Application of state-of-the-art neutronics methods toward the derivation of safeguard requirements for the pebble bed reactor

Abstract only, full text of publication follows: The development of neutronics methods for the design and analysis of the pebble bed reactor (PBR) at the Idaho National Laboratory (INL) is reviewed. Since the INL methods define the current state of the art in PBR analysis, this review is also a review of the state of the art. The main code within the INL PBR neutronics methods is PEBBED. The PEBBED code is a full core multigroup diffusion theory neutronics solver for the coupled neutronics-pebble flow equations with depletion, in cylindrical geometry. Currently the code includes an r-θ-z finite difference solver, an r-z nodal solver, and a hybrid r-z nodal/θ-FD solver. A fully nodal r-θ-z solver is being implemented. The PEBBED code includes thermal-hydraulics feedback treatment through calls to the THERMIX-KONVEK code. The PEBBED code is optionally embedded within a genetic algorithm shell for optimization applications. Its supporting codes include the COMBINE-7 spectrum code, which treats arbitrary levels of upscattering and all low lying resonances. Heterogeneity effects are treated using the newly developed PEBDAN code (with help from Prof. Kloosterman of Delft University, The Netherlands), which generates spatially-dependent Dancoff factors and takes into consideration the random packing of the fuel kernels in the pebbles and of the pebbles in the bed. Further developments are ongoing at the INL and at collaborating institutions (Prof. Rahnema at Georgia Tech and Prof. Hawari at North Carolina State University). For example, the treatment of non-multiplying and control zones through seamless embedded transport theory-based response functions and the improved treatment of spectral effects from depletion and extended zone effects are currently being implemented. A kinetic version of the PEBBED code, CYNOD, has also been implemented at the INL, which incorporates traditional thermal-hydraulic feedback. Under consideration is the incorporation of non-traditional feedback from material changes. PEBBED and its support codes have been applied to the design of the pebble bed version of the Next Generation Nuclear Plant (NGNP), showing that a 600 MWth passively safe design is possible. The code system was also used to demonstrate that the PBR concept is proliferation resistant. It was shown, however, that although its spent fuel would not be attractive to a would-be proliferant, it must be safeguarded for its fissile content. (author)