Characterization of Hanford N Reactor Spent Fuel and K Basin Sludges

Transcription

1 WHC-SA-2952 Characterization of Hanford N Reactor Spent Fuel and K Basin Sludges EOPJ m \ % m 0 S T ' Prepared for the U.S. Department of Energy Assistant Secretary for Environmental Management Westinghouse Hanford Company Richland, Washington Management and Operations Contractor for the U.S. Department of Energy under Contract DE-AC06-87RL10930 Copyright License 8y acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper. Approved for public release fflsraeuiioai OF m oocumr TO is inn Y SflAd I

2 Characterization of Hanford N Reactor Spent Fuel and K Basin Sludges B. J. Makenas R. P. Omberg D. J. Trimble R. B. Baker Date Published January 1996 To Be Presented at American Nuclear Society Waste Management '96 Tucson, Arizona February 25-29, 1996 Prepared for the U.S. Department of Energy Assistant Secretary for Environmental Management Westinghouse p.o BOX 1970 Hanford Company Richland, Washington Management and Operations Contractor for the U.S. Department of Energy under Contract DE-AC06-87RL10930 Copyright License By acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper. Approved for public release

3 CHARACTERIZATION OF HANFORD N REACTOR SPENT FUEL AND K BASIN SLUDGES ABSTRACT B. J. Makenas, R. P. Omberg, D. J. Trimble, and R. B. Baker Post Office Box 1970 M/S HO-40 Westinghouse Hanford Company Richland, Washington Over 2,000 tons of irradiated zirconium alloy clad uranium metal fuel are stored in the water-filled Hanford K Basins. Half of this N Reactor fuel is in open top aluminum and stainless steel canisters (in K East Basin) and half is in sealed vented canisters (in K West Basin). On the basin floor and associated with this fuel is an accumulation of sludge containing fuel, fission products, corroded structural material, and wind blown debris. Previous papers discussed plans and equipment for the sampling of gas and liquid from sealed canisters, sampling of sludge from the basin floor, and the movement of fuel to the Hanford hotcells. These sampling activities have now been accomplished. Various lessons have been learned from the execution of various sampling campaigns at the K Basins, and analytical characterization data resulting from the examinations continue to support decisions for the storage and disposal of fuel and sludge. 1. Data obtained from the sampling of gas and liquid in sealed canisters include radionuclide concentrations in the liquid and the degree of displacement of the nitrogen canister cover gas by significant quantities of hydrogen. The macroscopic condition of fuel and the volume of in-canister sludge, observed when selected canisters are opened has been shown. Inferences on the degree of fuel corrosion in canisters can be drawn. 2. The recent sludge sampling campaign for the K East floor has been much more comprehensive than any previous attempts with respect to both quantity of sludge retrieved and the number of physical/ chemical properties addressed. Sludge composition and properties are now known as a function of both basin location and of the position of various layers within the sludge. 3. Fuel elements from the K West Basin have been examined both visually and metallographically at the Hanford hotcells. Detailed observations on the condition (including corrosion) of damaged and undamaged elements have been made. 4. A control!ed-atmosphere furnace has been installed in the Hanford hotcells. This new capability has allowed small sections of fuel elements to be subjected to the dewatering and conditioning processes envisioned for the fuel now stored in K Basins. Moisture reduction and hydrogen evolution have been monitored as a function of temperature and system pressure (vacuum and flowing gas scenarios) With this system, oxide layers have been applied to fuel samples to reduce the chemical reactivity of the exposed surfaces and the stability of these protective layers has been assessed. 1

4 INTRODUCTION Characterization is in progress for the N Reactor fuel stored in the Hanford K Basins. These activities 1 support the strategy for removal of fuel from the basins and storage of fuel in a dry condition at an area remote from the Columbia River. This strategy currently consists of placing fuel in a Multi-Canister Overpack (MCO), drying the fuel while it resides in the MCO and conditioning some portion of the fuel to reduce its chemical reactivity. 2 Characterization includes the examination of fuel, canisters, and associated sludge. It consists firstly of in-basin activities such as visual examination, sludge depth measurements, and sampling of gas and liquid in canisters. Secondly characterization encompasses the examination of samples of fuel and sludge which have been removed from the basins and shipped to laboratories. This paper presents observations made in the basins during the most recent attempts to ship samples from the basins and data obtained in the laboratory hotcells. EXAMINATION OF CANISTERS AND FUEL Of the 2,000 metric tons of uranium metal spent fuel at the two Hanford K Basins, roughly half is stored in sealed aluminum and stainless steel canisters in the K West Basin. Each water-filled canister contains 14 fuel assemblies (14 inner elements and 14 outer elements distributed between two barrels). In March of 1995 three canisters were opened in K West Basin and three fuel elements were shipped to the Hanford hotcells. In-Basin observations were made during the fuel retrieval operations and the fuel was subsequently utilized in detailed hotcell examinations and conditioning tests summarized in the following sections. Specific canisters to open were chosen through a review of the data base of available fuel inventory and through sampling of canister gas and water to identify presence of uncontained fission products (which implies damaged fuel in a given canister). Only those canisters which were stainless steel, which showed evidence of gas generation and which contained fairly long fuel elements were considered candidates for opening due to emphasis, for this first shipment, on evaluating particular corrosion mechanisms. Gas and Water Samples From K West Canisters In order to identify canisters likely to contain failed fuel, liquid samples were obtained from 10 canisters (20 barrels). Also, in a few cases, gas samples were obtained from the sealed barrels. (Note canisters nominally are water-filled and have a 2.5 inch nitrogen gas space when they are first loaded into K West Basin). Gas and water samples obtained in this manner were first evaluated by a mobile laboratory which provided immediate analysis for fission products i.e., cesium in the water and krypton in the gas. Subsequently gas and water samples were sent to laboratories at Hanford for additional identification of chemical species. Analysis of cesium in K West canister water did lead experimenters to the desired fuel elements for hotcell examination. The most badly damaged element removed from the canisters was indeed from the canister with the highest cesium content (approximately 0.5 curies). Hydrogen gas was found to be the major constituent of all of the gas samples. Thus the original 2

5 nitrogen cover gas had been largely replaced. Krypton gas concentrations correlated well with hydrogen which in turn correlated with cesium concentrations in" the liquid. Thus krypton is a credible indicator of corrosion but krypton and hydrogen concentrations do saturate as the original nitrogen covergas is replaced. Reaction of Basin Water with Fuel After three canisters were opened in the K West Basin, no energetic reaction was observed between' the newly introduced basin water and the fuel. After three fuel elements were placed in shipping containers the associated containers were capped with inverted graduated cylinders to trap bubbles and indicate any further reaction (i.e., hydrogen production). At the end of 2 to 5 days of monitoring, no accumulation of gas was found in any of the three cylinders even though at least two of the resident elements we-re failed with obvious exposed fuel. Observations of Sludge During Fuel Handling When canisters lid valves were opened during the canister flooding operation a continuous stream of bubbles was observed. Often accompanying this stream was a distinctly separate stream of reddish-brown liquid which appeared to have the character of a suspension and which was easily. distinguishable from the surrounding basin water. When canister lids were opened to remove fuel, large amounts of sludge were not observed. For two of the canisters no significant sludge was visible in the canisters even when fuel elements were removed. No sludge was seen to accompany the fuel elements retrieved from these two canisters during their transfer to shipping containers. For the fuel element retrieved from a third canister, sludge was seen trailing behind the element during its transfer to a shipping container. Removal of the target element from this latter canister stirred up sufficient flocculent sludge to fill the canister barrel to within a few inches of the top for several hours. Variations in observed sludge content may however be linked to variations in flooding and opening procedures. Visible Condition of the K West Fuel The retrieval of fuel went as follows: 1. When the first of three canisters was opened, it was found that a known fuel element breach (identified in the loading video tapes circa 1983) had not greatly deteriorated, if at all, since the time it was placed in the canister. This outer fuel element, with obvious missing fuel/cladding piece, was selected for hotcell examination and moved to a shipping container. Subsequent hotceil examinations did in facx identify some additional corrosion which can be attributed to in-canister storaae. 3

6 2. The second canister was opened based on its high cesium concentration. One outer element with split cladding (Figure 1, inset) was easily visible. The fuel in this element certainly had reacted with the canister water (i.e., reaction was not apparent in video tapes made during original loading) and the element was selected for hotcell examination. 3. The third canister (Figure 1) was opened based on written records of a breached element and on a moderate cesium content measured in its water. An inner element was selected for shipping to hotcells based on the desire for an intact element and on a visible dent in the end cap of the selected element. Metallographic Examination for Hydrides Emphasis during the metallographic examination of the three K West elements was placed on the identification of uranium hydride due to its association with reported pyrophoric events. Specimens were cut using a tungsten-carbide slitting saw mounted on a remote-operated milling machine. Argon gas was used to cool the fuel element during the cutting, to provide an inert cover and thus, preserve the fuel microstructure as near to the as-received condition as possible. Cut samples were mounted, polished, attack-polished using chromic acid, and examined. The presence of uranium hydride was detected by a heat-tinting technique which delineates the oxidized hydride inclusions from the uranium matrix. Uranium hydride inclusions were found to be randomly distributed throughout the fuel matrix. It is not possible at this time to unambiguously determine the source of the hydrides. Possible sources include: (1) residual hydrogen which has been in the uranium alloy since fabrication, (2) hydrogen present as a result of corrosion processes and subsequent migration within the fuel, and (3) hydrogen present as a result of diffusion through the cladding. Thermodynamically, the zirconium alloy cladding is a favorable sink for hydrogen and therefore hydrogen diffusion through the cladding is the least likely source of hydrogen in the fuel. Fuel Conditioning Fuel samples (with cladding intact) from the K West elements were tested in a controlled atmosphere furnace to determine drying characteristics, dehydriding behavior, and oxide film formation for exposed uranium surfaces. These experiments are the first attempt to study a process whereby at least some of the fuel in K Basins will be conditioned in an oxygen containing atmosphere to reduce chemical reactivity through application of an oxide layer. Specimens were taken from a corroded fuel element but from areas which were not immediately adjacent to the principal corrosion sites. They were placed in a furnace system located in a hot call and tested according to the following heating cycles: A fuel drying step in which the fuel is dewatered for approximately 10 hours (free water removal) either at 373 K in dry argon or at 323 K in vacuum. The fuel is then dried at 573 K under dry argon {or vacuum) for approximately 24 hours (to remove water of hydration and partially decompose any uranium hydride present). 4

7 A fuel passivation step, in which the fuel is exposed to a 98% argon-2% oxygen atmosphere at temperatures ranging from 423 K to 523 K for about 10 hours. The objective is to create if possible, an adherent passive oxide film on any exposed uraniummetal or hydride surfaces. In the testing runs to date, this step was conducted at near atmospheric pressure. The evolution of moisture during the tests was monitored continuously by a moisture monitor while the total water release during the experiment was measured by trapping in a drying column. Hydrogen and oxygen in the off-gas stream were monitored by a gas chromatograph. Solid residues (spalled oxides) were analyzed by X-ray diffraction. Specimen temperature, hydrogen in the off-gas stream, moisture concentration in the off-gas stream, and the oxygen concentration are presented in Figure 2 for a typical two step drying cycle (Figure 2a) followed by a conditioning cycle (Figure 2b). The results indicate an increase in the moisture content of the gas stream for the low temperature drying cycle during the transient heat-up of the specimens, with the hydrogen concentration below detection limit. During the higher temperature drying cycle, both hydrogen and moisture concentration peaked during the transient heat-up of the specimens. The moisture eventually decreased to the detection limit of the moisture monitor, but the hydrogen maintained a steady concentration in the off-gas stream. Figure 2b shows the initial depletion of oxygen in the gas stream that occurs during the conditioning cycle. The total oxygen pick-up by the specimens was estimated by integration of the oxygen depletion curve and by specimen weight change. The results of the furnace testing indicate evolution of both moisture and hydrogen from the fuel specimens. The probable sources of the observed moisture are absorbed water on the metal specimen, oxide layers, and accompanying sludge. The hydrogen, however, could be from uranium hydride decomposition and/or reaction. Recent tests with defueled cladding have ruled out cladding as a hydrogen source. The low limit of solubility of hydrogen in uranium cannot account for the level of hydrogen measured but can contribute a small fraction to the amount of hydrogen measured. The depletion of the moisture source even though hydrogen is being released, and the moisture response to the oxygen addition seems to diminish the likelihood of uranium reaction with gas as the principal hydrogen source. SAMPLING OF BASIN FLOOR SLUDGE Associated with Spent Nuclear Fuel is an accumulation of particulate layered material which is generally called sludge. Sludge is found on the basin floors, in canisters, and in the basin pits which are used for miscellaneous tasks such as cask handling. In fact, numerous different types of sludge have been identified depending on which basin, canister type, or pit location that the particular sludge is found. Each type of sludge is a unique nonhomogeneous mixture possibly containing corroded fuel, debris such as windblown sand or insects, rack and canister corrosion products, and/or 5

8 fission products. All of the various sludges will need to be transported away from the K Basins and disposed with different types of sludge possibly having markedly different disposal paths. Characterization of sludge found on the K East Basin floor and that currently found in one K East Basin pit has been' completed and is discussed below. The central problem addressed in the current sludge characterization effort is "What is an acceptable way to retrieve sludge from the K East Basin (and Weasel Pit) and to process, transport, and store the material until a permanent repository becomes available?" the first part of this effort (i.e., retrieval, transportation, and processing of the sludge) will possibly require the specification, design, and fabrication of sludge handling/ processing/dewatering equipment or the procurement of similar commercial services. Sludge will, for example, need to be pumped and dewatered with devices such as filters and hydrocyclones. Such an effort will requ-ire the knowledge of various physical parameters (i.e., fluid viscosity, particle size, etc.) which could be used directly to design apparatuses and/or could be utilized to specify simulants which can be used for evaluation of candidate equipment. Knowledge of the Special Nuclear Materials content of the sludge will be necessary to maintain accountability of material which leaves the Basins. The second part of the problem is to designate a storage method whereby sludge can be stored away from K Basins in a more environmentally acceptable area. Two prime alternatives for storage have been identified. These are (1) transferring of sludge to Hanford double shell waste tanks and ultimate disposition along with other tank wastes, or (2) processing the sludge into a form appropriate for solid waste disposal. In these two cases the chemistry of the sludge must be determined, either to ensure compatibility of sludge with any non-fuel waste encountered in the tanks or to ensure that sludge does not contain chemicals which are incompatible with storage as solid waste. Measurements of sludge depths in the K East Basin and Weasel Pit were reported previously^ and have shown that the floor is covered with sludge to a depth of 5 to 19 cm (2 to 7.5 inches), with the Weasel Pit containing sludge approaching a meter in depth. Some of the fuel canisters in K East Basin have screened bottoms and slotted sides, and all of the K East canisters have open tops. This means that fuel corrosion products (uranium oxide and fission products mostly) found in canisters have, to some extent, mixed with the expected wind blown debris and corrosion products (from aluminum canisters and steel racks) in basin areas which are in close proximity to canisters. A campaign to retrieve 20 representative samples of sludge from K East Basin and Weasel Pit has been completed. Locations for sampling were chosen to span a diversity of expected sludge constituents and to supply information needed for sludge removal in a statistically valid manner. Equipment was designed and utilized, Figure 3, which assured that full representative core samples of material were collected from only specific localized areas. Sludge was found to contain significant iron, aluminum, and uranium and to be flocculent. Most particles are in the sub-micron range with most of the volume in 10 to 50 micron sizes. 5

9 CONCLUSION Canisters in the K West Basin do contain failed fuel. 'However the first shipping campaign demonstrated conclusively that serious deterioration of this fuel, even when failed, is not universal. Removal of this fuel, even when failed, is possible with the proper tools. For the specific type of fuel targeted by this examination, deterioration to rubble had not occurred and in fact large amounts of'sludge were not observed. That sludge which was observed in canisters had a definite flocculent character and took some time (hours) to settle. Corrosion of fuel is certainly occurring in some of the damaged fuel elements but it is also possible for some exposed fuel to have undergone only minimal corrosion. The predominant constituent of the canister cover gas sampled was hydrogen produced either from radiolysis or more likely from corrosi-on. A good discriminator for finding canisters with failed fuel appears"to be the concentration of cesium in the barrels. Chemical and physical property data to facilitate transfer of sludge from K East Basin floor have been obtained. Campaigns to recover fuel samples from the K East Basin canisters and sludge from inside of canisters from both basins are planned for the near future. REFERENCES 1. B. J. MAKENAS, et al., In-Situ Characterization of Metal Fuel Stored in the Hanford K Basins, Waste Management 1995, Tucson, Arizona, J. C. FULTON, et al., Overview of the Spent Nuclear Fuel Project at Hanford, Waste Management 1995, Tucson, Arizona, B. J. MAKENAS, et al., DOE Spent Nuclear Fuel Challenges and Initiatives, Salt Lake City, Utah, page 326,

10 Figure 1. Canister of N Reactor Fuel Opened After Over a Decade of In-Pool Storage. (An element sent for hotcell examination is shown in the inset. Note split cladding.) Figure 2. Data From the Fuel Drying and Conditioning Apparatus. (Note the release of moisture and the release of hydrogen from the sample and the depletion of oxygen from the nominal 2% 0 2 gas stream during conditioning.) a 3 <u nd Drying Cycle '< io. 200 K ' \ 1 _ Specimen Temperature - I / " 1st Drying Cy de j * %»\ - I i 1 Down I mm I Hydrogen '. - «. u-^» ->-, \ "^Moisture V _i E CL Q.,^, 300 C o at <u 250 i_ o L_ 3 D <S 200 I <U Q. <U E 150 S- 100 o 5 fn stu. 70 mg 0 2 Depletion 600 f*i\\i<fsh* ^wya\.aw Q..Q. c v I O c. <D g a o 2 Time (Minutes) Time (Minutes) DRYING CONDITIONING

11 Figure 3. A Sludge Core Isolation Tube in 0.7 m (30 Inches) of Sludge Found in the K East Weasel Pit. (The sludge contained in the tube was pumped to a sample container and shipped for analysis. The isolation tube ensured a representative sample of all sludge layers.) 9

12 DISTRIBUTION Number of copies ' ONSITE 7 Westinqhouse Hanford Company R. B. Baker L. A. Lawrence B. J. Makenas R. P. Omberg D. J. Trimble Central Files OSTI CZ~) HO-40 HO-40 HO-40 R3-85 HO-40 A3-88 A3-36 Distr-1

13 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.