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Performance assessment calculations for the proposed high level radioactive waste repository at Yucca Mountain, Nevada, were conducted using the Nuclear Regulatory Commission Total-System Performance Assessment (TPA 3.2) code to test conceptual models and parameter values for the source term based on data from the Peña Blanca, Mexico, natural analog site and based on a model for coprecipitation and solubility of secondary schoepite. In previous studies the value for the maximum constant oxidative alteration rate of uraninite at the Nopal I uranium body at Peña Blanca was estimated. Scaling this rate to the mass of uranium for the proposed Yucca Mountain repository yields an oxidative alteration rate of 22 kg yr−1, which was assumed to be an upper limit on the release rate from the proposed repository. A second model was developed assuming releases of radionuclides are based on the solubility of secondary schoepite as a function of temperature and solution chemistry. Releases of uranium are given by the product of uranium concentrations at equilibrium with schoepite and the flow of water through the waste packages. For both models, radionuclides other than uranium and those in the cladding and gap fraction were modeled to be released at a rate proportional to the uranium release rate, with additional elemental solubility limits applied. Performance assessment results using the Peña Blanca oxidation rate and schoepite solubility models for Yucca Mountain were compared to the TPA 3.2 base case model, in which release was based on laboratory studies of spent fuel dissolution, cladding and gap release, and solubility limits. Doses calculated using the release rate based on natural analog data and the schoepite solubility models were smaller than doses generated using the base case model. These results provide a degree of confidence in safety predictions using the base case model and an indication of how conservatism in the base case model may be reduced in future analyses.

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A mathematical study was carried out for a stirred reactor in which the feed and product streams were periodically interrupted. The cycled reactor proved superior to a continuous stirred-tank reactor for first-and second-order reactions, and surpassed a plug-flow reactor for those cases in which the reaction rate passes through a maximum, such as autocatulytic and exothermic adiabatic reactions. Temperatureprogrammed cycled reactors were also studied, but appeared to have l i t t l e advantage over conventional reactors.Cyclic processes have stirred up great interest among chemical engineers because of their ability to improve the performance of various conventional unit operations such as distillation, absorption, and extraction, However, cyclic operation of reactors has led to some confusion with some reports indicating process improvement while others indicate the opposite.The confusion regarding cyclic reactors s t e m s entirely from a problem of nomenclature. The terms pulsed, cycled, and periodic have been applied to such systems, sometimes having similar and sometimes quite different meaning. Those cyclic or pulsed operations in which sinusoidal or square-wave disturbances are superimposed on the steady feed rate to a continuous stirred-tank reactor (CSTR) ( 5 ) are to be differentiated from those treated here, in which the constant feed rate is periodically interrupted, resulting in short batch reaction steps (6). The two systems present e'ntirkly hiffdrent exberihent'al dnd hathematical problems and cannot be compared in any simple way. A third type of system, a backmix reactor with steady feed and naturally occurring internal oscillations (chemical oscillators), should also be clearly differentiated.The present study i s an elaboration of that by Fang and Engel ( 6 ) in which the feed and product streams of a stirred reactor were periodically turned on and off, resulting in cyclic behavior (controlled cycling). GENERALOne of the chief advantages of a backmix reactor over batch and plug-flow reactors i s its ability to avoid sensitive reactant and product concentrations lying between the initial and final concentrations. The controlledcycled stirred-tank reactor (CCTR) has this same capability because it operates in a series of short batch reaction steps under such desired operating conditions that the initial and final concentrations during a batch step will meet the required concentration criteria. At the end of each batch reaction, part of the reactor contents i s dumped and fresh feed i s added to reestablish the normal reactor volume. After several such operating cycles, the CCTR will achieve pseudo steady state conditions where each reaction cycle is a duplicate of the previous one.In general, a distributed feed plug-flow reactor can also avoid certain critical concentration ranges, and there i s a Superficial similarity between distributed feed reactors and the CCTR. However, it should be clear that although adjacent segments of a distributed feed reactor may have identical concentration patterns...