A Mechanistic Model of Spent Fuel Dissolution, Secondary Mineral Precipitation, and Np Release
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ABSTRACT A mechanistic spent fuel dissolution model has been developed, based on the general reaction-transport code of AREST-CT. It considers the dissolution of spent fuel under flow conditions. The kinetic reactions of spent fuel dissolution and precipitation of schoepite, uranophane, soddyite, and Na-boltwoodite are included in the model. The results of model prediction are compared against the results of drip-tests that simulate the conditions that may occur in the Yucca Mountain Repository. Comparison shows that the modeling results match the laboratory observations very well and no contradiction has been found. It indicates that the model is a reasonably good representation of the real system. After validation, the model was used to investigate the release rate of Np from the dissolution of secondary uranyl minerals by examining various degrees of Np incorporation into secondary uranyl minerals. The predicted Np concentration in the aqueous phase is 3 orders of magnitude lower than the upper-bound of the Np solubility range currently used in DOE performance assessment analyses. It suggests that the Np solubility range currently used is too conservative and could be replaced with more realistic values. INTRODUCTION As water seeps into failed waste packages (WPs), it will react with commercial spent nuclear fuel (CSNF). At this point, CSNF will be subject to aqueous dissolution and secondary-uranium minerals will precipitate. Since both laboratory experiments [1] and theoretical studies [2] show
that Np would be incorporated into secondary uranyl minerals (co-precipitation), it is conceivable that the releases of Np could be controlled by the formation and dissolution of secondary minerals. The formation/dissolution of potential secondary phases are determined by many factors, including the pH and other chemical parameters of the water seeping into waste packages, the partial pressure of C0 2 , temperature, and hydrogeological conditions. This paper presents a mechanistic model of Np release from secondary phase dissolution by considering those controlling factors. The model is built on the general reactive-transport simulator AREST-CT, which was developed at Pacific Northwest National Laboratory [3, 4] and modified at CRWMS M&O [5]. The model consists of two sub-models. One is the spent fuel dissolution and secondary mineral formation for the first 1,000 years after WPs are breached, named the Step- 1 submodel thereafter. The other is Np release from secondary-phase dissolution following the disappearance of CSNF, named the Step-2 submodel. STEP-1 SUBMODEL
Configurations The model treats a WP of CSNF as a 1-D column, as shown in Fig. 1. The length of the column is 1.56 m, the inner diameter of 21-PWR waste packages. Dripping water enters the 471 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
column at the top and exits it at the bottom. The volume fraction of
Incoming Water
CSNF is 14%, equal to the waste-form volume in a waste package
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divided by the void space of
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