Decommissioning of nuclear facilities and management of radioactive materials are some of the most complex and costly challenges in the nuclear industry. A key aspect of this process is the handling of contaminated concrete and building materials. Concrete structures within the containment of nuclear installations can amount to thousands, or even tens of thousands, of tonnes requiring decommissioning and release. This is particularly true for structures in close proximity to the reactor, such as the bio-shield or the support structure of the reactor, which might be significantly contaminated and activated. Furthermore, all concrete and building structures within nuclear installations must undergo radiological characterisation, proper removal, sorting and appropriate release. Given the large volume of material at each site, optimising this step can significantly reduce overall costs, time, and the amount of radioactive waste generated, for the operator.
The standard approach for remediating such materials involves multiple in-situ measurements on the standing structures as pre-measurements. This is followed by removal, potential crushing, and final release or characterisation measurements. Depending on specific site conditions, it may be beneficial to crush the material at the earliest possible stage to simplify transport and facilitate processing as bulk material.
The Free Release Measurement System (FREMES) developed by NUKEM Technologies Engineering Services GmbH offers a solution by integrating radiological characterisation and sorting into a single automated process.
Conducting characterisation and sorting in a single automated step can significantly enhance efficiency. In this scenario, time-intensive pre-measurements can be minimised while still ensuring a comprehensive evaluation of the contamination, including the determination of nuclide vectors. This technology significantly enhances efficiency, reduces costs, and ensures compliance with regulatory standards, making it a transformative approach for the decommissioning of nuclear sites.
Comprehensive pre-measurements are not required for 100% of the structures. Instead, the FREMES processes the demolished and crushed building materials, delivering results in a single step by:
- Data Evaluation: The acquired data is immediately analysed to determine the specific nuclide activity for each separable portion, including uncertainty values in compliance with ISO 11929.
- Classification: The results are compared against regulatory limits and classified into different categories based on customer’s requirements, typically Free Release (FR), Conditional Release (CR), Radioactive Waste (RW), and material suspected of containing hot spots (HS).
- Sorting: Based on the characterisation, the material is directed to its designated storage container via sorting belts.
- Documentation: The system automatically archives all measurement and operational data and generates the corresponding documentation in format defined by the client.
FREMES in use
The FREMES has been successfully implemented in a number of large-scale nuclear decommissioning projects, such as the remediation of NUKEM’s own legacy fuel element fabrication site in Hanau, Germany, and FBFC International’s facility in Dessel, Belgium. Those consisted of materials suspected of uranium contamination, such as those typically found on enrichment and fuel fabrication plants.
Here, the system successfully determined the total uranium content in the contaminated soil using direct gamma spectrometry to measure the U-235 signature, combined with known uranium enrichment factors. The patented system’s radiological measurement in these projects delivered:
- Certified direct proof of material release for applications in Germany,
- One of two redundant and diverse measurements required for material release in Belgium, as mandated by the Belgian nuclear authority (the second component being independent sample measurements).
- Sorting the materials into different activity/waste categories and deposition into corresponding storage containers.
- Providing all necessary measurement results and documentation required for release or disposal as waste, for regulatory compliance.
Bulk material characterisation and sorting
The FREMES combines advanced measurement technology with an automated transport system to enable continuous and precise characterisation of bulk materials, such as concrete debris. The only requirement is that the material must be reduced to a sufficiently small grain size to allow for efficient transport and accurate measurement as bulk material.
The system is composed of several key components:
- Conveyor belts: Continuously transporting material under the detectors.
- Detectors: High-purity germanium detectors measure the gamma radiation emitted by the material.
- Weighing system: An integrated weighing system records the material‘s weight.
- Software: Manages the entire process, from data acquisition to evaluation and material sorting.
The core measurement process of the FREMES involves the following steps:
- Material preparation: The material is mechanically processed (e.g. crushing, sieving) and then buffered, forming a continuous stream with a defined geometry suitable for measurement.
- Spectrum acquisition: The material is virtually divided into sorting portions, with gamma spectra captured the detectors and weight recorded for each portion.
In the Belgian project, the system processed approximately 45,000 tonnes of material, enabling the release of a substantial part of it. Sorting portions of around 100 kg were achieved with throughputs of approximately 10 –15 tonnes per hour, ensured by continuous technical supervision and system optimisation. The project was completed within the agreed quality, time and budget constraints, and the site was officially cleaned and released from nuclear supervision by royal decree in 2022.
As disposal costs for higher waste categories increase exponentially (usually by a factor of 10 per category), this precise sorting process significantly reduced overall costs – bringing the final expenditure to about 12% of the estimated cost for conventional processing methods, a demonstrable saving.
Remediation of concrete and building rubble
The application of FREMES to concrete structures and buildings follows a similar approach to the projects presented above. In general, the system is highly flexible and can be tailored to specific needs:
Material preparation, including tasks such as crushing and the removal of steel reinforcement, can be carried out either by NUKEM or the customer. This process can take place within controlled areas and existing buildings or be conducted outside of them.
Measurement and sorting containers can be customised to provide the required accuracy and throughput for specific needs. These containers can be designed as either containerised or free-standing systems.
Estimates for achievable detection limits of the system, depending on material throughput, for the most important gamma-emitting nuclides in radiological decommissioning.
In this design, the material separation size is approximately 100 kg at a bulk density of 2.0 g/cm3. This represents a typical example and exact parameters need to be determined from the details of any specific application.
The number of sorting categories and exits can be adjusted. While NUKEM’s standard design involves sorting into three categories, required sorting and limits can, of course, be modified to suit the requirements of the client.
Material sorting is carried out by comparing activity results against legal limits (including safety margins) for key nuclides detectable by the gamma spectrometry system.
Other nuclides (e.g. those specified by nuclide vectors provided by the customer) can be incorporated into the evaluation process. The accuracy of this determination this determination specifically, the detection limit for their specific activity is designed to meet the lowest legally required sorting limits, with an adequate margin of conservatism. The detection limits for the most important nuclides in the current standard design are shown in Figure 1.
The exact values depend on the operational parameters of the sorting process. There are three main design parameters that influence the system’s design:
- The detection limits and their target values, defined by legal sorting limits
- The maximum material throughput
- The smallest material separation size
Not all of these parameters can be optimised simultaneously without negatively affecting at least one of the others. Therefore, for each specific application, an optimal configuration must be selected.
For the NUKEM design used in Dessel, the following parameters where applied:
- Detection limit: Below 20 Bq/kg U-235, corresponding to below 0.5 Bq/g U-Total
- Maximum material throughput: 10 –15 tonnes per hour
- Material separation size: Approximately 100 kg
NUKEM provides the optimal design and solution for the customer’s specific application, typically in the form of the following key project steps:
- General feasibility study on legal requirements and priorities in process parameters
- Detailed study for on-site implementation and waste streams
Additionally, there are several optional enhancements to the system’s capabilities, which can be integrated into the FREMES, if required by the client, such as:
Alpha- and Beta counting measurements of the material surface: These can be used primarily to verify the presence of nuclides detected through gamma activity determination and known nuclide vectors. Since Alpha and Beta radiation are short ranged, the effectiveness of these measurements depends heavily on the specific measurement circumstances.
Optical detection methods to detect conventional contamination or non-mineral components of the material.
NUKEM can integrate additional detection systems capable of optically identifying materials such as remaining wall paint, plastic parts, heavy metal contamination, oil or chemical residues. The system can simultaneously characterize and sort the material based on both radioactive content and these additional contaminant factors in a single measurement step.
The future of FREMES
The FREMES represents a significant advancement in the decommissioning of nuclear facilities. By integrating radiological characterisation and sorting into a single automated process, FREMES addresses the critical need for efficient handling of large quantities of potentially contaminated concrete and building materials. The system‘s ability to continuously and precisely measure and sort bulk materials, such as concrete debris, offers several key benefits:
- Efficiency and cost reduction: The streamlined process reduces the need for extensive pre- measurements and optimizes the overall decommissioning workflow.
- Flexibility: FREMES is designed to be highly adaptable, allowing customisation to meet specific project requirements, including material preparation, measurement and sorting equipment, and the number of sorting categories.
- Regulatory compliance: The system ensures compliance with legal limits for radioactive materials, providing thorough documentation and facilitating regulatory approval for the release or disposal of materials.
- Reduction of waste disposal costs: By accurately sorting materials into categories such as Free Release, Conditional Release, and Radioactive Waste, FREMES minimises the amount of material classified as radioactive waste, thereby reducing disposal costs.
The successful application of FREMES in several large-scale decommissioning projects, including those involving uranium contamination, demonstrates its effectiveness and reliability. Indeed, earlier this year NUKEM was commissioned by the Swiss nuclear power plant Mühleberg to conduct a feasibility study to assess the use of the FREMES.
The feasibility study will provide critical documentation to support Mühleberg during the qualification of the proposed method by the Swiss Federal Nuclear Safety Inspectorate (ENSI).
FREMES offers a robust solution for the decommissioning of nuclear facilities, combining advanced measurement technology with automated sorting to enhance efficiency, reduce costs, and ensure regulatory compliance. Its inherent flexibility and adaptability make it a valuable tool for addressing the diverse challenges associated with the remediation of concrete and building materials in nuclear installations.
