Phenomenology of Graphite Burning in Air Ingress Accidents of HTRs

Air ingress with graphite burning belongs to the accident scenarios in HTRs with potentially severe consequences. This paper gives an overview of basic phenomena of graphite burning like ignition conditions and moving reaction fronts. The pioneering graphite burning experiments of Don Schweitzer are successfully reevaluated. Ignition conditions are examined, and it is underlined that burning depends not only on graphite properties but also on the heat balance in the whole graphite arrangement. In graphite-moderated reactors, ignition occurs at about 650°C for small air flow rates: this means that normal operation temperatures in HTRs always allow for ignition. Fuel behaviour in air ingress, as determined in the KORA facility, is discussed: up to about 1300°C modern TRISO fuel is stable in air, but from 1500°C a complete, fast destruction is observed. Exemplary calculations on massive air ingress by chimney draught performed with REACT/THERMIX are outlined. For a hot bottom reflector there is a substantial time span before fuel is attacked. Because severe air ingress in well-designed HTRs belongs to beyond design basis accidents, the knowledge is fairly good. Concerning protecting measures, a more detailed examination of thick SiC layers is proposed

Bedeutung des Prozessreifegrads für die Patientenorientierung im Gesundheitswesen

Aim Excellent organization of the internal processes is essential for a well-functioning health care system. Particularly, the process of treatment and interaction with the patients is important because the perception of treatment and interaction has a direct relation to the patients’ satisfaction. The latter has a strong influence on the recovery of the patients. The impact of the maturity level of the treatment and interaction process on patient orientation is investigated at the example of an extensive survey of hospital managers in Ukraine. Method The analysis is based on an empirical data collection conducted with hospital managers and their deputies in a large Ukrainian city, which is acknowledged as a ‘model city’ with regard to a comprehensive health care reform in the country. By means of the Business Process Maturity Model (BPMM) we have tested our hypothesis that a high level of process maturity leads to a high level of patient orientation. For this purpose, we have developed a scale for measuring the level of process maturity and have checked the method on validity and reliability. A total of 79 hospital managers and deputies, who represent all public health care facilities of the city (hospitals, primary care clinics, etc.), participated in the study. Results The study reveals a clear relationship between the level of maturity of the treatment and interaction process and the level of patient orientation of the respective health care facility. The variance, which is explained by the independent variable process maturity level, is 16.3 %. All formative dimensions have a strong significant influence on the construct “patient orientation”. Likewise the maturity level of the treatment and interaction process shows a strong significant influence on patient orientation. Conclusion For health care facilities it is of great importance to achieve a high maturity level of the treatment and interaction process to live up to the claim of being patient oriented. The maturity of the process can be tested using the questionnaire developed in the course of our study

Synthetic Photoswitchable Neurotransmitters Based on Bridged Azobenzenes

Photoswitchable neurotransmitters of ionotropic kainate receptors were synthesized by tethering a glutamate moiety to disubstituted C2-bridged azobenzenes, which were prepared through a novel methodology that allows access to diazocines with higher yields and versatility. Because of the singular properties of these photochromes, photoisomerizable compounds were obtained with larger thermal stability for their inert cis isomer than for their biologically activity trans state. This enabled selective neuronal firing upon irradiation without background activity in the dark

Vorträge zum 4. Darmstädter Geotechnik-Kolloquium am 13. März 1997

Mit dem Mitteilungsheft Nr. 37 publizieren das Institut und die Versuchsanstalt für Geotechnik der Technischen Hochschule Darmstadt die Beiträge zum 4. Darmstädter Geo- technik-Kolloquium mit den folgenden Themenschwerpunkten: - Internationale Projekte/Projekterfahrungen - Möglichkeiten und Grenzen der Kombinierten Pfahl-Plattengründung (KPP) - Rechtsfragen in der Geotechni

Fission Product Transport and Source Terms in HTRs: Experience from AVR Pebble Bed Reactor

Fission products deposited in the coolant circuit outside of the active core play a dominant role in source term estimations for advanced small pebble bed HTRs, particularly in design basis accidents (DBA). The deposited fission products may be released in depressurization accidents because present pebble bed HTR concepts abstain from a gas tight containment. Contamination of the circuit also hinders maintenance work. Experiments, performed from 1972 to 88 on the AVR, an experimental pebble bed HTR, allow for a deeper insight into fission product transport behavior. The activity deposition per coolant pass was lower than expected and was influenced by fission product chemistry and by presence of carbonaceous dust. The latter lead also to inconsistencies between Cs plate out experiments in laboratory and in AVR. The deposition behavior of Ag was in line with present models. Dust as activity carrier is of safety relevance because of its mobility and of its sorption capability for fission products. All metal surfaces in pebble bed reactors were covered by a carbonaceous dust layer. Dust in AVR was produced by abrasion in amounts of about 5 kg/y. Additional dust sources in AVR were ours oil ingress and peeling of fuel element surfaces due to an air ingress. Dust has a size of about 1  m, consists mainly of graphite, is partly remobilized by flow perturbations, and deposits with time constants of 1 to 2 hours. In future reactors, an efficient filtering via a gas tight containment is required because accidents with fast depressurizations induce dust mobilization. Enhanced core temperatures in normal operation as in AVR and broken fuel pebbles have to be considered, as inflammable dust concentrations in the gas phase

A safety re-evaluation of the AVR pebble bed reactor operation and its consequences for future HTR concepts

The AVR pebble bed reactor (46 MWth) was operated 1967-88 at coolant outlet temperatures up to 990°C. A principle difference of pebble bed HTRs as AVR to conventional reactors is the continuous movement of fuel element pebbles through the core which complicates thermohydraulic, nuclear and safety estimations. Also because of a lack of other experience AVR operation is still a relevant basis for future pebble bed HTRs and thus requires careful examination. This paper deals mainly with some insufficiently published unresolved safety problems of AVR operation and of pebble bed HTRs but skips the widely known advantageous features of pebble bed HTRs. The AVR primary circuit is heavily contaminated with metallic fission products (Sr-90, Cs-137) which create problems in current dismantling. The amount of this contamination is not exactly known, but the evaluation of fission product deposition experiments indicates that the end of life contamination reached several percent of a single core inventory, which is some orders of magnitude more than precalculated and far more than in large LWRs. A major fraction of this contamination is bound on graphitic dust and thus partly mobile in depressurization accidents, which has to be considered in safety analyses of future reactors. A re-evaluation of the AVR contamination is performed here in order to quantify consequences for future HTRs (400 MW_{th]). It leads to the conclusion that the AVR contamination was mainly caused by inadmissible high core temperatures, increasing fission product release rates, and not - as presumed in the past - by inadequate fuel quality only. The high AVR core temperatures were detected not earlier than one year before final AVR shut-down, because a pebble bed core cannot yet be equipped with instruments. The maximum core temperatures are still unknown but were more than 200 K higher than calculated. Further, azimuthal temperature differences at the active core margin of up to 200 K were observed, probably due to a power asymmetry. Unpredictable hot gas currents with temperatures > 1100°C, which may have harmed the steam generator, were measured in the top reflector range. After detection of the inadmissible core temperatures, the AVR hot gas temperatures were strongly reduced for safety reasons. Thus a safe and reliable AVR operation at high coolant temperatures, which is taken as a foundation of the pebble bed VHTR development in Generation IV, was not conform with reality. Despite of remarkable effort spent in this problem the high core temperatures, the power asymmetry and the hot gas currents are not yet understood. It remains uncertain whether convincing explanations can be found on basis of the poor AVR data and whether pebble bed specific effects are acting. Respective examinations are however ongoing. Reliable predictions of pebble bed temperatures are at present not yet possible. [...

HTR2008-58336 A SAFETY RE-EVALUATION OF THE AVR PEBBLE BED REACTOR OPERATION AND ITS CONSEQUENCES FOR FUTURE HTR CONCEPTS

ABSTRACT The AVR pebble bed reactor (46 MWth) was operated 1967-1988 at coolant outlet temperatures up to 990°C. Also because of a lack of other experience the AVR operation is a basis for future HTRs. This paper deals with insufficiently published unresolved safety problems of AVR and of pebble bed HTRs. The AVR primary circuit is heavily contaminated with dust bound and mobile metallic fission products (Sr-90, Cs-137) which create problems in current dismantling. The evaluation of fission product deposition experiments indicates that the end of life contamination reached several percent of a single core inventory. A re-evaluation of the AVR contamination is performed in order to quantify consequences for future HTRs: The AVR contamination was mainly caused by inadmissible high core temperatures, and not -as presumed in the past -by inadequate fuel quality only. The high AVR core temperatures were detected not earlier than one year before final AVR shut-down, because a pebble bed core cannot be equipped with instruments. The maximum core temperatures were more than 200 K higher than precalculated. Further, azimuthal temperature differences at the active core margin were observed, as unpredictable hot gas currents with temperatures > 1100°C. Despite of remarkable effort these problems are not yet understood. Having the black box character of the AVR core in mind it remains uncertain whether convincing explanations can be found without major experimental R&D. After detection of the inadmissible core temperatures, the AVR hot gas temperatures were strongly reduced for safety reasons. Metallic fission products diffuse in fuel kernel, coatings and graphite and their break through takes place in long term normal operation, if fission product specific temperature limits are exceeded. This is an unresolved weak point of HTRs in contrast to other reactors and is particularly problematic in pebble bed systems with their large dust content. Another disadvantage, responsible for the pronounced AVR contamination, lies in the fact that activity released from fuel elements is distributed in HTRs all over the coolant circuit surfaces and on graphitic dust and accumulates there. Consequences of AVR experience on future reactors are discussed. As long as pebble bed intrinsic reasons for the high AVR temperatures cannot be excluded they have to be conservatively considered in operation and design basis accidents. For an HTR of 400 MWth, 900°C hot gas temperature, modern fuel and 32 fpy the contaminations are expected to approach at least the same order as in AVR end of life. This creates major problems in design basis accidents, for maintenance and dismantling. Application of German dose criteria on advanced pebble bed reactors leads to the conclusion that a pebble bed HTR needs a gas tight containment even if inadmissible high temperatures as observed in AVR are not considered. However, a gas tight containment does not diminish the consequences of the primary circuit contamination on maintenance and dismantling. Thus complementary measures are discussed. A reduction of demands on future reactors (hot gas temperatures, fuel burn-up) is one option; another one is an elaborate R&D program for solution of unresolved problems related to operation and design basis accidents. These problems are listed in the paper

Shielding examinations for the European Spallation Source ESS: Update for use by EURISOL

The Moyer model used in shielding design of the ESS accelerator is outlined in detail and relevant results are presented. General guidelines for shielding of a large spallation based system are presented, which take into account accidental beam losses and enhanced beam losses during start-up, too

Deliverable D3: decommissioning issues

Final report upon completion of deliverable D