Mean first passage time for nuclear fission and the emission of light particles

The concept of a mean first passage time is used to study the time lapse over which a fissioning system may emit light particles. The influence of the "transient" and "saddle to scission times" on this emission are critically examined. It is argued that within the limits of Kramers' picture of fission no enhancement over that given by his rate formula need to be considered.Comment: 4 pages, RevTex, 4 postscript figures; with correction of misprints; appeared in Phys. Rev. Lett.90.13270

Evaporation of light particles from a hot, deformed and rotating nucleus

The dependence of the transmission coefficient on the deformation, the collective rotation and excitation energy of the compound nucleus emitting light particles is introduced in the framework of Wei{\ss}kopf's evaporation theory. The competition between fission and particle evaporation is treated by a~Langevin equation for the fission variable coupled to the emission process. Detailed calculations are presented on the decay of different Gd and Yb isotopes at an excitation energy of about 250~MeV. These calculations demonstrate the importance of the effects of nuclear deformation and of the initial spin distribution on the evaporation.Comment: 22 pages in LaTeX and 26 PS-figures include

Statistical fluctuations for the fission process on its decent from saddle to scission

We reconsider the importance of statistical fluctuations for fission dynamics beyond the saddle in the light of recent evaluations of transport coefficients for average motion. The size of these fluctuations are estimated by means of the Kramers-Ingold solution for the inverted oscillator, which allows for an inclusion of quantum effects.Comment: 12 pages, Latex, 5 Postscript figures; submitted to PRC e-mail: [email protected] www home page: http://www.physik.tu-muenchen.de/tumphy/e/T36/hofmann.htm

On nuclear transport at small excitations

Numerical computations of transport coefficients at low temperatures are presented for shapes typically encountered in nuclear fission. The influence of quantum effects of the nucleonic degrees of freedom is examined, with pair correlations included. Consequences for global collective motion are studied for the case of the decay rate. The range of temperatures is specified above which this motion may be described as a quantal diffusion process.Comment: 8 pages, LaTex, 2 postscript figures; final version with small editorial changes, Phys. Rev. Lett. 82 (1999) 4603; e-mail: [email protected]; [email protected]

Other in situ bioremediation techniques; literature study in the framework of the project "In Situ Bioremediation" in Asten

Aan de hand van literatuurgegevens zijn resultaten en kosten onderzocht van praktijksaneringen waarbij de bodem doorstroomd werd met lucht. De saneringstechnieken waarbij zuurstof via de lucht aan de bodem wordt toegevoerd, zijn bodemventilatie, bodemluchtextractie, bioventing en persluchtinjectie. Deze technieken werden vergeleken met het waterafgiftesysteem in Asten waarbij waterstofperoxyde toegepast is. De locaties, waarop deze technieken toegepast worden, zijn over het algemeen goed doorlaatbare zandgronden. De verontreiniging die zich op deze locaties bevond, besloeg het hele scala koolwaterstoffen. Uit de resultaten die in de literatuur gevonden werden, blijkt dat de saneringen over het algemeen zo'n twee jaar in bedrijf gehouden worden. In sommige gevallen worden hoge verwijderingssnelheden (tot ongeveer 20 mg.kg exp. -1 grond.dag exp. -1) bereikt. Nergens in de literatuur wordt vermeld dat de streefwaarden behaald konden worden. Bij de sanering in Asten zijn lagere restgehaltes behaald, maar na een langere saneringsduur. Deze locatie had waarschijnlijk ook gesaneerd kunnen worden met behulp van bioventing, waarbij de grondwaterspiegel verlaagd had moeten worden. Waarschijnlijk zou de saneringsduur korter zijn geweest omdat zuurstoflimitering een kleinere rol zou hebben gespeeld. Verwacht wordt echter dat de uiteindelijke restgehaltes hoger zouden liggen. Indien de kosten van de verschillende technieken onderling vergeleken worden, blijkt bioventing de meest kosteneffectieve saneringstechniek te zijn.In developing in situ remediation most of the focus used to be on techniques using infiltration water to supply oxygen to the location. Later, techniques were developed in which soil was flushed with air to enhance the oxygen availability to microorganisms. The aim of the study reported here was to examine the results of remediation where soil is flushed with air, as reported in the literature, and to examine the costs of such systems. In the study these techniques were compared with water-supply systems, particularly those using hydrogen peroxide, e.g. bioremediation in Asten (Noord-Brabant). The remediation techniques which use air to supply oxygen to the soil are soil ventilation, soil vapour extraction, bioventing and air sparging. Soil ventilation and soil vapor extraction are physical remediation techniques which can be employed only in the vadoze zone for removal of low molecular organic pollutants. Bioventing removes pollutants mainly by microbiological conversions in the vadoze zone. Air sparging can be employed only in the saturated zone, where the pollutants can be stripped by air, as well as converted by microorganisms. In general, these techniques are used on sandy soils, which are very permeable. The whole range of hydrocarbons can be remediated. According to the literature, remediations were generally kept in operation for two years. At some sites high removal rates, up to 20 mg.kg-1 soil.day-1, were reached. The literature did not mention anything about the remaining levels of pollution dropping to target-value level. Although sometimes only 5% of the original amount of pollution remains in the soil, the lowest levels obtained are found just below the former B value. Therefore, the remediation levels reached should always be viewed in connection with the remediation time and the amount of pollution originally present. At the remediation site in Asten, where a water-supply system with hydrogen peroxide dosing is used, lower remaining levels were obtained after a longer remediation time. This location could problably also be remediated using a bioventing technique if the groundwater table were lowered. The remediation time would probably be shorter because oxygen would be available to a greater extent. However, the final pollution levels in the soil are expected to be higher. Due to the lower soil-water content in the pores, the mineralization process would terminate earlier. It is possible that these results would not be obtained in the vicinity of the groundwater table. Another possibility might be natural attenuation after lowering the contamination levels with an active remediation technique. The Dutch target values can be expected to be attained, but only after a very long remediation time. Bioventing appears to be the most cost-effective remediation technique available.DGM/B