Application and development of numerical methods for the modelling of innovative gas cooled fission reactors

Innovative gas cooled reactors, such as the pebble bed reactor (PBR) and the gas cooled fast reactor (GFR) offer higher efficiency and new application areas for nuclear energy. Numerical methods were applied and developed to analyse the specific features of these reactor types with fully three dimensional calculation models. In the first part of this thesis, discrete element method (DEM) was used for a physically realistic modelling of the packing of fuel pebbles in PBR geometries and methods were developed for utilising the DEM results in subsequent reactor physics and thermal-hydraulics calculations. In the second part, the flow and heat transfer for a single gas cooled fuel rod of a GFR were investigated with computational fluid dynamics (CFD) methods. An in-house DEM implementation was validated and used for packing simulations, in which the effect of several parameters on the resulting average packing density was investigated. The restitution coefficient was found out to have the most significant effect. The results can be utilised in further work to obtain a pebble bed with a specific packing density. The packing structures of selected pebble beds were also analysed in detail and local variations in the packing density were observed, which should be taken into account especially in the reactor core thermal-hydraulic analyses. Two open source DEM codes were used to produce stochastic pebble bed configurations to add realism and improve the accuracy of criticality calculations performed with the Monte Carlo reactor physics code Serpent. Russian ASTRA criticality experiments were calculated. Pebble beds corresponding to the experimental specifications within measurement uncertainties were produced in DEM simulations and successfully exported into the subsequent reactor physics analysis. With the developed approach, two typical issues in Monte Carlo reactor physics calculations of pebble bed geometries were avoided. A novel method was developed and implemented as a MATLAB code to calculate porosities in the cells of a CFD calculation mesh constructed over a pebble bed obtained from DEM simulations. The code was further developed to distribute power and temperature data accurately between discrete based reactor physics and continuum based thermal-hydraulics models to enable coupled reactor core calculations. The developed method was also found useful for analysing sphere packings in general. CFD calculations were performed to investigate the pressure losses and heat transfer in three dimensional air cooled smooth and rib roughened rod geometries, housed inside a hexagonal flow channel representing a sub-channel of a single fuel rod of a GFR. The CFD geometry represented the test section of the L-STAR experimental facility at Karlsruhe Institute of Technology and the calculation results were compared to the corresponding experimental results. Knowledge was gained of the adequacy of various turbulence models and of the modelling requirements and issues related to the specific application. The obtained pressure loss results were in a relatively good agreement with the experimental data. Heat transfer in the smooth rod geometry was somewhat under predicted, which can partly be explained by unaccounted heat losses and uncertainties. In the rib roughened geometry heat transfer was severely under predicted by the used realisable k − epsilon turbulence model. An additional calculation with a v2 − f turbulence model showed significant improvement in the heat transfer results, which is most likely due to the better performance of the model in separated flow problems. Further investigations are suggested before using CFD to make conclusions of the heat transfer performance of rib roughened GFR fuel rod geometries. It is suggested that the viewpoints of numerical modelling are included in the planning of experiments to ease the challenging model construction and simulations and to avoid introducing additional sources of uncertainties. To facilitate the use of advanced calculation approaches, multi-physical aspects in experiments should also be considered and documented in a reasonable detail.

Congenital upper limb defects in Finland 1993-2005

Background: Interest in limb defects has grown after the thalidomide tragedy in the 1960s. As a result, congenital malformation registries, monitoring changes in birthprevalence and defect patterns, have been established in several countries. However, there are only a few true population based studies on birth prevalence of upper limb defects. The burden of hospital care among these children, specifically in terms of the number of admissions and total time spent in hospital, is also unknown. Aims and Methods: This study is based on information gathered from the Finnish Register of Congenital malformations (FRM) and the Finnish Hospital Discharge Register (FHDR). A total of 417 children born between 1993 and 2005 with an upper limb defect were gathered from the FRM. The upper limb defects were classified using the International Federation of Societies for Surgery of the Hand -classification that enables comparison with previous and future studies. Birth and live birth prevalence, sex and side distribution, frequency of associated anomalies as well as the proportion of perinatal and infant deaths according to the different subtypes were calculated. The number of hospital admissions, days spent in hospital, number and type of surgical operations were collected from the FHDR. Special features of two subgroups, radial ray defects (RRD) and constriction band syndrome (CBS), were explored. Results: Upper limb defects were observed in 417 of 753 342 consecutive births and in 392 of 750 461 live births. Birth prevalence was 5.5 per 10 000 births and 5.2 per 10 000 live births. Multiple anomalies or a known syndrome was found in 250 cases (60%). Perinatal mortality was 139 per 1000 births and infant mortality 135 per 1000 live births (overall Finnish perinatal mortality <5 per 1000 births and infant mortality 3.7 per 1000 live births). Altogether, 138 infants had RRD and 120 (87%) of these had either a known syndrome or multiple major anomalies. The proportion of perinatal deaths in RRD group was 29% (40/138) and infant deaths 35% (43/123). Fifty-one children had CBS in upper limbs. Fifteen of these (29%) had other major anomalies associated with constriction rings. The number of hospital admissions per year of children with congenital upper limb defects was 11-fold and the time spent in hospital 13-fold as compared with the general paediatric population. Conclusions: Birth prevalence of congenital upper limb defects was 5.5 per 10 000 births and 5.2 per 10 000 live births. RRD was especially associated with other major anomalies and high mortality. Nearly one third of the children with CBS also had other major anomalies suggesting different aetiologies inside the group. The annual burden of hospital care of children with congenital upper limb defects was at least 11-fold as compared with the general paediatric population.