Screening soil24 October 2018
A free release measurement system has been used to characterise and sort soil during remediation of a redundant nuclear fuel factory in Belgium. Felix Langer, Jörg Feinhals and Marina Sokcic-Kostic give an overview of the project.
WHEN DECOMMISSIONING BUILDINGS AND RELEASING areas of a site for re-use, large amounts of bulk materials have to be measured for contamination and cleared. These measurements may be required, as soil may be contaminated with nuclear fuel or activated material, or because the bulk waste needs to be minimised before disposal as nuclear waste.
NUKEM has completed a project at FBFC International’s former fuel element factory at Dessel, Belgium as part of its decommissioning. The purpose was to measure and check for potential contamination in the soil.
First the site and adjacent areas were pre-screened and, based on the results, an excavation plan was generated and a few thousand tonnes of soil were prepared for further processing. After this screening, the material handled by NUKEM’s Free Release Measurement System (Fremes) is not expected to consist of highly contaminated soil. Nevertheless, potentially suspicious soil is excavated and analysed to verify the absence of contamination. Using Fremes, all suspicious material is characterised, the amount of nuclear waste to be stored is minimised and all the information obtained is recorded. DMT provides experience in bulk material handling, optimising the machinery and solutions used.
Fremes performs the task of radiological characterisation and sorting of suspicious soil. It was designed using NUKEM’s experience from remediation of its own nuclear fuel factory in Hanau, Germany, where a similar system was used. The design has been updated and is optimised for intermediate volumes, easy transport and quick installation. Now Fremes consists of three containers. One houses the measurement equipment, one the sorting mechanism and the third container is the office, containing electronics, controls and the operator workstation.
Material is sorted based on a comparison of the mass specific activity with two thresholds, for a given nuclide vector. The key nuclide of the vector is uranium-235, where gamma emissions at 185.7keV can be detected up to a specific activity of 20Bq/kg at maximum throughput.
The total activity derived via the nuclide vector is then compared to two thresholds. If the material is below 1Bq/g U-total, it is considered exempt material. If the material is between 1Bq/g and 10Bq/g, it is stored for subsequent handling, and if the material is above 10Bq/g, it is classified as nuclear waste.
The material is transported to Fremes via an array of conveyors and a drum sieve. The soil is provided by trucks and fed into a larger feeder hopper to buffer the discontinuous material stream into a continuous stream. A sieve prevents larger objects from cluttering the conveyor belts and sorts them out. After screening by Fremes, conveyor belts transport the characterised and sorted material to its destination.
The soil to be characterised is buffered, and shaped into a form as close to the MCNP-simulated measurement geometry as possible. The radiological characterisation is performed using two collimated HPGe detectors. Directly measured U-235 is combined with data on the self- absorption of the soil. The material is also checked for its homogeneity of mass-specific activity and checked for the presence of hot spots.
After characterisation, the material is classified and directed by the sorting system into three streams according to its specific activity of uranium. The material streams are then handed over to belt conveyors, and directed onto the piles or into the containers. Contaminated material is directly filled into Big Bags.
Representative samples are taken from the material streams continuously, to be able to compare Fremes’s results with external analyses. The sampled material is filled into 200-litre drums, which are characterised by an ISOCS system. The results of ISOCS and Fremes are compared, and the difference is within an acceptable margin, the results are approved.
Fremes has just one operator, whose main responsibility is materials handling and handling of characterised material. Characterisation and sorting is monitored by the operator, minimising human error during operation.
Extensive commissioning of the measurement and automation has been performed to demonstrate the required precision and reliability of the results. Prepared contaminated material characterised by Fremes was found to be in accordance with external measurements, so it became an approved process for exempting bulk material.
All tests were accompanied and overseen by FBFC International’s experts. Each step was discussed, performed in the presence of experts and documented. Minor modifications have been made to improve the general performance of the system, reduce potential downtime and increase the safety of operation.
The Belgian authorities were kept informed during the process. During major tests, the authorities were present and attended.
Authority-related activities were performed by FBFC International. Periodical meetings with all involved participants ensured distribution of all necessary information. Several modifications and improvements were direct results of fruitful discussions with technicians, experts and managers. Workshops on site provided quick solutions to issues arising during the commissioning phase. FBFC International’s management of other companies and parties on site resulted in efficient provision of other needs, such as larger amounts of sand or operators.
Based on the experience gathered, different versions of the system can be adapted for similar uses. If required, the throughput can be increased to up to 100t/h per system. The modular design of the measurement system carrier can be used to add different measurement systems, such as alpha or beta detectors or systems for detecting organic and toxic materials. The automatisation and evaluation software is adaptable to different applications. All of the main components are rugged for construction site toughness or are encapsulated in hermetic and temperature controlled casings. The system can be adapted for various decommissioning activities or to minimise the amount of contaminated material by extracting the non-contaminated part, therefore reducing subsequent handling efforts.
NUKEM’s experience in decommissioning projects such as its fuel element factory in Hanau, proved invaluable in providing the customer with a suitable solution. The project management, technical expertise and philosophies of all the parties are working together. The design of the system provided was adjusted and optimised to provide the most suitable solution. Technical difficulties have been encountered and overcome during the commissioning, underlining the challenge and complexity of the undertaking.
The project on FBFC International site started with industrial characterisation of suspicious material in February 2018.
Author information: Felix Langer, Project manager, operations at NUKEM Technologies Engineering Services GmbH; Jörg Feinhals, Head of the radiation protection and radwaste unit at DMT GmbH & Co. KG; Marina Sokcic-Kostic, Principal engineer I&C, electrical systems, and radiation monitoring systems at NUKEM Technologies Engineering Services GmbH
Dessel: a history
The FBFC International site produced components and nuclear fuel assemblies for the Western European reactor market from 1974 to April 2012. In 1975 and 1976, prior to the commissioning of the Romans plant in France, Dessel manufactured the first cores of the first two French pressurised water reactors Fessenheim 1&2. In 1987, the industrial production of MOX fuel began using rods originating first from Belgonucléaire, then from Cadarache, then MELOX.
In April 2012 production of uranium fuel assemblies was terminated and operations began to dismantle the workshops. Disassembly of the production equipment was carried out from 2013-2014 and from 2015-2016 cleanup of buildings 1, 2, 3 and 5 was completed. July 2016 saw the start of inspections at buildings 1 and 2 to check cleanup operations had eliminated traces of contamination.
In October 2017, the Belgian Federal Agency for Nuclear Control gave its approval for the demolition of buildings 1 and 2 at the FBFC International plant in Dessel.