Transportable Calorimeter Measurements of Highly Enriched Uranium

Transcription

1 . Transportable Calorimeter Measurements of Highly Enriched Uranium \ C. Rudy, D. S. Bracken, R. Cech, M. Craft, J. McDaniel, and D. Fultz Los Alamos National Laboratory Safeguards Science and Technology Group, NS-5, MS E540 Los Alamos, NM USA Presented at the nstitute of Nuclear Materials Management 38th Annual Meeting Phoenix, Arizona This is apreprintof a paper intendedfor p u M i in ajoumal ar proceedings. Becausedmges may be made betore publication,thiipreprintis made availablewith theunderstandc'ng that it will not be cited ar reproducedwithout the permission of the author.

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3 DSCLAMER 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 spccific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, m m- mendation, 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.

4 Transportable Calorimeter Measurements of Highly Enriched Uranium* C. Rudy, D. S. Bracken, P. Staples, and L. Carrillo Los Alamos National Laboratory, Los Alamos, NM R. Cech Global Manufacturing Solutions, Franklin, OH M. Craft, and J. McDaniel University of Dayton Research nstitute, Dayton, OH D. Fultz Mound Applied Technologies, Miamisburg, OH Abstract A sensitive calorimeter has been combined with a small temperature-controlled water bath to compose a transportable system that is capable of measuring multikilogram quantities of highly enriched uranium (HEU). The sample chamber size, 5 in. in diameter by 10 in. high, is large enough to hold sufficient HEU metal or high-grade scrap to provide a measurable thermal signal. Calorimetric measurements performed on well-characterized material indicate that the thermal power generated by 93% 235Usamples with 1.0%234U can be measured with a precision of about 1%(1 sigma) for 4-kg samples. The transportable system consists of a twin-bridge calorimeter installed inside a 55-gal. stainless steel drum filled with water with heating and cooling supplied by a removable thermoelectric module attached to the side. sotopic measurements using high-resolution gamma-ray measurements of the HEU samples and analysis with the FRAM code were used to determine the isotopic ratios and specific power of the samples. This information was used to transform the measured thermal power into grams of HEU. Because no physical standards are required, this system could be used for the verification of plutonium, 238Puheat sources, or large quantities of metal or other high-grade matrix forms of HEU. ntroduction Twin-bridge calorimeters are the most sensitive form of water-bath Calorimeter. They are routinely used for calibrating 238Puheat standards and the most accurate calorimeter measurements. This type of calorimeter requires more space than other designs. n addition they have generally been constructed with large water baths, typically 200 gal. in volume. A large transportable calorimeter, using 125 gal. of water has been used for verification measurements. Recently at EG&G Mound Applied Technologies a smaller water bath with good temperature control was developed and mated with a sensitive twin-bridge calorimeter. The bath requires 35 gal. of water for temperature control. The smaller water system is capable of maintaining temperature control so that small thermal powers can be measured with the calorimeter. This system was transported to Los Alamos National Laboratory (LANL)for further testing and development. This calorimeter can be used for verification measurements of plutonium and is sensitive enough to measure multikilo ram uantities of highly enriched uranium (HEU) metal or scrap from the heat generated by the 4Uisotopic component. Highlyenriched uranium with 1 % 234Ugenerates about B a q *Thiswork is supported by the US Department of Energy, Office of Nonproliferation and National Security, Office of Safeguards and Security.

5 1/1OOOthe thermal power of 239Pu.Previous work' indicated that sufficient thermal power would be generated by the component so that multikilogram quantities of HEU could be assayed by calorimetry combined with gama-ray jsotopic measurements. The assay technique for HEU would parallel the technique used for calorimetric assay of plutonium described in American National Standards nstitute (ANS)NP This paper describes the performance of the transportable system in measuring quantities of HEU ranging from 1 to 4 kg. Transportable Calorimeter Hardware The twin-bridge calorimeter described here was built at Mound and was originally designed for use with a large 200-gal. static water bath. The sensitivity of the calorimeter is such that a 1-W heat source generates a bridge potential difference of 28,025 k 208 pv. The calorimeter accepts a cylindrical container 5 in. in diameter arid 10 in. tall (volume = 3.2 L). ts sample chamber volume is large enough to hold multikilogram quantities of uranium oxide, metal, or scrap. The combination of high sensitivity and large sample volume makes it a good candidate to test whether the temperature of a small-volume water bath can be maintained and allow for accurate measurements of plutonium- or HEU-bearing materials. A 55-gal. steel drum3constructed of 16 gauge type 304 stainless steel was selected as the water-bath container. The use of a standard drum was chosen because of its relatively low cost and because its cylindrical shape would minimize its exposed surface to external temperature variation. nitially it was planned to have an insulating blanket falbricated that would wrap around the drum and cover the top to insulate the bath from its environment. The bath control system was later shown to have a robust control capability, and the blanket was riot needed for normal laboratory room environments. Another reason for the drum size was that it allowed the calorimeter to be easily shipped to other facilities or rolled to different locations iusing wheels placed on the bottom of the drum. The lid of the drum was modified by cutting an opening to accept the calorimeter. The top of the calorimeter is sealed so that the water volume is filled to the top. The system is sealed so that only small quantities of water need to be added, about 0.5 L per month. A picture of the calorimeter system is shown in Fig. 1. Fig. 1. Loading 61 sample in the transportable calorimeter.

6 * + listed uncertainties are the standard deviations of the average. Average of 4112,4047 and 4047 g ' =e L 0 n 0 m U i E ' :48:00 f 12:00:00 baseline 13:l2:OQ 14:24:00 Time Fig :36:00 16:48:00 Bridge potential generated by 4 kg of HEU (93.17% 235U) measured with a transportable calorimeter, HEU Measurements-Gamma-Ray Spectroscopy The uranium isotopic values for all samples were measured by gamma-ray spectroscopy using a coaxial Ge detector. The isotopic measurements are used to determine the sample effective specific power which is used to calculate the sample mass from the calorimeter measurement. Details of this calculation may be found in references 1 and 2. Peak analyses and relative efficiency determinations were done using the gamma-ray analysis code PC-FRAM? Gamma-ray measurements re uirin from 1 to 2 h measurement time were performed on the HEU samples to obtain the 234U,'5U, i d 238Uisotopic composition. The gamma-ray data used to determine the uranium isotopics covers the

7 A temperature control system was designed so that it would both actively heat and cool the bath water and maintain a reference temperature at a set point of 25.O0O0Cf 0.00loC. Water is circulated into and out of the water bath and into a manifold that acts as a heat exchanger using a 1/6 hp magnetically coupled pump. The heat exchange manifold was fabricated with common 0.75-in. hard copper whose shape was then modified into a flat oval 0.25 in. thick. The flattened sides allowed for more efficient heat transfer to the cooling and heating plates. The tubing was then constructed so that the water made four passes through the heal. exchanger. The water temperature is changed by using a thermoelectric cooling device with an integral electrical heater? The system was modified to allow standard 120-V AC control signals generated by a Hart model 2100 digital bath controller to be used with this system. The circuit allows either 100% cooling or 100% heating. When the controller calls for heat, the heat is turned on and the cooling is turned off. When the controller turns off the heat, the cooler automatically turns on. This allows for strong corrective response by the control system because the heating and cooling are never working simultaneously as is the case of baths that use compressors and are cooling 100% of the time. n systems incorporating a compressor, the heating is less effective because it has to overcome the cooling before it has any effect and thercfore requires much greater heating power to get comparable results. The heat exchanger system and Hart controller hang on the side of the 55-gal. drum and are connected to the water bath with two hoses. The controlled water leaves the manifold and returns to the bath. nside the bath, a 90-in. stainless steel hibe that is bent to form a 90 turn redirects the returned water to flow between the two calorimeter tubes. The flow of water causes circulation around the two tubes and from the top to the bottom of the bath. Temperature input to the Hart controller is by a thermistor that is placed through a fitting on the top plate of the calorimeter. The thermistor reads the temperature of the bath water at a point approximately 8 in. below the top and between the calorimeter tubes. HEU Measurements-Calorimetry Four low-power HEU samples were measured in order to test the performance of the transportable system. Three 1-kg uranium oxide standards packaged in one-quart cans with varying 235U enrichments were measured by calorimetry and gamma-ray spectroscopy. The fourth sample to be measured consisted of eight HEU metal disk standards with a uranium mass totaling almost 4-kg packaged in two storage cans, The calorimeter was calibrated using 238Puheat sources with thermal powers ranging from to W. Three calorimeter measurements were performed on the 4kg sample each taking about 8.5 h. The signal due to sample power reached 80% to 90% of the final equilibrium value in this time period. The bridge potential data from the later part of the runs were fit to a single exponential in order to estimate the final equilibrium value. For the 4-kg sample, the halflife for the single exponential curve approach to equilibrium ranged from 1.O to 1.1 h. The insulating packing material used with this sample added to the equilibrium time. The equilibrium bridge potential calculated for the three runs for the 4-kg HEU sample were 222.5,219, and 219 pv. The bridge potential generated by HEU for one on the runs is shown in Fig. 2. The 235U enrichments and uranium masses of these reference materials are given in Table. The uranium mass and isotopic composition were determined by destructive analysis techniques. n the same table are listed the results of the calorimetry/gamma-ray measurements. The precision of the calorimeter measurement improved witlh increasing thermal power. The thermal power of the uranium oxide standards ranged from 1 to 1.6 mw and precision of the measurements from 12% to 18% relative standard deviation (RSD). The average thermal power of the 4-kg sample was k mw. For this sample the precision of a single power measurement was lower, 0.92% RSD.

8 energy range from 120 kev to 1001 kev 235U,and 232U/238U progeny gamma rays were used to define the relative efficiency curve. The relative efficiency as a function of energy for the detector/sample was based on the principle of internal or intrinsic self-determination dependent upon the emitted gamma-ray spectra from each of the samples. The specific functional form used for the isotopic ratio was determined relative efficiency curve may be found in Reference 5. The 234U/235U using the ratio of peak areas kev 234U/143.76keV 235Uand the u5u/238uratio was determined using the ratio of peak areas kev 235U/1001keV 234mPa. The relative efficiency curve was extrapolated to 120.9keV to obtain the relative efficiency for the peak. Small biases were expected in the assay results in that the 236Uisotopic fraction was not used in the calculation. ts isotopic fraction was estimated to be less than 1%and the specificpwer 100 times smaller than 234U. Empirical isotopic correlations using the measured u4u, 23 U, and *U fractions may be used to provide an estimate of the 236Ufraction as for 242h.6 The isotope u2u and its progeny isotopes were at too low a concentration to contribute to the sample thermal power. The precision of the average 234Uisotopic composition for the 4-kg sample was 0.7 1%. For the three 1-kg uranium oxide samples, the largest uncertainty component was the variation due to the calorimeter measurements. For the 4-kg metal sample, the uncertainties in the calorimeter measurement and the specific power from the gamma-ray isotopic measurement were about comparable. Conclusion These measurements demonstrate that accurate measurements of low thermal power may be made with a transportable calorimeter requiring relatively small volumes of water. Such calorimeter systems may be used for the recertification of 238Puheat standards, and the preparation of secondary HEU standards of metal or high-grade scrap for other more rapid NDA techniques. Plutonium in quantities as low as 1 g/lcould also be characterizedby this transportable system for a variety of matrix types. The experiments with the transportable calorimeter combined with high-resolution gamma-ray spectroscopy demonstrated a new passive method for accurately measuring large quantities(> 4 kg) of HEU. The technique of using calorimetry/gamma-ray spectroscopy for verifying the plutonium content of materials can now also be applied to multikilogram quantities of HEU. References 1. C. R. Rudy, L. J. Satkowiak, W. W. Rodenburg, and J. A. McDaniel, CalorimetridGammaRay Assay of High Enriched Uranium, Nucl. Mater Manage XX (Proc. ssue), (1993). 2. Plutonium Bearing Solids--Calibration Techniques for Calorimetric Assay, American National Standard for Nuclear Materials, ANS N Manufactured by the Sharpsville Container Co. 4. Manufactured by ThermoElectric Cooling America Corporation. 5. T. A. Kelley, T. E. Sampson, and D. DeLapp, PC/FRAM: Algorithms for the Gamma-Ray Spectrometry Measurement of Plutonium sotopic Composition, A N S Fifth nternational Conference on Facility Operations (Safeguards nterface, Jackson Hole, Wyoming, September 24-29, 1995), Los Alamos National Laboratory document LA-UR R. Gunnink, Use of sotopic Correlation Techniques to Determine 242PuAbundance, Nucl. Mater Manage. 9(2), (1980).