The research for technological improvement and innovation in sodium-cooled fast reactor is a matter of concern in fuel handling systems in a view to perform a better load factor of the reactor thanks to a quicker fuelling/defueling process. An optimized fuel handling route will also limit its investment cost. In that field, CEA has engaged some innovation study either of complete FHR or on the optimization of some specific components. This paper presents the study of three SFR fuel handling route fully described and compared to a reference FHR option. In those three FHR, two use a gas corridor to transfer spent and fresh fuel assembly and the third uses two casks with a sodium pot to evacuate and load an assembly in parallel. All of them are designed for the ASTRID reactor (1500 MWth) but can be extrapolated to power reactors and are compatible with the mutualisation of one FHS coupled with two reactors. These three concepts are then intercompared and evaluated with the reference FHR according to four criteria: performances, risk assessment, investment cost, and qualification time. This analysis reveals that the “mixed way” FHR presents interesting solutions mainly in terms of design simplicity and time reduction. Therefore its study will be pursued for ASTRID as an alternative option.
In the framework of the French Act of June 28, 2006, about nuclear materials and waste management, a Generation IV and actinides incineration demonstration prototype is to be commissioned in the 2020 decade [ core design with the objective of reducing the probability of core meltdown and/or limiting the energy release during potential accidents, development of innovative third shutdown system, and improvement in core monitoring; improvement of decay heat removal (DHR) systems performances, with the development of an efficient system through the reactor vessel and the integration of DHR heat exchangers in intermediate heat exchangers; development of a strategy in support of the limitation of core melting consequences including the R&D in support to the development of the core catcher; development of innovative heat exchangers for a gas-based energy conversion systems (ECS) as an alternative to the classical steam cycle; development of innovative fuel handling systems (FHS).
In terms of economy, Generation IV systems shall be competitive, for the same overall performance, compared to other sources of energy at the time they will be put into operation [ After a learning period, the reactor must demonstrate a high load factor (e.g., up to 90%). The investment cost of the prototype shall be minimised, with technical options compatible with future commercial reactors deployment. This option is particularly relevant in FHS selection which can influence several parts of the reactor block design: from the primary vessel diameter until the balance of plant and plant layout. The ratio of FHS (including external vessel storage tank (EVST), Casks, and civil engineering) in the total reactor investment cost is estimated from 15% to 20%.
From 2007 to 2009, R&D investigations in FHS aim to review design options [
Before defining the several routes chosen in the past and that could be investigated for the future, a review of the different options has been carried out using the fast reactor database and recent technological development in SFR design. The considered options concern fuel handling systems (under rotating plugs), transfer assemblies options between reactor vessel and external storage, and also, in the particular case of fuel handling through gas corridor, fuel handling in the EVST. The work performed is a characterisation of solutions, a performance review, and an analysis of the main advantages and drawbacks of the options compared to a so-called Starting Reference Solution (SRS) based upon well-known French SFR options or some option already envisaged in French project, that is, EFR reactor [ The primary in-vessel FHS is composed of two rotating plugs (Superphenix and EFR option). A direct lift charge machine is placed in the centre of the Above Core Structure (ACS) (EFR option). It is used for removal and insertion of core components belonging to the inner handling zone. A fixed arm charge machine is placed on the large rotating plug (Phenix and EFR option). It is used for removal and insertion of core assemblies belonging to the outer handling zone. Furthermore, it forms the link between the load-unload station in the reactor and the direct lift charge machine using intermediate put-down/take-over positions at the inner core zone boundary. The load-unload station in the reactor is an equipment supported by the reactor (Phenix and Superphenix option). The fuel assembly evacuation is performed using a sodium pot for its permanent cooling (Phenix and Superphenix options). A fuel handling cask leads to evacuate fuel assembly from the primary vessel to the in-sodium external vessel storage tank (Rapsodie and EFR option, but, was designed with no sodium pot in both cases, only gas cooling system and with low residual power spent fuel). There is no rotor system (exchange new/spent fuel assembly), neither in the reactor vessel nor in the external storage.
The SRS option is represented in Figure
View of the fuel handling route called SRS.
Starting from the SRS route, several innovations were selected either on some specific and targeted study on a single component or on a global approach on fuel handling route (FHR) from the primary vessel to the EVST. In a first step, a large survey has been performed on innovative ideas without constraints or restrictions regarding maturity and cost level. Then, technological feasibility conclusion study is presented for each option, and a criteria grid analysis has been performed to highlight innovative options to persue for ASTRID. The following options have been investigated concerning the single route optimization: Above Core Structure (ACS) designed in two parts (one in the small rotating plug, SRP, and the other in the large rotating plug, LRP), Pantograph Arm Machine associated with a slit ACS, design of the “Dual Location Rotor.” design of the “Simultaneous Handling of two fuel assemblies.”
A second review [
This option is investigated in order to mutualize the necessary equipment to twin nuclear plant units, for the development of an industrial fleet of commercial SFR [
View of the fuel handling route shared by two nuclear vessels.
The transfer lock/charge and discharge ramp solution, which is reliable and for which there is a significant feedback [
Technological solutions characteristics constituting this mixed way fuel handling route are described below.
Sodium progressive tilting proposed.
The fuel transfer pot moving along the ramp comprises the following main components described in Figure
Intravessel fuel transfer pot and ramp (1: massive common section, 2: gripper cradle, 3: embedded handles, 4: translation wheel, 5: holding counterwheel, and 6: chain fastening).
Detailed view of the ramp/gas corridor interface rotating part (1: primary circuit isolation valve, 2: ramp continuity rail, 3: inclined rotating part, 4: pot-holding truck in upper position, 5: chain guide and reel, 6: sealed secured hoist 7: biological shielding, 8: tight passage and motor access, 9: metallic envelope, 10: gas corridor link/exit interface, 11: dripping path and sodium recovery, 12: rotating part base, 13: isolation valve motorization unit, 14: rotating part motorization and drive unit, and 15: set-down and pick-up of the sodium pot).
The pot translation with horizontal transfer mechanisms takes place using a cable/wheel system driven by two synchronized motors, placed outside. Above the access to the external storage unit, a hoist lifts up and down the pot holder to set down and pick up the sodium pot in the external storage vessel.
An isolation valve is located in the lower part of the access door to the external storage pit, and another one is located on the upper roof of the EVST. The pot-holder truck accessing the EVST moves along vertical rails to a low position in the vessel, to handle the assembly using the grabbing arm coupled to its rotating plug. With the geometry described in this study, it is possible to store approximately 340 assemblies for a main vessel diameter of 6.3 meters. The extrapolation to a twin commercial reactor can be obtained by symmetry of the gas corridor, since the pot transfer between the gas corridor horizontal transfer mechanism and the lift truck of the external storage unit can take place with arrival from the left or from the right.
Figure
Vertical cross-section of the mixed way fuel handling route.
Kinematics for the evacuation of an irradiated assembly in sodium pot (1: assembly set-down in the pot, 2: pot moving up the ramp, 3: arrival at the upper stop of the rotating transfer lock, 4: rotation of the transfer lock in position, arrival of the corridor transfer mechanism in position, truck moving down the ramp, and pot set-down, 5: translation of the corridor transfer mechanism, 6: docking of pot holder associated with the temporary cooling pit and descent in the pit, 7: arrival in the EVST + pot lifting using the pot holder up to detachment and disconnection, 8: pot-holder descent to the lower position of EVST, and 9: assembly fuel handling and set-down in storage unit using the fixed arm).
The cooling of the sodium pot containing a fuel assembly in the gas corridor (from the primary sodium exit to the entry into the EVST) must be studied according to the external design of the pot (pot designed to enhance heat convection with fins and forced ventilation of the transfer lock and corridor). The failure and blockage modes during displacements must also be investigated, especially with return to a safe cooling state to be ensured in any configuration (in primary vessel, in external storage unit, or in temporary storage pit with passive cooling system), and fast enough to avoid fuel failure. Depending on the time to return to safe position, a residual power value per fuel assembly authorised for evacuation may be determined. The blockage risk of the transfer mechanism during translation in the corridor, loss of power or drive motor failure, and the breaking of drive cables of the transfer mechanism must be studied and remedied using backed-up systems to return to a safe cooling position for the pot. A fuel handling rate may also be defined using this kinematic chain, according to the values usually taken for truck and horizontal transfer mechanism movement speeds, valve opening, pot dripping time, and so forth.
The main new points to be qualified for this option are the pot thermal hydraulic with the hottest assembly in sodium; the management of sodium drips and aerosols in the inert atmosphere of the gas corridor and the rotating transfer lock; hoist mechanisms embedded in the rotating transfer lock; and demonstration of return to a safe state in case of transfer mechanism movement failure.
This concept is slightly similar to the previous one. It has been studied in collaboration with Cea and Comex Nucléaire engineering nuclear company. This alternative solution presents potential advantages: fuel handling was possible without takeover position in the primary vessel: no fuel handling arm, only vertical lifts; minimisation of mechanisms in the vessel, two direct lift machines make fuel handling more flexible and restrict rotating plugs movements; no fastening and unfastening pot requirements because of a single transfer system use for both operations.
The elements forming this fuel handling option are shown in Figure
Steps of the three-rotating-plug solution in detail (1: the assembly is extracted from the core using one of the two direct lift machines, 2: the rotating plug movements position the direct lift machine above the horizontal transfer mechanism pot (HTMP), 3: the assembly is lifted down into the HTMP, 4: the HTMP is tilted in the exit ramp, 5: the HTMP is driven back up the ramp to its position in the transfer truck, 6: the HTMP is tipped vertically into the transfer truck, 7: the transfer truck moves along the corridor to its position above the external storage vessel, 8: the HTMP is lifted down vertically into the EVST, 9: the assembly is picked up by a fuel handling arm and removed from the HTMP, 10: the fuel handling arm positions the assembly above its storage location, and 11: the fuel handling arm sets the assembly down in its location).
Two plugs/three plugs kinematics comparison.
Detail of the fitted direct lift machines.
Horizontal transfer mechanism pot transfer system.
View of the ramp system.
The transfer corridor is fitted with two rails guiding the truck and sealed penetrations for manual intervention in the event of an incident on the drive system units positioned above the truck. The thickness of the corridor protects operators against radiation. The assemblies are cooled down by argon at a maximum temperature of 50°C in the transfer corridor.
The main new points to be qualified for this option are as follows: thermal hydraulic features of the pot (in sodium at handling temperature, irradiated, and new assembly), management of drips and behaviour in sodium aerosol atmosphere (mock-up representative of part of the corridor, outside biological protection), and failure modes for the horizontal transfer mechanism displacement.
This option has been studied in collaboration with Bertin/CNIM mechanical systems designer company. This preliminary design presents the following advantages: no mechanism in the vessel during the reactor operation, optimized cooling of the assembly during the transfer using the cask (through integrated active cooling systems), and the transfer lock is ensuring continuity of the confinement of the reactor building. This solution can be considered as an innovative evolution of the SRS option.
The equipment required for the primary fuel handling is described as Figure
Main components of the Bertin/CNIM fuel handling route.
Retention of the direct lift machine at the centre of the ACS.
Details of the running rails and cask holding structure.
A cask support structure, integral with the SRP, locks the cask when the truck is removed. The cask/structure link is ensured by two locking hooks and holds the cask in the event of an earthquake.
Overview of the transfer cask.
The lower part of the pot comprises a plug valve sealing the pot. This valve is controlled by a rack located at the pot guiding tube. When the pot descends into the vessel, the toothed shaft of the valve engages in the rack and rotates the pot. The bearings of the journals of the plug valve are specially designed to be disengageable from the top of the pot to open the valve even in the event of seizure (Figure
Plug valve at the bottom of the pot.
In this cask the spent fuel must be continuously cooled. The cooling system architecture is done by argon flowing in a circuit in the cask and cooled down by the units outside the cask by blowers (set with two redundant cooling units). The cooling units are installed on a floor integrated into the cask with mechanical uncoupling.
Cross-building airlock.
The two main points to qualify are the cask featuring direct extraction with its sodium pot with opening in the lower part:
the plug valve qualification: operation in air, sodium, and temperature environments, behaviour from aerosol deposits, ageing of the bearings and the seals, remote controller, and degraded mode operation, the guiding tube system qualification with integrated grab, the truck behaviour when loaded, the isolation valves of the reactor vessel qualification (in sodium aerosols); the reactor building qualification exit airlock.
The advantages and drawbacks of each solution are listed in Table
Advantages and drawbacks of the reference solution plus the innovative solution investigated.
Advantages | Drawbacks |
---|---|
SRS option | |
(i) System’s simplicity |
(i) |
|
|
Mixed way—ramp and transfer lock and gas corridor | |
(i) Limited reactor containment openings |
(i) |
|
|
Three rotating plugs, ramp and gas corridor | |
(i) Improved fuel handling duration |
(i) |
|
|
Cask and direct fuel handling | |
(i) No mechanism inside the primary vessel |
(i) |
Cost and risks aspects of the three FHR compared to the SRS option.
Criterion proposed | SRS | Mixed way | 3 rotating plugs, ramp and gas corridor | Cask and direct fuel handling |
---|---|---|---|---|
Manufacturing cost analysis | = | = |
|
|
Development and qualification | Medium | Medium | Medium | Very long |
Technical rest (of not achieving performance) | Medium | Medium | Medium | high |
Safety risks | Medium | low | low | high |
2 | 1,5 | 1,5 | 3 |
A graphical representation in Figure
Graphical representation of the assessment of solutions.
In Figure
Performance of the options (“radar” diagram).
Thus, based on the analysis above, the following orientations are recommended. Studies on the mixed way—ramp and transfer lock and gas corridor—are pursued due to advantages of this solution, in order to go in detail in its potentiality, especially in the improvement of the load factor and its simplicity in designing the cask. The option on the three rotating plugs, ramp and gas corridor is not retained due to technological breakthrough. The option on cask and direct fuel handling provides technological difficulties and is not retained.
This study has revealed that the corridor option has to be reconsidered even if the SRS remains the reference option for ASTRID reactor. Advantages of this mixed way option such as load factor improvement and design simplicity have to be confirmed. The study of a complete fuel handling route (from the primary vessel until the EVST) is interesting in the way that it helps providing new ideas (e.g. the gas corridor or the cross-building airlock). Some specific innovative aspects can therefore be pointed out even if the whole FHR appears to be too challenging for a SFR reactor. Nonetheless the SRS version also involves technological issues, especially the sodium pot cask which has to transfer an assembly at a high residual power (around 40 kW). As the continuation of this study, effort will be put on the mixed way—ramp and transfer lock and gas corridor. In parallel the thermal and mechanical aspects of the sodium pot cask have to be performed.
Above Core Structure
Advanced Sodium Technological Reactor for Industrial Demonstration
Balance of plant
Core Cover Structure
Decay heat removal
Energy conversion system
External vessel storage tank
Fuel handling system
Fuel handling route
Intermediate heat exchanger
Horizontal transfer mechanism pot
In-service inspection and repair
Large rotating plug
Research and development
Rotating plug
Specific component optimization
Sodium-cooled fast reactor
Small rotating plug
Starting reference solution.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Bertin/CNIM and COMEX Nucléaire engineering companies for their works and contribution in bringing alternative and innovative solutions to systems which were so far from their current works. In particular the authors provide special thanks to MM. G. Rainaud and D. Dumont for their works on the cask and direct fuel route and MM. D. Roulet and M. Macia and their engineering team for their work on the three rotating plugs, ramp and gas corridor route. M. Saez (CEA) kindly accepted to read this text at all the steps and made important comments, observations, and suggestions now fully integrated in the present version.