A Review on Radiation Damage in Concrete for Nuclear Facilities: From Experiments to Modeling

Concrete is a relatively cheap material and easy to be cast into variously shaped structures. Its good shielding properties against neutrons and gamma-rays, due to its intrinsic water content and relatively high-density, respectively, make it the most widely used material for radiation shielding also. Concrete is so chosen as biological barrier in nuclear reactors and other nuclear facilities where neutron sources are hosted. Theoretical formulas are available in nuclear engineering manuals for the optimum thickness of shielding for radioprotection purposes; however they are restricted to one-dimensional problems; besides the basic empirical constants do not consider radiation damage effects, while its long-term performance is crucial for the safe operation of such facilities. To understand the behaviour of concrete properties, it is necessary to examine concrete strength and stiffness, water behavior, volume change of cement paste, and aggregate under irradiated conditions. Radiation damage process is not well understood yet and there is not a unified approach to the practical and predictive assessment of irradiated concrete, which combines both physics and structural mechanics issues. This paper provides a collection of the most distinguished contributions on this topic in the past 50 years. At present a remarkable renewed interest in the subject is shown

Gamma-ray shielding properties of heavyweight concrete with Electric Arc Furnace slag as aggregate: An experimental and numerical study

This study investigates, both experimentally and numerically, the radiation shielding properties of two types of heavyweight concretes, one containing barite and one made with Electric Arc Furnace (EAF) slag as coarse aggregates. Fresh and hardened concrete properties are preliminary analyzed and compared to a conventional mix containing natural aggregates, namely: fresh and hard bulk density, consistency, compressive and tensile strength, elastic modulus. Three specimens per each mix are subsequently irradiated with both a high-activity (8.97 TBq) and a low-activity (280 kBq) 60Co gamma-ray source to measure their linear attenuation coefficients. Gamma-ray shielding characteristics are also numerically analyzed via a Monte Carlo code assessing radiation transport calculations. Experimental and numerical results indicate that EAF concrete has comparable shielding properties to baritic one, and it allows a certain decrease in thickness of the radiation shield, if compared to an ordinary concrete, however it has superior mechanical properties than the other studied mixtures

On the hydration of unsaturated barriers for high-level nuclear waste disposal

The paper addresses the topic of the effects of hydration and temperature on the final state of an engineered barrier composed of compacted bentonite. In particular the distributions of water content and dry density are examined. Those issues are explored with reference to a long-duration field test that reproduces at full scale the behaviour of an unsaturated compacted bentonite barrier subjected to hydration and thermal ef-fects. The state of the barrier is discussed at two different stages using data from two separate dismantling op-erations. It is observed that the state of the barrier is not homogenous even after reaching saturation. It is shown that coupled THM analyses are able to predict satisfactorily the state and evolution of the barrier throughout.Postprint (published version

Radiation damage assessment for the concrete shielding of SPES, a nuclear facility for the selective production of exotic species

The research activity moves from a partnership between the Department of Structural and Transportation Engineering of the University of Padova and the National Institute of Nuclear Physics (INFN), National Laboratories of Legnaro (Padova), aimed at the design of the SPES project (Selective Production of Exotic Species), for the construction at the INFN laboratories of Legnaro of a next-generation nuclear facility, dedicated to the production of a special kind of radioactive ion beams called exotic species. Their study is expected to provide advances in physics and astrophysics and a possible medical application of these beams is foreseen, as well. The topic of our study is the concrete shielding surrounding such a facility and the deeper understanding of the physical aspects involved in the radiation exposure of concrete to nuclear radiation generated by the fission reactions due to a primary proton beam impinging a uranium-carbide target. The study can be ideally subdivided into three phases: a) a bibliographic research within the scientific production in the field of nuclear physics and concrete behaviour (articles and manuals); b) the numerical implementation and validation within an ad hoc finite element code of a damage law, experimentally based, for concrete under neutron radiation; c) the application of the numerical model developed so far to the SPES project, provided the boundary conditions for the exposed concrete have been defined, in accordance with the design work specifics of the facility. This phase has required the combined use of the Monte Carlo technique and the FEM model. The previous bibliographic investigation has lead mainly to define the status of the art of the shielding materials employed in nuclear engineering and collect experimental data of irradiated concrete samples. As for the first point, the use of some improved mixtures has been envisaged to be of interest in the common practice so that some special concretes have been considered, in addition to ordinary Portland concrete; they differ basically for the presence of hydrous aggregates in the mixture, which are able to retain their water content even at high temperature, or heavy aggregates (iron-based or barytes aggregates, mainly). The former are known to assure good shielding properties against neutrons, since the atoms of hydrogen in the water molecules can easily absorb a huge amount of the incident kinetic energy of neutrons in a few scattering events; the latter provide, generally, good shielding properties against gamma rays, unsought by-product of atom-neutron reactions. The second point of the study allowed us to better understand the degradation mechanisms for concrete under an irradiated environment and quantify the threshold value for neutron fluence, marking the beginning of the macroscopic decay in the mechanical properties, in terms of compression and tension strength and Young modulus. The required damage law has been defined based on the enveloping curve of several empirical data describing the behaviour of the Young modulus of exposed concrete, with respect to that proper of the virgin material, in function of the neutron fluence; this macroscopic parameter for defining radiation damage has been chosen in agreement with the effective stress theory by Kachanov. The damage law here introduced has been implemented in a research FEM code able to solve the coupled hygro-thermo-mechanical problem in multiphase porous systems like concrete. Concrete is modeled by the code in its visco-elastic behaviour, taking into account damage effects due to mechanical loads, thermal loads and, thanks to the upgrade, a surrounding radiation field. The validation of the code has been accomplished by reproducing a particular irradiation test found in the scientific literature and involving serpentinitic concrete samples subject to two different neutron fluences; the overall stress-strain trend numerically simulated is found in good agreement with the data of the empirical investigation. A comparative analysis has followed the validation phase, aimed at studying the shielding performance of ordinary concrete, with respect to that of the improved mixtures, for a given radiation field, which is assumed to follow the simplified one-dimensional model known as diffusive model for thermal neutrons; a similar model is defined for high energy neutrons, i.e. fast neutrons, based on the so-called two-group theory. The one-dimensional simplification above mentioned is meant to be acceptable if the attenuation of radiation is considered to happen along the thickness of a wall, which is the geometry under analysis in the following: as explained later on, a portion of the directly impinged wall has been modeled with the FEM code and the attenuation process has been studied along the beam direction. The incident flux being equal, the comparative analysis has shown higher damage values for concrete specimens under fast neutrons than thermal and, as expected, better shielding characteristics have been found for the special concretes than ordinary Portland. The third phase of our work has focused the target room, the most critical area of the facility under design for the National Laboratories of Legnaro, dedicated to the fission reactions; it has been modeled in a Mote Carlo environment through a special software developed by CERN and INFN of Milan; the statistical tool is able to handle 3D radiation transport calculations and it has been exploited for our purposes to define the radiation and the temperature field from the design specifics of the SPES facility. The combined use of the Monte Carlo technique and the FEM code, upgraded to take into account the radiation exposure effects on concrete, has allowed us to identify in the thermal aspect, i.e. the temperature rise in the shielding due to radiation energy deposition, the most severe factor for prescribing a work scenario consistent with concrete durability. The numerical have allowed us to quantify an admissible irradiation profile of up to 6-7 months per year, for five years, under the design characteristics of the accelerating system and of the primary beam, not considering any protective device, such as outer metallic liners working as coats for the biological shield or the presence of a cooling systems inside the walls.L’attività di ricerca si inserisce nell’ambito di una collaborazione del Dipartimento di Costruzioni e Trasporti dell’Università di Padova con l’Istituto Nazionale di Fisica Nucleare (INFN), Laboratori Nazionali di Legnaro (Padova), finalizzata alla messa a punto del progetto SPES (Selective Production of Exotic Species), per la costruzione nei suddetti laboratori di un impianto nucleare di ultima generazione per la produzione di speciali fasci di ioni radioattivi, detti specie esotiche, a scopi di ricerca in campo fisico, astrofisico e auspicabile applicazione in campo medico. L’argomento di studio è il rivestimento in calcestruzzo per un impianto di questo tipo e le problematiche connesse all’irraggiamento da neutroni a seguito delle reazioni di fissione nucleare generate dalla collisione di un fascio primario di protoni su un elemento bersaglio in uranio-carbonio. Lo studio può suddividersi idealmente in tre fasi: a) una prima fase di ricerca nella letteratura scientifica di settore (articoli e manuali); b) una successiva fase di implementazione numerica e validazione in apposito codice agli elementi finiti di una legge di danno da radiazione sul calcestruzzo, basata su sperimentazione; c) una terza fase di applicazione del modello numerico al caso di studio, il progetto SPES, con la necessaria definizione delle condizioni al contorno per il calcestruzzo esposto, dovute alle condizioni di progetto del macchinario. Ciò ha richiesto un uso congiunto della tecnica Monte Carlo e del modello FEM. L’iniziale indagine bibliografica ha coinvolto la definizione dello stato dell’arte sui materiali di schermatura impiegati in campo nucleare e la raccolta di dati sperimentali di irraggiamento neutronico su campioni in calcestruzzo. Il primo punto ha permesso di considerare, a fianco del calcestruzzo ordinario, l’impiego di altri impasti migliorati per la presenza o di aggregati idrati, in grado di ritenere il loro contenuto d’acqua anche ad alte temperature, o di aggregati pesanti (di natura ferritica o baritica soprattutto); la prima caratteristica garantisce una buona capacità schermante nei confronti dei neutroni, essendo in grado l’idrogeno contenuto nelle molecole d’acqua di assorbire dopo pochi eventi di scattering una grande aliquota dell’energia incidente di un neutrone; la seconda caratteristica è indice di una buona prestazione schermante nei confronti dei raggi gamma, indesiderato prodotto secondario delle reazioni atomi-neutroni. Il secondo punto ha condotto alla comprensione dei meccanismi di deterioramento del calcestruzzo sotto un ambiente irraggiato e alla quantificazione della soglia di flusso neutronico oltre la quale si hanno le prime manifestazioni macroscopiche di perdita di resistenza del materiale, valutata in termini di resistenza a compressione, a trazione e modulo elastico. La legge di danno ricercata è stata definita come la curva di inviluppo del decadimento del modulo elastico di calcestruzzo esposto, rispetto al materiale vergine, proveniente da diversi tests, in diverse condizioni sperimentali, che, tuttavia, hanno permesso di identificare un trend univoco, in funzione del flusso di neutroni, di questo parametro macroscopico (il modulo elastico), scelto in accordo alla teoria dello stress effettivo di Kachanov. La legge è stata implementata in un preesistente codice FEM che numericamente risolve il problema termo-igro-meccanico accoppiato per i mezzi porosi multifase, come si configura il calcestruzzo. Il materiale è qui modellato nel suo comportamento visco-elastico danneggiato, in cui le forme di danno possibili provengono dal carico meccanico, dal carico termico e, grazie all’upgrade prefissato, dal campo di radiazione nucleare. La validazione è stata fatta sulla base di una prova di irraggiamento reperita in letteratura per calcestruzzo serpentinitico, sottoposto a due diversi flussi neutronici; nel complesso la risposta del materiale irraggiato è ben colta dal modello numerico, in termini di legame tensioni-deformazioni. A questa ha seguito un’analisi comparativa sulla bontà di schermatura del calcestruzzo ordinario, rispetto a provini fatti di impasti migliorati individuati anch’essi da letteratura, nell’ipotesi che il campo di radiazione spazialmente segua un modello semplificato monodimensionale, nella fattispecie noto in letteratura come modello diffusivo per i neutroni a bassa energia o termici; un modello analogo è definibile per i neutroni ad alta energia o veloci, sulla base della cosiddetta teoria dei due gruppi, che assume la suddivisione dello spettro neutronico reale in due soli livelli energetici, in una logica di semplificazione computazionale della teoria generale del trasporto della radiazione nella materia. L’evoluzione monodimensionale è accettabile se si analizza l’attenuazione della radiazione lungo lo spessore di una parete uniformemente investita, geometria che non si discosta dal caso di studio, il quale considera una porzione di parete orientata in direzione del fascio, per la quale la sezione trasversale è la faccia esposta e l’attenuazione avviene lungo lo spessore. A parità di flusso incidente, l’analisi comparativa ha messo in luce valori di danno superiori in presenza di neutroni veloci, rispetto a quelli dati da neutroni termici e ha permesso di quantificare le migliori prestazioni degli impasti speciali, rispetto al calcestruzzo ordinario. Successivamente si è preso in considerazione l’elemento sensibile dell’impianto in progetto per i Laboratori Nazionali di Legnaro, ovvero il vano ospitante l’elemento bersaglio e sede delle reazioni di fissione. La geometria dell’impianto è stata ricreata in un codice Monte Carlo di ricerca, un brevetto CERN-INFN di Milano in grado di effettuare calcoli 3D di trasporto della radiazione, allo scopo di ricavare i campi di radiazione e temperatura attesi per SPES nelle condizioni di lavoro di progetto per il macchinario. L’uso congiunto della tecnica Monte Carlo con il codice FEM, modificato per tenere in conto gli effetti del campo di radiazione nel materiale, ha consentito di definire l’aspetto termico, ossia lo sviluppo di calore interno al materiale per effetto del deposito di energia da radiazione, il fattore limitante per descrivere uno scenario di lavoro compatibile con la durabilità del calcestruzzo. Le simulazioni hanno condotto alla definizione di un profilo di irraggiamento ammissibile di cicli continuativi al più di 6-7 mesi all’anno, per un quinquennio, alle specifiche di progetto del sistema di accelerazione e del fascio primario sull’elemento fissile, in assenza di ulteriori provvedimenti o dispositivi di attenuazione del fronte termico, come ad esempio predisposizione di liners metallici all’intradosso delle pareti direttamente investite della camera di fissione o impianti di raffreddamento annegati in parete

A Review on Radiation Damage in Concrete for Nuclear Facilities: From Experiments to Modeling

Concrete is a relatively cheap material and easy to be cast into variously shaped structures. Its good shielding properties against neutrons and gamma-rays, due to its intrinsic water content and relatively high-density, respectively, make it the most widely used material for radiation shielding also. Concrete is so chosen as biological barrier in nuclear reactors and other nuclear facilities where neutron sources are hosted. Theoretical formulas are available in nuclear engineering manuals for the optimum thickness of shielding for radioprotection purposes; however they are restricted to one-dimensional problems; besides the basic empirical constants do not consider radiation damage effects, while its long-term performance is crucial for the safe operation of such facilities. To understand the behaviour of concrete properties, it is necessary to examine concrete strength and stiffness, water behavior, volume change of cement paste, and aggregate under irradiated conditions. Radiation damage process is not well understood yet and there is not a unified approach to the practical and predictive assessment of irradiated concrete, which combines both physics and structural mechanics issues. This paper provides a collection of the most distinguished contributions on this topic in the past 50 years. At present a remarkable renewed interest in the subject is shown

Dynamic stability of an elastic beam with visco-elastic translational and rotational supports

Purpose – The purpose of this paper is to show how to find the regions of dynamic instability of a beam axially loaded and visco-elastically constrained at its ends by Kelvin-Voigt translational and rotational units variously arranged according to different configurations, by using the equation of boundary frequencies. Design/methodology/approach – With respect to visco-elasticity the time variable is present as a parameter so that the above-mentioned exact approach is exploited to draw three-dimensional diagrams of the dynamic component of the periodic load and its frequency, varying with time and with the viscosity parameter m characterizing the restraints. Findings – For not rigidly constrained configurations a peculiar asymptotic tendency is recognizable in both cases. Research limitations/implications – The study allows for identifying the influence of visco-elastic restraints in the response of a beam under a dynamic axial load. Dynamic excitation occurs in several fields of mechanics: dynamic loads are encountered in structural systems subjected to seismic action, aircraft structures under the load of a turbulent flow and industrial machines whose components transmit time-dependant forces. Practical implications – Visco-elasticity accounts for possible vibration control solutions planned to improve the dynamic response of the rod; they can consist of layers of visco-elastic material within the body of the modelled element or local viscous instruments affecting the boundary conditions; the latter is the application this paper focuses on. Originality/value – With this paper a calculation procedure to get an exact solution for particular static configurations of the beam is followed in order to define the influence of visco-elastic restraints under a dynamic axial load; the responses are given in terms of boundary frequencies domains and are supposed to be useful to learn the behaviour in time and in dependence of the intrinsic viscosity of the restraints