6. Conclusions
In the following, the main results obtained in this work, as well as the future perspectives that the research contributed to open are summarised.
Status of the researches towards the LFR
To play an essential role, future nuclear energy systems will need to provide:
manageable nuclear waste, effective fuel utilization, and increased environmental benefits;
competitive economics;
recognized safety performance;
secure nuclear energy systems and nuclear materials.
These challenges are the basis for defining the goals of next-generation nuclear energy systems.
The Lead Fast Reactor, LFR, meets all the requirements defined for the Generation IV systems; in fact, it is top-ranked in sustainability and in proliferation resistance and physical protection and it is rated as good in safety and economics.
In particular, sustainability requires both the nuclear fuel to be available in the long term and the fuel cycle to be compatible with the environment. Conservation of resources encourages increasing fuel utilization beyond the level attained in current fuel cycles.
If compared to the existing nuclear power plant, the LFR systems will allow to increase about 160 times the effective fuel utilization, making possible the employment of the nuclear energy for 1000 years more by the present uranium resources.
In Europe, several efforts are spent in the frame of the R&D activities aimed to support the design and technical demonstration of the LFR systems.
In particular, the ELSY project aims at the demonstration that it is possible to design a competitive and safe lead fast power reactor using simple engineered features. Synergic links have been created between ELSY and the other European Research Activities, like IP-EUROTRANS and VELLA, especially in the frame of system design, HLM technology, materials, component development, coolant chemistry control, feasibility demonstration.
In Italy, the ENEA Brasimone Research Centre has wide competencies and relevant infrastructures in these fields and it is strongly involved in the ELSY, EUROTRANS (DEMETRA) and VELLA Projects.
Several facilities were built up, like CHEOPE, LECOR, CIRCE and NACIE, aimed to investigate the phenomena related to the HLM technologies, fundamental for the LFR development.
The results that are expected for the next years should create the conditions for initiating a significant development program including the necessary steps of design and construction of a Demo; the full power operation is foreseen around the year 2018.
Effectiveness of Gas Enhanced Circulation
The possibility to enhance the circulation of liquid metals by gas injection is an important design option for both normal operation and accident conditions (e.g., operating the Decay Heat Removal System) of an LFR, as well as of an ADS nuclear system.
The gas-lift technique allows for the primary coolant circulation with a plant simplification with respect to the use of mechanical pumps. The reliability of this technique was never proved on a significant scale before ENEA tests.
The experimental campaign on gas enhanced circulation has been successfully accomplished in the CIRCE facility. All the obtained results confirmed that it is possible to establish a steady liquid metal flow rate, higher than 200 kg/s; the available experimental data allow characterizing the gas lift technique.
In quantitative terms, the relationship between the gas flow rate and the entrained LBE flow rate follows a power law, when a gas flow rate in the range from 1 Nl/s to 7 Nl/s is injected. Actually, the liquid flow rate is only slightly dependent on temperature (at least in the investigated range, from 200 to 320 °C); then, it was possible to establish flow characteristic curves independent from temperature.
The two-phase flow in the riser was studied in this work and data concerning the flow quality, the void fraction and the pressure drops were collected. In the riser, both the friction and the acceleration pressure drops were evaluated. As a result, it is possible to conclude that, under the tested conditions, the pressure drop along the riser is essentially due to the gravimetric head, while the acceleration term is negligible. Then, the void
fraction and the slip ratio distributions along the riser were estimated and the two-phase flow was characterized.
Different models were considered, comparing the behaviour of the system with their predictions; the separate flow model seems to be in good agreement with the experimental evidence.
Finally, a circulation performance analysis was carried out and an optimum gas enhanced circulation performance was estimated.
The gas-lift technique demonstrated a good reliability, a medium efficiency but a low pressure head (about 40 kPa). The main advantages of this kind of pumping system are a very high simplicity and the possibility of avoiding issues of erosion and corrosion as could occur with mechanical pumps.
For application in a pool-type nuclear power plant, where higher pressure head are required, the gas lift seems anyway not applicable. For an HLM pool facility, built up for thermal-hydraulic and component tests, however, the gas lift technique seems to be a suitable pumping system, because the high simplicity and good reliability needed in a experimental plants.
For the application in the CIRCE facility for the ICE test section, therefore, where low pressures head are required, the gas lift technique was preferred, because of the good reliability and the very low mechanical-structural effort required with respect to the mechanical pumping system.
Design of the Integral Circulation Experiment and of the Related Tests
The Integral Circulation Experiment, ICE, will be carried out to reproduce the thermal hydraulic behaviour of a HLM pool-type nuclear reactor primary flow path, adopting Lead or LBE as working fluid.
In order to achieve the above mentioned goal, the ICE test section has been designed to be similar to a HLM primary system as practicable as possible. In this aim, the ICE test section, to be placed inside the CIRCE multipurpose facility located at the ENEA Brasimone Research Centre, couples an appropriate heat source, with a heat exchanger.
The heat source was designed to have high thermal performance, similar to the one expected for a LFR or ADS systems. In particular, the envisaged thermal power is 800 kW,
the core power density is 137 W/cm3, the fuel power density is 500 W/cm3 and the pin heat flux is 1 MW/m2; all these values are typical of HLM fast reactors.
Several CFD simulations were performed to evaluate the cooling of the heat source, calculating the LBE velocity and the clad temperature along the sub-channels; the obtained results agree with those of the simpler analytical design procedure adopted in the heat source design.
The reference heat sink designed for the ICE test section is a LBE-pressurized water heat exchanger.
To promote the primary circulation, the gas lift technique was chosen; several calculations were made to confirm that the available driving force under gas enhanced circulation flow regime allows obtaining the envisaged nominal LBE flow rate.
The oxygen control will be achieved by means of a chemistry control system; the system was designed in collaboration with the IPPE (Russia); in this case, it will be the first time that this kind of system will be qualified on a large-scale pool facility.
Thanks to the above mentioned features, the experiment will allow for an in depth investigation of the thermal-hydraulic behaviour of a HLM pool system.
Several numerical simulations were performed applying the RELAP5 system code; a modified version of this code was adopted to take into account the thermal and physical properties of the LBE and to better calculate the HTC inside the fuel bundle and HX tube bundle. The obtained results allow to estimate the gas flow rate needed to promote the LBE flow during the steady state enhanced circulation tests, to define the test procedure for the start-up of the system, as well as to simulate the behaviour of the system during the foreseen transients.
By the data to be collected by the ICE activity it will possible to obtain the knowledge necessary to understand the thermal hydraulics behaviour of a HLM pool system, to qualify and manage the chemistry control system, to test the main components and to evaluate the structural material performance. The ICE activity will strongly contribute to the demonstration of the HLM pool-type nuclear reactor feasibility.
By the end of this year the ICE test section will be assembled and introduced inside the CIRCE facility and by the spring of the 2009 the experimental campaign will start.
The NACIE loop was also designed and built-up by the Brasimone Research Centre. To carry on this activity, several calculations were made adopting a CFD code for thermal hydraulic analyses and a finite element code for structure-mechanical stress analyses.
In a first phase the NACIE loop will support the ICE test section design. In fact, due to the high thermal performance required for the ICE Heat Source, prototypical pin elements have been realized in collaboration between ENEA and Thermocoax SAS, adopting two different technological solution.
The aim of the NACIE loop is to house a high flux bundle, made by four prototypical pins and to test and qualify the elements in order to define which will be the adopted element for the ICE bundle. These tests were foreseen by the end of 2008 and already started; when the tests will be completed, the delivery of the ICE bundle will take place.
Preliminary experimental tests have been performed by the NACIE loop, testing the Data Acquisition System, the Power Supply System and starting the thermal characterization of the prototypical pins. In particular, feeding only one pin, steady state natural circulation tests have been run with a thermal power of 25 kW.
The preliminary results show that the NACIE loop behaviour agrees with the design specifications and that the thermal behaviour of the prototypical pin matches with the numerical simulations performed by the supplier.
After the high flux bundle qualification tests, the low flux bundle will be installed on the NACIE loop and a new experimental campaign will start. Its aim is to characterize the natural circulation and gas enhanced circulation flow regimes in a HLM loop. Several experimental data on natural circulation heat transfer coefficient in a rod bundle assembly will be available, covering the knowledge gap existing in this field.
On the NACIE facility a chemistry control system similar to the one envisaged for the ICE test section was installed, allowing to obtain a characterization of the components in a HLM loop. For the NACIE loop a double-wall LBE-water HX was designed and installed; it will be tested and characterized, allowing getting preliminary data on this kind of HX; a similar one is foreseen as DHR in ELSY, where a helium gap is adopted.
Finally, several operational and accidental transients will be simulated, allowing to establish a reference experiment for the benchmark of commercial codes when employed in HLM loop.
