In-toto non-destructive assay methodology for the elimination of metallic waste from particle accelerators after melting.

Melting of metallic waste reduces the waste volume, allows more accurate radiological characterization, and minimizes handling at the waste production site. This paper proposes a new non-destructive assay methodology to radiologically characterize low- and intermediate-level (LILW) waste before melting. A non-destructive assay technique is developed and qualified using geometry optimization technique and sample analysis after melting. Additionally, we present an operational methodology to predict the activity values of the major gamma emitters based on the average dose rate measurements.

Radiological characterisation for the clearance of burnable waste produced at CERN.

Burnable waste produced at CERN during upgrading, maintenance and dismantling campaigns may be contaminated with radioactive nuclides produced through activation of accelerator components. Here, we present a methodology for the radiological characterisation of burnable waste, which takes into account the wide range of potential activation conditions (beam energy, material composition, location, irradiation and waiting time). Waste packages are measured using a total gamma counter, with the sum of clearance limit fractions estimated using the fingerprint method. Gamma spectroscopy was found to be unsuitable for classifying this waste due to the long counting times required to identify many expected nuclides, but was retained for quality control purposes. Using this methodology, a pilot campaign was performed in which we were able to clear 13 m3 of burnable waste as conventional non-radioactive waste.

Qualification of the activities measured by gamma spectrometry on unitary items of intermediate-level radioactive waste from particle accelerators.

In the frame of maintenance, upgrade and dismantling activities, activated equipment are removed from the accelerator complex and require characterization in view of their disposal as radioactive waste. The characterization process consists of a series of radiation measurements, complemented by analytical studies, which quantify the activity of radionuclides inside an object. A fraction of the radioactive waste produced at CERN presents contact dose-rates higher than 100 µSv/h, and can therefore be classified as LILW Waste ("Low and intermediate level radioactive waste"). These objects, due to the activation mechanisms, are often subject to large activity heterogeneities. The quantification of gamma-emitting radionuclides is typically performed by gamma spectrometry, under the assumption of homogeneous distributions of activity within an object. However, this assumption can lead to underestimating the activity value of such radionuclides. In this article we perform a gamma spectrometry qualification in order to quantify the impact of assuming homogenous distribution.

Qualification of gamma spectrometry measurement for the radiological characterization of mixed VLLW cables in particle accelerators.

In the framework of maintenance activities in particle accelerators, such as upgrades and dismantling, a large number of activated equipment are removed from the accelerator complex and require characterization in view of their disposal as radioactive waste. In particular, cables can be of different types. This feature induces variations of the efficiency calibration curves due to the variation of the material composition, source distribution and density. Hence, quantifying the activities of the gamma-emitting radionuclides can be quite challenging for mixed cables. In this article, we propose a new qualification methodology, based on gamma spectrometry, in order to assess the activity results uncertainties of gamma-emitting radionuclides. This new methodology is developed to define the envelop efficiency calibration curves and allows for the establishment of more accurate activity values with their corresponding uncertainties.

An enhanced characterization process for the elimination of very low level radioactive waste in particle accelerators.

The elimination of very low level waste towards the French national repository requires their radiological characterization to estimate the radionuclide inventory and the associated activities within a waste package. Such characterization is performed by means of activation calculations and measurements. Two elimination projects have been identified at CERN, to dispose of bulk metallic waste and cables activated in the CERN accelerator complex. Based on the experience gained over the last 4 years, we develop a large scale elimination process to dispose of such types of activated equipment. A program for quality controls has therefore been developed through a novel software tool whose purpose is to compute the radiological data required by the repository for the acceptance of the waste as well as performing quality controls.

Classification of radiological objects at the exit of accelerators with a dose-rate constraint.

Maintenance activities and operations of high-energy particle accelerators can lead to the collection of radioactive equipment as well as waste materials. In order to ensure their proper classification as radioactive or non-radioactive, one has to quantify the activities of radionuclides produced. According to the regulatory requirements in Switzerland, these activities need to be compared with nuclide-specific clearance limits. In particular, a new set of clearance limits was introduced by the Swiss authorities in January 2018, leading to more conservative values for a number of relevant radionuclides. We describe in this paper a new methodology based on dose-rate measurements to classify potentially radioactive objects at the exit of the CERN accelerator complex. This methodology concerns the specific material compositions typically found at CERN and takes into account the latest clearance limits introduced by the Swiss authorities.

Generation of low-energy neutrons cross-sections for the Monte Carlo code FLUKA and the deterministic code ActiWiz.

Activation of material is of interest for waste treatment and hazard assessment. In particular, activation of printed circuit can lead to the production of radionuclides at an isomeric state, for example, coming from silver. In particle accelerators, the production of silver isomeric states mainly come from low energy neutrons, below 20 MeV. The quantification of activation and associated doses at CERN is based on the FLUKA and ActiWiz codes. In the FLUKA release 2011.2c, all branching ratios for isomer production were set at 50% by default. The present work provides a set of nuclide- and energy-dependent branching ratios, extracted from the library EAF-2010. In the ActiWiz release 3.3, the library JEFF3.1.1 was used for low energy neutron cross-sections. This study provides a new set of neutron cross-sections extracted from JEFF3.3, ENDFB/VIII.0 and EAF-2010 for future update of ActiWiz.

A Bayesian framework to update scaling factors for radioactive waste characterization.

Nuclear power plants and research facilities commonly employ the so-called scaling factor (SF) method to quantify the activity of difficult-to-measure (DTM) radionuclides within their radioactive waste packages. The method relies on the establishment of a relationship between an easy-to-measure (ETM) radionuclide, called key nuclide (KN), and difficult-to-measure radionuclides, after the collection of a representative sample from the waste population. The distribution of the scaling factors, as well as the parameters defining the distribution, can change over time. Therefore, the accuracy of the calculated activity of the DTM radionuclides depends on the capacity of the scaling factor method to follow the time evolution of the waste population. In practice, waste producers collect periodically new samples from the waste population and check the variation and the validity of the scaling factors. In this article, we present a simple Bayesian framework to update scaling factors when a new data set becomes available. The method is tested and validated for radioactive waste produced at CERN (European Organization for Nuclear Research) and can be easily implemented for waste of different origin.

Spectrum and Yield to Dose Conversion Coefficients for Beta Skin Doses Linked to the Q System.

Monte Carlo simulations are a state-of-the-art method to calculate dose coefficients and could be used with the Q system for radioactive material packaging. These simulations often take a long time to converge with sufficient precision. Furthermore, if multiple sources have to be taken into account, many weeks of calculations may be needed. In order to reduce the calculation time, this paper proposes a new method based on a transfer function to instantly compute Q values associated with beta skin doses. The method developed in this paper can be applied to compute beta skin dose and easily could be extended to other particles and different depths in organs with various kinds of shielding configurations between source and target.

A new approach to characterize very-low-level radioactive waste produced at hadron accelerators.

Radioactive waste is produced as a consequence of preventive and corrective maintenance during the operation of high-energy particle accelerators or associated dismantling campaigns. Their radiological characterization must be performed to ensure an appropriate disposal in the disposal facilities. The radiological characterization of waste includes the establishment of the list of produced radionuclides, called "radionuclide inventory", and the estimation of their activity. The present paper describes the process adopted at CERN to characterize very-low-level radioactive waste with a focus on activated metals. The characterization method consists of measuring and estimating the activity of produced radionuclides either by experimental methods or statistical and numerical approaches. We adapted the so-called Scaling Factor (SF) and Correlation Factor (CF) techniques to the needs of hadron accelerators, and applied them to very-low-level metallic waste produced at CERN. For each type of metal we calculated the radionuclide inventory and identified the radionuclides that most contribute to hazard factors. The methodology proposed is of general validity, can be extended to other activated materials and can be used for the characterization of waste produced in particle accelerators and research centres, where the activation mechanisms are comparable to the ones occurring at CERN.

Radiological characterization of printed circuit boards for future elimination.

Electronic components like printed circuit boards (PCBs) are commonly used in CERN's accelerator complex. During their lifetime some of these PCBs are exposed to a radiation field of protons, neutrons and pions and are activated. In view of their disposal towards the appropriate final repository, a radiological characterization must be performed. The present work proposes a general characterization procedure based on the definition of a reference chemical composition, on the calculation of the corresponding radionuclide inventory and on the measurement of a tracer radionuclide. This method has been validated with real-life cases of electronic boards which were exposed to the typical radiation fields in CERN's accelerators. The activation studies demonstrate that silver is the key element with respect to the radiological characterization of electronic waste due to the production of Ag-110m and Ag-108m. A sensitivity analysis shows that the waiting time is the main parameter affecting the radionuclide inventory. Results also indicate that, as is the case of other families of radioactive waste, an accurate assessment of the radiological inventory of PCBs would require the precise knowledge of their chemical composition, as well as the radiation field to which they were exposed.

Radiation protection challenges in the management of radioactive waste from high-energy accelerators.

The European Laboratory for Particle Physics (CERN) has operated high-energy accelerators for fundamental physics research for nearly 60 y. The side-product of this activity is the radioactive waste, which is mainly generated as a result of preventive and corrective maintenance, upgrading activities and the dismantling of experiments or accelerator facilities. Prior to treatment and disposal, it is common practice to temporarily store radioactive waste on CERN's premises and it is a legal requirement that these storage facilities are safe and secure. Waste treatment typically includes sorting, segregation, volume and size reduction and packaging, which will depend on the type of component, its chemical composition, residual activity and possible surface contamination. At CERN, these activities are performed in a dedicated waste treatment centre under the supervision of the Radiation Protection Group. This paper gives an overview of the radiation protection challenges in the conception of a temporary storage and treatment centre for radioactive waste in an accelerator facility, based on the experience gained at CERN. The CERN approach consists of the classification of waste items into 'families' with similar radiological and physical-chemical properties. This classification allows the use of specific, family-dependent techniques for radiological characterisation and treatment, which are simultaneously efficient and compliant with best practices in radiation protection. The storage was planned on the basis of radiological and other possible hazards such as toxicity, pollution and fire load. Examples are given of technical choices for the treatment and radiological characterisation of selected waste families, which could be of interest to other accelerator facilities.

Shielding of proton accelerators.

This paper discusses some of the methods that can be employed for calculating shielding of proton accelerators, showing that a simple analytical model is often useful for a first estimate before going into complex Monte Carlo simulations. In particular what we call the Monte Carlo 'hybrid' approach, which employs source terms and attenuation length data calculated by Monte Carlo simulations under generic geometrical conditions, with a point-source line-of-sight model is discussed. Examples are given of the application of this method to the shielding calculations of two versions of the CERN SPL (2- and 3.5-GeV energy), comparing its results with Monte Carlo simulations of the full geometry.

Radioactive waste management and decommissioning of accelerator facilities.

During the operation of high-energy accelerators, the interaction of radiation with matter can lead to the activation of the machine components and of the surrounding infrastructures. As a result of maintenance operation and during decommissioning of the installation, considerable amounts of radioactive waste are evacuated and shall be managed according to the radiation-protection legislation. This paper gives an overview of the current practices in radioactive waste management and decommissioning of accelerators.

Shielding design for the front end of the CERN SPL.

CERN is designing a 2.2-GeV Superconducting Proton Linac (SPL) with a beam power of 4 MW, to be used for the production of a neutrino superbeam. The SPL front end will initially accelerate 2 x 10(14) negative hydrogen ions per second up to an energy of 120 MeV. The FLUKA Monte Carlo code was employed for shielding design. The proposed shielding is a combined iron-concrete structure, which also takes into consideration the required RF wave-guide ducts and access labyrinths to the machine. Two beam-loss scenarios were investigated: (1) constant beam loss of 1 Wm(-1) over the whole accelerator length and (2) full beam loss occurring at various locations. A comparison with results based on simplified approaches is also presented.