• double wall bayonet tube SG

The configuration of the Lead cooled Fast Reactor (LFR) pointed out in the framework of the European Lead cooled SYstem (ELSY) and Lead-cooled European Advanced DEmonstration Reactor (LEADER) EC founded projects deals with the compact pool type reactor in which the steam generators (SG) are located inside the reactor tank. In this scenario, the steam generator design plays an important role since it represents the interface between the nuclear island and the secondary system and one of the most impacting incidents connected to this is the Steam Generator Tube Rupture (SGTR) that may propagate to the near-bough tubes (which are located in the same vessel of the reactor core). In the framework of the LEADER project, an innovative configuration of SG has been proposed: the super-heated steam double wall once through bayonet type with leakage monitoring. This conceptual design was studied since 60’ for Sodium Reactor application. An example of facility that operates with this concept is CIRCE (ENEA Brasimone), nevertheless the application is limited to the heat exchange function.

The single tube vertical unit consists of three concentric tubes. Starting from the smallest one (feed-water tube), the water crosses it in down-flow. At the end of this tube, it enters the second concentric tube (annular riser) in up-flow where it starts to boil due to the heat exchange with the liquid lead that flows in counter-current at the tube outer surface. The tube design allows the achievement of super-heated steam. The liquid lead is not in direct contact with the second tube. A third concentric tube, that creates an annulus, separates it from the steam-water sides realizing the so called double wall.

The study of this configuration is motivated by safety improvement. In fact, it allows the double physical separation between lead and water sides. Furthermore, by means of gap pressurization (with Helium), a leakage check system should be introduced between the double wall in order to prevent incident scenarios. On the other hands, the monitor-ability of the pressurized gap has to be demonstrated and the thermal efficiency of the unit has to be improved. In particular two main design issues are of great importance from the economic point of view. The first deals with monitor-ability of leakages: since a gas as helium is required and it has low conductivity, the gap between the tube should be minimized and a porous heat transfer enhancer should be introduced to reduce, as much as possible, the SG volume (tube length or tube number). The second deals with the thermal insulation of the feed-water descending tube. In fact, the water enters at its minimum temperature at the top of the SG, it is above the active length and it exchanges heat with the superheated stem that is leaving the SG unit. Therefore, in order to avoid steam condensation at the feed-water tube outer surface this tube should be designed with sandwich wall: steel - thermal insulator – steel.

This doctorate is co-financed by ANSALDO and ENEA and is directly connected to LEADER and Accordo Di Programma (ADP) national project. The activity aims to support the design of the double wall bayonet tube bundle SG with leakage monitoring and to investigate its TH performance both analytically and experimentally (into a representative prototype scaled down in power). The activity started in 2011 and is still ongoing. At present time the following objectives are reached and are presented in this thesis:

• Assessment of the TH performance of the bayonet tube by means of RELAP-5.
• Design, construction, operation and disposal of the TxP (Tubes for Powders) facility devoted to powders conductivity measurement into an annular geometry. Its aim was to qualify the conductivity of powders for their application in the annular gap between the tubes that separate the fluids both for application as heat transfer enhancer (as for the ALFRED SG) and to both to accommodate a give temperature drop (as for the HX of facilities under operation at Brasimone).
• Design and commissioning of the HERO (Heavy liquid mEtal pRessurized water cOoled tubes) test section based on the results achieved during the experimental campaigns in TxP. This test section is actually located in CIRCE and aims to investigate the TH behavior of a bundle of seven tubes that represent, as much as possible, the ALFRED SG tubes (1:1 in length).

Eight main sections and four attached appendixes constitute the structure of this work.

The first three sections have to be intended as introduction to the activity. Justification of the activity, framework and objective are given in section one. GEN-IV systems and LEADER project are briefly described in section 2. The third section presents the ALFRED SG. It includes also the historical development of double wall SG in the nuclear technology and a brief description of the HXs that have been operated or are under construction at ENEA CR Brasimone that make use of double wall concept.

The fourth and fifth sections are theoretical activities conducted in support to the R&D of the steam generator bayonet tube. In particular, section four is aimed to assess the TH performance of a single tube of the ALFRED SG by fixing, in a theoretical way (which is described in section five), some modeling issues still not defined in the ALFRED design (i.e. the powder material to be introduced between the double wall and its modeling). Section five aims to point out empirical models to treat the conductivity of powder media and to individuate candidate materials to be acquired and tested with the intent to design a double wall bayonet tube bundle test section. This last section includes an investigation on insulating materials for their application to the the feed-water tube.

Section six constitutes the core of this activity and has to be intended as a first step on R&D in support to the design of the bayonet tube steam generator with particular reference to the heat transfer enhancer porous medium placed between the double wall. The Tubes for Powders (TxP) facility has been designed (by ENEA), constructed (by LIMAINOX), instrumented (by ENEA) and operated to test the conductivity of powders both under un-pressurized air environment and under pressurized helium atmosphere. This process takes more than two years and gives rise to two main experimental campaigns. The first set of tests have been conducted to qualify the HXs of the facilities under operation at the Brasimone Research center while the second campaign aimed to support the design of the HERO (Heavy liquid mEtal pRessurized water cOoled tubes) test section.

The design of the HERO test section is still on-going. At present time the SG bayonet tube unit has been designed, constructed (by CRIOTEC), instrumented and connected to CIRCE (section seven). The secondary loop is under design phase.

Conclusions are finally given in section eight.