Approaches to the Characterization of Graphite UGR Blocks Based on Beta-emitting 14С and 36Сl Radionuclides. Testing at RBMK-1000-type Reactors

This paper presents some methodological approaches to the characterization of graphite blocks based on difficultto-measure 14C and 36Cl radionuclides during graphite stack dismantlement at uranium-graphite reactors (UGR) developed by JSC PDC UGR. It presents the testing results of the developed methods applied during RBMK-1000 graphite stack block characterization at the Leningrad NPP.

Approaches to graphite block characterization by gamma radionuclides and their packaging during UGR stack dismantlement

This paper presents technical and methodological approaches developed by PDC UGR to characterize the removed graphite blocks by gamma-emitting radionuclides during graphite stack dismantlement and to enable their packaging. It overviews the experimental testing of the characterization method and its findings. It also presents container design option allowing to achieve the maximum filling efficiency.

Analysis of a severe beyond design basis accident for the EGP-6 reactor of the Bilibino NPP. Radioactive source term determination

Analysis was performed of dynamic phase of severe accident of the EGP-6 reactor of the Bilibino NPP, due to uncontrolled reactivity insertion initiated by withdrawal of two pare of automatic control rods with followed by full failure of reactor emergency protection system. This initial event leads to promt increasing of reactor core power up to 450% of nominal value with short period, coupled with rise of temperature of fuel, pressure and temperature of coolant. These factors lead to crisis of heat exchange with subsequent ruptures tubes of fuel assemblies and coolant blow down into graphite stack. All its lead to rise of pressure in reactor shell and damage of it, outflow of steam-water mixture through up-reactor area to ventilation system, communication corridors and reactor hall and further – to atmospheric release. Transient processes were calculated using code RELAP5/Mod3.2. It was considered stages of processes of fuel damage and evaluated dynamic of a number and degree of damaged fuel assembles. They were grouped on burn-up and for each group it was performed analysis of dynamic of damage values. Further it was considered processes of yield of fission products from damaged fuel with models, based on experimental data on yield of fission products from fuel material, used in assembles of Bilibino NPP fuel type (fuel tubes with steel cladding, where fuel material is grits of uranium dioxide in magnesium), under condition of severe accident, especially performed in SSC IPPE. Transport of fission products with steam and air up to release points was evaluated with models, based on experimental data of fission product transport through graphite stack under conditions of severe accident, also especially performed in SSC IPPE. Evaluation of source term was performed in accordance with accident dynamic and assumed modes of release for conservative and most possible approaches. It was noted good self-protection property of EGP-6 reactor under severe beyond design basis accident condition.

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).