SYNTHESIS OF THE TECHNICAL FEASIBILITY EVALUATION OF
SPENT FUEL CONDITIONING IN BELGIUM
C. Dierick, M. Detilleux
TRACTEBEL Energy Engineering
G. Nuyt
BELGONUCLEAIRE
L. Janssen, G. Demazy
SYNATOM
ABSTRACT
At the end of 1993 the Belgian government decided that both reprocessing and direct disposal have to be considered as equal options in the back-end policy for spent fuel in Belgium. As a result of this decision Synatom, the Belgian utilities' subsidiary in charge of the nuclear fuel cycle for all PWR reactors in Belgium, entrusted the engineering companies Tractebel and Belgonucléaire to develop a dedicated spent fuel container, to perform a feasibility study of an appropriate encapsulation process and to perform a preliminary design study of a complete spent fuel conditioning plant.
The feasibility and design studies of the Open Cycle project started mid 1994 and came to an end, as scheduled, by the end of 97. The design of the spent fuel container was presented at the ICEM'97 Conference in Singapore in a paper called "Development of a container for spent nuclear fuel disposal in clay". The present paper is dealing mainly with the demonstration of the feasibility of the process and a brief description of the design of the conditioning facilities.
INTRODUCTION
The preliminary design study of the spent fuel conditioning plant covers the conditioning itself, i.e. the development of the concept of a sand-filled bottle containing a fuel assembly, and the evaluation of the technical feasibility of the conditioning process, involving the evaluation of adequate processes and the design of the industrial facilities suitable for implementing these processes.
INTERFACES UPSTREAM AND DOWNSTREAM OF THE OPEN CYCLE
The various options available for spent fuel storage prior to conditioning are not reviewed in the present study. We just bear in mind that this storage is to allow the assemblies to cool down enough before their deep burial in clay strata.
The studies performed are based on the assumptions that resulted of vitrified waste burial concepts, which Niras/Ondraf, the national organization for management of radwaste, extrapolated to spent fuel conditioned in bottles. It is further assumed that transfer and burial of the conditioned bottles will take place in the same operation period. For this reason the conditioning plant will be provided with only a small buffer-storage facility for the conditioned bottles. In turn, in the present philosophy, the conditioning area can be geographically separate from the place of transfer and deep burial.
Spent fuel storage, either wet or dry, upstream of the conditioning facility, and subsequent transfer and deep burial of the conditioned bottles, are outside the present scope of the studies for the OPEN CYCLE project.
LIMITS OF THE CONDITIONING SITE : FUNCTIONS COVERED BY THE STUDY
All the functions to be performed during operation of the spent fuel conditioning plant (place figure 1 here) and which have been the subject of the conceptual design study are listed below.
Basic functions
The assembly conditioning building comprises several functional zones :
Safety Functions
It must be ascertained that all the operations described above take place under the required conditions of safety. Based on a safety analysis and the relevant regulations, appropriate structural and functional barriers are planned to prevent any unacceptable releases of radioactive matter to the environment, this during normal operation as well as during accidental situations (structures and buildings are earthquake and/or aircraft crash resistant, areas with cascades of underpressure, airtight locks, ...).
In particular, functional barriers in the unloading cell are designed so as to be able to treat in routine operation 1% leaking assemblies (conservative assumption).
Furthermore, in-depth studies have been performed regarding passive and active protection systems.
A radioprotection system has been developed, comprising a network of radiation monitoring equipment for normal radiation monitoring, and an independent self-supporting network meeting the "Safeguards" requirements (see 2.3) imposed by the relevant inspection organisations (Euratom, IAEA). The radioprotection system also comprises structural barriers (shielded walls and doors, leaded glazing, ...).
A fire detection and protection system has been designed for the main conditioning building.
Particular attention was devoted to the safety-constraints relating to the various spent fuel storage areas, with detailed investigation of the criticality aspects in these areas and of the evacuation of heat from these areas through natural convection circuits.
Finally, the safety analysis which led to the above measures being adopted also allowed to assess the radiological impact of the operation of this site on the environment, based on radioactive releases calculated for normal operation conditions as well as for incidental operation in cases such as the dropping of a fuel assembly and the total loss of ventilation (appropriate combination of key scenarios cover the consequences of the other imaginable scenarios). In the same context, dose rates at the limit of the site as a consequence of radioactivity sources in the conditioning building have been examined as well.
Surveillance Functions
The rules of the Non-Proliferation Treaty on Nuclear Arms demand guarantees against any diversion of nuclear materials. To this effect, a comprehensive system of surveillance means has been developed. It comprises radiation detectors and surveillance cameras so that the fuel assemblies can be traced throughout the conditioning processes and so that the Material Inventory can be established at any time of the fuel present in the building. Anti-intrusion measures are also part of this study.
Other
Auxiliaries
Operating such a site requires that a number of general auxiliary functions be secured.
Utility systems and equipment such as diesel generators for electrical redundancy, boilers for steam production, supply of process gases, a fire water pumping station, etc... are housed in a utilities building. Also, a smaller separate building houses the high-voltage cubicle and the transformers.
An office building and a guard house are also provided at the site.
Decontamination of Transport Containers
After the assemblies are unloaded, the required operations are inspection, rinsing and internal and external decontamination, checking and maintenance and possibly repair of the transport containers, before reuse or their removal to a dismantling facility. These operations take place in a Decontamination, Maintenance and Repair Workshop (called ADER) adjacent to the conditioning facilities building. The interventions are quite limited in scope, but are necessary to allow reuse to be made of the transport containers.
The in-depth decontamination of these containers was not part of the present study, nor was the dismantling of their used internals (the neutron-absorbing baskets). However, the storage of used baskets is addressed.
Treatment of Secondary Effluent and Waste
A Secondary Effluent and Solid Waste Treatment centre (TES) was created to achieve the desired autonomy of the conditioning site. Accordingly, the TES building comprises storage facilities for drums (for the solid waste) and tanks (for the liquid effluent), as well as evaporation, compaction and encapsulation installations.
The exit interface of the TES building is provided with a storage area for conditioned drums prior to their transfer to a final repository or deep burial site.
DESIGN CONSTRAINTS FOR THE CONDITIONING SITE
Areas of Uncertainty
Apart from the fact that the site is unknown and neither is its precise time of coming into operation, a number of uncertainties were identified in the course of the study.
Generally, these uncertainties led to prudent approaches, the taking into account of envelope values, worst cases, conservatism and redundancies. As for the uncertainties relating to the future evolution of technology and regulations, it should be borne in mind that the design is based on proven technology and considering the present regulations.
Guaranteed Throughput
The installation is to have a conditioning capacity of 800 bottles per year (200 working days). This quantity is based on the daily rate considered for the deep burial of the vitrified waste canisters (4 per day). Moreover, expressed in quantities of UO2, this rate of conditioning/burial is comparable to that adopted elsewhere (e.g. 350 t/year of UO2 in the German scenario).
The technical solutions proposed for conditioning, and particularly the choices of operating two conditioning chains in parallel and of multiplying the buffer storages, are founded mainly on considerations aimed at guaranteeing the throughput:
Multiple Functionality of the Building
A particular requirement relates to it having to be possible to condition spent fuel assemblies of various types (UO2, MOX 1.5 %) and sizes. Moreover, depending on the transport scenario, it shall be possible to take delivery and process various types of transport containers (TN12, TN13, TN24D, TN24XL), including containers that comprise defective assemblies or assemblies originating from the BR2 or BR3 research reactors. For these reasons, sufficient flexibility has been built-in in the design of the installations (exchangeable flanges and positioning cones, envelope heights for the cells, adaptable container and bottles seats, multiple-position unbolting plates, ...
Also, specific arrangements are included to take into account unplanned interventions.
DESIGN OF THE BOTTLE AND OF THE PROCESSES AT THE HEART OF THE CONDITIONING INSTALLATIONS
A single-place container, adopted as the reference package for deep burial, was studied in detail. The necessary studies were undertaken in a R & D context in order to validate the bottle concept and the associated conditioning techniques. Demonstration of the feasibility of the core processes of the installations required considerably more effort than for the other operations since, for the latter, useful experience was already available.
Conceptual Design of the Bottle
A first study performed at the very start of the project was devoted to the definition of the bottle design. This study resulted in a thin-wall concept for the bottle, provided with dished ends, i.e. one at the bottom, the other to be welded on as the cover after introduction of the spent fuel assembly. The cover has a plug that is to be welded in order to seal the bottle once it has been filled with sand. The bottle is fitted with bogies for easy moving in the envelope tubes in the tunnels at the burial site. The bottle has a welded ring on its outside, for accurate positioning of the welding equipment during the bottle closing and sealing operations. A guiding basket is provided inside the bottle, so that during the assembly insertion operations the assembly can be oriented and centred as required (place Figure 2 here).
Particular choices made during this first study on the bottle design relate to the material of which the bottle is made (AISI 316 L), its wall thickness (about 10 mm), the filling material (sand), the dimensional characteristics of the various bottles and the handling and transfer systems (grab head and bogies).
Construction of a Bottle Prototype
A prototype bottle was built at real diameter, though with a length reduced to some 2 m, and adapted for laboratory tests. The test bottle was not fitted with the accessories referred to above, but was provided with glazed view ports so as to visually monitor the filling with sand during the tests.
The basket inside the bottle was the type designed for this project.
A prototype assembly was also built, differing from a real assembly only by its length. A number of guide tubes of the assembly skeleton were replaced by Plexiglas tubes so as to visually monitor the filling process inside the assembly with an endoscope.
Choice of the Appropriate Sand
At a first stage, several types of sand were examined on the bases of their geological origin and the availability and homogeneity of supplies. Following these investigations, three types of sand were retained for further tests and full characterisation in the laboratory.
With a view to validation of the technological properties, deemed essential to securing a reliable conditioning process, the fluidity and the hygroscopic evolution of the sands were verified. Indeed, for the adopted process it is crucial that the sand should flow easily, and there must be total confidence that the sand will not spontaneously rehydrate during storage.
Also, the sand that proved the most suitable following the above tests, was subjected to oedometric tests in order to assess the impact of settlement (density) on the elasticity of the sand matrix within the bottle.
Analysis of the Mechanical Strength of a Filled Bottle
Based on the tensile properties of the sand and of the steel envelope of the bottle, a theoretical model was developed in order to compute the behaviour of the filled bottle under the geological loads. The computations show that bottle deformation (with a 9.53 mm wall thickness) remains within the elastic domain.
Sand Filling Tests
Filling tests were carried out using the bottle containing the dummy assembly, under various vibratory regimes, i.e. with two vibrators fixed to the outer wall of the bottle that applied various frequencies and amplitudes. The regime that achieved the maximum sand density was retained as the optimal reference regime.
Vibration was applied during and after the filling of sand in the bottle, and its impact on sand settlement was quantified and compared to the case of filling without vibration.
It results from the tests that this vibrating regime is instrumental to achieving the appropriate density and elasticity of the sand matrix.
Complete filling of the bottle, i.e. the absence of air pockets among the elements embedded in the sand, was verified by means of differential weight measurement, visual inspection through the view ports of the bottle walls and through endoscopic examinations inside the assembly skeleton.
Conceptual Design of Sand Filling Systems
Given the importance of these systems to the demonstration of the feasibility of the conditioning process, a detailed study addressed the systems for sand transport and storage in silos, the equipment and systems for dosing, filling, sand filling rate and level controls, positioning and centring of the bottle and the filling hole. Particular attention was devoted also to rendering the bottle contents inert so as to prevent corrosive matter being formed.
The above concept, and in particular the design of the appropriate equipment was performed with the help of specialist firms in order to ascertain to the greatest possible extent the feasibility of these concepts (place figure 3 here).
Welding Tests
Studies were started regarding the various welding techniques in order to identify the appropriate technique for circumferential welding of the bottle cover and for welding of the plug that seals the filled bottle. Among the evaluated techniques (MIG, TIG, Plasma), the Plasma technique was selected for a series of tests on representative samples.
The tests demonstrated the feasibility of Plasma welding of plugs (of 10 mm and, as an alternative, 13 mm thickness). The quality of the welds performed on samples was verified through ultrasonic tests, x-rays and metallographic sections. The results, again obtained with the collaboration of a specialist firm, look promising.
The first steps were taken to validate the welding procedures and equipment, identify the main welding parameters and evaluate the methods for the final checking of the leak-tightness of the bottle in the real conditioning environment.
Finally, the welding process has been integrated in the lay-out of the conditioning installations.
The Research & Development efforts described above made it possible to demonstrate the feasibility of the core processes of the installation. Further studies would confirm the choices made, improve the reliability of the technical solutions adopted, and would allow the tuning of the proposed techniques in a constraining nuclear environment.
CONCLUSIONS
As regards conditioning, the feasibility evaluation attained its objectives, which were the demonstration of the technical feasibility of providing industrial facilities for spent fuel conditioning according to the adopted spent fuel conditioning concepts.
Based on experience available regarding construction and operation of nuclear installations in Belgium and abroad, implementation of the means that can meet the functional requirements whilst also meeting the applicable regulations and the specific precautions required for this type of installation did not reveal any fundamental problem that could not be overcome through existing technology and know how.
As concerns the crucial part of the installations, being the bottling of the spent fuel assemblies, which involves filling the bottles with sand and then sealing them by welding, various Research & Development type approaches were successfully made in order to validate these critical operations, since these are original compared to other concepts explored abroad.
The proposed solutions were submitted for advice to several foreign experts (of Germany's GNS, of Sweden's SKB and of Britain's EWE, the latter under a design review contract with Synatom). All of these experts issued a positive advice about the systematic and substantiated approach of the problems within a comprehensive and in-depth preliminary design. This strengthened the confidence of Synatom and its partners that the design is based on principles that are both sound and realistic.
A parliamentary debate on the different options in the back-end policy for spent fuel in Belgium is scheduled for 1998. In the mean time, Synatom decided to continue with its partners Tractebel and Belgonucléaire to further validate the proposed conditioning methodology.
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