Activities on advanced fast LMFR and technology

The experience gained with fast reactors has revealed no fundamental problems with reactor physics, operation of various equipments and sodium technology. This experience will be useful in upgrading the design of next generation LMFRs. The objectives of the development of advanced LMFRs are to achieve better economics relative to alternatives, to find optimal solutions for the back-end fuel cycle and to achieve a high degree of safety and reliability.

The final goal relating to the objective of achieving of a very high degree of safety is to design a reactor system which, not only during normal operation but also in case of an accident, could exclude any radiological impact that would require evacuation of the public.

The following aspects are considered among the most important [21]:

— Assure stability of the reactor core under all modes of normal and abnormal operating conditions. Minimization of excess reactivity and sodium void effects, the reactor's strong negative power and reactivity feedback with increased temperature, the large margin to reactor coolant boiling at its operating temperatures, low pressure system with large thermal inertia and sufficient safety margins are key aspects;

— Assuring the heat removal from the core and the reactor under all upset conditions, by assuring the ability to limit coolant temperatures below boiling, and fuel element clad temperature below prescribed limits without the need for rapid operator action, i.e.

having a reasonably long grace period;

— Take advantage of passive safety systems to provide safety related functions without reliance on operator action or on external mechanical and/or electrical power signals;

— Minimize the burden on the operator of a nuclear power plant. Extensive consideration is being given to man-machine interfaces.

In considering the objective to increase safety and reliability margins, advanced LMFRs are being designed using all previous experience of being both good and not so good. Notable examples of the innovative safety characteristics are: passive decay heat removal systems;

passive (inherent) reactor shutdown and stabilization by thermal and reactivity response characteristics of the reactor even under extremely unlikely accident conditions.

As to the argument that some LMFRs faced reliability issues, it should be noted that this would be true for any other reactor line at the initial stage of development, as LMFRs are. Full industrial development of fast reactors has not been completed yet. It is simply too early at the prototype stage of development to more general view of LMFR technology and economics, particularly in the present situation where antinuclear environmentalists trend to discourage investors from entering this field with the required resources. Other reactor technologies, including water cooled reactors, achieved high reliability and lower generating costs when their respective large scale introduction had taken place. One cannot say that this will not happen in the case of LMFRs.

The early development of experimental and prototype liquid metal fast reactors was conducted to a large extent on a national basis. However, for advanced LMFRs, international cooperation begins to play a greater role and the Agency promotes international cooperation

— interalia — in its development. Especially for R&D incorporating innovative features, international cooperation can play an important role allowing a pooling of resources and expertise in areas of common interest to help to share the high costs of development.

To support the IAEA's functions of encouraging LMFR development, the IAEA promotes technical information exchange and cooperation between Member States with fast reactor development program, offers assistance to Member States with an interest in exploratory or research program, and publishes reports available to all Member States interested in the current status of LMFR development. Experience gained from R&D, operation and construction of fast reactors has been reviewed periodically by the TWG-FR.

The role of the IAEA as a truly international forum for cooperation and exchange of information cannot be overemphasized. About 130 various specialist meetings, seminars and symposia have been organized so far by the IAEA on the advice of the TWG-FR (formerly IWG-FR) and about 3 900 specialists from more than 25 countries have participated in these events. Some examples of topics of Specialists and Technical Committee Meetings held on the advice of the TWG-FR/IWG-FR include:

— Sodium combustion and its extinguishment-techniques and technology;

— Steam generators for LMFBRs, fuel failure mechanisms;

— Fuel and cladding interaction;

— Properties of primary circuit structural materials including environmental effects;

— Leak detection and location in LMFBR steam generators;

— Sodium fires, design and testing;

— LMFBR steam generator integrity and reliability with a particular reference to leak development and detection;

— Repair of FR steam generators: acoustic/ultrasonic detection of in-sodium water leaks,

— Propulsion reactor technology for civilian applications;

— Sodium removal and disposal from LMFRs in normal operation in the framework of decommissioning;

— Operational and decommissioning experience with fast reactors;

— Conceptual design of advanced fast reactor;

— Absorber materials control rods and design of shutdown systems for advanced liquid metal fast reactor;

— Influence of low dose irradiation on the design criteria of fixed internals in fast reactors;

— Use of fast reactors for actinide transmutation, and

— Creep-fatigue damage rules for advanced fast reactor design.

Research on LMFR during the last decades has significantly improved understanding of LMFR designs, technology and safety. Pool type design concept was chosen for all small, medium and large size advanced reactors: CEFR (China), PFBR (India), KALIMER (Republic of Korea), BN-800 and BN-1800 (Russian Federation), and EFR-1500 (European Fast Reactor, Fig. 4), except for DFBR-660 and JSFR (Japan) which uses the loop-type concept.

1-hung safety vessel; 2-debris tray; 3-core; 4-IHX; 5-above core structure; 6-secondary loop;

7-primary pump; 8-hot collector; 9-cold collector; 10-grid plate; 11-core support structure;

12-spring bearing; 13-anchered safety vessel FIG. 4. EFR: cut of reactor.

The pool reactor placed in a guard vessel has been shown to have very attractive safety characteristics, resulting to a large extent from a liquid metal cooled reactor being a low pressure system with large thermal inertia. This type of design practically excludes unfavourable consequences of failures in the external radioactive systems and loss of reactor coolant [22]. One of the outstanding achievements of the EFR programme is to make firm and

reliable cost estimates. Construction of a reactor to the EFR design may not be possible in the near future, but a well validated way forward to commercial utilisation of fast reactors has been established. This way is generally consistent with other studies, and it indicates that the goal of competitive fast rectors may be within reach (Fig. 5).

FIG. 5. Generating cost comparison EFR vs. advanced PWR.

Findings of recent work have led to improvement in LMFR designs and safety. For example, an advanced design of the system: main vessel - safety vessel - vault was developed for EFR (Fig. 6).

FIG. 6. EFR: safety vessel and vault [22].

The main vessel is completely surrounded by a leak tight safety vessel anchored to the surface of the concrete vault. A layer of metallic insulation covers the inside surface of the safety vessel, which reduces both the heat losses and the thermal cycle skin fatigue. A layer of sodium resistant concrete is provided between the safety vessel and the structural concrete of the vault. The structural concrete is kept cool by the vault cooling system. The top entry loop type design was selected for the Japanese DFBR because of the following considerations [23]:

— Major primary components such as the intermediate heat exchanger (IHX) and the pumps are outside of the reactor vessel, and this facilitates maintenance and repair;

— The system has flexibility to introduce such innovative technologies as the electromagnetic pump integrated component, which is needed for commercialization of the FR; and

— Experience gained at the prototype MONJU must be fully utilized. Considering that the top entry system is quite a new concept, the conceptual design study, the evaluation study of commercialization prospects and the water hydraulic tests using models of thermalhydraulic properties specific to the top entry system were conducted.

In progress of the loop type LMFR design development, Japan is now in a position to embark on an in-depth study of an advanced plant configuration - a compact loop type LMFR design JSFR (Fig. 7).

FIG. 7. Bird’s eye view of JSFR-1500 NSSS [3].

To achieve the economic target, several innovative technologies and LMFR design improvement measures have been adopted [3]. The reduction of plant material is accomplished by adopting the following technologies:

— Shortening the piping length and reduction of the number loops by adopting 12 Cr steel which has low thermal expansion with high strength;

— Development of integrated intermediate heat exchanger (IHX) with mechanical pump.

Although there are differences in conceptual design approaches, a number of common topics could be identified among the conceptual designs. These include improvements with regard to safety margins, design simplifications and cost reduction.

Safety margins improvements include the consideration of core catcher design for excluding the recriticality, cooling capabilities, passive backup reactivity shutdown, decay heat removal systems and a strong inherent negative reactivity feedback with temperature rise. Economic competitiveness should be improved by the following design efforts: the optimization of the number of cooling loops and equipments, and achievements of reducing its weights as well as the building volumes, findings of structural materials, fuel technology and core design to achieve high burnup, and limiting the number of safety graded systems.

The development of simple, reliable, efficient and flexible systems and components is a primary objective in the design of advanced fast power reactors. Systems and components for developmental design (advanced reactor designs which range from moderate modification of existing designs to entirely new design concepts) in general require much extensive testing and demonstration to verify component and system performance. Key issues are scaling effects for simulating plant configuration, design life and interactions among different systems.