Raimondas Pabarčius

Design and Tests of a Device for the Generation of Controlled Condensation Implosion Events

A test facility has been constructed of which the purpose is to explore the characteristics of a thermo-hydraulic component (a “pulser”) where condensation implosion events can be initiated in a controlled and reproducible manner. This paper summarizes experiments conducted in a horizontal cylindrical pulser. For design purposes, a modified Jakobs number criterion was defined. It is an index measuring the capability of the pulser water to condense all of the initial pulser steam that takes into account the heat capacity of the pulser metal and heating of water by condensation. The tests showed that in order to achieve a rapidly growing condensation rate that exceeds the inertial time constant of the vapor phase, the modified Jakobs number has to approach a value of 5. For a 0.027 m3 volume cylindrical pulser, this can be achieved when the steam contains residual amounts of air (on the order of 0.02 to 0.03 mole fraction). The paper presents experimental results characterizing the dynamic response of the pulser as a function of the liquid side turbulence.

The Concept and RELAP5 Model of Thermal-Hydraulic System, Employing a Rapid Condensation for Coolant Circulation

The rapid condensation event is mostly considered a dangerous and undesirable side effect in thermal-hydraulic systems. This work demonstrates a different viewpoint, where condensation implosion is employed to perform mechanical work. Previous experimental study of the condensation implosion event, briefly presented in this article, showed that condensation implosion can be induced intentionally. These results were used as the basis for further investigations. In this work, a concept of the thermal-hydraulic system has been developed, where condensation-implosions-generated pressure difference could be used as a driving force. Numerical study has been performed to investigate the operation of the developed conceptual thermal-hydraulic system. A thermal-hydraulic computer code RELAP5 was selected for modeling the system operation. The RELAP5 code was found not able to predict the condensation implosion; therefore, a modified heat transfer model was implemented into the code. This modification allowed simulating the condensation implosion artificially in the thermal-hydraulic system and modeling the system response to the event. Final results show that a proposed circulation principle is possible and such a thermal-hydraulic system can operate.

Analysis of Void Reactivity Coefficient for 3D BWR Assembly Model

The effect of BWR fuel assembly 3D model on void reactivity coefficient (VRC) estimation is investigated. VRC values were calculated for different BWR assembly models applying deterministic T-NEWT and Monte Carlo KENO-VI functional modules of SCALE 6.1 code package. The difference between deterministic T-NEWT and Monte Carlo KENO-VI simulations is negligible (0.18 pcm/%). The influence of the assumed more detailed coolant density profile was estimated as well. VRC increases with the application of a larger number of coolant density values across fuel assembly height. It was shown that the coolant density profile described by 6 values per height could be considered sufficient from prospect of VRC estimation, as a more detailed density profile has impact below 1% on total assembly void effects. VRC values were decomposed to values for individual nodes and isotopes, since decomposition provides useful insights to describe the overall behaviour of VRC in detail.

Void Reactivity Coefficient Analysis during Void Fraction Changes in Innovative BWR Assemblies

The study of the void reactivity variation in innovative BWR fuel assemblies is presented in this paper. The innovative assemblies are loaded with high enrichment fresh UO2 and MOX fuels. UO2 fuel enrichment is increased above existing design limitations for LWR fuels (>5%). MOX fuel enrichment with fissile Pu content is established to achieve the same burnup level as that of high enrichment UO2 fuel. For the numerical analysis, the TRITON functional module of SCALE 6.1 code with the 238-group ENDF/B-VI cross section data library was applied. The investigation of the void reactivity feedback is performed in the entire 0–100% void fraction range. Higher values of void reactivity coefficient for assembly loaded with MOX fuel are found in comparison with values for assembly loaded with UO2 fuel. Moreover, coefficient values for MOX fuel are positive over 75% void fraction. The variation of the void reactivity coefficient is explained by the results of the decomposition analysis based on four-factor formula and neutron absorption reactions for main isotopes. Additionally, the impact of the moderation enhancement on the void reactivity coefficient was investigated for the innovative assembly with MOX fuel.

Assessment and benchmarking of the impact to gamma dose rate employing different photon-to-dose conversion factors using MCNPX code at the decommissioning stage of Ignalina Nuclear Power Plant

A comparative study was performed to reveal the impact of several photon-to-dose conversion factors for gamma dose rate calculations when applied to heterogeneous environment in the case of decommission stage of the Ignalina Nuclear Power Plant. The following set of conversion factors were investigated by employing the Monte Carlo N-particle transport (MCNPX) code derived from the recommendations given in ICRP-21, ICRP-74 and ANSI/ANS-6.1.1 standards (1977 and 1991), based on the experiments performed for gamma radiation dose rate measurements inside the deplanting Emergency Core Cooling System tank. MCNPX precise simulation and the benchmark between the conversion coefficients highlighted the impact to the results for the selected case of this investigation. The results revealed that the data from the ANSI/ANS-6.1.1 1991 publication are reliable for various dose and shielding calculations in the case of decontamination of radioactive equipment and similar applications since it showed a statistically satisfied agreement between the simulation results and experimental data. These tendencies suggest that the radiological protection system currently adopted in NPP during the decommissioning stage can be characterised using ANSI/ANS-6.1.1 1991 standards with respect to gamma dosimetry.

Study of Controlled Condensation Implosion Events

At LEI (Lithuanian Energy Institute) an experimental program has been initiated to investigate the ‘condensation implosion’ phenomena that can occur for horizontally stratified liquid-vapour flow conditions. The goal is understand the critical boundary conditions sufficiently so that the phenomenon can be controlled and initiated at will. After a reliable ‘pulser’ is developed, the follow up goal is to implement this unique component in a thermal-hydraulic system designed to perform certain tasks, e.g. to pump water or to transport energy passively in a downward direction. Experimental data obtained to data has shown that pulsers can be designed in which the vapour-liquid interface perturbation required for the initiation of condensation implosions is generated internally and depends solely on the rate at which liquid is supplied to the pulser. Data is presented which documents the conditions required for transition from a smooth to a wavy interface, and subsequently to an exponentially increasing surface distortion that culminates in a ‘condensation implosion’. The importance of the shear-stress generated by the condensation rate is illustrated.Copyright

Uncertainty and Sensitivity Analysis of Void Reactivity Feedback for 3D BWR Assembly Model

Uncertainty and sensitivity analysis of void reactivity feedback for 3D BWR fuel assembly model is presented in this paper. Uncertainties in basic input data, such as the selection of different cross section library, manufacturing uncertainties in material compositions, and geometrical dimensions, as well as operating data are considered. An extensive modelling of different input data realizations associated with their uncertainties was performed during sensitivity analysis. The propagation of uncertainties was analyzed using the statistical approach. The results revealed that important information on the code predictions can be obtained by analyzing and comparing the codes estimations and their associated uncertainties.