Primary Circuits of the Kozloduy NPP shutdown reactor units


A complex sequence of chemical decontamination operations of the Primary Circuits and associated equipment of Kozloduy NPP four VVER-440 reactor units was completed in mid-2022 as part of the project for decommissioning of the shutdown nuclear facilities. The main objective of the operations was to reduce the dose rates as low as reasonably possible, thereby facilitating the dismantling activities and reducing the need of expensive remotely operated instrumentation.

The process consists in removal of radioactive deposits by chemical processes, and as result of the decontamination operations on the Primary Circuits, the activity has been reduced as follows:

Kozloduy NPP, Unit No.

Unit 3

Unit 4

Unit   1

Unit 2

Start – Finish Dates

5-25 Feb 2022

17 Mar-2 April 2022

11-28 May 2022

12-26 June 2022

Decontamination Factor (DF)





Activity Removed





SG Contact Dose Rate (Average), µSv/h

from 2,303 to 80

from 1,802 to 26

from 1,021 to 21

from 1,377 to 25

The results obtained will help to lower the waste category of the Primary Circuit components and thus decrease the volume of waste requiring interim storage as an intermediate-level waste (ILW).

The activities were carried out by means of an installation, so-called “Spider”, which is inserted into the reactor and interconnected by special equipment, thereby providing circulation of chemical solutions intended to decompose and remove precipitates and deposits on the interior of the pipelines. The system is in ‘dynamic equilibrium’ – at the same time as the chemical solutions are injected, radioactive metal ions are continuously released through ion-exchange columns mounted behind biological shielding “screens”, allowing work to be performed in a low-radiation working environment. The “Spider”, which is placed into the reactor, is almost

8 metres high and weighs nearly 27 tonnes, including the biological protection “shield”, which allows the work in the reactor to be carried out under conditions of lower dose uptake by the personnel, fully complying with the ALARA principle. The operations, implemented by an international team of about 30 specialists, started in early February and ended at the end of June 2022.

The installation was already used in other European Nuclear Power Plants – Bohunice V1 NPP in Slovakia with the same type of reactors and Ringhals NPP in Sweden. State Enterprise Radioactive Waste chose the same strategy implemented for decontamination of the Primary Circuits of the Slovak Nuclear Power Plant under a project funded by the Bohunice International Decommissioning Support Fund (BIDSF) and completed at the end of 2017. The successful implementation thereof ensures a significant reduction of the risks associated with the project. The synergy between the Slovak and Bulgarian Nuclear Decommissioning Programmes confirmed the full applicability of the technology to achieve such state of the nuclear facility that is appropriate for the installation, testing, commissioning, and operation of the decontamination equipment, including experience and knowledge sharing. In fact, the technology transferred to Kozloduy NPP previous experience with similar applications of decontamination operations during decommissioning and in the case of operational and shutdown facilities, gained from activities mainly in France, Belgium, Spain, Sweden, and the USA.

To ensure that the final waste packages – Reinforced Concrete Containers (RCC) – meet the acceptance criteria for disposal at the National Disposal Facility, SERAW processes the waste generated during the decommissioning process in compliance with proven technologies, by applying the principle of minimizing radioactive waste and provisions of the respective regulatory documents, restrictions and conditions.