[en] When a plasma is bounded by an electron-emissive wall, the sheath which forms in its vicinity will accelerate any released electrons into the plasma and this will give rise to the beam-plasma instability. The development of this instability has been modelled using a 1-D particle-simulation code developed for this purpose. When simulating the instability the sheath region is not represented in the code and the parameters for the sheath and associated secondary-electron beam have been obtained from a separate calculation. Equations, generalised to include current flows to the wall under equilibrium conditions, have been derived for this purpose and their solutions obtained. The thermal fluctuation energy of the most unstable mode in the sheath has also been estimated. While pursuing the equilibrium calculations a hypothetical plasma system was discovered for which the choice of boundary condition was not clear, which has led to the derivation of a new criterion for sheath formation. (author)

[en] Bounding calculations of thermal transients following a hypothetical (Category V) loss of coolant accident in two international thermonuclear experimental reactor (ITER) design options, EU-I and ITER-fusion energy advanced tokamak (FEAT), have been carried out for a variety of assumptions concerning heat-transfer to and heat rejection from, the cryostat. It is shown that, even when unrealistically conservative assumptions are made in illustrative calculations, the temperatures do not approach melting point for any part of the plant structure. In all cases, maximum temperatures are reached within 1 year. It is also concluded that for the baseline scenario for ITER-FEAT in which helium gas is assumed to be present in the cryostat, convectively transferring heat between components, the outboard first-wall temperature never exceeds 530 deg. C. A significant improvement could still be made if the surface emissivity of the thermal shields could be increased by some means during the post-accident period

[en] Refinements to consequence modelling for hypothetical accidents in Fusion Power Plants have been explored. This leads to improved accuracy and a reduction in some of the conservatism inherent in previous calculations. Assumptions made in previous analyses for the Safety and Environmental Assessment of Fusion Power (SEAFP) are examined, with particular emphasis given to aerosol modelling within the containment and dispersion and dose calculations. By employing a more realistic treatment of the time dependence in the aerosol model and introducing a procedure for accounting for the effects of wind meander, it is shown how to obtain results which may be used to adjust previously derived dose estimates. Further analysis assesses the possibility of aerosol particle removal by filtering in cracks in the containment barriers, as material leaks into the environment. The potential for mitigation by this mechanism has been neglected in previous calculations

[en] We report here improvements to a code, FUSCON, developed for the SEAFP-2 (Safety and Environmental Aspects of Fusion Power) study to model containment transport and release to the environment. The previous version of FUSCON did not include heat transfer to the wall, accounted for condensation for only a limited set of conditions, and hence placed restrictions on some of the containment parameters investigated. The improvements described involve modelling of post-accident convective transport in the containment system. This is a good basis for enhancement of the model to include mass transfer effects associated with condensation. Three-dimensional, computational-fluid-dynamics, finite-element calculations have guided the derivation of a computationally economic model of convective containment transport. The efficiency of the model will ensure that FUSCON is still sufficiently fast-running to be used to study confinement system optimisation and selection

[en] It is found for conditions relative to fusion safety studies that there is a range of initial aerosol masses (up to ∼1000 kg) within which the aerosol particle growth dynamics are dominated by Brownian agglomeration. Above this range, the dominant growth mechanism is gravitational agglomeration. Previously, monodisperse aerosol modelling has been reported (W.E. Han 1996) which enables the accurate prediction of aerosol behaviour up to initial masses of ∼1000 kg. Here monodisperse modelling is extended to deal with initial aerosol masses above 1000 kg, enabling the prediction of aerosol transport and the release to the environment in the gravitationally dominated agglomeration regime for hypothetical accidents involving mobilised radionuclides. It is shown that increasing the initial aerosol mass in the Brownian dominated agglomeration regime results in increased release to the environment. However, increases in initial mass in the gravitationally dominated regime soon lead to a maximum in release to the environment; as a result, further increases in initial mass produce smaller releases to the environment. (orig.)

[en] The modelling of the transport and depletion of aerosols in fusion power plant containment volumes is an important part of the analysis of postulated accidents. This has hitherto been performed either by very over-simplified and over-conservative calculations or by the use of large codes which are time-consuming and inflexible to use, and do not readily lead to understanding. These large codes make quite heavy demands on computer resources because they have to calculate agglomeration and removal rates, as well as number densities of aerosol particles with a distribution of sizes. This paper describes a method of taking these particle growth and mass depletion effects into account by transforming the problem into one of calculating the behavior of a monodisperse aerosol of constant size which is defined by the geometry of the containment system and the total mass of aerosol present initially. Results were obtained for a number of test cases and compared with calculations for the same cases carried out using ITHACA. The agreement between the two methods was found to be excellent. Further calculations for a more comprehensive range of conditions were also carried out with the proposed model and the results presented. These extended calculations illustrate the fact that even significantly damaged containments (signified by a high leak rate) can, depending on the conditions, bring about retention of nearly all the aerosol by gravitational settling

[en] Highlights: •Energy and power demands implied by start-up and attainment of operating conditions for a pulsed DEMO are explored. •The current drive performance of the associated auxiliary power system is also explored. •Requirements for transformer recharge, and for energy storage to smooth the plant output, are derived. •Cost of the power supply required for transformer recharge increases as dwell time is reduced, rising sharply below 500 s. •Energy storage costs increase with dwell time, but remain much lower than power supply costs at 2000 s, for assumptions used. -- Abstract: The energy and power demands implied by start-up and attainment of operating conditions for a pulsed DEMO have been explored. The information gained has been used to establish the requirements both for transformer recharge and for an energy storage system designed to ensure the plant effectively maintains an uninterrupted 1 GW supply to the grid. It was determined that a minimum of approximately 150 MW of auxiliary heating would be required to access the desired operating conditions for the burn phase, and the current-drive potential of a heating system of this capacity was assessed to determine how the pulse length could be extended beyond purely inductive operation. The overall peak power and energy demand associated with the start-up phase of each pulse of output power was determined for a range of dwell times. These two quantities determine the costs of the subsystems required for the power provision during the transformer recharge phase and the energy storage needed for constant plant output. We are therefore able to produce a simple estimate of such costs as a function of dwell time. For falling dwell times, costs are found to rise sharply below around 500 s, but they remain relatively flat as dwell times increase beyond around 1000 s, over the range considered

[en] The paramount reason for pursuing fusion R and D is that fusion power stations potentially have major safety and environmental advantages over alternatives for bulk electricity generation. However, these advantages must be secured in an economically acceptable form. This paper discusses the safety, environmental and economic objectives for fusion, and considers how they might best be reconciled and achieved. The main results of the SEAFP study are briefly presented. The fundamental reasons for the very favorable results are explained, distinguishing between results stemming from generic factors, and results which might alter as a result of design changes. Systems code calculations are used to assess the broad design features which improve economic performance. Scoping calculations are given which indicate the changes in S and E performance which would arise from design changes made to improve economic performance. The changes which improve both S and E and economic performances, those that improve economics without significant impact and S and E and vice versa, and those that remove the potential for significant conflicts between economic and S and E objectives, are noted. The implications of the above for the definition of attractive fusion demonstration power plants (DEMOs), and for the plasma physics and technology R and D programs leading up to these, are discussed

[en] The potential and charge particle distributions are calculated for a collisionless sheath bounding a plasma adjacent to a wall which emits secondary electrons under both floating and current-carrying conditions. The corresponding conditions at the sheath-plasma boundary are determined in terms of the parameters secondary electron beam density/plasma density and beam velocity/plasma electron thermal speed. This ''steady state initial condition'' is then examined for instability to excitation of the beam-plasma instability using particle-in-cell computer simulations. Results are given and summarized in the appropriate parameter space showing when the instability can be classified as weak, strong, or violent. The work is related to other work on the beam-plasma instability - theoretical, computational and experimental -and on large amplitude electron plasma waves. (author)

[en] Analyses have been performed of the potential consequences to the public of hypothetical loss-of-coolant accidents in conceptual fusion power plant designs. In order to establish upper bounds to the consequences of such events, a case has been studied in which total loss of all active cooling has been assumed, with no remedial intervention for the duration of the accident sequence. The analyses are based on three conceptual power plant designs, two of them similar to those assumed in the earlier safety and environmental assessment of fusion power (SEAFP) study (Raeder et al., 1995), with updating of assumed structural materials. The three models studied provide a broad range of design options. In all cases the decay-heat driven temperature transients are well below the level at which structural melting would begin. Based on conservative assumptions, mobilisation, release and dose calculations show that potential maximum doses to the public are very far below the levels at which evacuation might be considered