Features of methods for monitoring the fuel cladding tightness in lead-cooled fast breeder reactors

To timely detect failed fuel elements, a reactor plant should be equipped with a fuel cladding tightness monitoring system (FCTMS). In reactors using a heavy liquid-metal coolant (HLMC), the most efficient way to monitor the fuel cladding tightness is by detecting gaseous fission products (GFP). The article describes the basic principles of constructing a FCTMS in liquid-metal-cooled reactors based on the detection of fission products and delayed neutrons. It is noted that in a reactor plant using a HLMC the fuel cladding tightness is the most efficiently monitored by detecting GFPs. The authors analyze various aspects of the behavior of fission products in a liquid-metal-cooled reactor, such as the movement of GFPs in dissolved and bubble form along the circuit, the sorption of volatile FPs in the lead coolant (LC) and on the surfaces of structural elements, degassing of the GFPs dissolved in the LC, and filtration of cover gas from aerosol particles of different nature. In addition, a general description is given of the conditions for the transfer of GFPs in a LC environment of the reactor being developed. Finally, a mathematical model is presented that makes it possible to determine the calculated activity of reference radionuclides in each reactor unit at any time after the fuel element tightness failure. Based on this model, methods for monitoring the fuel cladding tightness by the gas activity in the gas volumes of the reactor plant will be proposed.

Gas detection in sodium cooled fast reactors: determination of a transfer function

To keep the dependability of Sodium Cooled Fast Reactor, the “clean sodium concept” is demanded, which means that the sodium is free from contamination. The release of fission products is searched for by a contamination measuring system. We need to have a comprehensive description of cladding failures and the detection of contamination, including the failure occurrence on the fuel pin, the transfer process through the sodium and cover gas, the measurement efficiency, etc. We aim to identify the important parameters of physical phenomena, with modelling and simulations based on the return of experiments from past reactors such as PHENIX. There have been a total of 15 open pin failures in PHENIX reactor. Through studying these detected signals, we can get a better physical explanation and description of the evolution of failures. The detection system is related to different stages of the evolution of fuel pin, with different types of fission products, various release mechanism and physical properties. During the evolution of the failed fuel pin, gaseous fission products is released on the first stage of failure and the gas detection system is aimed for the gaseous fission products monitoring. We proposed a quantitative modeling of transfer function to describe the time broadening of the gas release from the fuel pin to the detector. The result matches well with PHENIX experiment data, with the same order of magnitude of the time broadening and the same shape of exponential decreasing. A determination of the transfer function of this gas detection system is validated in this paper.

Safety Analysis of Gas Pressure in a Subcritical Assembly for Molybdenum-99 Production System

The Subcritical Assembly for Molybdenum-99 Production system is a subcritical system fueled by uranium nitrate, which utilizes the Kartini reactor’s beam port as the neutron source. One of the problems in using uranium nitrate fuel involves the radiolysis reactions and gaseous fission products that form in the cavity above the Subcritical Assembly for Molybdenum-99 Production fuel tube, resulting in a buildup of pressure. To address this issue, this study examined the total accumulated gas pressure in each Subcritical Assembly for Molybdenum-99 Production tube contributed by gaseous fission products and water radiolysis by neutron and gamma radiation during 7 days of operation. Examinations were performed by combining the Subcritical Assembly for Molybdenum-99 Production and Kartini reactor geometry to obtain the burnup power using a tally within the Monte Carlo N-Particle eXtended code. Subcritical Assembly for Molybdenum-99 Production system was then simulated for 7 days with the obtained burnup power with the same code. Outputs from the code were then calculated and analyzed to determine the total accumulated pressure on each fuel tube from each of the pressure contributors. This research showed that the maximum accumulated pressures were 0.45 atm and 0.5 atm for Kartini reactor’s power of 100 kW and 110 kW, respectively. These pressures are lower than the atmospheric pressure; hence, the current Subcritical Assembly for Molybdenum-99 Production system can be operated safely for 7 days.

Study of hydrogen generation and radionuclide release during wet damaged oxide spent fuel storage

The paper describes the outcomes of the experiments to study hydrogen and gaseous fission products accumulation during simulations of the wet damaged VVER-440 SNF storage in air-tight canisters with the water drained and no drying conducted. Physical and chemical processes occurring during the damaged oxide SNF storage in wet environment are discussed. The experiments were carried out in two stages: 1) preliminary soaking of fine fuel particles in water in an air-tight canister, 2) water draining and keeping the wet SNF in the air-tight canister. The experiments were conducted one after another using the same SNF canister and differing only in the SNF soaking temperature, i.e. 25 and 80 °С. The radionuclide release into the liquid phase during the SNF storage under water was studied. Uranium and cesium isotopic concentrations were found to reach steady values when the SNF is kept under water for more than a month. The kinetics of hydrogen and gaseous fission product accumulation in the gaseous phase during wet storage of the spent fuel in the air-tight canister with the water drained coincide for both experiments. The kinetics demonstrate an abrupt decrease of the hydrogen and gaseous fission product accumulation rate in 46 hours. The data obtained can be applied for development and verification of the damaged SNF behavior models during SNF storage in wet environment under radiolysis.