1
1 INTRODUCTION
1.1 General Background
Nowadays the energy supply plays a crucial role in the international landscape both from economical and political point of view. In fact, the capability to keep the present level of welfare in the developed countries and to improve life conditions in developing countries depends critically on energy production and supply. A more rational use of the natural resources together with more efficient energy production systems are needed considering that, even in a recessive economic phase, the energy demand increase is about 2-3% per year and, in the first half of the XXI century, the energy market should shift its barycentre just to the developing countries where the energy demand might reach the 70 - 90% of the world’s demand [1]
Nuclear energy has an important role in the world’s electricity production, with 900 operating nuclear reactors all over the world. Around 440 of them are large power reactors that are distributed over 30 countries and provide about 16% of the world’s total base-load electricity. About 480 are small reactors aimed at powering submarines and ships or at research materials structure and production of radioactive isotopes in 56 countries [2].
One of the most important assets in producing nuclear energy is the absence of greenhouse gases, which is a key issue considering the consequences of global warming. Furthermore, nuclear power provides an improved degree of energy independence and security of supply. Nevertheless, a larger commercial use of nuclear energy is opposed mostly by controversial arguments about three main topics: nuclear reactor potential risk on public and environment, costs and nuclear wastes.
The problem of public acceptance of nuclear energy production has arisen after the Three Mile Island accident and especially after the Chernobyl accident. A clear and correct information about nuclear energy related issues and the demonstration of economic competitiveness, efficiency and reliability of this technology are key factors to promote public acceptance.
Costs might be significantly reduced by a more rational use of fuel, which implies the adoption of fuel cycles allowing a larger utilization of the raw material from the current utilization fraction of 1-2% to 33 % [3], and the development of technologies capable of utilizing 60 - 70% of the uranium [4], despite of the fact the largest share of costs in nuclear industry are due to construction costs and interests expenses [5,6]
Furthermore, the sustainable use of fuel is also connected to the problem of nuclear wastes, which might be solved by adopting the so called “Double strata concept” (see Fig. 1.1, [7]) which currently is regarded as one of the most suitable solutions for reducing the amount of actinides in wastes. This concept includes two steps for achieving the optimization of the fuel cycle. In the “First Stratum”, the fuel cycle is typical of the current reactors such as fabrication, core loading and utilization, discharging and possible reprocessing, with the plutonium recovery and the subsequent utilization as MOX (Mixed Oxid Fuel).
In the “Second Stratum” the Minor Actinides, the Pu not recovered in the “First Stratum” and the fission products are partitioned and transmuted in an ADS (Accelerator-Driven System) reactor, as can be seen in Fig. 1.1 [7].
Fig. 1.1 - Comparison between the homogeneous recycling (left) and the double strata concept (right).
This kind of hybrid systems, which are schematically shown in Fig.1.2 [8], consists of a subcritical reactor (keff < 1) coupled with a high intensity proton
accelerator (beam power from 1 - 10 mA and proton energy ranging from 0.5 -1 GeV) with high power spallation target (more than 1 MW). They should be able to transmute Pu, Minor Actinides, such as Np, Am, Cm, and other long lived fission products partitioned before in short lived or stable products, thus reducing their radiotoxicity. As a consequence, their adoption represents a possible contribution to the solution of the problem of nuclear wastes and at a same time provides a more rational fuel utilization.
Fig.1.2 - Schematic diagram of ADS
It is clear that solutions to these problems represent opportunities for a more extended use of the nuclear energy source, but they need a strong effort focused on research and development of new technologies. In order to coordinate and guide these activities and more generally to develop next generation’s nuclear
3 energy systems, also taking into account the knowledge and the experience gained from the past reactor generations, the Generation IV International Forum (GIF), lead by U.S. Department of Energy’s Office - Nuclear Energy, Science and Technology, was established [2,9].
The GIF has proposed the roadmap from current nuclear systems to the so called Generation IV systems [9] in order to enhance sustainability, economics, safety and reliability, proliferation resistance and physical protection, thus increasing the attractiveness of nuclear energy. Once these goals have been defined, GIF has selected, among 100 nuclear reactor concepts, the six most promising systems on the basis of the key areas mentioned above and, obviously, of their technical feasibility. Three of the six selected reactors are fast neutron reactors because of their capabilities in better exploiting the fuel. This choice implies a cooling system able to safely remove heat without appreciable “softening” the neutron energy spectrum; thus, the choice of coolant plays a crucial role in developing these reactors.
Currently, the most promising coolants are liquid metals (sodium, lead and lead bismuth eutectic) because they have several advantages concerning thermal and nuclear properties (i.e., high thermal conductivity and low neutron absorption cross section [10,11]) with respect to other coolants like water or gas. In addition, they allow operating at low pressure, thus reducing the likelihood of LOCA (loss of coolant accident). Nevertheless, they have also some drawbacks, like opacity which poses problems in systems inspections or corrosion and high temperature erosion problems, which arise in structures in direct contact with them. Furthermore, sodium is strongly chemically reactive with air and water, leading to the need to avoid contacts inside the reactor.
It must be pointed out that a lot of experience was already gained in liquid metal technologies. In fact, the first fast neutron reactors (CLEMENTINE, which was built in USA in 1945-46 [12,13] and BR-2 built in USSR in 1956 [14]) were cooled by liquid mercury. Russian experience showed that mercury was not an attractive coolant because it is dangerous for the men's health. US experience concluded that mercury produced a significant corrosive impact on the materials of the construction that, finally, was the main reason for abandoning the use of mercury in fast reactors.
After mercury, sodium has been considered for cooling fast reactors for its appealing characteristics. Two sodium fast reactors have been built in France, respectively Phénix reactor [15],which was closed in 2010, and the demonstrator plant on industrial scale Superphénix (SPX-1), which was, shut down in the late ‘90 [16,17]. Russia has built BN 600 reactor, which is still successfully in operation [18] Since sodium atomic weight is relatively low and it works partially like a moderator, as well as its chemical reactivity with water and air, lead and lead-bismuth have gained new interest in the scientific community because of their potential advantages compared to sodium. Therefore, LBE (Pb 55%-Bi 45%) and lead technologies have been studied since the ‘60s in USSR and then in Russia and applied in nuclear submarines [19]. Problems arisen in sodium fast reactors, in particular in SPX-1 and in MONJU, another sodium fast reactor built in Japan in 1985 [20], have practically blocked the development program of liquid metal reactors, decreasing their appeal and reducing the research focused on LMFR topics. The renewed interest in LMFRs, stimulated by the GEN IV Forum, has brought the scientific community to deal with these key topics again. Further problems arose because, despite the experience gained, a lot of analytical tools
and documentation was lost, as well as most of people who developed tools and design concepts are no more involved in this branch of research.
Among the most important safety concerns for these reactors is the interaction between liquid metals and fuel (FCI, Fuel-Coolant Interaction), in case of sodium cooled reactors, and between heavy metals and water (CCI, Coolant-Coolant Interaction), in the case of lead or LBE reactors. These interactions may occur following a core disruptive accident in sodium cooled reactors and to rupture of tubes of the steam generator placed in the reactor’s liquid metal pool in HLMR. Despite of their low likelihood of occurrence, both FCI and CCI represent a threat for the integrity of reactor’s structure. In fact, during a FCI or a CCI, a huge amount of heat might be transferred from the material at higher temperature to the colder material (sodium or water) on a time scale ranging from a few milliseconds to seconds. If the interaction occurs in the millisecond range, it can lead to energetic steam explosions, where the heat transfer between involved materials is so intense and rapid that, generally, the timescale for heat transfer is shorter than the timescale for pressure relief. This can lead to the formation of shock waves which can damage the inner structures of reactors and in HLMR might cause a sort of “chain effect”, damaging other steam generator’s tubes. The rapid heat transfer rate also allows the hotter material to solidify and vaporizes the colder material; this vapour tends to migrate towards the cover gas region, thus compressing the gas there contained and increasing pressure (expansion phase), resulting in another threat for structure integrity.
The phenomenology of these interactions, due to the short timescale, material phases and components, is extremely complex and requests careful experimental studies in order to understand it completely and to develop models which can be used in simulations. Concerning lead and LBE technologies, the research Centre of ENEA Brasimone has been strongly involved in R&D activities since the early decades of these studies. In particular, an experimental program has been carried out for ADS studies [21] in the DEMETRA Domain of IP-EUROTRANS, under the aegis of 6th EU-Framework Program, while an experimental campaign was dedicated to R&D for the ELSY reactor [22]. Regarding sodium fast reactors, the availability of past experimental campaigns allows analyzing them in a new perspective, for instance simulating them with updated code versions and trying to fill the knowledge gaps that inevitably are still present.
1.2 Motivation for the present work
As previously mentioned, the studies concerning FCI and CCI request a considerable effort in analysing, simulating and understanding the phenomena involved in the addressed interactions. In particular, one of the most important involved aspects is the evaluation of the energy released in such interactions, in order to estimate the potential loads and the resulting damage on reactor structures.
A strong connection and close cooperation between the experimental and simulation activity is needed and may be fruitful in order to achieve these goals. In this aim, a scientific cooperation between ENEA Brasimone research centre, for the experimental part, and the University of Pisa, for theoretical analysis and the simulation by the SIMMER [23] code, has been set up regarding the HLM activities. The present doctoral work was developed in the frame of this cooperation,
5 addressing two specific aspects that represent major challenges for safety analyses of future reactors cooled by molten metals:
• the interaction of water and LBE during postulated steam generator tube failures;
• the expansion phase of bubbles in sodium cooled fast reactors, generated by the interaction of fuel with the coolant.
The two phenomena share the characteristic of belonging to dynamic scenarios requiring an attentive analysis in terms of energy releases to understand their impact on structural integrity of the reactor. This is the reason why they could be both studied by the use of the SIMMER III code, a tool specifically intended for such studies [23]. The availability of experimental data related to these phenomena during the development of the research motivated the specific choice to address these phenomena by applying the SIMMER III code, in order to contribute to its validation.
In particular, LBE-water experiments were performed for the IP-EUROTRANS and ELSY campaigns, making possible the qualification of the SIMMER III code on the basis of these experimental data. This activity has been aimed at a further characterization of the interaction between LBE and water in order to understand which conditions might lead to more or less serious consequences for reactors. The use of existing tools for analysing energy releases, coupled with the SIMMER III code, and the development of new ones based on existing theoretical models constituted a further contribution provided by the study. This topic plays a crucial role because simulations tools are the only means available to assess energy releases in reactors given the unavailability of representative experimental data.
Experiments dealing with the injection of a high pressure gas into a stagnant liquid pool are of interest for studying in depth characteristic phenomena taking place during the expansion phase of a CDA and for testing and benchmarking codes used for their analyses. Further investigations concerning the mechanisms involved in this phase of the accident evolution have been carried out through an experimental campaign called SGI that was performed in 1994 in former Forschungszentrum Karlsruhe (FZK), now KIT [4]. This experimental campaign has been simulated again with SIMMER III, thus contributing to its further qualification by the numerous experimental data available, except for the energy release again. Therefore, the assessment of the energy release has been performed on the basis of SIMMER III qualification similarly to what have been done for LBE -water interaction. Tools based on the existing theoretical models have been set up also for this subject and applied to this campaign and to the LBE - water experiments.
1.3 Outline of the present work
The present work is divided into six chapters, which are arranged in two parts. The first part is devoted to presenting the phenomenological background and a literature review of the relevant problems to be dealt with.
In particular, Chapter 2 provides an overview of the characteristics of liquid metals, underlining advantages and disadvantages of their adoption as coolants in nuclear power plants (NPPs). Chapter 3 concerns a review of the state-of-the-art of interaction phenomena. Firstly, the interaction phenomenology is described and then attention is focused on models available in literature for energy release
evaluation. A short comparison between FCI and CCI and a brief overview on the main features of the SIMMER III code are also included in this chapter.
The second part of the work concerns the simulation activities performed in relation to the addressed experimental data. In particular, Chapter 4 describes the activity carried out in cooperation with the ENEA Brasimone Research Center, aiming at studying and analysing in depth physical phenomena and possible consequences of LBE-water interactions, covering a wide range of operating conditions. Two experimental campaigns, one in the frame of DEMETRA domain of the IP-EUROTRANS Project [21] and one in the frame of ELSY Program [22], are considered. Also in this case, after a preliminary qualification activity of the SIMMER III code, the evaluation of energy releases has been performed, by application of dedicated tools developed in connection with SIMMER III. Chapter 5 reports on the simulation activity of the experimental campaign SGI (acronym of “Schnelle Gas Injektion”) carried out at FZK (now KIT) [24] and aimed at studying the injection of gas in a stagnant water pool under different boundary conditions. After a preliminary analysis of simulation results, the development of a SIMMER III post processing tool for energy assessment is described, discussing the relevant results and conclusions obtained from their use. Finally, Chapter 6 summarizes the main conclusions drawn from the current work and presents some recommendations for future researches.
7
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