INSPECTION OF STATE OF SPENT FUEL ELEMENTS STORED IN RA REACTOR SPENT FUEL STORAGE POOL

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1 CH01$0025 INSPECTION OF STATE OF SPENT FUEL ELEMENTS STORED IN RA REACTOR SPENT FUEL STORAGE POOL V. G. ADEN, S. YU. BULKIN, A. V. SOKOLOV Research and Development Institute of Power Engineering, P. Box 788, Moscow, Russia and M. V. MATAUSEK, Z. VUKADIN VINCA Institute of Nuclear Science, Belgrade, Yugoslavia ABSTRACT About five thousand spent fuel elements from RA reactor have been stored for over 30 years in sealed aluminum barrels in the spent fuel storage pool. This way of storage does not provide complete information about the state of spent fuel elements or the medium inside the barrels, like pressure or radioactivity. The technology has recently been developed and the equipment has been manufactured to inspect the state of the spent fuel and to reduce eventual internal pressure inside the aluminum barrels. Based on the results of this inspection, a procedure will be proposed for transferring spent fuel to a more reliable storage facility. 1. Introduction The 6.5 MW heavy-water moderated and cooled research reactor RA was designed and constructed by specialists from former USSR. It was in operation from 1959 to 1984, when it was shut down for reconstruction [1]. Due to both technical and political reasons the reconstruction of the reactor has not been completed. In the course of reactor operation about 7000 fuel elements were used: until 1976 fuel elements with uranium metal having initial enrichment of 2% 235 U, and afterwards fuel elements with uranium dioxide in aluminum matrix having initial enrichment of 80% 235 U. Both fuel types have the same geometry and dimensions and approximately the same initial content of 235 U. Fuel elements is 11 cm high, fuel is in tubular form with 1 mm thick inner and outer aluminum cladding. Currently, the RA reactor spent fuel is stored in three different ways: - In the original stainless steel channel-type spent fuel containers, filled with demineralized water and submerged in ordinary water in four mutually connected basins which form the RA reactor temporary spent fuel storage pool; - Densely repacked in sealed aluminum barrels, also filled with demineralized water and submerged in ordinary water in the storage pool; repackaging of the spent fuel was introduced already during the 1960's in order to increase the storage capacity; - In fuel channels of the drained reactor core. The state of the fuel stored in the channel-type stainless steel containers could be relatively easily inspected [2]. Water samples have been taken from about 10% of the total number of these containers. Increased radioactivity of these samples indicates that at least one fuel element is leaking in most of the channel-type containers. Similar situation can be expected in aluminum barrels. Besides, due to corrosion, the barrels could fail as a result of increased internal pressure and radioactive substances could be released to the pool water and eventually to the neighboring environment. 124

2 In order to reduce a potential danger, a joint Russian - Yugoslavian project was initiated [3]. The immediate goal of the project is to perform underwater drilling of aluminum barrels and inspect the state of the repacked spent fuel elements by analyzing water and gas samples taken from these barrels. Based on the results of this inspection, a procedure will be proposed for transferring spent fuel to a more reliable storage facility. Preparatory actions, as removal of sludge from the bottom of the RA reactor spent fuel storage pool and decreasing the turbidity of the pool water, have been successfully performed by Yugoslavian side [4]. Major requirements during implementation of the present project are safety and reliability. All operations are to be remotely controlled in order to avoid unnecessary and even unpermitted irradiation of personal involved. 2. Problems Related to Fuel Elements Storage in Aluminum Barrels Configuration of an aluminum barrel is presented in Fig. 1. The barrel has a cylindrical shell welded to the bottom. Inside this shell there are thirty tubes, having holes which make possible to level up the water inside the barrel. Each tube can receive up to six fuel elements. The upper lid is sealed with a rubber gasket and bolted to the central rod, which is also welded to the bottom plate of the barrel. All barrel components are made of aluminum alloys, except the stainless steel bolt. Before being sealed, a barrel is filled with approximately 11 liters of demineralized water in such way that an air gap, about 3 cm high, is left between the water level and the upper lid. There are 30 barrels containing altogether about uranium metal spent fuel elements. They are located in the annex to basin 4, Fig. 2, arranged in two layers, one on the top of other. Upper layer of the barrels Protective Sign in a form of an arrow Special ^grapple Channel-type spent fuel containers Fig 1. Aluminum barrel. a) b) Fig 2. Basin 4 of the spent fuel storage pool. a - Current arrangement of the barrels in the annex to basin 4. b - Arrangement of the barrels after replacement. During more than thirty years of storage, as a result of corrosion of a relatively large surface of aluminum in contact with water inside a barrel, and eventual gaseous fission products release from the leaking fuel, pressure inside the barrels might have increased. Calculated value of internal pressure under which a barrel fails is about 10 bar. In case of barrel failure under this pressure, about 20 liters of gaseous matter (mainly hydrogen) would be released to the working room where the storage pool is located, while the water from the failed barrel would mix with the pool water. 125

3 Water samples taken from some channel-type stainless steel containers had rather high radioactivity [2]. In the container where a fuel element which failed already during reactor operation was stored, water radioactivity was 1.86 Ci/1. In three other channel-type spent fuel containers radioactivity of water samples was between 1.54xlO" 2 Ci/1 and 0.75xlO~ 4 Ci/1. Radioactivity of water in aluminum barrels could be several times higher, since volume of water per a fuel element in a barrel is approximately 20 times less than in the channel-type container. Most probable maximum radioactivity of water in the barrels is 2x10"' Ci/1. All what was said above indicates the necessity to inspect promtly the state of spent fuel inside the aluminum containers and to elaborate a procedure to remove this fuel from the temporary storage pool. Having in mind that corrosion inside the aluminum barrels, as well as corrosion of the barrels, will proceed with time, higher reliability of the spent fuel storage can only be achieved by placing this fuel into a dry storage facility. To implement this task the following activities should be accomplished: - Releasing the possible overpressure in aluminum barrels in order to increase safety of spent fuel storage under the current conditions; - Determination of water and spent fuel characteristics inside the barrels; - Opening of the barrels and removal of spent fuel elements; - Separation of the leak tight fuel elements from the failed ones; - Placing of leak tight spent fuel into containers for dry storage; - Creation of the additional ecological barrier for the failed spent fuel elements by placing them into special cans for dry storage. RDIPE jointly with other Russian organizations has developed technology [5] and equipment to fulfill the activities described in first two items. The equipment has recently been manufactured partially in Russia and partially in Yugoslavia. 3, Venting of Aluminum Barrels and Determination of Spent Fuel Characteristics inside the Barrels For releasing the possible overpressure from aluminum barrels, and for determining characteristics of the spent fuel inside these barrels, the following devices have been designed and manufactured: - Special grapple; - Dismountable protective container; - Drilling device; - Control panel; - Sampling device; - Support frame for mounting and operating the equipment. Before being installed at the spent fuel storage pool, the above equipment is tested at special stand in the RA reactor hall. The aim of testing is to confirm the technical characteristics of the equipment (the grapple weight-lifting capacity, the protective container leak-tightness, etc.), to check the remote control and to train the personnel to perform the planned operations. The technology of barrels treatment incorporates the following stages: - Temporary removal of channel-type spent fuel containers from basin 4 to transport channel and replacement of all barrels from the annex to the bottom of the basin 4 (elevation m) Fig. 2. Before the barrels are replaced, the upper metal structures had to be cut off along entire rout of the grapple movement. When the barrels were first placed into the storage pool, they were held by the central bolt. Now this way of handling can not be used because of the possible deterioration of bolt - 126

4 central rod connection due to corrosion. To move the barrels a special grapple, Fig. 3, which ensures that a barrel can not be dropped or opened during the operation, was designed and manufactured. - Installation of the support frame over the annex to basin 4, Fig.4. Protective container base is fastened to the support frame. The immersion depth of the protective container base is selected so that the required protection layer of water is ensured, which reduces the dose rate for the personnel involved to an acceptable level. Measurements of gamma-activity at the upper level of the barrels gave the following results: at the barrels top surface ~ 200 R/h, between two neighboring barrels > 1000R/h. - Replacement of a barrel to the protective container base. - Installation of the protective container cover, with built in drilling device, over the barrel and sealing of the protective container. - Drainage of water from the protective container into the storage pool by pressurized inert gas. Drainage is required in order to prevent mixing of water from the barrel with water from the pool. - Drilling of 15 mm aperture in the barrel's lid. Drilling device is operated by a pneumatic drive. Location of the aperture to be drilled is selected so to ensure that sampling device can be placed between the tubes inside the barrel. A sign, in form of an arrow, painted on the side surface of each barrel before they were loaded with spent fuel, can be used for determining the drilling location. If this sign is unavailable or unclear, ultrasonic inspection should be used to identify the drilling location. - Measuring the pressure inside the protective container, gas sampling and its analysis. After the gas from the barrels enters the internal space of the protective container, the pressure should not exceed 1.5 bar. - Releasing the gas from the protective container and blowing down of the protective container by inert gas. - Taking samples of water from the barrel at three different levels by using the capillary method. Figure 4 shows position of valves on the control panel for all technological operations. Ventilation system Drilling device Steps of technology Valves position OPEN CLOSED [.Draining of water from protective container 2.DrilIing of the barrel cover and pressure measuring 3.Gas sampling 4.Gas release into ventilation system S.Blowing of the container cavity by gas 2,5,7 1 2,1 2,3 2,3,6 1,3,4,6 2,3,4,5,6,7 1,3,5,6,7 1,4,5,6,7 1,4,5,7 Fig 3. Special grapple. Fig 4. Technological system for barrels drilling, ventilation and sampling. 127

5 - After venting of the barrel is completed and gas and water samples are taken, aperture in the barrel's lid is sealed by a rubber plug or a glued stopper, the protective container is opened by a reverse procedure and the barrel is transferred to the bottom of the basin 4 for further short term storage. - The same procedure is repeated for the next barrel. If radioactivity of the water from the barrel is found to be the same as in the storage pool, it means that it contains no failed fuel elements. Such barrel can be opened by unscrewing the upper bolt or, if the bolt is corroded and can not be unscrewed, holes can be drilled in the barrel's lid-around the bolt in order to lift the lid. The spent fuel elements can than be transferred to dry storage. If water in a barrel is radioactive, the only conclusion is that it contains at least one failed fuel element. In this case the leak tight sampling system should be used to substitute the water inside the barrel before the barrel is opened. The fuel elements should then be visually inspected and tested on leak-tightness at the appropriate vacuum facility or using a sipping test. Leak-tight spent fuel elements can be directly transferred to the dry storage, while the failed spent fuel elements should first be put in additional protective cans. 4. Conclusions The present way of storing the research reactor RA spent fuel in sealed aluminum barrels situated in the temporary storage pool at the reactor building does not satisfy the safety requirements. In order to get detailed information about the state of the stored spent fuel, a technology has been developed and special equipment has been designed and produced for venting the aluminum barrels and taking gas and water samples from the barrels. Conclusions about further treatment of the RA reactor spent fuel will be made depending on the results of the accomplished work. 128 References [1] M. V. Matausek, N. Marinkovic, Z. Vukadin, «Research Reactor RA at VINCA Institute of Nuclear Sciences. Ageing, Refurbishment and Irradiated Fuel Storage», Proc. Of a Topical Meeting RESEARCH FACILITIES FOR FUTURE OF NUCLEAR ENERGY World Scientific Publishing, p.69, (1996). [2] M.V. Matausek, Z. Vukadin, S. Pavlovic, T. Maksin, Z. Idakovic, N. Marinkovic, «Detection of Fission Products Release in the Research Reactor «RA» Spent Fuel Storage Pool», Proc. Annual Meeting on Nuclear Technjlogy, Aachen, (1997). [3] D. Litai, «Travel Report: IAEA Expert mission to prepare detailed plans for priority remedial action related to the spent fuel pool at the RA Reactor at VTNCA Institute, February)), IAEA, [4] M.V. Matausek, M. Kopecni, Z. Vukadin, I. Plecas, R. Pavlovic, O. Sotic, N. Marinkovic «Current Activities on Improving Storage Condition of the Research Reactor RA Spent Fuel - Part II» Transactions, 2 nd International Topical meeting on Research Reactor Fuel Management, Belgium, pp , [5] The Technological Rout for Conducting Inspection of Barrels with Spent Fuel Elements in the Storage Pool at the RA Nuclear Reactor, RDIPE, Moscow, 1998.