The safety assessment of a geological repository for used nuclear fuel must ensure that future generations are shielded from radiation from fission products, in particular those released by re-criticality events. An investigation is required to understand whether or not criticality can actually be achieved. In fulfilling this end, this study assesses the uncertainty in the composition and total mass of precipitates forming in the far-field due to variation in transport parameters. The Latin Hypercube Sampling technique is employed to generate an accurate, random distribution of variables employed in the transport model and to assess the uncertainty of attaining a critical mass. The average characteristics of the damaged fuel from the Fukushima Daiichi reactor cores is used as the reference waste form. Results are compared to the minimum critical masses of previous studies to assess the criticality safety margin.

We discuss the challenge of selecting materials for nuclear applications and outline the need for comprehensive databases to assist scientists and engineers in choosing materials that meet interdependent physical, chemical, and nuclear criteria. In conventional engineering, chemical and physical properties and the electronic structure of materials are typically the primary considerations; nuclear applications must also consider the nuclear physics characteristics of a material. Development of databases that correlate physical, chemical, and nuclear properties would accelerate and facilitate innovations in nuclear design.

The RBWR-TR is a thorium-based reduced moderation BWR (RBWR) with a high transuranic (TRU) consumption rate. It is charged with LWR TRU and thorium, and it recycles all actinides an unlimited number of times while discharging only fission products and trace amounts of actinides through reprocessing losses. This design is a variant of the Hitachi RBWR-TB2, which arranges its fuel in a hexagonal lattice, axially segregates seed and blanket regions, and fits within an existing ABWR pressure vessel. The RBWR-TR eliminates the internal axial blanket, eliminates absorbers from the upper reflector, and uses thorium rather than depleted uranium as the fertile makeup fuel. This design has been previously shown to perform comparably to the RBWR-TB2 in terms of TRU consumption rate and burnup, while providing significantly larger margin against critical heat flux. This study examines the uncertainty in key neutronics parameters due to nuclear data uncertainty. As most of the fissions are induced by epithermal neutrons and since the reactor uses higher actinides as well as thorium and 233U, the cross sections have significantly more uncertainty than in typical LWRs. The sensitivity of the multiplication factor (keff) to the cross sections of many actinides is quantified using a modified version of Serpent 2.1.19 [1]. Serpent [2] is a Monte Carlo code which uses delta tracking to speed up the simulation of reactors; in this modified version, cross sections are artificially inflated to sample more collision, and collisions are rejected to preserve a "fair game." The impact of these rejected collisions is then propagated to the multiplication factor using generalized perturbation theory [3]. Covariance matrices are retrieved for the ENDF/B-VII.1 library [4], and used to collapse the sensitivity vectors to an uncertainty on the multiplication factor. The simulation is repeated for several reactor configurations (for example, with a reduced flow rate, and with control rods inserted), and the difference in keff sensitivity is used to assess the uncertainty associated with the change (the uncertainty in the void feedback and the control rod worth). The uncertainty in the RBWR-TR is found to be dominated by the epithermal fission cross section for 233U in reference conditions, although when the spectrum hardens, the uncertainty in fast capture cross sections of 232Th becomes dominant.

The resource-renewable boiling water reactor (RBWR) is an innovative boiling water reactor that has the capability to breed or to burn transuranium elements (TRUs). Core characteristics of the RBWR of the TRU burner type were evaluated by two different core analysis methods. The RBWR core features an axially heterogeneous configuration, which consists of an internal blanket region between two seed regions, to achieve the TRU multi-recycling capability while maintaining a negative void reactivity coefficient. Axial power distribution of the TRU burner core tends to be more heterogeneous because the isotopic composition ratio of fertile TRUs to fissile TRUs becomes larger in the TRU burner-type core than in the breeder-Type core and the seed regions need to be axially shorter than that of the breeder-Type core. Thus core analysis of the TRU burner-type core is more challenging. A conventional diffusion calculation using nuclear constants prepared by two-dimensional lattice calculations was performed by Hitachi, while the calculation using nuclear constants prepared by three-dimensional calculations and axial discontinuity factors was performed by the University of Michigan to provide a more sophisticated treatment of the axial heterogeneity. Both calculations predicted similar axial power distributions except in the region near the boundary between fuel and plenum. Both calculations also predicted negative void reactivity coefficients throughout the operating cycle. Safety analysis was performed by Massachusetts Institute of Technology for the all-pump trip accident, which was identified as the limiting accident for the RBWR design. The analysis showed the peak cladding temperature remains below the safety limit. Detailed fuel cycle analysis by University of California, Berkeley, showed that per electrical power generated, the RBWR is capable of incinerating TRUs at about twice the rate at which they are produced in typical pressurized water reactors.

The Cl35(n,p) and Cl35(n,α) cross sections at incident neutron energies between 2.42 and 2.74 MeV were measured using the Berkeley High Flux Neutron Generator. The cross sections for Cl35(n,p) were more than a factor of 3 to 5 less than all of the values in the neutron absorption data libraries, while the Cl35(n,α) cross sections are in reasonable agreement with the data libraries. The measured energy-differential cross section is consistent with a single resonance with a width of 293(46) keV. This result suggests that, despite the high incident neutron energy, any attempt to model (n,x) cross sections in the vicinity of the N=Z=20 shell gap requires a resolved resonance approach rather than a Hauser-Feshbach approach.