Band 69, Heft 4

The two-phase flow is a kind of dissipative structure in multiphase hydrodynamics. A drift-flux model with four equations is adopted to investigate the natural circulation bifurcation, in which the sub-cooled boiling, condensation, and void fraction etc. have been taken into account. The bifurcation spectrum of a natural circulation system with low steam quality and low system pressure is presented and the physical mechanisms of bifurcation are described in detail. Bifurcation characteristics of the two-phase natural circulation indicate typical static instability observed in HRTL-5 simulating the geometry and system design of the 5 MW nuclear heating reactor operating at low steam quality and low system pressure. Analysis shows: (1) bifurcation phenomenon can occur in a two-phase natural circulation under certain thermal and geometrical condition; (2) with changing, namely increasing and decreasing, control parameters, the tracks of the work points of the natural circulation system are not identical, which shows the basic characteristic of the bifurcation phenomenon; (3) the bifurcation phenomenon of the natural circulation delay and disappear with increasing system pressure and decreasing heat flux.

The experiments were performed on the test loop HRTL-5, which simulates the geometry and system design of the 5 MW nuclear heating reactor. In a wide range of inlet sub-cooling various flow instabilities were observed at p = 0.1 MPa and p = 1.5 MPa. Because of the different geometry design and operating conditions between the heating reactors and the boiling water reactors, the flow behavior presents great difference. Analysis shows: (1) under heating reactor conditions, sub-cooled boiling, condensation and void flashing are the fundamental thermodynamic processes; (2) sub-cooled boiling, condensation, void flashing and the compressibility of the steam space play an important role in the flow instabilities of the natural circulation system; (3) sub-cooled boiling instability, flashing instability, and flow excursion are the special instabilities at nuclear heating reactor conditions.

In this study, the transient in which the pool water is lost throughout one or more of the main coolant pipes which are supposed to be broken guillotine-like is investigated for the TR-2 research reactor in Istanbul. The applicability of the methods used for other similar types of research reactors is shown. Decrease of the pool water level until the top of the core, and from the top to the bottom of the core are examined as two successive phases of the accident. Finite difference scheme and integral methods are employed to solve energy equations and the results of both methods are compared. The finite difference solution uses an explicit form for the analysis of the first phase, and a moving boundary approach for the second phase. The integral method is based on the assumption that the temperatures appearing in the energy equations have the same profiles during the transient as the steady state ones. Analyses are done both for nominal and hot channel, and the results of both methods are observed to be in agreement.

A detailed three-dimensional MCNP model of the ETRR-2 research reactor has been developed for the analysis of the neutronic parameters of the reactor. The model was used to determine the partial control plates reactivity worth and the behavior of the worth with time. The results of the present model were compared with the experimental measurements for partial control plates reactivity worth and with the design calculations for control plates behavior with time. The comparisons indicate satisfactory agreement.

The paper presents the results of a thermal hydraulic experiment conducted at the PMK-2 test facility within the IMPAM-VVER project. The test was dedicated to a loss-of-coolant event during the cool-down phase of the plant, when neither core flooding tanks nor safety injections systems are automatically available. Though fuel rod dryout occurs, the activation of one of four lines of low pressure emergency injection is sufficient to re-establish reliable core cooling. In the cooling-down situation, the primary circuit is pressurised with nitrogen, therefore, non-condensable gas can enter the primary circuit from the pressurizer during the blow-down phase. The transport of the nitrogen along the primary circuit was studied with a novel type of a local void probe equipped with an integrated micro-thermocouple. RELAP5 was found to be able to predict the behaviour of the non-condensable gas in the primary circuit. Despite of some delay in the prediction of the time of the appearance of the gaseous phase in the begin of the process, most details of the transient were well reproduced by the code.

Since 1991 the national corporation WISMUT GmbH has been charged with the rehabilitation of the sites of former Uranium mining and milling in East Germany. Since decommissioning and site remediation are carried out at places with enhanced dose rates and high concentrations of airborne radioactivity (Rn-222, long-lived alpha emitters), a large part of the employees is classified as occupational exposed radiation workers. Thus, in 2002, 1,073 workers were engaged with relevant works in radiation protection areas aboveground (radiation exposed workers of category B; mean effective dose: 0.3 mSv; maximum dose: 2.2 mSv), and 514 workers still prepared by underground activities the flooding of the Uranium mines (category A; mean effective dose: 0.9 mSv; maximum effective dose: 4.9 mSv). The paper presents the measures taken by WISMUT to optimise the occupational exposure. The need for implementing appropriate ALARA methods in the field of occupational exposure is last but not least caused by the fact that for a number of employees the integral exposure over the total working life time has exceeded already the 400 mSv figure.

The concept of exemption has been widely used in radiological protection. It is part of the regulator's arsenal for applying legislative requirements in a graded fashion, to avoid the expenditure of effort on situations where the return in terms of improvement in protection would otherwise be trivial. Nevertheless, it still remains a controversial matter. It is not so much the dosimetric criteria for exemption that cause debate; it is more the way in which the concept is used; the scenarios employed to calculate derived activity concentrations, and its relationship to the somewhat analogous concept of exclusion. A particular problem has been the full integration into the legislative system of protection of exposures to radiation from sources of natural origin and the degree to which exemption is a relevant concept for dealing with such situations. The purpose of this paper is to attempt to provide some clarity on the two concepts and their practical implementation with a view to encouraging international harmonization and avoiding further unnecessary debate.

The ETRR-2 replacement cooling tower capability has been evaluated by the thermal acceptance test performed in June 2003. All instruments used were calibrated prior to the test. The measured data are collected at regular intervals in accordance with the acceptance test code for water cooling towers of the Cooling Tower Institute recommendations. Both the characteristic curve and the performance curve methods were used to evaluate the tower capability. The test results yield a tower capability of about 105% and so the tower is thermally accepted.