Various types of chemical stimulation methods have been considered. High pH fluids seem to be a logical choice for some wellbore and/or reservoir stimulations. However, forming of secondary deposits and creating of new reservoir damages due to chemical reactions between the rock material and these stimulation fluids make it advisable not to consider any of these high pH fluids in high temperature geothermal reservoirs. Fluids having a neutral pH can be successfully used in chemical stimulation methods only in a very few and rare instances. Low pH fluids, i.e. acids, have by far the best chance to be used for these chemical stimulation jobs. The major part of this report is concerned with acidizing techniques for geothermal injection wells. The acidizing techniques to be used for an experimental and the routine stimulation of injection wells are described. Some recent field experiences and the results of some laboratory work are described.

The natural circulation tests of the Fast Flux Test Facility (FFTF) demonstrated a safe and stable transition from forced convection to natural convection and showed that natural convection may adequately remove decay heat from the reactor core. The COBRA-WC computer code was developed by the Pacific Northwest laboratory (PNL) to account for buoyancy-induced coolant flow redistribution and interassembly heat transfer, effects that become important in mitigating temperature gradients and reducing reactor core temperatures when coolant flow rate in the core is low. This report presents work sponsored by the US Department of Energy (DOE) with the objective of checking the validity of COBRA-WC during the first 220 seconds (sec) of the FFTF natural-circulation (plant-startup) tests using recorded data from two instrumented Fuel Open Test Assemblies (FOTAs). Comparison of COBRA-WC predictions of the FOTA data is a part of the final confirmation of the COBRA-WC methodology for core natural-convection analysis.