This work was produced as part of a multidisciplinary study of the corrosion of zirconium alloys undertaken by a consortium of universities working in the MUZIC program; Oxford, Manchester and The Open University. The objective of the project as a whole was to further the understanding of the mechanisms of the breakaway oxidation process and to characterise these corrosion processes within a number of fuel rod cladding materials. This thesis describes laser 3D atom probe characterisation of the nano-scale chemical redistribution of oxygen and other solutes that occurs at the metal-oxide interface during corrosion, and a large body of technique development that was required to achieve this goal. The development of the metal-oxide interface of ZIRLO, a Zr-Nb-Sn-Fe-O alloy, is followed by generating 3D atomic scale reconstructions at four different stages of corrosion. The formation of a sub-oxide ZrO layer is seen during pre-transition oxide development. The ZrO interfacial layer is consumed by the rapid formation of oxide after the breakaway transition. After transition the chemistry of the interface is similar to the early pre-transition case, although an oxygen-saturated layer of metal adjacent to the interface formed during corrosion remains. The ZrO interfacial layer (Zr-ZrO-ZrO₂) and the region of oxygen-saturated material ahead of the metal-oxide interface alter the distribution of minor alloying additions such as niobium and iron. The ZrO layer increases the acceptance of niobium into the oxide, which is otherwise seen to be rejected at the Zr-ZrO2 interface along with iron. Niobium is seen to precipitate out of solution as nano-scale particles near the interface after around 100 days of corrosion. This is not seen in the bulk metal matrix of the corroded material due to the absence of other factors driving the process: the stress at the interface and a very high oxygen concentration in the metal ahead of the interface. The nano-scale niobium particles are found to be of a meta-stable composition. Iron is seen to redistribute in the corroded material and can be correlated with the local oxygen concentration. Similarities are seen in the behaviour of solutes within pre-transition ZIRLO and Zircaloy-4 (Zr-Sn-Fe-O). In both cases no redistribution of tin is seen at the metal-oxide interface. A Zr-Nb-Ti alloy with very poor corrosion resistance was also analysed in this way, and the similarities and differences with chemically-similar ZIRLO are discussed. The segregation of solutes to grain boundaries and solute clustering within the matrix are also examined before and after corrosion.

Examensarbetet behandlar problematik som uppstår vid härdsmältningsförlopp i en kärnkraftsreaktor av typen kokvattensreaktor. Resultatet ska användas som riktlinjer till strategier som utvecklas av Severe Accident Management Guidelines (SAMG) där arbetets uppdrag är ett delmoment i framtagning av strategier för att bemästra de olika fenomen som uppstår vid härdsmälta.   Syftet med arbetet är att undersöka begränsningar för att bevara reaktortanken intakt vid haveri, genom att undersöka den minsta mängd vatten som behövs för att undvika tankgenomsmältning. Undersöka fallen som leder till härdsmälta och gruppera dem efter händelsesekvenser. Undersöka metall/vatten-reaktionen som uppstår då härden blir över 800°C och undersök om fallen kan grupperas i händelsesekvenser.  Metoder som används i arbetet är PSA-dokumentation, händelseutvecklingsträd, teoretiska beräkningar och MAAPv5.03. Resultatet beskriver att grupperingar av fallen som slutar i härdsmälta och grupperingar av metall/vatten-reaktionen hos de olika fallen kan genomföras. Resultatet beskriver även ett minsta flöde som kan föras in i reaktortanken för att hindra tankgenomsmältning och flöden upp till 100 kg/s så det finns resultat för olika flöden beroende på vilka kylmedel som är tillgängliga.  Slutsatsen av arbetet är att fall kan grupperas efter händelsesekvenser och påverkan hos metall/vattenreaktion, grupperingarna sparar tid vid ett haveriförlopp. I varje grupp kunde det svåraste fallet beräknas för minsta flöde för att klara tankgenomsmältning och flöden upp till 100 kg/s.  Framtida arbeten bör undersöka trycket och vätgasen som skapas vid vatteninmatning samt dess påverkan på reaktorinneslutningen. / The master thesis deals with problems that arise during nuclear meltdown in a nuclear powerplant of the type boiling water reactor. The work will be used as guidelines for strategies developed by Severe Accident Management Guidelines (SAMG), this master thesis is a sub-element in the development of strategies for mastering the various phenomena that arise during a meltdown.  The purpose of the work is to investigate limitations for maintaining the reactor tank intact during the meltdown by, examining the minimum amount of water needed to avoid the meltdown getting through the reactor tank. Examining the cases that lead to meltdown and group them according to the event sequences. Examine the metal/water-reaction that occurs when the core becomes over 800°C and examine if the cases can be grouped into event sequences.  Methods used in the master thesis is PSA-analysis, event development threes, theoretical calculations and MAAPv5.03.  The result describes the groupings of the cases ending in meltdown and the groupings of the metal/water-reaction of the various cases. The result also describes a minimum flow that is required to prevent meltdown of getting through the reactor tank and flow up to a 100 kg/s.  The conclusion of the master thesis is that cases can be grouped according to event sequences and the influence of the metal/water-reaction, the groupings save time in the event of a breakdown. In each group the most difficult case was calculated so that the lowest flow to prevent the meltdown from getting through the reactor tank was presented among with different flows up to 100 kg/s. Future work should investigate the pressure and hydrogen gas created by the water input and its influence on the reactor inclusion.