PREPARING AND REMOVING SPENT NUCLEAR FUEL FROM RESEARCH REACTORS OF NATIONAL RESEARCH CENTRE KURCHATOV INSTITUTE FOR REPROCESSING

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1 E9 PREPARING AND REMOVING SPENT NUCLEAR FUEL FROM RESEARCH REACTORS OF NATIONAL RESEARCH CENTRE KURCHATOV INSTITUTE FOR REPROCESSING YU.A. ZVERKOV, V.P. EVSTIGNEEV, S.G. SEMENOV, A.D. SHISHA National Research Centre Kurchatov Institute, Rehabilitation Department, Moscow, Russia A.A. DROZDOV National Research Centre Kurchatov Institute, Institute of Nuclear Reactors, Moscow, Russia Abstract Spent nuclear fuel removal activities in the National Research Centre Kurchatov Institute were resumed in Spent fuel on NRC KI site is stored in form of research reactor spent fuel assemblies, as well as in form of fuel elements and their fragments. Accumulated fuel assemblies and fuel elements are stored in reactor storage pools or in separate storage facilities arranged in form of buried wet or dry tanks. Some spent fuel assemblies and fuel elements stored there are resized, deformed, or have defective claddings. Conditions of fuel assemblies and fuel elements transportation to reprocessing plant are set by OST state regulations. Spent fuel assemblies shipping rules require the types of fuel contained in fuel elements to be identified. To this end, different methods are used for fuel elements identification, such as those based on fuel element weight, U-235 enrichment, X-ray characteristic spectra, etc. These rules prescribe delivery in leak-tight transport canisters for fuel elements with cladding defects resulting in fuel contact with water of storage facilities. This requires development of the repackaging procedure for defective fuel assemblies. Two types of shipping casks were used to transport spent fuel for reprocessing: an old-generation shipping cask TUK-19 and a new-generation one TUK-128. Compared with TUK-19, TUK-128 is a dual-purpose shipping cask, because it can be used for both transportation and storage of fuel assemblies. It is also larger and can accommodate up to 20 fuel assemblies (instead of 3 4 in TUK-19). For spent fuel loading into these shipping casks the following technological approaches were implemented at the NRC KI site: direct loading performed in the centralized spent fuel dry storage facility; loading via a shielding box arranged in the MR reactor central hall; and underwater loading performed in the reloading pool arranged in the room of the spent fuel wet storage facility. By now more than 500 spent fuel assemblies of MR, IR-8, VVR-2 and OR research reactors were prepared and removed for reprocessing to the Mayak enterprise. In 2012 this work will continue. 1. INTRODUCTION Many years of research and development activities on the National Research Centre Kurchatov Institute site included creation and operation of 12 research reactors, about 20 critical and subcritical experimental facilities, hot material research laboratories for irradiated fuel exploration, and some other unique physical installations [1]. Their long operation in the framework of different research programs resulted in the Kurchatov Institute onsite storage facilities now containing significant amounts of spent nuclear fuel (SNF) from research reactors. SNF is stored in form of spent fuel assemblies (SFAs) as well as in form of fuel elements and their fragments. The accumulated SFAs and fuel elements are stored in reactor storage pools or in storage facilities arranged in form of buried wet or dry tanks. Some SFAs and fuel elements stored there are resized, deformed, or have defective claddings. Conditions of SFAs and fuel elements transportation to reprocessing plant are set by OST state regulations. SFA shipping rules require the types of fuel contained in fuel elements to be identified. To this end, different methods are used for fuel elements identification, such as those based on fuel element weight, U-235 enrichment, X-ray characteristic spectra, etc. International Atomic Energy Agency 1

2 These rules prescribe delivery in leak-tight canisters for fuel elements with cladding defects resulting in fuel contact with water of storage facilities. This requires developing of the repackaging procedure for defective SFAs. Up to 1990, SNF was regularly removed to the Mayak enterprise for radiochemical reprocessing. In , because of financial difficulties, SNF was not removed from the NRC KI site. In 2004, after a 13-year interval, annual removal of SNF was resumed [2]. In , three shipments of SNF from the NRC KI site to the Mayk enterprise were arranged and performed. As result, 192 SFAs of MR reactor were shipped for reprocessing in TUK-19 shipping casks. In 2006, Rosatom prescribed the Kurchatov Institute to organize and perform SNF removal in new-generation TUK-128 shipping casks. In these casks, 40 SFAs from IR-8 reactor were delivered to the Mayak enterprise for reprocessing. In , five shipments of SNF from the NRC KI site to the Mayk enterprise were arranged and performed [3]. As a result, the total of 272 SFAs of VVR-2 and OR reactors was shipped for reprocessing in TUK-19 shipping casks. 2. MR SPENT FUEL ASSEMBLY REMOVAL TECHNOLOGY The technology of direct SNF loading into shipping casks was used for SFAs of MR reactor [2]. These SFAs were stored in the centralized SNF dry storage facility. In , three shipments of MR reactor SFAs were arranged and performed. TUK-19 shipping casks were used for removing these SFAs to reprocessing plant. TUK-19 is a one-purpose shipping cask, because it s used for transportation of research reactor SFAs only. It can accommodate 3 or 4 SFAs. TUK-19 shipping casks are usually transported in container carriages allowing installation and transportation of eight casks each. A container carriage with eight TUK-19 casks inside can be transported by railway. Each MR SFA shipment included delivery of two container carriages with sixteen empty TUK-19 casks by a special train to the NRC KI site and their installation in a special working area arranged near the Institute s railway line. Here a truck crane removed dumpers and extracted TUK-19 casks from container carriages; then a forklift loader successively delivered TUK-19 to the dedicated area near the centralized SNF storage facility building, where the truck crane installed these casks on a dedicated accumulator rack. During the loading process a truck crane installed TUK-19 casks on a transportation trolley, which delivered them to the room of centralized SNF storage facility. In this room the protective lid was removed from TUK-19 and then all loading operations were carried out. The technology of direct MR SFA loading into TUK-19 casks included the following operations performed mostly by remote control means using the regular cathead crane with lifting capacity of 0.5 ton, special manipulators and the video monitoring system. Installing the transportation trolley with TUK-19 at the special working place; Extracting SFAs from a cell of SNF storage facility; Identifying and examining the technical condition of extracted SFAs; Loading SFAs into TUK-19; Installing and sealing the protective lid on TUK-19 loaded with SFAs; Monitoring TUK-19 leaktightness and surface radiation contamination; Transporting TUK-19 by the transportation trolley to the dedicated area near the centralized SNF storage facility; Installing TUK-19 on an accumulator rack using the truck crane. After completion of all loading operations, the forklift loader successively delivered TUK-19 casks to the working area near the Institute s railway line. Here the truck crane loaded TUK-19 casks into container carriages of a special train. 2

3 3. IR-8 SPENT FUEL ASSEMBLY REMOVAL TECHNOLOGY The technology of SNF loading into shipping casks via a shielding box was implemented for SFAs of IR-8 reactor [2]. New-generation TUK-128 shipping casks were used for removing SFAs of IR-8 reactor to reprocessing plant. It was the first use of these shipping casks, so this shipment was considered as a trial one, with a particular goal of confirming TUK-128 efficiency. Compared with TUK-19, TUK-128 is a dual-purpose shipping cask, because it can be used for both transportation and storage of research reactor SFAs. It is also larger and can accommodate up to 20 SFAs (instead of 3 4 in TUK-19). TUK-128 casks are transported in ISO-containers allowing installation and transportation of a single cask each. ISO-containers with TUK-128 inside can be transported by railway or truck. Two empty TUK-128 shipping casks in two ISO-containers were delivered by trucks to the NRC KI site and installed in a working area specially arranged near the Institute s railway line. Here a truck crane removed dumpers and extracted TUK-128 from ISO-containers; then a forklift loader delivered TUK-128 to the dedicated area near the MR reactor building. Here a truck crane installed TUK-128 casks on a special transportation trolley, which delivered them to the MR central hall. In the MR central hall the overhead crane with a lifting capacity of 10 tons installed TUK-128 casks at the dedicated working place and then the protective lids were removed from casks. All loading operations were mainly carried out using remote control means. Loading operations included the installation of a guide cone on the TUK-128 cell the fuel assembly should be loaded into. SFAs of IR-8 reactor were previously stored in the reactor storage pool. Therefore, SFAs of IR-8 were delivered to the MR central hall by an onsite transfer cask loaded from below. SFAs from this onsite transfer cask were placed in the protective box arranged in the MR central hall and then loaded into TUK-128. After completion of loading operations, loaded TUK-128 casks were checked for leaktightness and surface radiation contamination, and were transported by a trolley to the dedicated area near the MR reactor building. Then a forklift loader successively delivered TUK-128 to the working area near the Institute s railway line, where a truck crane loaded them into ISO-containers, which were in turn installed on the railway carriage making part of a special train. So, the shipment of SNF from the Institute s research reactors to reprocessing plant in TUK-128 shipping casks can be considered successful. In the same time, insufficient development of the standard reloading equipment, as well as too heavy container (about 10 tons) requiring heavy-lift process equipment, should be considered as TUK-128 drawbacks. 4. VVR-2 AND OR SPENT FUEL ASSEMBLY REMOVAL TECHNOLOGY The technology of underwater SNF loading into shipping casks was used for SFAs of VVR-2 and OR research reactors [3]. In , five shipments of these SFAs to the Mayk enterprise were arranged and performed Preparatory activities VVR-2 and OR research reactors make part of the experimental equipment of the Gas Plant Complex situated on a separate Kurchatov Institute site. SFAs from VVR-2 and OR are stored in three reactor storage pools and in the separate building of SNF wet storage facility. For VVR-2 and OR conditions the best option of SFA loading into shipping casks under a layer of water was selected and implemented. To this effect, TUK-19 shipping cask was selected in view of its smaller size/weight and narrow space available for loading. Besides, a special reloading pool was arranged in the room of SNF wet storage facility building (fig. 1), where all loading operations were 3

4 performed. The reloading pool was filled with distilled water and equipped with water purification (clearing) system. In addition, this reloading pool was equipped with a special support for installing TUK-19 inside. FIG.1. Reloading pool outlook. A special crossbeam was also produced for operating with TUK-19 casks in the SNF storage facility room Fuel assembly loading operations sequence Each VVR-2 and OR SNF shipment included delivery of two container carriages with sixteen empty TUK-19 shipping casks to NRC KI site via railway, by a special train. A truck crane extracted TUK-19 casks from container carriages and then special trucks transported them to the Gas Plant site, where a truck crane installed them on a specially arranged accumulator rack. In the course of loading, TUK-19 casks were extracted from the accumulator rack by the truck crane and then they were successively delivered to the SNF storage facility using the forklift loader. SFAs from VVR-2 and OR previously stored in reactor storage pools or in the SNF wet storage facility were extracted and delivered into the room of SNF storage facility using the cathead crane with a lifting capacity of 5 tons, the onsite transfer container and the forklift loader. The technology of underwater SFA loading into shipping casks included the following operations: 4

5 Preparing TUK-19 for installation into the reloading pool and loading SFAs inside it (removing the protective lid, installing SFA support rack into the shipping cask s basket and fixing the cask on the upgraded crossbeam for subsequent lowering into the pool); Lowering TUK-19 into the reloading pool and installing it on the special underwater support; Loading SFAs into the onsite transfer container and delivering them to SNF storage facility; Lowering the onsite transfer container with SFAs inside into the reloading pool; Extracting SFAs from the onsite transfer container and loading it into TUK-19 (fig. 2); Installing the protective lid on TUK-19 loaded with SFAs inside the reloading pool (fig. 3); Removing loaded TUK-19 from the reloading pool; Sealing loaded TUK-19 protective lid, and monitoring its leaktightness and surface radiation contamination; Transporting loaded TUK-19 by the forklift loader to the working area near the SNF wet storage building; Installing loaded TUK-19 by a truck crane on the dedicated accumulator rack. FIG. 2. Spent fuel assemblies loaded into TUK-19 underwater in the reloading pool. FIG.3. Protective lid installed on TUK-19 underwater in the reloading pool. 5

6 After completion of all loading operations, the truck crane extracted TUK-19 casks from the accumulator rack and then special trucks successively transported them to the special working area arranged near the Institute s railway line. Here the truck crane again loaded TUK-19 casks into container carriages of a special train Methods of fuel assembly identification by fuel type VVR-2 and OR research reactors used square fuel assemblies consisting of 16 or 11 fuel elements. A square fuel assembly is sized mm, has total length of 700 mm and weighs about 3.5 kg. Besides, VVR-2 reactor also used rectangular and triangular fuel assemblies respectively consisting of 8 or 4 fuel elements. Fuel elements have external diameter of 10 mm and active part of 500 mm. Fuel elements are clad in aluminum alloy and have fuel kernels made either of UO 2 MgO ceramics enriched by 10% of U-235 (so-called EK-10 type), or of UAl alloy with 36% enrichment (so-called S-36 type). As a result, SFAs containing fuel elements of different types are identical both in form and size, and it s impossible to visually determine the types of fuel contained in fuel elements. The rules of SFA transportation to reprocessing plant are set by OST state regulations. These rules require the types of fuel contained in SFA fuel elements to be identified. To this end, two special methods were developed for fuel elements identification, such as based on analysis of: 1) fuel element weight and U-235 enrichment; 2) X-ray characteristic spectra. The first method includes weighing fuel elements previously extracted from SFAs in storage pools and then comparing their weight with that of non-irradiated elements made of known fuel. This method takes into account that the average difference between S-36 and EK-10 element weights makes about 30 grams. Special tools were developed for extracting fuel elements from SFAs and weighing them under water. The weighing was performed using the electronic balance with 2% precision. This method turned out to be the promptest because it was easier to implement and assured radiation safety of the operating personnel thanks to protective water layer in the storage pool. If necessary, verification of SFAs parameters was also carried out by X-ray spectrum method [4]. This spectrometric method is based on measuring the radiation spectra of both the nuclear spent fuel itself and its fission products. Comparing the counting rates in the peak of total absorption of the characteristic uranium emission line with the Cs-137 emission line allows S- 36 and EK-10 fuel elements with different burnup levels to be identified with a maximum error of 20%. In order to perform these measurements in the SNF storage building, an experimental facility was built, including the necessary high-resolution spectrometry devices, as well as the protective box made of lead and intended for installing spent fuel assemblies or elements inside it Procedure of fuel assembly dismantling for extraction of fuel elements Preliminary examination of SFAs from VVR-2 and OR stored in the storage facilities showed that some fuel assemblies have structural defects, changed sizes or forms, as well as surface contamination with organic coolant, which did not allow such SFAs to be loaded directly into shipping cask baskets. In the same time, the results of spectrometric analysis of storage waters demonstrated that fuel elements had no direct contact with water, which allowed such fuel elements to be loaded into shipping cask baskets. Taking these circumstances into account, in the preparation of VVR-2 and OR spent fuel for removal to the "Mayak" enterprise had some specific features, and both standard SFAs and defective ones containing EK-10 and S-36 fuel elements were actually selected for delivery. Defective VVR-2 and OR reactor SFAs selected for delivery were dismantled into separate EK-10 and S-36 fuel elements. SFA dismantling procedure was carried out in the 6

7 pool of one reactor storage. It took much labor and time because of extensive manipulations needed with the defective fuel assembles. Successive extraction of fuel elements from the defective assemblies was performed by remote-control tools with special fingered bushing by gripping the top plug of each element (fig. 4). FIG. 4. Fuel elements extracted from the spent fuel assembly with a collet-grip tool. The first fuel element extracted was identified as belonging to S-36 or EK-10 type by its weighing, and then this and the next extracted elements were placed into metal cans for the corresponding fuel type for temporary storage in the pool. If the extraction by the way described above was impossible, the personnel placed the upturned fuel assembly in the special metal vessel established in one of the storage cells and tried to push fuel elements by special tools through the assembly s bottom lattice (fig. 5). FIG. 5. Fuel elements pushed out from the upturned spent fuel assembly with a special tool. Extracted fuel elements selected for delivery were reloaded into special transport canisters. Each transport canister is not leaktight and can be loaded with a certain number of fuel elements of corresponding type. After loading transport canisters were closed by lids, marked and then loaded into corresponding TUK-19 for transportation to reprocessing plant, as described above. 5. CONCLUSIONS Long operation of research reactors in the Kurchatov Institute resulted in onsite storage facilities now containing significant amounts of SNF in form of spent fuel assemblies and fuel elements. SNF removal from the NRC KI site to the Mayk reprocessing enterprise was resumed in Two types of shipping casks were used to transport SNF for reprocessing: 7

8 old-generation TUK-19 casks and new-generation TUK-128 casks. For SNF loading into these shipping casks, the Kurchatov Institute has actually implemented the following technological approaches: direct loading in the centralized SNF storage facility; loading via a shielding box arranged in the MR reactor central hall; and underwater loading in the reloading pool. In , more than 500 spent fuel assemblies from MR, IR-8, VVR-2 and OR research reactors were prepared and shipped for reprocessing in TUK-19 and TUK-128 shipping casks. Plans are to remove for reprocessing all the SNF from VVR-2 and OR reactors in Then SNF removal from other Kurchatov Institute s nuclear installations will start. REFERENCES [1] RYAZANTSEV, E.P., et al., Decommissioning of nuclear and radiation-hazardous facilities in RRC Kurchatov Institute, Atomnaya Energiya 87 (1999) (in Russian). [2] VOLKOV V.G. et al. Management of spent nuclear fuel from research reactors in RRC Kurchatov Institute, Atomnaya Energiya 106 (2009) (in Russian). [3] VOLKOV, V.G., et al., Preparation and removal of spent nuclear fuel from VVR-2 and OR research reactors of RRC Kurchatov Institute to reprocessing, ibid, pp (in Russian). [4] POTAPOV, V.N., et al., Spectrometric method for assessment of spent nuclear fuel parameters, ibid, pp (in Russian). 8