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stay within inventory. Tellurium, an antimony daughter, is somewhat similarly though perhaps less intensely deposited. Ruthenium-106 was found more strongly deposited in the core than in the heat exchanger and most strongly at the gas-liquid interface in the pump bowl (as was "Sb). Because over a dozen half-lives had elapsed before counting, the 95Nbdata are doubtless less precise but appear similar to the ruthenium pattern. Generally these data show somewhat higher intensities of deposition than reported9 for surveillance specimens; turbulence and mass transfer rates may have been less for surveillancespecimens. The mass transfer coefficients cited by Briggs" indicate that deposition should vary with flow conditions from region to region, and also should depend on sticking factors presumably characteristic of the surface material and substance transported. Mass transfer coefficients for xenon atoms in various regions of the MSRE were given as: core center 15%, 0.3 ft/hr; core average, 0.06 ft/hr; heat exchanger, 0.7 ft/hr; piping, 1.2 ft/hr; reactor vessel, 0.6 ft/hr; and 0.02-in. bubbles in salt, 2 ft/hr. The same proportions between regions should hold for colloidal particles (<1 p) of similar size, shape, and density. There is nothing in the data presented here to show directly whether deposition occurred by an atomic or particulate mechanism. The relative scatter of the data does not permit us to relate them to the mass transfer coefficients of the regions. 6.4 METAL TRANSFER IN IW3RE SALT CIRCUITS

E. L. Compere

E. G. Bohlmann

Cobalt-60 is formed in Hastelloy N by neutron activation of the minor amount of 59C0(0.09%)put in the alloy with nickel; the detection of 6oCoactivity in 9. F. F. Blankenship et al., "Examination of the Fourth Set of Surveillance Specimens from the MSRE," MSR Program Semiannu. Progr. Rept. Feb. 28, 1970, ORNL-4548, pp. 104-111. 10. R. B. Briggs, Estimate o f the Afterheat by Decay of Noble Metals in MSBR and Comparison with Data from the MSRE, MSR-68-138(Nov. 4,1968).

bulk metal serves as a measure of its irradiation history, and the detection of 6oCo activity on surfaces should serve as a measure of metal transport from irradiated regions. Cobalt-60 deposits were found on segments of coolant system radiator tube, heat exchanger tubing, and on core graphite removed from the MSRE in January 1971. The activity found on the radiator tubing (which received a completely negligible neutron dosage) was -160 dpm/cm2. This must have been transported by coolant salt flowing through heat exchanger tubing activated by delayed neutrons in the fuel salt. The heat exchanger tubing exhibited subsurface activity of about 3.7 X lo8 dpm/cc metal, corresponding to a delayed neutron flux in the heat exchanger of about 1 X 10". If metal were evenly removed from the heat exchanger and evenly deposited on the radiator tubing throughout the history of the MSRE, a metal transfer rate at full power of about 0.0005 mil/year is indicated. Cobalt40 activity in excess of that induced in the heat exchanger tubing was found on the fuel side of the tubing (3.1 X lo6 dpm/cm2) and on the samples of core graphite taken from a fuel channel surface (5 X lo6 to 3.5 X lo' dpmlcm'). The higher values on the core graphite and their consistency with fluence imply that additional activity was induced by core neutrons acting on 6oCoafter deposition on the graphite. The reactor vessel (and annulus) walls are the major metal regions subject to substantial neutron flux. If these served as the major source of transported metal, and if this metal deposited evenly on all surfaces, a metal loss rate at full power of about 0.3 mil/year is indicated. Because deposition occurred on both the hotter graphite and cooler heat exchanger surfaces, simple thermal transport is not indicated. Thermodynamic arguments preclude oxidation by fuel. One mechanism for the indicated metal transport might have 10% of the 1.5 w/cc fission fragment energy in the annular fuel within a 30-1.( range deposited in the metal, and a small fraction of the metal sputtered into the fuel. About 0.4% of the fission fragment energy entering the metal resulting in such transfer would correspond to the indicated reactor vessel loss rate of 0.3 mil/yr. If this is the correct mechanism, reactors operating with higher fuel power densities adjacent to metal should exhibit proportionately higher loss rates.