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[en] In the last years the Multileaf Collimator (MLC) has been developed for some linear accelerators; it allows to conform the target volume in an easier way with respect to the beam shaping blocks. Recently, a linear accelerator Varian 2100C/D, provided with the MLC, has been installed in our Institute. In order to be able to use this accessory in the clinical practice, a work project has been elaborated: we were interested in the knowledge of the absorbed dose per M.U. at the treatment depth, in different geometrical configurations, for any MLC field, positioned in an arbitrary zone of the irradiating field. The work we carried out could be divided schematically in three points: 1) A theoretical model for dose calculation: Starting from a model which calculates the dose values for irregularly shaped fields, an algorithm specific for the MLC has been elaborated. The formula we thought is ideally divided in two components: the first takes into account the dose variation due to the primary collimators, which usually define an asymmetric field with respect to the central axis; the last component of the formula introduces some factors which take into account of the changes produced by the MLC. 2) MLC dosimetry: Specific measurements for the MLC have been performed. Their aim is to display the effect of the MLC on the dose delivered per M.U.. In particular we studied both the scattering towards the measurement point and towards the monitor chambers and the influence of the primary collimating system on the scattered radiation. We also investigated the effect of the reduction of the scattering volume due to the MLC insertion in the irradiation field. 3) Dosimetric data elaboration: The experimental measurements were elaborated trying to emphasize the impact of the measured quantities on the dose delivered per M.U.. Moreover it was necessary, in order to modify as less as possible the algorithms in use before the introduction of the MLC, to refer the different parameters, specific of the new configuration, to the standard geometrical one used for the linac calibration
Journal
[en] Dynamic wedge technique consists in the generation and the delivery of customized wedge-shaped dose distributions using computer controlled dose rate and collimator motion, in such a way that no external beam modifier is used to shape the wedged field. Physical wedges have a number of adverse practical and dosimetric attributes. These problems can be removed with the introduction of the Dynamic Wedge. Commissioning of the Dynamic Wedge requires the development of new data acquisition techniques, since the traditional method of scanning across a water tank with a single dosimeter is not valid for the Dynamic Wedge. The bulk of the data was obtained using film exposed in a water phantom, supported by integrated ionization chamber measurements at appropriate points. This study describes the amount of work required prior to clinical application of a Dynamic Wedge System (Dynamic Wedge transmission factors, effects on depth dose, isodose profiles, ...). In particular, we will make reference to the 6MV photon beam and Dynamic Wedge 60 deg. generated by the Varian 2100C/D linear accelerator (in fact we found that the Dynamic Wedge 60 deg. has the roughest behaviour). Finally, the effect of the backscattering radiation towards the monitor chambers, during the movement of the collimator jaw across the radiation field, will be examined
Journal
[en] As a part of the Italian Project for an Organic Moderated Nuclear Power Plant, the National Commission for Nuclear Energy (CNEN) has installed a critical facility, known as ROSPO, planned to perform criticality studies of the nuclear characteristics of cores to be tested in a 60 MW(t) prototype reactor (PRO). The first core loading consists of MTR-type fuel elements, with flat, 90% enriched, stainless-steel clad plates; the main physical question to be answered concerns the possibility of obtaining, with this core, a proper reactivity excess, in order to compensate for temperature and poisoning effects, and to ensureta convenient lifetime to the power reactor. The variable parameter is the uranium/stainless steel ratio in the fuel element, and the experiments are performed by studying cores where some of the 16 active plates of each fuel element are substituted by dummy (stainless steel) plates. For three of these cores (0,3 and 5 dummy plates per element respectively), detailed calculations have been carried out to obtain flux plots, reactivity effects and keff versus number of elements. The calculation was based upon a modified two-group theory that accounts for fissions in the epithermal zone. The following codes were used: THESIS for thennal, and MUFT 4 for epithermal spectra; FLIP 1 (transport theory) and TUT-T5 (Monte Carlo) for fluxes in the unit cell; PDQ-O2 coupled to WANDA 4 (diffusion theory in xy and slab geometries) for reactivities and macroscopic fluxes. The results are shown and compared with those obtained for the same cores by AGIP Nucleare and by the CNEN Bologna Computation Centre; in spite of the analogy of the methods used, large discrepancies were observed, which are thought to derive from differences in the libraries of cross-sections. This seems to confirm the necessity of critical experiments as the most valid help in choosing and checking calculation methods, nuclear codes and libraries. The measurement of keff versus the number of fuel elements, performed for the three meritiohed uranium/stainless steel ratios in ROSPO allows a detailed analysis of these problems. (author)
[fr]
Dans le cadre d'un projet de réacteur de puissance ralenti par un fluide organique, la Commission italienne de l'énergie atomique a installé un ensemble critique désigne par les lettres ROSTO; cet ensemble doit permettre d’étudier les caractéristiques nucléaires de coeurs qui doivent être mis à l'essai dans le prototype de réacteur de 60 MW(t) (PRO). La première charge du coeur consiste en éléments combustibles qui sont du type employé dans les réacteurs d'essai des matériaux, et se compose de plaques d'uranium enrichi à 90%, gainées d'acier inoxydable; au point de vue de la physique des réacteurs, la principale question est celle de savoir si ce coeur permet d'obtenir un excédent de réactivité suffisant pour neutraliser les effets de température et d'empoisonnement et pour garantir une durée normale de fonctionnement de la génératrice nucléaire. Le paramètre variable est le rapport uranium/acier inoxydable de l'élément combustible; aux fins des expériences, on remplace plusieurs des 16 plaques actives composant chaque élément du coeur par des plaques factices fén acier inoxydable). Trois de ces coeurs (zéro, trois et cinq plaques factices par élément combustible,, respectivement) ont fait l’objet de calculs détaillés en vue de déterminer les courbes du flux, les effets sur la réactivité et keff en fonction du nombre d'éléments. Ces calculs sont fondés sur une théorie à deux groupes modifiée dè'manière à tenir compte des fissions dans la zone épithermique. A cet effet, on s'est servi des codes suivants: THESIS’, pour le spectre des neutrons thermiques, et MUFT 4 pour celui des neutrons épithermiques; FLIP 1 (théorie du transport) et TUT-T5 (Monte-Carlo) pour les flux dans la cellule; PDQ-02 combiné avec WANDA 4 (theórie de la diffusion dans les geometries xy et les géométries de plaque pour les réactivités et les flux macroscopiques. L’esauteurs indiquent les résultats et les comparent à ceux qui ont été obtenus par ces mêmes coeurs par AGIP Nucleare et par le Centre de calcul électronique de la Commission italienne de l'énergie atomique à Bologne; malgré l'analogie des méthodes employées, on a observé des écarts importants qui'sont probablement dus à dès différences dans les bibliothèques de sections efficaces. Ces constatations semblent confirmer que les expériences critiques sont indispensables du fait qu'elles permettent mieux que tout autre moyen de choisir et de vérifier les méthodes de calcul, les codes nucléaires et les bibliothèques. Gráce aux mesures de keff en fonction du nombre d'éléments combustibles qui ont été faites dans ROSPO pour les trois rapports uranium/acier inoxydable mentionnés, il est possible de soumettre ces problèmes à une analyse détaillée. (author)[es]
Como parte del proyecto italiano de una central nucleoeléctrica con reactor de moderador orgánico, el Comitato Nazionale per l'Energia Nucleare ha construido la instalación critica ROSPO, destinada a la realización de estudios de las características nucleares, en el estado crítico, de cuerpos de reactor que luego se ensayarán en un reactor prototipo de 60 MW(t), el PRO. La primera carga del cuerpo consta de elementos combustibles del tipo MTR (reactor de ensayo de materiales), con placas planas, enriquecidas al 90 por ciento y revestidas de acero inoxidable; la principal incógnita física se refiere a la posibilidad de obtener, con dicho cuerpo, un exceso de reactividad apropiado, a fin de compensar los efectos de temperatura y de envenenamiento y de asegurar que el reactor de potencia tenga una duración aceptable. El parámetro variable es la relación uranio/acero inoxidable en el elemento combustible; los experimentos se ajecutan estudiando cuerpos en los que algunas de las 16 placas activas de que consta cada elemento combustible se reemplazan con placas simuladas (de acero inoxidable). Para tres de estos cuerpos (de 3 y 5 placas simuladas por elemento, respectivamente) se efectuaron cálculos detallados a fin de obtener diagramas de flujo, de los efectos de reactividad y de kef en función del número de elementos. El cálculo se basó en una teoría de dos grupos modificada, que tiene en cuenta las fisiones en la zona epitér- mica. Se utilizaron las siguientes claves: THESIS para los espectros térmicos, y MUFT 4 para los epitérmiros; FLIP 1 (teoría del transporte) y TUT-T5 (Monte Cario) para los flujos en la celda unitaria; PDQ-O2 combinada con WANDA 4 (teoría de la difusión en geometrías xy y de placas) para las reactividades y los flujos macroscópicos. Los autores exponen los resultados y los comparan con los obtenidos para los mismos cuerpos por la AGIP Nucleare y por el Centro de cálculo del Comitato Nazionale, en Bolonia; a pesar de la analogía de los métodos utilizados, se observaron grandes discrepancias, que provienen aparentemente de diferencias en los valores catalogados de las secciones eficaces. Este hecho parece confirmar la necesidad de llevar a cabo los experimentos críticos, como medios auxiliares de gran valor para la elección y verificación de los métodos de cálculo, claves nucleares y catálogos de datos nucleares. La determinación de kef en función del número de elementos combustibles, realizada para los tres valores de la mencionada relación uranio/acero inoxidable con la instalación ROSPO, permite analizar detalladamente estos problemas. (author)[ru]
Kak chast' ital'janskogo proekta jadernoj silovoj ustanovki s organicheskim zamedlitelem Nacional'nyj komitet po atomnoj jenergii smontiroval kriticheskuju ustanovku, izvestnuju pod nazvaniem ROSPO, prednaznachennuju dlja provedenija kriticheskih issledovanij jadernyh harakteristik aktivnyh zon, podlezhashhih ispytaniju v prototipnom reaktore (PRO) moshhnost'ju 60 mgvt. Pervaja zagruzka aktivnoj zony sostoit iz toplivnyh jelementov tipa MTR s ploskimi plastinami 90%-nogo obogashhenija v obolochke iz nerzhavejushhej stali. Osnovnoj fizicheskij vopros, na kotoryj neobhodimo otvetit', svjazan s vozmozhnost'ju poluchenija v jetoj aktivnoj zone sootvetstvujushhego izbytka reaktivnosti v celjah kompensirovanija temperaturnogo jeffekta i jeffekta otravlenija i obespechenija podhodjashhej prodolzhitel'nosti kompanii jenergeticheskogo reaktora. Izmenjajushhimsja parametrom javljaetsja otnoshenie uran —nerzhavejushhaja stal' v toplivnom jelemente, i v nastojashhee vremja provodjatsja opyty putem izuchenija aktivnyh zon, v kotoryh nekotorye iz 16 aktivnyh plastin kazhdogo toplivnogo jelementa zameshhajutsja maketami (nerzhavejushhaja stal'). Dlja treh iz jetih aktivnyh zon (3 i 5 plastin-maketov na kazhdyj toplivnyj jelement sootvetstvenno) provedeny podrobnye raschety s cel'ju poluchenija grafikov potokov, izuchenija jeffektov reaktivnosti i KJef v zavisimosti ot kolichestva jelementov. Raschet byl osnovan na izmenennoj teorii dvuh grupp, kotoraja uchityvaet delenija v nadteplovoj zone. Primenjalis' sledujushhie kody rascheta: THESIS—dlja spektrov teplovyh nejtronov i MUFT 4 —dlja spektrov nadteplovyh nejtronov; FLIP-1 (teorija perenosa) i TUT-T5 (Monte Karlo) —dlja potokov v edinichnoj jachejke; PDQ-O2, svjazannaja s WANDA-4 (teorija diffuzii v geometricheskih formah hu i forme plastin) dlja izmerenija reaktivnosti i makroskopicheskih potokov. Privodjatsja rezul'taty, kotorye sravnivajutsja s rezul'tatami, poluchennymi dlja teh zhe aktivnyh zon institutom AGIP Nucleare i vychislitel'nym centrom v Bolon'ja. Nesmotrja na shodstvo primenennyh metodov, otmechalis' bol'shie rashozhdenija, kotorye, kak polagajut, voznikajut iz-za razlichij v dannyh o poperechnyh sechenijah. Jeto obstojatel'stvo, po-vidimomu, podtverzhdaet neobhodimost' kriticheskih opytov kak naibolee jeffektivnogo sredstva pri vybore i proverke metodov rascheta, jadernyh kodov i sobrannyh dannyh. Izmerenie Keff , zavisimogo ot kolichestva toplivnyh jelementov, vypolnennoe dlja treh upomjanutyh otnoshenij uran—nerzhavejushhaja stal' v ROSPO, daet vozmozhnost' provesti podrobnyj analiz jetih problem. (author)Source
International Atomic Energy Agency, Vienna (Austria); 494 p; Apr 1964; p. 401-422; Symposium on Exponential and Critical Experiments; Amsterdam (Netherlands); 2-6 Sep 1963; IAEA-SM--42/77; ISSN 0074-1884; 
; 9 refs., 8 figs., 10 tabs.
; 9 refs., 8 figs., 10 tabs.
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