RELAP5-3D Three-Dimensional Analysis Based on PHÉNIX Dissymmetric Transient Test

Before the final shutdown of the PHÉNIX fast reactor, the CEA carried out a final set of experimental tests to gather data and additional knowledge on relevant sodium fast reactors (SFR) operation and safety aspects. One of these experiments conducted was the dissymmetrical configuration test, which was selected as benchmark transient within the H2020 SESAME project. ENEA and Sapienza University of Rome are participating in the benchmark using the RELAP5-3D© code. The thermal hydraulic analysis focuses on adequate core cooling prediction in accidental scenario. With the goal of investigating asymmetric thermal hydraulic behavior inside of the reactor pool, two different nodalization approaches have been applied for the RELAP5-3D model, which adopt the same geometrical scheme for the primary flow path, with the exception of the hot and cold pools and the core bypass. The first scheme has been developed using vertical parallel pipes with cross junctions for the hot and cold pools and an equivalent pipe to reproduce the core bypass. The second model includes a multidimensional (MULTID) component, which simulates the pools and provides a detailed nodalization of the core bypass. This study aims at assessing whether the two modeling approaches are equally capable to predict the asymmetrical temperature evolution over the test, caused by the azimuthal asymmetry of the boundary conditions. Blind calculation results are presented and discussed. The paper will be a first step toward the RELAP5-3D code assessment against the experimental results collected as part of the PHÉNIX dissymmetric test.

Estimation of the inventory of 14C and other key radionuclides in irradiated RBMK-1500 graphite based on limited measurements and full 3D core modeling

ABSTRACTIgnalina NPP contains two Units with RBMK-1500 reactors. After shutdown, several Unit 1 systems and equipment were radiologically characterized and dismantled. The highest volume of reactor structures is attributed to the graphite stack of the reactor core, radiological characterization of which has not yet been performed. The stack can be visualized as a vertical cylinder 8 m high and 14 m diameter, made up of 2488 columns where each column is made up from several graphite blocks. The total mass of the graphite stack blocks is about 1700 tonnes. Therefore, the main goal of work reported in this paper was to estimate the inventory of 14C and other key radionuclides in the irradiated graphite by a combination of activity measurements and full 3D reactor graphite stack neutron activation modeling. Obtained results show that, based on the combination of modeling and measurement techniques, the total inventory of 14C in graphite stack is estimated at 3.22×1014 Bq at 9 years after Unit 1 reactor final shutdown. 14C activity is the highest among the analyzed radionuclides; the second highest is 60Co (~6 times lower).

Analysis of Chronobyl NPP Actual Costs Using International Structure for Decommissioning Costing

The paper presents the analysis of actual costs for the decommissioning of Chornobyl NPP units 1, 2 and 3 in compliance with the International Structure for Decommissioning Costing ISDC (according to IAEA recommendations) in 2002–2015. The results obtained demonstrate the importance of the soonest possible reconstruction of the infrastructure of ensuring site life sustaining activities immediately after the final shutdown of power units to save resources. It is also a good illustration of the need to accumulate significant costs in NPP operation to create decommissioning infrastructure.

Modelling of RBMK Spent Nuclear Fuel Characteristics

After the final shutdown of Ignalina NPP, total amount of spent nuclear fuel is approximately 22 thousands of fuel assemblies. Radionuclide content and its characteristics in spent fuel are initial data for analysis of various safety related areas such as shielding, thermal analysis, radioactive releases and other processes. Experimental investigations of radionuclide content and characteristics in spent nuclear fuel are complicated and expensive, therefore numerical evaluation methods are widely used. Numerical modelling of spent RBMK fuel characteristics was performed using TRITON code from SCALE 6.1 system. Activities of fission products and actinides, gamma and neutron sources, decay heat obtained with TRITON code are compared with previous modelling results obtained using SAS2H sequence from the former SCALE 4.3 version. Some evaluated parameters are compared with published experimental data for RBMK spent nuclear fuel.

Modelling of the Radiological Contamination of the RBMK-1500 Reactor Water Purification and Cooling System

This paper presents modelling results on the RBMK-1500 reactor water purification and cooling system (PCS) components contamination at Ignalina NPP Unit 1. The modelling was performed using a computer code LLWAA-DECOM (Tractebel Energy Engineering, Belgium), taking into consideration PCS components characteristics, parameters of the water flowing in circuits, system work regimes, and so forth. During the modelling, results on activity of PCS subsystems and components’ deposits and nuclide composition of deposits at the moment of the final shutdown of the reactor, as well as activity decay of the most contaminated PCS components’ deposits and dose rates after the final shutdown of the reactor, were obtained. Significant difference of contamination levels was revealed among PCS subsystems and subsystems components. The subsystem of nonpurified water is the most contaminated in PCS, and the activity of the least contaminated component in this subsystem is only 1.42% compared to the activity of the most contaminated component. The most contaminated and the least contaminated components of the purified water subsystem comprise 28.33% and 0.86% of activity, respectively, compared to the activity of the most contaminated PCS component.

Belgian Regulatory Framework for Decontamination and Decommissioning: Lessons Learned and New Initiatives

Belgium can rely on significant experience in the field of decontamination and decommissioning of nuclear facilities. Several projects are ongoing and include research reactors (BR3, Thetis), uranium and MOX fuel fabrication plants (FBFC International, Belgonucleaire), fuel reprocessing facilities (Eurochemic) and radwaste processing facilities (Belgoprocess). Additional projects are expected in the coming years with the planned final shutdown of the oldest nuclear power reactor units. Two national authorities are involved in the decontamination and decommissioning process of nuclear facilities. The FANC (together with its subsidiary Bel V) is concerned for all matters related to nuclear safety and radiation protection, while NIRAS/ONDRAF is concerned for all matters related to radioactive waste and fuel management and financial provisions. These attributions ensure that all safety and material concerns are addressed and that both the licensees and the national authorities bear their own responsibilities. They rely on an existing regulatory framework covering both the procedural and the technical aspects of the decontamination and decommissioning activities. However, opportunities for regulatory improvement were raised after some recent events in Belgium, among which the bankruptcy of a nuclear company producing radioisotopes, involving numerous additional interested parties in a complex judiciary context. Amendments in the current regulations are considered to increase the prerogatives of the authorities regarding the management of radioactive waste by a licensee, the transfer of an operating license from an operator to another, and the general decommissioning strategy of a facility. Furthermore, a dedicated “waste and decommissioning” working group within WENRA defined new reference levels applying to the decontamination and decommissioning of nuclear facilities. Belgium committed to include these requirements explicitly in its national legislation, even though most of them were already included in the existing decommissioning authorizations. Amendments will cover the safety provisions inherent to the decontamination and decommissioning phase, such as the decommissioning strategy, the qualification of techniques, the experience feedback valorization, the periodic safety reviews, the radioactive waste management, or the final characterization of the sites. Additionally, requirements regarding the authorization process will be detailed, such as the content of the licensee’s application file or the structure of the safety report covering the decontamination and decommissioning phase. These changes will contribute to a more secure regulatory framework for all interested parties.

Activities ONDRAF/NIRAS Related to the Decommissioning of Nuclear Facilities

Since 1980, the Agency is responsible by law for the safe management of all radioactive waste produced in Belgium, including decommissioning wastes. By the law of 11 January 1991 and the implementing Royal Decree of 16/10/1991, ONDRAF/NIRAS has been entrusted with a mission concerning the decommissioning of nuclear facilities. This mission involves the collection and assessment of data concerning decommissioning forecasts for nuclear facilities, the approval of facilities’ decommissioning programmes, the establishment — in consultation with operators — of financing conditions for decommissioning, as well as the implementation of these programmes on request by the operator, or in the case of its failure to do so. This is the case for the company Best Medical Belgium SA located at Fleurus (MDS Nordion SA, till April 2011), which produced radioisotopes for medical applications and went bankrupt in 2012. These installations have been entrusted to ONDRAF/NIRAS. A plan of action was developed for taking-over the operations in the framework of remediation and decommissioning. Steps have been taken to integrate his new role as a nuclear operator. The installations of Best Medical Belgium SA are now referred to as the “O/N - Site Fleurus.” Nuclear facility operators, or any person requesting to operate a nuclear facility, are obliged to provide ONDRAF/NIRAS, under their responsibility and in due time, with all the necessary information concerning these facilities’ decommissioning forecasts, the nature, quantities and dates of transfer of the resulting waste, and the financing conditions for these facilities’ decommissioning. In order to make the necessary funds available for decommissioning a nuclear facility when it will be shut down, operators are obliged to establish provisions during the facility’s active life. These provisions are calculated in such a way that the total amount established at the time of the final shutdown covers all costs resulting from the facility’s final phase, namely the preparation of decommissioning, decontamination, more or less long-term maintenance, dismantling, treatment, conditioning and disposal of waste produced during this phase. In order to face to multiple and repeated evaluation processes, the Agency, already in the early 90’s, started with the implementation of its own integrated data processing system, recording the physical and radiological inventories of nuclear facilities, and allowing the evaluation of the quantities of decommissioning materials and wastes as well as of the decommissioning costs of these facilities. The cost evaluations cover all decommissioning activities from final shutdown of the facility until final release from nuclear control, as well as conventional demolition and site restoration if required. Beside the information related to the inventories, the database involves so called auxiliary tables integrating unit prices of the various decontamination and dismantling techniques, unit rates for radioactive waste processing, interim storage and final disposal.