Narrative

6.39.1. Preparation for decommissioning

Problem encountered

The preparation for decommissioning of old nuclear facilities at the ÚJV Řež, a. s. (Nuclear Research Institute Řež) (ÚJV), Czech Republic, started in 1996. These old facilities were designed and installed in the 1950s and 1960s. Many of the facilities have been out of operation for a long time.

Collection of design and operational documentation, records, procedures, equipment and radioactive waste inventories was the first preparation step. During the preparation for decommissioning, the following unknowns were identified:

— Lack of documentation, working procedures and operational records;

— Design documentation that did not correspond to the actual state;

— Many upgrades were made without proper documentation;

— Lack of inventory of equipment, materials, radioactive waste, etc.

The preparation also included interviews with the actual and former or retired staff.

In the framework of the first stage, studying the existing data led to the identification of information that was lacking. Then, characterization of sources of ionizing radiation and radioactive contamination using dosimetric measurement and radiochemical analyses was performed. During characterization, many inaccessible environments were identified.

Analysis and solution found

According to the standard procedure valid at the ÚJV, all design documentation should have been archived in the central archive and in the respective buildings. Many documents had been lost or discarded in the central archive in the past. However, many documents have been found in the respective buildings. A thorough analysis has to be conducted to ascertain which documentation is relevant, because several versions or non-relevant documents exist.

The interviews with the actual and former or retired staff were important, but were sometimes not reliable.

A thorough analysis of what information is relevant needs to be carried out. It was decided not to try to access inaccessible environments if there was a risk of release of radionuclides or loss of integrity.

Lessons learned

Unknowns are inevitable in decommissioning, as was demonstrated in the decommissioning of old facilities at ÚJV where many unknowns were identified. Collection of information, interviews with staff and extensive physical and radiological characterization are necessary during preparation and execution of decommissioning.

The approach for how to use the collected information must be well balanced. If not, the risk can be underestimated (impacts on safety) or overestimated (impacts on budget and time schedule). A proper documentation management system is necessary for the entire lifetime of the facility.

6.39.2. Decommissioning of a special sewage system

Problem encountered

A special sewage system was used for transfer of liquid radioactive waste from various facilities to a radioactive waste processing facility. The system consisted of a stainless steel pipe network, of total length 410 m, situated in an underground concrete corridor accessible through shafts. The decommissioning procedure started with removal of soil and opening of the corridor. During excavation works, it was discovered that the pipeline was partially laid in another location. This unknown had almost no impact on decommissioning, but in the case of excavation work in the past, the system could have been damaged.

Analysis and solution found

As this unknown had almost no impact on decommissioning, it was more important for the next decommissioning work at ÚJV.

Lessons learned

This was the first case where a major discrepancy with the documentation during decommissioning of old ÚJV facilities was found. It was taken into account during the preparation and execution of decommissioning of other facilities.

6.39.3. Decommissioning of technology for radioactive waste treatment

Problem encountered

The old radioactive waste treatment technology comprised the evaporation unit, storage tanks and a set of mixed bed filters. The technology started operation in 1962, and was shut down in 1992. The evaporation unit consisted of a heater heated by steam and a separator. With regard to the design documentation, the separator should have been simply put into an opening in the concrete floor during installation. Instead, the separator was captured in concrete. Two sedimentation tanks, each of volume 25 m³, were used for sedimentation of liquid radioactive waste. The tanks were also captured in concrete, which was in contrast to the design documentation.

Analysis and solution found

As it was not removable in one piece, the described equipment was partially dismantled by oxyacetylene cutting and a nibbler prior to removal.

Lessons learned

The discrepancy between the design documentation and the real state is a frequent unknown during decommissioning, and must be taken into account at all times.

6.39.4. Decommissioning of gloveboxes

Problem encountered

Two laboratories, called Alpha halls, contained eight sets of wall boxes and a number of gloveboxes. The boxes were significantly contaminated by alpha particle emitting radionuclides (U, Np, Pu and Am). The total volume of the boxes was approximately 80 m³.

The main unknown was the insufficient information about the equipment and contamination inside the boxes.

Measurements and analyses were performed to obtain the information. As the boxes were contaminated by alpha particle emitting radionuclides, sampling was planned.

Analysis and solution found

The sampling was identified as a risk operation because of the possibility of the loss of box integrity after opening. The sampling will be done during decommissioning when tight confinement will be installed over the boxes.

Lessons learned

Sometimes, the sampling means the start of the real decommissioning activities. Then, it is better to perform it together with other decommissioning activities when all equipment and personnel are available and safety measures are of the necessary level.

6.39.5. Decommissioning of a hot cell

Problem encountered

A hot cell was used for experiments in the reprocessing of spent fuel during the period 1969–1974. The cell was heavily contaminated. The hot cell, made from cast iron of thickness 100 mm, had been partially decontaminated in the past and then its internal and external surfaces were coated to prevent the contamination from spreading.

The main unknowns were the lack of design documentation and almost no information on what was really inside the hot cell. In the late 1970s, the equipment was removed. In the early 1980s, the hot cell was partially decontaminated. The last record was: “Decontamination of the hot cell will be finished next year.” Instead of this, the facility was abandoned.

Analysis and solution found

No one knew what was behind the door. That was why a survey had to be performed before decontamination.

Decontamination of the hot cell started with removal of the fixative coating by a paint remover at first, and then dry ice blasting was applied for decontamination. Subsequently, the hot cell was removed from its mounting.

During floor decontamination, contamination of subsurface layers was identified. Only a part of the contamination was removed to avoid breaching the integrity of the building that was still used. The range of the remaining contamination was recorded, and will be used for decommissioning of the building in the future.

Lessons learned

In this case, deferred and not properly planned decommissioning was the main problem. This was also accompanied by lack of design and operational documentation.

The decommissioning must be properly planned and performed as soon as possible after the shutdown. If this is not possible, all information about the facility (including the remaining contamination) must be collected and properly stored.

6.39.6. Release of waste into the environment