SPENT FUEL STORAGE IN BELGIUM R. Vermeyen Tractebel Energy Engineering Belgatom 1. Introduction The Belgian utility Electrabel presently operates 7 PWR nuclear power plants, distributed over two sites (Doel and Tihange), with a total output of 5909 MW. This amounts to about 58 % of Belgium s power production. The spent fuel of the first three units (Doel 1 and 2 and Tihange 1) was sent to Cogema in La Hague (France) for reprocessing. For the other units (Doel 3 and 4 and Tihange 2 and 3), Synatom, Electrabel s subsidiary in charge of the nuclear fuel cycle for all PWR reactors in Belgium, decided to study the possible solutions for a temporary storage of the spent fuel. Moreover, at the end of 1993, the Belgian government decided that both reprocessing and direct disposal had to be considered as equal options in the back-end policy for spent fuel in Belgium. The resolution further requested that the performance of a second contract concluded in 1990 (for reprocessing services after 2000) be postponed for five years and to use this period to evaluate the once-through route for spent fuel management. As a result of this decision, Synatom entrusted Belgatom to develop the appropriate solutions to solve both issues. On the one hand, as the available storage capacity was becoming short in the existing spent fuel pools, an additional interim storage capacity had to be provided. A technical-economic analysis was performed by Belgatom. Considerations of design and safety criteria as well as flexibility, reversibility, technical constraints, global economic aspects and construction time led to adopt dry storage with dual purpose casks (in operation since end 1995) for the Doel site and wet storage in a modular pool for the Tihange site (in operation since 1997). On the other hand, for the longer term (final disposal), Synatom required to develop a dedicated spent fuel flask, to perform a design study of an appropriate encapsulation process and to prepare a preliminary feasibility study of a complete spent fuel conditioning plant The present document describes the two types of interim storage facilities currently in use in Belgium. The emphasis will be placed on the dry storage facility and its licensing and qualification process. The long term storage flask will be briefly presented in the final chapter.
2. The Interim Spent Fuel Storage Facilities General Several solutions of the interim storage of spent fuel assemblies can be imagined: Reracking the existing pools Consolidation of the spent fuel assemblies Storage pools in a bunkered building Dry storage in dual purpose casks (storage and transportation) in a light or in a bunkered building Dry storage in canisters located in a bunkered building Dry storage in vaults The first two solutions were quickly eliminated. Any desirable expansion of the storage capacity would be very limited with re-racking because the existing racks are already of the high density type. Similarly, consolidation offers too little expansion capability. Moreover, this technique was deemed insufficiently proven. Before launching the design process of the new interim storage facilities of Doel and Tihange, the main criteria to be met were listed on the basis of which the ideal type of facility could be determined. The criteria are as follows : Leaktightness : Confinement of the fission products of the fuel assemblies under normal and accident conditions was one of the major concerns in the choice of the solutions. Ideally, the target that would be considered under normal operating conditions is the zero release concept. For accident conditions (the most significant one being an aircraft crash), the target was to limit the release to the same value as the limit imposed for the LOCA. Reversibility : Since a definitive decision not to reprocess the fuel has not been made yet and because of the temporary character of the storage in these facilities, it is important to be able to easily retrieve the spent fuel assemblies after a few years. From this point of view, the best solutions are dual purpose (transport and storage) casks and storage pools. Flexibility : Since eventually a decision may be made to restart fuel reprocessing, the solution must be as flexible as possible. The modular character of the solution is an advantage in this respect. Monitoring : Preference was given to systems that permit permanent monitoring so that no radioactivity release would occur during storage.
Cask Overall Dimensions : In the case of storage in casks or in canisters that could eventually be sent to the reprocessing facility, attention had to be paid to avoid too bulky devices that could be incompatible with the existing plants, with the reprocessing facility, or with the requirements for standard rail transportation on the European network. The large bulk/capacity ratio of concrete casks is a drawback of this latter solution. Aircraft Crash Resistance : Since Belgium is a small country with a high population density and a rather high level of air traffic, the storage facilities have to be able to withstand an aircraft crash and special provisions have to be made for radiation shielding of the public. The ideal solution would therefore be one which is inherently safe. Otherwise, special hardening, such as a bunkered protection building, should be provided. In this respect, dual purpose metallic casks have the advantage that they need only a little adaptation to be qualified for this type of load. Owing to the fact the types of aircraft taken into consideration for the Belgian sites are heavy commercial and military aircraft, concrete silos are not directly suitable because it is not possible to guarantee their integrity in the direct vicinity of the impact. In addition, their mass, even if a certain number of them are linked together, is not high enough to ensure the overall stability of the modules. In the case of pools, the building has to be bunkered. This solution has already been implemented for the last four Belgian nuclear plants. Its major drawback, in addition to the price, is that such a building can hardly be extended, and thus the final size has to be decided at the start of the construction. Construction Time : This criterion can be of some importance in some countries. In Belgium, this was not a leading factor. The main factors affecting the construction time are the licensing process and the construction arrangements. From the point of view of construction, the more modular the system, the quicker the facility will be available. Qualified design : At the time of the decision, none of the techniques, except those of the pools and the metallic dual purpose casks, had been qualified to withstand a large aircraft crash. Therefore, preference was given to the qualified techniques already mentioned which were well known in Belgium. With these criteria in mind and through a thorough economical comparison, the ideal solution for both the Tihange and the Doel site was determined : dry storage in metallic dual purpose casks in a non-bunkered concrete building at the Doel site wet storage in pools in a bunkered concrete building at the Tihange site. The Doel Dry Storage Facility Initially, the Doel facility was designed for the storage of 53 casks. The construction started in May 1994 and the commissioning was granted in September 1995. An extension increasing the capacity to 165 casks was commissioned in March 1998.
The fuel casks are aircraft crash resistant and have a capacity of 20 to 37 fuel assemblies depending on their design. The maximum enrichment reached is up to 4,25 % with an average mean burn-up of 50,000 MWd/t and 55,000 MWd/t for an individual assembly. The Tihange Wet Storage Facility For the Tihange site, the following peculiarities had to be taken into account : Impossibility to accomodate a high capacity fuel cask in the fuel building of Tihange 1 Necessity to modify the fuel evacuation pools to interface with high capacity fuel casks An eventual pool extension was anticipated at the beginning of the Tihange site, with the necessary additional cooling capacity which was not the case at Doel. Taking into account the issues listed above and using several discount rates, Belgatom calculated the discounted cost of both solutions for each site. The calculation showed that a break-even point exists beyond which the pool solution is cheaper. In function of these break-even points and the uncertainties related to the needed capacity, the pool solution was chosen at Tihange. The facility was commissioned in July 1997. It is designed to store 3702 fuel assemblies in eight identical pools. 3. Design Basis And Environmental Impact The following design criteria apply to both facilities : Aircraft crash resistant o Type : military fighter o Weight : 14 600 kg o Impact Speed : 150 m/s o Impact Surface : 2,6 m² Acceptable gas and aerosol release under accident conditions 2 : < 0,02 Sv within one week at the site boundary Dose rate from a cask (Doel site) : o Surface dose rate : < 2mSv/h o At a distance of 2 m from the cask : < 100 µsv/h o Accident conditions (1 m from the surface of the cask) : < 10 msv/h 2 The addressed accident conditions are : aircraft crash, fire, earthquake and cask drop.
Dose rate from the whole facility under normal operating conditions at the site fence : < 1 msv/y (an additional request of the Belgian Safety Authorities is that the annual dose rate for potential inhabited areas outside the site fence should not exceed 100 µsv/y). These requirements necessitated the addition of boron to the concrete used for the roof of the building (0,5 wt %) Criticality keff < 0,95 in pure water (including an uncertainty margin of 2σ) Earthquake resistance : o Doel : Not required for the building, since it is allowed to collapse after such an event. For the casks, the aircraft crash resistance qualification includes seismic qualification. o Tihange : 0,17g horizontal; spectrum : site specific Lifetime : 50 years Additional criteria for dry storage : Fuel rod temperature (on the rod outer surface) o Normal conditions : < 300 C o Accident conditions : < 380 C Fire resistance : 600 C for 60 minutes Drop resistance : 9 m with shock absorbers (transport conditions), 2,5 m without shock absorbers (storage conditions) 4. Licensing Approach And Qualification Programme For the Tihange solution, the design of the new facility is very similar to that of the pools of the existing units. The licensing approach is classical and no particular issues were raised during the licensing process. However, for the Doel solution, three issues needed clarification : Cask qualification after an aircraft crash : In order to avoid having to bunker the building, it was necessary to demonstrate the aircraft crash resistance of the casks. This was performed successfully in a test using a 1/3 scale mockup. A missile sized to model the fuselage of an F-4 Phantom fighter hit the model at a speed of 150 m/s on the anticrash lid at the height of the primary lid. This represents, according to the calculations, the worst load on the gaskets. After the impact, no leak could be detected. Cask qualification after a free drop : o The casks will be handled without shock absorbers throughout the storage facility. The crane of this facility is not single failure proof and therefore a drop from a height of 2,5 m has to be assumed. o The test was performed using the same 1/3 scale mockup as for the aircraft crash test. After the drop, the measured leak was still far below the allowed limit.
Cask cooling after collapse of the building : o The building surrounding the cask is neither earthquake nor aircraft crash resistant. o It was demonstrated that after such an event, even in the case of the collapse of the building, the cooling capability was sufficient to keep the fuel cladding temperatures below the limits allowed in the case of an accident. 5. Spent Fuel Conditioning In View Of Final Disposal The studies performed by Belgatom together with Synatom have proven the feasibility of a fuel conditioning process in view of its disposal in Boom clay layers as an alternative to fuel reprocessing. If requested by the customer, a solution fitted to other host rocks should also be elaborated by Belgatom. In the case of the Boom clay layers, which are the reference host rock for Belgium, the feasibility studies addressed two main topics: the spent fuel flask and the conditioning facility. Spent Fuel Flask A spent fuel flask has been developed that meets all the presently known requirements related to its final disposal in the Boom clay layers, the reference host rock for disposal of high level radioactive waste at present studied by Niras/Ondraf, the national agency responsible for management of radwaste in Belgium. Among others, the flask complies with the following design criteria: safety and sub-criticality control corrosion resistance in the clay host rock limited thermal output of the flask, imposed by the host rock characteristics compatibility with the disposal concept in small horizontal tubes. The developed flask concept consists of a non-shielded, thin-walled, AISI-316L stainless steel container enclosing one complete PWR fuel assembly. The void volume of the flask is filled with dry sand, compacted by vibration, in order to withstand the host rock pressure and to guarantee sub-criticality control. The air inside the flask is replaced by an inert gas. To facilitate the horizontal emplacement of the cylindrical flask in the disposal galleries, two sets of bogies are fixed to the shell of the flask. The external diameter of the flask is about 36 cm and its length varies from 320 cm to 510 cm, depending on the size of the fuel elements. Its total loaded weight varies from 1 to 2 tons. MOX fuel as well as UO 2 fuel would be conditioned using the same type of flask. Recently a study has been started in order to check the feasibility of a flask containing four fuel assemblies instead of one. This solution has been temporarily abandoned because of fuel criticality problems in the very long term.
Spent Fuel Conditioning A conceptual design and feasibility study has been performed to work out a spent fuel conditioning plant able to encapsulate the spent fuel discharged from the Belgian nuclear power reactors. This study covered the development and the technical feasibility of an adequate conditioning process as well as the design of the industrial facilities suitable for implementing this process. The concept to be developed had to fulfil the following design criteria : the conditioning facility is considered to be built on a new site (independent of the existing nuclear sites), all the spent fuel elements are shipped to the site in transport casks, conditioning of the spent fuel takes place after minimum 50 years of preliminary cooling, the annual throughput of the plants: 800 PWR spent fuel elements (about 400 thm/year), different sizes of fuel assemblies are to be considered, UO 2 as well as MOX fuel must be encapsulated, total operational lifetime of the plant is about 20 years, final disposal of the conditioned flasks at the same rhythm as the production process (disposal of 4 flasks per day), sufficient flexibility of the plant operation towards the disposal process, guaranteed by different intermediate storage areas in the conditioning facility (i.e. a total storage capacity of 400 conditioned disposal flasks corresponding to half a year of production), cooling by natural convection, all crucial buildings are designed to withstand an aircraft crash. Based on those criteria a preliminary feasibility study has been carried out. Different buildings have been designed providing the technical support in order to independently operate the site. Detailed drawings of the most important parts of the plant have been established. The most crucial part of the operations consists of the conditioning process itself. It can be divided in the following steps: arrival of the transport casks containing the spent fuel assemblies unloading and buffer storage of the spent fuel assemblies conditioning of the assemblies : two parallel conditioning lines should enhance the flexibility of the production process loading of one assembly in an empty flask welding of the cover of the flask
inertizing and filling of the flask with sand through an opening in the flask cover welding of the cover plug decontamination and intermediate storage of the filled flask destorage of the filled flask, final check, decontamination if necessary loading of the transport casks and shipment to the final disposal site. Besides the main conditioning process, all other necessary functions have been provided for on the site (cask cleaning and maintenance, secondary waste treatment, auxiliaries, etc.). 6. Conclusions Whilst waiting for a decision concerning the reprocessing of the spent fuel and with the preoccupation of keeping open both alternatives, Belgatom and Synatom had to tackle two problems: the interim storage and the direct disposal of the spent fuel. On the one hand, Belgatom and Synatom have implemented interim spent fuel storage facilities through a careful examination of all possible solutions. On the other hand, since 1994, Belgatom and Synatom are intensively contributing to the back end cycle studies, by investigating spent fuel conditioning with a view to direct disposal as a valid alternative to reprocessing.