UNIVERSITÀ DI PISA Scuola di Dottorato in Ingegneria “Leonardo da Vinci” Corso di Dottorato di Ricerca in SICUREZZA NUCLEARE ED INDUSTRIALE Tesi di Dottorato di Ricerca NUCLEAR SAFETY OF RBMK REACTORS Carlo Parisi Anno 2008 UNIVERSITÀ DI PISA Scuola di Dottorato in Ingegneria “Leonardo da Vinci” Corso di Dottorato di Ricerca in SICUREZZA NUCLEARE ED INDUSTRIALE Tesi di Dottorato di Ricerca NUCLEAR SAFETY OF RBMK REACTORS Autore: CARLO PARISI Firma___________________ Relatori: Prof. Francesco D’Auria (UNIPI) Firma_____________ Dr. Gianni Petrangeli Firma_____________ Prof. Eugenius Uspuras (LEI) Firma_____________ Dr. Sergei L. Soloviev (NIKIET) Firma_____________ Anno 2008 ACKNOWLEDGMENTS This PhD thesis was developed at the San Piero a Grado Nuclear Research Group of the University of Pisa. So, my first thank is due to the leader of this Group and thesis supervisor, prof. F. D’Auria. He gave me the opportunity to perform this job in a fruitful and stimulating environment, and he always supported and encouraged the research activities. I appreciated also his example in looking at the RBMK technology without any prejudice. I would like to give also special thanks to the other PhD thesis supervisors, Dr. G. Petrangeli, prof. E. Uspuras and Dr. S. L. Soloviev, for their collaboration, suggestions and for the trust they showed in my activities. Dr. V. Malofeev from Kurchatov Institute and Prof. K. N. Ivanov from the Pennsylvania State Univeristy helped me a lot with their advises on the neutron transport calculations. I am very grateful to them. A special mention is due to the NIKIET staff, led by Dr. B. Gabaraev, for their kind hospitality and assistance provided during my stays in Russia. I would like to thank also Dr. B. Ivanov, from Westinghouse Electric and Dr. E. Vanagas, from the Lithuanian Safety Authority for the useful conversations I had with them on the RBMK HELIOS models. This work would not be possible if I did not have the opportunity to work in the serious and at the same time friendly environment of the San Piero a Grado Nuclear Research Group. For this reason I am grateful to all my present colleagues and to the whole original team of persons that from 2004 to 2006 contributed with dedicated work to the unquestionable success of the TACIS Project R2.03/97, from which this PhD work originated. My final thanks and dedication of this work should go to my parents, and especially to my father. During all these years he staunchly supported and encouraged me to perform and finalize the PhD activities, reminding, when difficulties seemed discouraging, that “..non est ad astra mollis e terra via”. i ii SOMMARIO La presente Tesi di Dottorato esamina il livello di sicurezza dei reattori di potenza raffreddati ad acqua bollente e moderati a grafite (reattori RBMK), mediante l’uso di codici di calcolo best-estimate termoidraulici e neutronici accoppiati. La disponibilità di tali sofisticati strumenti ha reso possibili, infatti, analisi dettagliate e realistiche anche per questo tipo di impianti nucleari, notoriamente complessi e comunemente denominati “reattori di tipo Chernobyl” (in riferimento al gravissimo incidente occorso ad uno di questi impianti nel 1986). Parte delle attività della presente Tesi di Dottorato, si sono svolte nell’ambito del Progetto TACIS R2.03/97 “Software development for the RBMK and WWER reactors”, coordinato dal Gruppo di Ricerca Nucleare San Piero a Grado dell’Università di Pisa in collaborazione con i progettisti e gli esercenti russi di tali impianti (NIKIET, Kurchatov Institute, RosEnergoAtom, oggi EnergoAtom Concern OJSC). Le attività di ricerca hanno contemplato lo sviluppo di una complessa nodalizzazione termoidraulica dell’unità 3 dell’impianto nucleare di Smolensk, la sua validazione ed il suo successivo accoppiamento con un modello di cinetica neutronica tridimensionale. Il codice utilizzato è il RELAP5-3D. I calcoli di neutronica hanno richiesto, preliminarmente, lo sviluppo di librerie di sezioni d’urto macroscopiche. Tale attività, è stata svolta in collaborazione con la Pennsylvania State University, ed ha comportato l’utilizzo del codice di trasporto deterministico HELIOS. Dopo la validazione dei modelli sviluppatii, si sono eseguite analisi di transitori a piena potenza, focalizzandosi su quelli particolarmente critici per la sicurezza (per esempio, la rottura di un collettore dei canali di potenza o del sistema di raffreddamento delle barre di controllo). Una speciale enfasi è stata data, anche mediante l’uso del codice Montecarlo MCNP5, allo studio del transitorio contemplante il bloccaggio di un canale di potenza. L’ultima parte delle attività di dottorato si sono concentrate sull’analisi di un transitorio a bassa potenza, ricostruendo uno scenario incidentale estremo come quello occorso a Chernobyl. Le sezioni d’urto di cella per lo Xeno sono state calcolate con il codice al trasporto deterministico DRAGON. In conclusione, le analisi avanzate effettuate nell’ambito di questa Tesi di Dottorato, hanno confermato il migliorato grado di sicurezza di tali impianti, ottenuto grazie alle importanti modifiche effettuate a seguito dell’incidente di Chernobyl. iii ABSTRACT This PhD thesis is evaluating the safety level of the graphite-moderated boiling water cooled nuclear power reactors (RBMK reactors) by the use of best estimate three dimensional neutron kinetics coupled thermal-hydraulics codes. The availability of such sophisticated tools has allowed detailed and realistic analyses of these kind of reactors, also known as “Chernobyl-type” reactors. Chernobyl is the name of a RBMK reactor where, in 1986, a severe accident occurred, leading to the destruction of the plant and to a major release of radioactivity into the environment. Parts of the activities of this PhD thesis were developed in the framework of the European Union funded TACIS Project R2.03/97 “Software development for the RBMK and WWER reactors”. This project was awarded to the “Gruppo di Ricerca Nucleare San Piero a Grado” of the University of Pisa and managed by it in collaboration with the RBMK designers (NIKIET, Kurchatov Insititute) and the licensee (RosEnergoAtom, now EnergoAtom Concern OJSC). The research activities dealt with the development and the validation of a sophisticated thermal-hydraulic nodalization of the Smolensk-3 Nuclear Power Plant. This thermal-hydraulic model was then coupled with a three dimensional neutron kinetics model of the core. The code used was RELAP5-3D system code. Suitable RBMK cross sections libraries were developed in collaboration with the Pennsylvania State University, using the deterministic lattice physics code HELIOS. After the validation of the developed models, the most relevant transients for the plant safety at full power were calculated, e.g. the group distribution header rupture, the break of the control and protection system cooling circuit. A special emphasis was put in the simulation of the single fuel channel transient, using also the Monte Carlo code MCNP5. The last part of the PhD activities concerned the analysis of a low power transient. In particular, the Chernobyl extreme scenario was reconstructed. Xenon fuel cell cross sections were calculated using the deterministic transport code DRAGON. Finally, all the analyses performed in the framework of this PhD confirmed the upgraded level of nuclear safety of the RBMK reactors, obtained also as a consequence of the relevant hardware modifications implemented in the aftermath of the Chernobyl accident. iv CONTENTS ACKNOWLEDGMENTS............................................................................... I SOMMARIO................................................................................................ III ABSTRACT ................................................................................................IV CONTENTS. ................................................................................................V NOMENCLATURE .....................................................................................IX LIST OF SYMBOLS .................................................................................XIII LIST OF FIGURES ....................................................................................XV LIST OF TABLES....................................................................................XXII 1. INTRODUCTION ..................................................................................... 1 1.1. State of the Art on the RBMK Safety Analyses............................. 1 1.2. Scope 2 1.3. Objectives......................................................................................... 3 1.4. Structure........................................................................................... 3 2. THE DESCRIPTION OF THE RBMK ...................................................... 5 2.1. The Primary System ........................................................................ 8 2.1.1. The pressure boundary ............................................................ 8 2.1.2. The core ................................................................................... 15 2.1.2.1. The fuel .......................................................................................... 15 2.1.2.2. The Absorbers and the Power Control System......................... 18 2.1.2.3. The scram signals ........................................................................ 22 2.1.2.4. The fuel cell neutronic characteristics....................................... 23 2.1.2.5. Reactivity Control