substantiated by the sorption behavior reported by Milstead' for the cesium-nuclear graphite system. In particular Milstead has shown that at temperatures of 800 to 1100째C and concentrations of 0.04 to 1.6 mg of Cs per gram of graphite, cesium sorption on graphite follows a Freundlich isotherm (1.6 mg of Cs on 1 m2 of graphite surface corresponds to the saturation surface compound CsC'). Below this, a Langmuir isotherm is indicated. In the MSRE graphite under consideration, the 137Cs content was -10l6 atoms per gram of graphite, and the 133 and 135 chains would provide similar amounts, equivalent to a total content of 0.007 mg of Cs per gram of graphite. At 650째, in the absence of interference from other adsorbed species, Langmuir adsorption to this concentration should occur at a Cs partial pressure of about 2 X lo-'' atm. At this pressure, cesium transport via the gas phase should be negligible, and surface phenomena should control. To some extent, Rb, Sr, and Ba atoms also are indicated to be similarly adsorbed and likely to diffuse in graphite. It thus appears that for time periods of the order of a year or more the alkali and alkaline earth daughters of noble gases can be expected to exhibit appreciable migration in the moderator graphite of molten-salt reactors.
8. C. E. Milstead, "Sorption Characteristics of the CesiumGraphite System at Elevated Temperatures and Low Cesium Pressure," Curbon 7,199-200 (1969).
6.3 FISSION PRODUCT CONCENTRATIONS ON MSRE SURFACES E. L. Compere
E. G. B o h l m a ~
The recovery of segments of surfaces from the fuel circulating system of the Molten-Salt Reactor Experiment in January 1971 has permitted the direct comparison of the intensity of deposition of several fission product isotopes at a number of points around the circuit. Activity determinations were available on segments of a central graphite bar and a control rod thimble from the core center, on a segment of heat exchanger tube and shell, and on specimens cut from the pump bowl mist shield. Most of these determinations have been described by members of the Reactor Chemistry and the Metals and Ceramics Divisions in this and the preceding reports. In Table 6.3 we present these data expressed as atoms per square centimeter for comparison with each other, and with a maximum mean deposition intensity given by the MSRE inventory divided by total area of graphite and metal in contact with flowing fuel salt. Comparisons between graphite and metal for various isotopes in various core regions do not indicate any marked difference in affinity or sticking factor between the two substances, except that possibly 99Tcis more strongly deposited on metal. Deposits on graphite appear more intense at the bar center. For '"Sb, strong depositions occurred in almost all locations, particularly on metal - it appears necessary for the outer graphite regions to be lower than elsewhere to
Table 6.3. Fission product concentrationon graphite and metal surfaces in MSRE (1014atoms/cm2) lZsSb
Inventory/total flow area
Graphite Core center TOP Middle Bottom Heat Exchanger Tube Shell
6.9 22 9
2,4 6 4
6.13 40 11
17,26 100 50
Pump bowl Subsurface Interface