La maîtrise des risques : une approche indispensable dans le développement des études de tunnels en terrains diificiles

L’étude et la construction de tunnels longs et profonds et de tunnels en milieu urbain sont souvent associées au risque dérivant de la non conformité des informations géotechniques, d’un choix non adéquat de la méthodologie de construction et des incidents potentiels pendant la construction. La gestion de la plupart des risques peut avoir lieu à travers l’utilisation d’un Plan de gestion du risque (Risk Management Plan, RMP). Le RMP identifie et quantifie les risques et les problèmes, en sélectionnant et mettant en action les mesures pour mitiger et contrôler les risques et indique s’il existe un risque résiduel qui devra être partagé entre les sujets impliqués dans le projet. Le système d’aide à la décision pour les tunnels DAT (Decision Aids for Tunnelling), logiciel de support au processus de décision dans l’étude des tunnels et partie intégrante de l’analyse des risques, est utilisé pour simuler, de façon probabiliste, le processus de la construction en souterrain. Les résultats du DAT illustrent les effets des incertitudes liées à la géologie, à la méthode de construction, etc., en termes de temps et coûts pour les différentes solutions alternatives. La discussion du processus de développement d’un RMP est suivie par l’application des concepts de RMP à deux projets pris comme exemple : 1) le tunnel de base Maurienne-Ambin faisant partie de la liaison ferroviaire Lyon-Turin et 2) les tunnels des lignes C et S du métro de Porto au Portugal. Les résultats démontrent que le processus de gestion du risque est un instrument d’ingénierie pratique qui peut être appliqué aux études de tunnels garantissant des résultats satisfaisants et des bénéfices en ce qui concerne temps, coûts et sécurité

Pubblica Amministrazione. Introduzione.

Il testo introduce le principali tematiche attualmente in dicussione relative alla pubblica amministrazione italiana

Equivalence of pure propane and propane-TE gases for Tissue microdosimetric measurements

A tissue-equivalent proportional counter (TEPC) simulates micrometric volumes of tissue, if the energy deposited in the counter cavity is the same as that in the tissue volume. Nevertheless, a TEPC measures only the ionizations created in the gas, which are later converted into imparted energy. Therefore, the equivalence of the simulated diameter in two gases should be based on the equality of the mean number of ions pairs in the gas rather than on the imparted energy. Propane-based tissue equivalent gas is the most commonly used gas mixture at present but it has the drawback that its composition may change with time. From this point of view, the use of pure propane offers practical advantages: higher gas gain and longer stability. In this work, microdosimetric measurements performed with pure propane, at site sizes between 0.05 mg/cm2 and 0.3 mg/cm2, demonstrate that the response of a propane-filled detector in gamma and in neutron fields is almost the same if an appropriate gas density is used

MICRODOSIMETRIC SIMULATIONS OF CARBON IONS USING THE MONTE CARLO CODE FLUKA

Therapeutic carbon ion beams produce a complex and variable radiation field that changes along the penetration depth due to the high density of energy loss along the particle track together with the secondary particles produced by nuclear fragmentation reactions. An accurate physical characterisation of such complex mixed-radiation fields can be performed by measuring microdosimetric spectra with mini tissue-equivalent proportional counters (mini-TEPCs), which are one of the most accurate devices used in experimental microdosimetry. Numerical calculations with Monte Carlo codes such as FLUKA can be used to supplement experimental microdosimetric measurements performed with TEPCs, but the nuclear cross sections and fragmentation models need to be benchmarked with experimental data for different energies and scenarios. The aim of this work is to compare experimental carbon microdosimetric data measured with the mini TEPC with calculated microdosimetry spectra obtained with FLUKA for 12C ions of 189.5 MeV/u in the Bragg peak region

Mini-TEPC Microdosimetric Study of Carbon Ion Therapeutic Beams at CNAO

Mono-energetic carbon ion scanning beams of 195.2 MeV/u at the Italian National Centre for Oncological Hadrontherapy (CNAO) have been used to study the microdosimetric features of an “active” carbon ion beam used in hadrontherapy. A 30x30 mm2 area has been scanned by a 6 mm beam with scanning steps of 2 mm. A mini TEPC of 0.57 mm3 has been used to perform measurements in a water phantom at different depths on the beam axis. The detector small size allowed for measuring, with good spatial resolution, also inside the relatively small Bragg peak region and inside the distal edge, where the radiation quality varies quickly. In spite of the high event rate (up to ~ 105 s-1), no pile-up effects were observed. Results showed that the frequency-mean lineal energy scaled well with the absorbed dose. Moreover, the dose-mean lineal energy itself seemed to be a good descriptor of the radiation quality

Mini-TEPC Microdosimetric Study of Carbon Ion Therapeutic Beams at CNAO

Mono-energetic carbon ion scanning beams of 195.2 MeV/u at the Italian National Centre for Oncological Hadrontherapy (CNAO) have been used to study the microdosimetric features of an “active” carbon ion beam used in hadrontherapy. A 30x30 mm2 area has been scanned by a 6 mm beam with scanning steps of 2 mm. A mini TEPC of 0.57 mm3 has been used to perform measurements in a water phantom at different depths on the beam axis. The detector small size allowed for measuring, with good spatial resolution, also inside the relatively small Bragg peak region and inside the distal edge, where the radiation quality varies quickly. In spite of the high event rate (up to ~ 105 s-1), no pile-up effects were observed. Results showed that the frequency-mean lineal energy scaled well with the absorbed dose. Moreover, the dose-mean lineal energy itself seemed to be a good descriptor of the radiation quality

Microdosimetric measurements in the thermal neutron irradiation facility of LENA reactor

A twin TEPC with electric-field guard tubes has been constructed to be used to characterize the BNCT field of the irradiation facility of LENA reactor. One of the two mini TEPC was doped with 50 ppm of 10B in order to simulate the BNC events occurring in BNCT. By properly processing the two microdosimetric spectra, the gamma, neutron and BNC spectral components can be derived with good precision (~6%). However, direct measurements of 10B in some doped plastic samples, which were used for constructing the cathode walls, point out the scarce accuracy of the nominal 10B concentration value. The influence of the Boral® door, which closes the irradiation channel, has been measured. The gamma dose increases significantly (+ 51%) when the Boral® door is closed. The crypt-cell-regeneration weighting function has been used to measure the quality, namely the RBEμ value, of the radiation field in different conditions. The measured RBEμ values are only partially consistent with the RBE values of other BNCT facilities