Three experiments tested the hypothesis that implicit and explicit tasks involve distinct modes of processing. Ss observed rule-ordered letter strings and were asked either to memorize the strings or to try to discover the underlying rules. In Experiment 1, they then made well-formedness judgments of novel strings under long-deadline and short-deadline conditions. Rule-discovery Ss, but not memory Ss, were impaired by the short deadline. In Experiment 2, all Ss made "similarity" judgments of the novel strings instead of the traditional "rule-based" judgments; there were now no differences between the rule-discovery and memory groups. In Experiment 3, Ss explicitly instructed in the rules were significantly more impaired under short deadlines than were memory Ss. An analysis of decision times to individual strings for the rule-trained versus memory groups also showed qualitative differences consistent with the implicit-explicit distinction.We are grateful for the comments of Zoltan Dienes, Pawel Lewicki, and Pierre Perruchet. Thanks go to C. Michael Levy for comments on an earlier draft of this article. We also wish to thank Amie Alagood, Greg Prysock, Jodi Samuels, and Eric Vaden for data collection in Experiments 1 and 2.A copy of the program used in the experiments may be obtained by sending a 3.5-or 5.25-in. PC-compatible disk to either Carl W. Turner or Ira S. Fischler.

The deposition rate of calcium carbonate on a heat-transfer surface has been measured using a calcium-47 radiotracer and compared to the measured rate of thermal fouling. The crystalline phase of calcium carbonate that precipitates depends on the degree of supersaturation at the heat-transfer surface, with aragonite precipitating at higher supersaturations and calcite precipitating at lower supersaturations. Whereas the mass deposition rates were constant with time, the thermal fouling rates decreased throughout the course of each experiment as a result of densification of the deposit. It is proposed that the densification was driven by the temperature gradient across the deposit together with the retrograde solubility of calcium carbonate. The temperature dependence of the deposition rate yielded an activation energy of 79 ± 4 kJ/mol for the precipitation of calcium carbonate on a heat-transfer surface.

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Heat Exchanger Technology Branch, Atomic Energy of Canada Limited, Chalk River Laboratories, Chalk River, ONKOJ IJO, Canada D egradation of the thermal performance of steam generators (SCs) is a serious problem in nuclear power stations throughout the world (Lovett and Dow, 1991). In pressurized-heavy-water reactors (PWHRs), the reduced thermal performance of the SCs is manifested by an increase of the primary coolant reactor inlet header temperature (RIHT). In pressurized-light-water reactors (PWRs), which operate with fixed primary coolant temperature, the loss of thermal performance is manifested by a reduction of the steam pressure.Degradation mechanisms that may contribute to the loss of SC thermal performance include:fouling of the boiler tube inner surfaces (primary-side fouling); fouling of the boiler tube outer surfaces (secondary-side fouling); divider and thermal plate leakage that causes the coolant to bypass either the SC or the integral preheater; and fouling of the steam separators. The relative contribution of these various degradation mechanisms to the overall loss of thermal performance is still under investigation. Soulard et al.(1 990) examined the relative contributions of tube bundle fouling, divider plate leakage, and thermal plate leakage to the increase in RIHT at the Point Lepreau Nuclear Generating Station, and concluded that tube fouling contributes to a significant fraction of the loss of thermal performance. Corrosion products deposit on both the inner (primary) and outer (secondary) surfaces of the boiler tubes. Thus, a complete understanding of the reasons for the loss of thermal performance and the development of strategies to mitigate this loss requires a knowledge of the thermal resistance of tube deposits under primary-and secondary-side heat-transfer conditions. We present here the results of measurements of the thermal resistance of primary-side and secondary-side boiler tube deposits performed under single-phase forced convection and flow-boiling conditions, respectively.The results are discussed in terms of the physical properties of the deposit and the mode of heat transfer.A schematic of the loop used for the measurements of deposit resistance under primary-and secondary-side heat-transfer conditions has been reported elsewhere (Turner and Codin, 1994). The loop is a waterrecirculating facility that can operate at temperatures and pressures up to 320°C and 14 MPa, respectively. The loop water chemistry is controlled in the loop makeup tank. High-purity, deoxygenated water was used throughout the measurements. Residual hydrazine was maintained at a Measurements were made of the thermal resistance of porous deposits of various thicknesses under both single-phase forced convection and flow-boiling conditions. Both synthetic deposits and deposits on tubes removed from operating steam generators were used in this investigation. The thermal resistance was modeled as the sum of two components: one associated with conduction through the porous deposit and a second associated with...