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CHEMICAL CLEANING OF SOLIDIFIED LEAD FROM 316 AUSTENITIC STEEL A STRUCTURE CANDIDATE MATERIAL FOR LEAD-COOLED FAST REACTORS

Daniel Petrescu, Lucian Velciu

Abstract

Scopul lucrari este acela de a identifica o solutie de curatare specifica pentru inlaturarea plumbului solidificat de pe un material candidat pentru Reactorii Rapizi raciti cu Plumb (LFR) fara deteriorarea acestuia. Printre tehnologiile promitatoare de reactoare care au fost luate in considerare de Forumul International de Generatie IV (GIF), LFR a fost identificat ca o tehnologie cu un mare potential, deoarece acest sistem poate indeplini simultan mai multe obiective cheie, cum ar fi durabilitatea, neproliferarea, economia si siguranta. Unele dintre aceste obiective sunt realizate datorita proprietatilor inerente favorabile ale plumbului. Cu punctul de fierbere foarte ridicat, presiunea scazuta a vaporilor, capacitatea de ecranare ridicata, transferul de caldura ridicat, densitatea mare, sectiune scazuta de absorbtie a neutronilor si compatibilitatea buna cu aerul si apa, face ca plumbul sa extinda domeniul viabilitatii pentru proiectarea reactorului, pentru o configurație a sistemului foarte simplificata. Inspectia si reparatia sunt niste probleme importante pentru dezvoltarea sistemelor nucleare de generatie noua, deoarece contribuie in mare masura la siguranta intregului sistem. Pentru inspectia componentelor scoase din vasul reactoarului este foarte important sa se poata curata materialul structural de plumbul rezidual fara ca acesta sa fie afectat. Prin urmare, in aceasta lucrare au fost studiate cateva solutii chimice de curatare. Pentru a indeparta plumbul rezidual de pe suprafata materialelor candidate pentru reactoarele LFR, s-au testat mai intai probe de otel inoxidabil 316 in plumb lichid la 450 °C timp de 48 de ore. Dintre solutiile testate care si-au dovedit eficienta pentru inlaturarea plumbului au fost cateva amestecuri pe baza de acid acetic, apa oxigenata si alcool (etanol si propanol). Toate acestea au fost testate in doua conditii de temperatura: 20 0C si 75 0C. Viteza de curatare a fost semnificativ mai rapida pentru amestecurile care au avut temperatura mai mare. Alte solutii care au fost testate, dar s-au dovedit a fi total ineficiente sau au afectat otelul austenitic si stratul oxidic protector sunt acidul azotic, apa regala si solutia electrolitica de acid tetrafluoroboric. S-au efectuat o serie de analize pe toate probele de otel austenitic dupa fiecare perioada de imersie in solutiile de curatare chimica: microscopice, gravimetrice si microduritate Vickers. Analizele au aratat ca amestecurile de solutii cu acizi organici au fost mai potrivite pentru indepartarea plumbului de pe otelurile austenitice 316.

The aim of this study is to find a specific cleaning solution that removes solidified lead from a candidate material for Lead-cooled Fast Reactors (LFR) without damaging it.
Among the promising reactor technologies being considered by the Generation IV International Forum (GIF), the Lead-cooled Fast Reactor has been identified as a technology with great potential, because this system can fulfill simultaneous several key objectives like sustainability, non-proliferation, economics and safety. Some of these goals are achieved thanks to the favourable inherent properties of lead. With his very high boiling point, low vapour pressure, high shielding capability, high heat transfer, high density, small neutron absorption cross-section, good compatibility with air and water, lead permits to extend the viability domain for reactor design, opening new possibilities for a greatly simplified system configuration.
In service inspection and repair (ISIR) is an important issue in the development of the next generation nuclear systems as it contributes greatly to the safety of the system. For the inspection of components removed from the reactor vessel, it is very important to be able to clean the structural material from residual lead without damaging it. Therefore, in this paper, several chemical cleaning solutions were studied. In order to remove residual lead from the surface of candidate materials for LFR, first were tested 316 stainless steel specimens in liquid lead at 450 0C for 48 hours.
Among the tested solutions that proved their efficiency for lead removal were several mixtures based on acetic acid, hydrogen peroxide, and alcohol (ethanol and propanol). All of these were tested under two temperature conditions: 20 0C and 75 0C. The cleaning speed was significantly faster for the mixtures that had a higher temperature. Other solutions that were totally ineffective or had affected the austenitic steel and the protective oxide layer are nitric acid, aqua regia and the electrolytic solution of fluoboric acid.
A series of analyses were performed on all austenitic steel specimens after each immersion period in the cleaning solutions: microscopic, gravimetric and Vickers microhardness. These analyses showed that the solutions based on organic acids were more appropriate for removing residual lead from the 316 austenitic steels.

Keywords

lead-cooled fast reactor, chemical cleaning, austenitic steel

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