A Continuous, On-Line, Flow-Insensitive Hafnium Monitor in a Zirconium-Hafnium Separation Facility Using Californium-252 Neutron Activation Joseph A. Megy Chemical Research and Development, Teledyne Wah Chang Albany, Albany, Oregon 9732 1
Stephen E. Binney" Department of Nuclear Engineering, Oregon State University, Cowallis, Oregon 9733 I
A continuous on-line analyzer utilizing californium-252 activation analysis has been designed and installed to continuously monitor a process stream for hafnium at the low parts per million level. The monitored process stream contains hafnium and zirconium thiocyanates and dissolved water in a methyl isobutyl ketone solvent. The hafnium-178 atoms in the process stream are activated by neutrons from a 105 wg californium-252 source placed in a well in the process stream to produce hafnium-l79m. A sodium iodide detector placed in another well in the process stream 36 s downstream from the source responds to the y-ray emissior, from the hafnium179m decay and eventually drives a ratemeter with a 5-min time constant. The ratemeter output is continuously displayed on a strip chart recorder. The system has no moving parts and is relatively insensitive to flow variations and to interference from other elements in the process stream. This technique is also applicable to monitoring other elements with favorable nuclear properties.
Introduction Zirconium and hafnium are very similar chemically and invariably coexist in nature. The principal use of zirconium metal is as a fuel cladding material in nuclear power reactors. Hafnium in the cladding acts as a neutron poison due to its much higher neutron absorption cross section. At Teledyne Wah Chang Albany hafnium and zirconium thiocyanates are partitioned between an organic phase (methyl isobutyl ketone) and an aqueous phase in a continuous liquid-liquid countercurrent extraction separation process. A simplified schematic of the separation process is shown in Figure 1. Each of the columns (1-5) is six stories high and has an aqueous phase residence time of about 1h. Until recently, the control of this process was based on batch samples of the zirconium raffinate stream taken a t 2-h intervals. Since the sample analysis time was 2 h and the residence time in the separation process was about 3 h, there was up to a 5-h lag between process changes and determination of the change. Recently a continuous, on-line hafnium monitor was installed on the organic stream entering the bottom of column 3 as shown in Figure 1. The monitor has a 5-min time constant and gives a continuous analysis of the hafnium concentration in the process stream on a slowly moving strip chart recorder. The system, which utilizes the neutron activation technique with a californium-252 neutron source, is described in the next section. Advantages of this system include relatively low cost, simple operation, fast information readout, a high degree of reliability, and good sensitivity. Description of Monitor The neutron source was a 105-hg doubly encapsulated sample of californium-252 obtained under a user's agreement with the Californium-252 Demonstration Center operated for the Energy Research and Development Administration by IRT Corporation. It was placed in a well in the process stream that was to be analyzed (see Figure 2). The source was located in a 1-in. diameter Bondstrand well with a 0.060-in. zirconium liner such that it was cantered in the 8-in. process stream 436
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through which the solution flowed a t a rate of 3700 gal/h (3.9 1.h). The thermal neutron flux was calculated t o be 1.0 X lo6 n/cm2-s when averaged over the irradiation volume (Romanko and Dungan, 1963). The neutrons activated hafnium-178 atoms to produce radioactive hafnium-179m atoms, which have an 18.6-s half-life. Sufficient shielding was provided around the portion of the column containing the source to reduce the dose rate to 2 mrem/h a t 5 ft from the source, the distance of closest possible approach by personnel. This shielding, shown in Figure 2, was 24 in. high, 20 in. in diameter, and weighed about 200 lb. It consisted of Y4 in. of lead around the californium-252 capsule and 5 in. of Chevron petrolatum 110 with 10% boric acid contained in a fiber glass shell surrounding the column. The petrolatum has advantages over paraffin, which is frequently used, in that it does not crack because it does not set up hard, and its viscous molten state facilitates the on-site preparation of a uniform boric acid mixture. The radioactivity from hafnium-179m was measured by a 3 X 3-in. sodium iodide detector which was placed in a 4-in. diameter Bondstrand well with a 0.060-in. zirconium liner. The well was located in an 8-in. section of the process stream about 40 ft downstream from the source. The time required for the solution to flow from the source location to the detector location is about 36 s, or about two half-lives. Although this downstream ,distance allowed the hafnium-179m activity to decrease by a factor of 4,this distance was necessary to minimize the direct radiation from the source to the detector. The portion of the process stream containing the detector was wrapped with about l/4 in. of lead to reduce the background radiation further. Because of the possible combustion of the atmosphere from arcing of the high voltage supplied to the detector, the whole detector assembly was placed in an explosion-proof stainless steel container. A preamplifier/photomultiplier base was attached directly to the detector to amplify the detector output pulses while the rest of the nuclear instrumentation was located in the control room about 50 f t from the detector. The pulses coming to the control room were further am-
AQUEOUS PHASE
Hf ‘offmote
3
H A F N I U M CONCENTRATION, ppm
Figure 1. Separation plant flow schematic.
Figure 3. Sensitivity of system to counting statistics.
;Ppnedstrand ,Fiberglass
eye
U‘
Mild steel support structure
Figure 2. Californium-252source location and shielding.
plified before going to a single-channel analyzer. The window on the single-channel analyzer was set to count pulses corresponding to y-ray energies in the range of 130 to 260 keV, which included the 161- and 217-keV y rays from the radioactive hafnium-l79m. The background in this window, due largely to y rays directly from the californium-252 source and the Compton distribution from any other higher energy y ray, was 20.0 f 1.1counts/s. The pulses from the single channel analyzer were converted by a ratemeter into analog signals proportional to the count rate. The ratemeter was factory modified to give a 5-min time constant, which provided an adequate time response to variations in hafnium concentrations in the process stream and averaged the rapid output variations associated with the random nature of the hafnium decay over a longer time period. The zero suppression on the ratemeter was used to offset the background so that the ratemeter output was proportional to the net hafnium concentration. The analog signal from the ratemeter was displayed on a 0.5-in./h strip chart recorder. The recorder was adjusted so that full scale (100 countsh) corresponded to 1000 ppm of hafnium. All of the nuclear instrumentation used was standard, commercially available Nim bin (modular) equipment.
Total costs for this system (excluding design and engineering) was about $7000, which roughly breaks down to about $1000 for source container and shipping (the source itself is not included in this figure, as it was obtained on loan for this research project), $2000 for fabrication and shielding materials, and $4000 for instrumentation. Purchase of a source would involve about $1000 for the source itself and about $5000 for its encapsulation. Approximately two man-months were spent on this project.
Results The system has been on-line and in operation without downtime for almost 1year. Maintenance has been restricted to servicing the recorder and making bimonthly gain adjustments of the recorder to compensate for the radioactive decay of the californium-252 source, which has a 2.65-year half life. Although raffinate check samples are still being spectrographically analyzed every 2 h for hafnium, it is the on-line hafnium monitor that is used by the operators to control the separations plant. I t has been possible to recognize various patterns in the on-line hafnium monitor output as being responsive to various malfunctions in the separations process. The precision of the on-line hafnium monitor system is affected by the decay of the source, process stream flow variations, counting statistics of detected radiation due to hafnium-l79m, and counting statistics of detected radiation due to background sources. Since bimonthly gain adjustments are made to compensate for the decay of the californium-252 source, a f2.2% variation of the monitor response results from this effect. Sensitivity of the monitor response to flow variations is given in the Appendix (typically less than 1%).The remaining contribution to the error in the monitor response is due to counting statistics. The counting statistics are dependent only upon the number of pulses detected due to the hafnium-179m peak and the number of pulses detected due to background sources, the latter of which will vary according to the details of the system. For the on-line hafnium monitor, the error from counting statistics is shown in Figure 3 as a function of the net hafnium-179m count rate. The errors from each of these sources are independent and can be combined to give the overall counting error for the system. The curve in Figure 3 labeled “organic” represents the monitor already installed. A t this location, typical hafnium concentrations are about 150 ppm with a f 3 % error due to counting statistics. The curve in Figure 3 labeled “aqueous” represents a monitor proposed for installation on the aqueous Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 3, 1976
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Table I. Approximate Sensitivities" for an On-Line Process Stream Monitor
