The LINAC@LNK project

The first objective, radioactive waste characterization is an important and high-priority societal matter, as radioactive waste has to be characterized before it can be safely disposed of. In Belgium, ONDRAF/NIRAS is responsible for the management of radioactive waste coming from industry, medicine, research and nuclear power plants. One of their responsibilities is the establishment and operation of a near-surface disposal facility for category A waste, which comprehends low- and intermediate level short-lived radioactive waste. Prior to disposal, radioactive waste needs to be processed and conditioned into a solid, compact and chemically stable end product. It is conditioned in 400-L drums that contain both the waste form and any container and internal barriers, as absorbing and liner materials, which makes it hard to characterize. Currently, all drums are temporary stored at Belgoprocess, pending final disposal. Since these drums have been stored there for decades, it is extremely important to verify whether they are compliant with the category A waste conformity criteria.

Identification of the waste drum content and verification of its materials and internal structure are fundamental requirements to ensure a safe and final disposal. Compliance verification is performed by means of both destructive and non-destructive analytical methods. However, it is often not possible to verify a large number of waste drums since only expensive, time-consuming, destructive testing might be available for the necessary controls. To increase the systematic quality control, different installations have established non-destructive testing methods. Non-destructive testing methods are used in order to minimize the radiation dose to personnel, to avoid secondary radioactive waste production
and to minimize costs. With this method, the extent of the destruction of a waste drum can be reduced to the minimum. Furthermore, with destructive testing, there is always the essential question of taking a representative sample, especially for heterogeneous waste. In view of this, one of the main needs for the compliance verification of low-level radioactive waste is the development of new non-destructive methods especially for the verification of the compliance of conditioned (high-density) waste packages.

A number of non-destructive methods for quality and compliance checking of radioactive waste packages are currently being developed and tested. The EU-funded project CHANCE (Characterization of conditioned nuclear waste for its safe disposal in Europe) aims at identifying characterization methods across Europe and developing instrumentation for muon tomography, which is suited for large volume radioactive waste packages. The French Alternative Energies and Atomic Energy Commission (CEA) has set up a unique state-of-the art facility that uses megavolt-computed tomography (MV-CT) for the evaluation of waste packages.

MV-CT is an advanced imaging technique, suitable for conditioned waste inspection that provides crosssectional visualization of the waste packages. Several properties can be verified using this method, such as the position and shape of the different radioactive waste components, their homogeneity, density, the quality of immobilization matrices, the presence of voids, cracks, internal shields, metal structures, liquids, containers, etc. Medical CT scanners use energies high enough to traverse humans but incompatible with dense waste packages. Medical LINACs on the other hand, produce photons with high intensity and energy, suitable for this application. However, the use of MV-CT is currently not possible in Belgium due to a lack of expert facilities. LINAC@LNK proposes an innovative and multi-disciplinary approach to characterize waste packages from ONDRAF/NIRAS using MV-CT.

Beyond the provision of novel services for non-destructive waste characterization, LINAC@LNK proposes a state-of-the-art approach to another fundamental societal matter, our third objective, radiotherapy dosimetry. Cancer is the second leading cause of death worldwide, responsible for one in six fatalities, according to the World Health Organization (WHO). In 2020, there were over 18M cancer patients worldwide, of which 2.7M were new cases. It is estimated that 50 to 60% of the oncology patients undergo radiotherapy as a part of their treatment aiming to irradiate tumours with ionizing radiation and kill cancer cells while safeguarding healthy tissues. The latter, especially near the tumour, unavoidably receive a part of the radiation dose and may suffer damage, which can lead to side effects. The treatment outcome, i.e. from cure, permanent side effects to death, strongly relates to the delivered dose of ionizing radiation, hence it is paramount to ensure efficacy on malignant tissues and low toxicity on healthy ones, to eradicate cancer cells and to minimize the risk of severe side effects. The most fundamental step to achieve successful radiotherapy is to demonstrate routinely and consistently that the radiation dose planned for the treatment and the radiation dose actually received by the patient coincide to the maximum possible extent.

Radiation dosimetry refers to the measurement, calculation and assessment of the ionising radiation dose absorbed by the human body. The International Commission on Radiation Units and Measurements (ICRU) recommends an overall accuracy in radiotherapy dose delivery of 5%, which can be complex to achieve when all the uncertainties are considered.

LNK is the designated metrology laboratory in Belgium for the ionizing radiations, sub-field of dosimetry, and performs secondary standard calibrations and irradiations with ionizing radiations following the requirements of ISO-17025 quality assurance (QA) standard. LNK is a member of the Belgian Metrology Network (BELMET), of the European Association of National Metrology Institutes (EURAMET), a collaborative alliance of national metrological organizations from member states of the European Union (EU) and of the European Free Trade Association (EFTA) with the purpose of achieving higher efficiency by coordinating and sharing metrological activities and services. LNK is also member of the International Atomic Energy Agency (IAEA)/WHO Network of Secondary Standards Dosimetry Laboratories (SSDL Network) and has participated in multiple EU projects and initiatives to disseminate dosimetry practices.

At the LNK, calibration coefficients are determined using a Cobalt-60 gamma radiation source. However, clinical external beam radiotherapy is mostly delivered with megavolt (MV) photon or electron beams produced in medical LINACs, which have different characteristics. When an ionization chamber (IC) is used in a radiation beam of a quality different from the one of its own calibration, a beam-quality correction factor is necessary; adding to the uncertainties on measurements and consequently to patient dose. Ideally, the beam-quality correction factor should be directly measured for each dosimeter at the same quality as the clinical beam. However, this  currently is neither achievable at the LNK nor in most calibration facilities worldwide, as the laboratories do not have a LINAC to create the appropriate range of beam qualities used in radiotherapy.

A state of the art solution is to use a medical-grade LINAC for calibrations to ensure that radiotherapy equipment is properly calibrated for accurate, reliable and effective cancer treatment. Only a few installations around the world have this capacity being the National Physical Laboratory (NPL), United Kingdom (UK), the National Metrology Institute of Germany, PTB, the French National Metrology Laboratory for Ionizing Radiation, LNHB, and the laboratories in Austria of IAEA. In Belgium, such calibration capabilities are currently missing.