Use of axially graded burnable boron for hot-spot temperature reduction in a pressurized water reactor core

Shortly after the loading of a pressurized water reactor (PWR) core, the axial power distribution in fresh fuel has already attained the characteristic, almost flat shape, typical of burned fuel. At beginning of cycle (BOC), however, the axial distribution is centrally peaked. In assemblies hosting uniform burnable boron rods, this BOC peaking is even more pronounced. A reduction in the axial peaking is today often achieved by shortening the burnable boron rods by some 30 cm at each edge. It is shown that a two-zone grading of the boron rod leads, in a representative PWR cycle, to a reduction of the hot-spot temperature of approximately 70 °C, compared with the nongraded case. However, with a proper three-zone grading of the boron rod, an additional 20 °C may be cut off the hot-spot temperature. Further, with a slightly skewed application of this three-zone grading, an additional 50 °C may be cut off. The representative PWR cycle studied was cycle 11 of the Indian Point 2 station, with a simplification in the number of fuel types and in the burnup distribution. The analysis was based on a complete three-dimensional burnup calculation. The code system was ELCOS, with BOXER as an assembly code for the generation of burnup-dependent cross sections and SILWER as a three-dimensional core code with thermal-hydraulic feedback.

Unitary SK1 of unitary and graded division algebras

The reduced unitary Whitehead group $\SK$ of a graded division algebra equipped with a unitary involution (i.e., an involution of the second kind) and graded by a torsion-free abelian group is studied. It is shown that calculations in the graded setting are much simpler than their nongraded counterparts. The bridge to the non-graded case is established by proving that the unitary $\SK$ of a tame valued division algebra wih a unitary involution over a henselian field coincides with the unitary $\SK$ of its associated graded division algebra. As a consequence, the graded approach allows us not only to recover results available in the literature with substantially easier proofs, but also to calculate the unitary $\SK$ for much wider classes of division algebras over henselian fields.

Åldersblandning i skolan : elevers erfarenheter

Mixing ages in school classes became more and more common during the last dec-ades of the 20th century. From being a way to organise classes out of necessity they have now come to be something which is implemented on the basis of pedagogical arguments. The aim of this research has been to improve our knowledge of classes where pupils are not of the same age. A study of the pupils’ perspectives has been my main interest. (Age) homogeneous class can been looked upon as a result of the authorities’ deci-sion to have a fixed age for children to start school and their decision that certain courses should be completed within a defined period of time. Terms and the data concerning heterogeneous age groupings are ambiguous and cannot be fully understood without knowledge of national and sometimes even local contexts. Practices within age heterogeneous classes may differ greatly. A great deal of individual work takes place in age heterogeneous classes. Whether the class is non-mixed or mixed-aged does not seem to have a major im-pact on cognitive or non-cognitive abilities among the pupils, but there are suggestions that age heterogeneous classes might be disadvantageous to pupils in problematic situations. I am able to show that more than 30% of pupils in grades 1-3, close to 25% in grades 4-5, about 15% in grade 6 and a couple of percent of Swedish pupils in the later school years are taught in mixed-age groups. My own empirical research focuses on pupils’ experiences. My investigation has a ‘life-world’ oriented approach inspired by phenomenology. Pupils in grades 5 and 6 from three schools in three different socio-economic settings were interviewed. These pupils had experienced both mixed-age and single-age classes. The life-world of pupils seems to be something different from that encompassed by the philosophy about the advantages of mixing the ages in classes. Pupils find it diffi-cult to maintain or create relationships when only a few pupils of the same sex, who have started school at the same time, can be together in a class for a long time. Be-cause of the importance of social relationships almost every pupil in this investigation wished to be in a single-age class during the following year. It is the importance of common experiences rather than age that is central. Pupils stated that having things in common to study in their everyday schoolwork makes it easier to communicate and contributes to stable friendships. In my conclusion I focus on what it means to have relationships and how these are important for human identity. I also try to show how relationships are important in learning situations at school and for pupils’ opportunities to expand their knowledge.