Cassiano R. E. de Oliveira

Large sea, small tides: the Late Carboniferous seaway of NW Europe

Debate surrounds the extent of tidal influence in Palaeozoic shallow epi-continental seas. In the absence of analogical reasoning, numerical modelling provides a quantitative means of investigating tidality in the geological record. Herein a new finite element model, tested for accuracy on the present-day Mediterranean, is used to predict an exceedingly low tidal range (<10 cm) in the epi-continental seaway that covered much of NW Europe during the Late Carboniferous. This small bulge may have been amplified to c. 1 m in estuaries, leading to the localized deposition of cyclic rhythmites, agreeing with geological observations. Extremely low tidal ranges in ancient epi-continental seas may be one mechanism to prevent water-body mixing, enhancing stratification and promoting anoxia.

Coupled Space-Angle Adaptivity for Radiation Transport Calculations

Abstract This paper describes the development of a coupled space-angle a posteriori error analysis and adaptive method for radiation transport calculations based on the second-order, even-parity form of the transport equation discretized by a variational finite element–spherical harmonics method (FE-PN). Rigorous a posteriori error estimates for the global L2 norm in the even-parity angular flux are derived by utilizing duality arguments. Separate error components for the spatial and angular discretizations are obtained by the adaptive algorithm by first seeking convergence in the spatial variable and then by projecting the spatially converged solution onto the higher-order PN equation to estimate the angular truncation error. The validity of the developed coupled space-angle adaptive refinement strategy is assessed by comparing the developed error indicator with the true error for representative problems in one and two dimensions. The method of manufactured solutions and alternative transport solution methods are used to provide the true error. Comparisons indicate that the space-angle adaptivity framework is capable of guiding the FE-PN method toward the true solution.

Finite element-spherical harmonics solutions of the 3D Kobayashi benchmarks with ray-tracing void treatment

The finite element spherical-harmonics method is applied to the solution of the Kobayashi 3D benchmark problems. In particular, we evaluate the surface radiation exchange method based on raytracing which has been developed to circumvent the difficulty caused to the second-order, even-parity formulation by low-density regimes. This method brings several advantages which include obviating the need to explicitly solve the problem in voided regions and improving accuracy of the solution for a given order of angular approximation. Results produced by the computer code EVENT are presented for the six cases proposed. Comparisons with reference solutions show that the hybrid scheme can solve to reasonable accuracy most cases with relatively modest space-angle resolution. For the more difficult cases involving streaming in purely absorbing media, noticeable discrepancies were observed, but this indicated the need for a more judicious space-angle refinement (not attempted) rather than any deficiency of the hybrid scheme itself.

Multicycle Optimization of Advanced Gas-Cooled Reactor Loading Patterns Using Genetic Algorithms

Abstract A genetic algorithm (GA)–based optimizer (GAOPT) has been developed for in-core fuel management of advanced gas-cooled reactors (AGRs) at HINKLEY B and HARTLEPOOL, which employ on-load and off-load refueling, respectively. The optimizer has been linked to the reactor analysis code PANTHER for the automated evaluation of loading patterns in a two-dimensional geometry, which is collapsed from the three-dimensional reactor model. GAOPT uses a directed stochastic (Monte Carlo) algorithm to generate initial population members, within predetermined constraints, for use in GAs, which apply the standard genetic operators: selection by tournament, crossover, and mutation. The GAOPT is able to generate and optimize loading patterns for successive reactor cycles (multicycle) within acceptable CPU times even on single-processor systems. The algorithm allows radial shuffling of fuel assemblies in a multicycle refueling optimization, which is constructed to aid long-term core management planning decisions. This paper presents the application of the GA-based optimization to two AGR stations, which apply different in-core management operational rules. Results obtained from the testing of GAOPT are discussed.

An Investigation of Power Stabilization and Space-Dependent Dynamics of a Nuclear Fluidized-Bed Reactor

Abstract Previous work into the space-dependent kinetics of the conceptual nuclear fluidized bed has highlighted the sensitivity of fission power to particle movements within the bed. The work presented in this paper investigates a method of stabilizing the fission power by making it less sensitive to fuel particle movement. Steady-state neutronic calculations are performed to obtain a suitable design that is stable to radial and axial fuel particle movements in the bed. Detailed spatial/temporal simulations performed using the finite element transient criticality (FETCH) code investigate the dynamics of the new reactor design. A dual requirement of the design is that it has a moderate power output of ˜300 MW(thermal).

On an improved design of a fluidized bed nuclear reactor-I: Design modifications and steady-state features

Abstract This paper describes several modifications to the design of a fluidized bed nuclear reactor in order to improve its performance. The goal of these modifications is to achieve a higher power output, requiring an excess reactivity of 4% at maximum expansion of the bed. The modifications are also intended to obtain a larger safety margin when the reactor does not operate; a shutdown margin of 4% is required when the bed is in a packed state. The modifications include installing an embedded side absorber, changing the reactor cross-section area, and modifying the moderator-to-fuel ratio. The new design based on the modifications related to the aforementioned parameters achieves the desired shutdown margin and the excess reactivity. A model describing the coupling of neutronics and thermal/fluid dynamics is developed, and it is used to study the behavior of the reactor at steady conditions. The results show that the reactor can achieve a high output temperature of 1163 K and produce a thermal power of ~120 MW. Further, the results indicate that the power level of the reactor can be controlled easily by adjusting the flow of helium into the core without any further use of control rods or other active control mechanisms.

Criticality Behavior of Dilute Plutonium Solutions

Abstract Numerical modeling of criticality transients in dilute plutonium solutions is presented. The solutions concerned may be expected to have an overall positive temperature coefficient of reactivity at ambient temperatures. Simulations were performed using the FETCH coupled radiation transport-multiphase hydrodynamics code for the cases of step and ramp reactivity insertions. The code has been developed for modeling scenarios beyond the reach of experiment and has been extended to cover the coupling of radiolytic gas behavior and pressure. It is demonstrated that dilute plutonium criticality excursions may be terminated by radiolytic gas evolution provided that the gas is allowed to evolve freely, and that this may result in modest fission yields. However, it is also demonstrated that suppression of radiolytic gas bubbles by pressure may give rise to considerably energetic excursions with consequent large yields.

Discussion on large sea, small tides: the Late Carboniferous seaway of NW Europe

Roger Higgs writes: Wells et al . (2005 a , b ) produced an innovative and thought-provoking numerical model of tides in the Late Carboniferous seaway of NW Europe, predicting very low tidal ranges (<1 m), consistent with the scarcity of sedimentary evidence for (weak) tides reported in the corresponding rocks. Based on the model, Wells et al . (2005 a ) argued intuitively that tidal range was minimum during (glacioeustatic) highstands, and maximum during lowstands (and transgressions). I would argue the converse. During highstands, not only was the water volume (mass) greater, leading potentially to larger tides, but also any tide in the adjacent ocean would necessarily be passed into the seaway, adding to the locally generated tide. This is a weakness of the model: the oceanic tidal influence was ‘not incorporated for simplicity’ (Wells et al . 2005 a , p. 418). In contrast, during lowstands, the seaway became a freshwater lake (or chain of lakes), perched at the level of the outflow sill, and bordered by incised highstand prograded deltas and alluvial plains (‘Lake Bude’ of Higgs 1991, 2004). Disconnected from the global ocean, the lake would have had an extremely low tidal range (<5 cm), as in all lakes, because of the relatively small water volume (Talbot & Allen 1996). Indeed, no evidence for tides has been reported in Lake Bude deposits (Ross, Crackington and Bude Formations of Ireland and England; Higgs 1991, 2004, and references therein). However, given the (scarce) evidence reported elsewhere in the seaway, sedimentary structures indicative of weak tidal currents (e.g. ripples with foreset slack-water mud drapes; rhythmic lamination; opposed ripples) may eventually be discovered in the Lake Bude formations, probably associated with the volumetrically minor shale bands with brackish and marine fossils (Higgs 1991, 2004), representing transgressions and highstands, when …

Transient analysis and dosimetry of the Tokaimura criticality incident

Abstract This paper describes research on the application of the finite element transient criticality (FETCH) code to modeling and neutron dosimetry of the Tokaimura criticality incident. FETCH has been developed to model criticality transients in single and multiphase media and is applied here to fissile solution transient criticality. Since the initial transient behavior has different time scales and physics to the longer transient behavior, the transient modeling is divided into two parts: modeling the initial transient over a time scale of seconds in which radiolytic gases and free-surface sloshing play an important role in the transient—this provides information about the dose to workers; and modeling the long-term transient behavior following the initial transient that has a time scale over hours. The neutron dosimetry of worker A who received the largest dose during the Tokaimura criticality incident is also investigated here. This dose was received mainly in the first few seconds of the ensuing nuclear criticality transient. In addition to the multiorgan dosimetry of worker A, this work provides a method of helping to evaluate the yield in the initial phase of the criticality incident; it also shows how kinetic simulations can be calibrated so that they can be applied to investigate the physics behind the incident.

Validation of the 3D Finite Element Transport Theory Code EVENT for Shielding Applications

This paper is concerned with the validation of the 3D deterministic neutral-particle transport theory code EVENT for shielding applications. The code is based on the finite element-spherical harmonics (FE-PN) method which has been extensively developed over the last decade. A general multi-group, anisotropic scattering formalism enables the code to address realistic steady state and time dependent, multi-dimensional coupled neutron/gamma radiation transport problems involving high scattering and deep penetration alike. The powerful geometrical flexibility and competitive computational effort makes the code an attractive tool for shielding applications. In recognition of this, EVENT is currently in the process of being adopted by the UK nuclear industry. The theory behind EVENT is described and its numerical implementation is outlined. Numerical results obtained by the code are compared with predictions of the Monte Carlo code MCBEND and also with the results from benchmark shielding experiments. In particular, results are presented for the ASPIS experimental configuration for both neutron and gamma ray calculations using the BUGLE 96 nuclear data library.