Michal Koleška

Capabilities of the LVR-15 research reactor for production of medical and industrial radioisotopes

A typical reactor representing a flexible source of radionuclide production is the LVR-15. More detailed irradiation conditions available within one core configuration of this reactor will be provided. The most important parameter—neutron flux density in different energy intervals—will be determined experimentally by activation detectors and compared with calculated values. Experimental and calculated values will be used for the estimates of the radionuclide production in case of the selected radioisotopes.

Extended set of activation monitors for NCT beam characterization and spectral conditions of the beam after reactor fuel conversion

Since 2010 the LVR-15 reactor has been gradually converted from highly enriched fuel (36wt% (235)U) to low enriched fuel with the enrichment of 19.75wt% (235)U. Paper presents influence of the core pattern changes on the neutron characteristics of the epithermal beam. The determination of neutron spectrum free in the beam was done with a set of neutron activation monitors. After the reactor conversion the change in neutron spectrum is not provable as differences are in the range of measurement errors.

Determination of IRT-2M fuel burnup by gamma spectrometry.

A spectrometric system was developed for evaluating spent fuel in the LVR-15 research reactor, which employs highly enriched (36%) IRT-2M-type fuel. Such system allows the measurement of detailed fission product profiles. Within these measurements, nuclides such as (137)Cs, (134)Cs, (144)Ce, (106)Ru and (154)Eu may be detected in fuel assemblies with different cooling times varying between 1.67 and 7.53 years. Burnup calculations using the MCNPX Monte Carlo code data showed good agreement with measurements, though some discrepancies were observed in certain regions. These discrepancies are attributed to the evaluation of irradiation history, reactor regulation pattern and buildup schemes.

Thermal neutron filter design for the neutron radiography facility at the LVR-15 reactor

In 2011 a decision was made to build a neutron radiography/ facility at one of the unused horizontal channels of the LVR-15 research reactor in Rez, Czech Republic. One of the key conditions for operating an effective radiography facility is the delivery of a high intensity, homogeneous and collimated thermal neutron beam at the sample location. Additionally the intensity of fast neutrons has to be kept as low as possible as the fast neutrons may damage the detectors used for neutron imaging. As the spectrum in the empty horizontal channel roughly copies the spectrum in the reactor core, which has a high ratio of fast neutrons, neutron filter components have to be installed inside the channel in order to achieve desired beam parameters. As the channel design does not allow the instalment of complex filters and collimators, an optimal solution represent neutron filters made of large single-crystal ingots of proper material composition. Single-crystal silicon was chosen as a favorable filter material for its wide availability in sufficient dimensions. Besides its ability to reasonably lower the ratio of fast neutrons while still keeping high intensities of thermal neutrons, due to its large dimensions, it suits as a shielding against gamma radiation from the reactor core. For designing the necessary filter dimensions the Monte-Carlo MCNP transport code was used. As the code does not provide neutron cross-section libraries for thermal neutron transport through single-crystalline silicon, these had to be created by approximating the theory of thermal neutron scattering and modifying the original cross-section data which are provided with the code. Carrying out a series of calculations the filter thickness of 1 m proved good for gaining a beam with desired parameters and a low gamma background. After mounting the filter inside the channel several measurements of the neutron field were realized at the beam exit. The results have justified the calculated values. After the successful filter installing and a series of measurements, first test neutron radiography attempts with test samples could been carried out.

Comparison of various hours living fission products for absolute power density determination in VVER-1000 mock up in LR-0 reactor

Measuring power level of zero power reactor is a quite difficult task. Due to the absence of measurable cooling media heating, it is necessary to employ a different method. The gamma-ray spectroscopy of fission products induced within reactor operation is one of possible ways of power determination. The method is based on the proportionality between fission product buildup and released power. The (92)Sr fission product was previously preferred as nuclide for LR-0 power determination for short-time irradiation experiments. This work aims to find more appropriate candidates, because the (92)Sr, however suitable, has a short half-life, which limits the maximal measurable amount of fuel pins within a single irradiation batch. The comparison of various isotopes is realized for (92)Sr, (97)Zr, (135)I, (91)Sr, and (88)Kr. The comparison between calculated and experimentally determined (C/E-1 values) net peak areas is assessed for these fission products. Experimental results show that studied fission products, except (88)Kr, are in comparable agreement with (92)Sr results. Since (91)Sr has notably higher half-life than (92)Sr, (91)Sr seems to be more appropriate marker in experiments with a large number of measured fuel pins.

Numerical and experimental determination of neutron characteristics in irradiation rigs operated in LVR-15 research reactor

The LVR-15 reactor is a 10-MW research reactor mostly dedicated to material research and isotope production. Material testing can be performed in various irradiation loops and rigs. For specimen irradiation, several rig constructions can be used, including standard single-cell CHOUCA rigs or special dedicated multi-cell rigs. The temperature in the rigs is controlled by a temperature control system, which can be operated in a stable or pulsed mode, with regard to the rig design. Irradiation conditions in the rig are monitored by a set of various fluence detectors. From these detectors, neutron fluence and its energy distribution can be determined for the whole volume of irradiation samples. Besides measurement, irradiation conditions are calculated by the Monte Carlo code MCNPX, which provides a complete review of irradiation conditions including neutron fluence and its energy distribution in samples and detectors, radiation damage and radiation heating conditions for the rig. A set of experimental and theoretical characteristics for dedicated irradiation positions in the core reflector and in fuel will be provided.

Commissioning of the New Irradiation Device for Neutron Transmutation Doping

The LVR-15 reactor is a 10 MW tank type multi-purpose research reactor. The reactor is utilized for production of a wide scale of isotopes with the main focus on generators, testing of materials and chemical regimes of coolant in irradiation rigs and experimental loops (in-pile and out of pile), beam experiments and neutron transmutation doping. The current capacity for neutron transmutation doping was one irradiation facility dedicated to maximum 3 inch ingots. Recently, this capacity was increased by a new irradiation device. Although, the current market demand for neutron transmutation doping is for ingots with diameters up to 6 inch, occasionally even 10 inch, the new device was designed for smaller ingots. The main reason for having a 4 inch diameter maximum for this new device was the limited space in the reactor core. For commissioning purposes of the irradiation channel and for repetitive verification of irradiation parameters special dummy ingots were developed. These dummies can be equipped with three sets of activation detectors, so that the neutron fluence across the ingot can be determined. Within the commissioning, two sets of measurements using the dummy ingots were performed. The primary purpose of these measurements was to verify axial positioning of the irradiation device. The first measurement revealed some issues with accurate positioning of the irradiation device. After these issues were fixed, a new set of measurements using dummy ingots was performed. As a result of these measurements, an estimation of optimal irradiation position was refined. For this refined position a set of silicon ingots was irradiated. From the irradiation results of these ingots, a calibration of the online monitoring system could be done. The successful irradiation of this set of silicon ingots was the last test of the device commissioning; the positioning of the irradiation device and its performance was verified.

Commissioning of the new irradiation device for NTD

The LVR-15 reactor is a 10 MW tank type multi-purpose research reactor. The reactor is utilized for production of a wide scale of isotopes with the main focus on 99Tc generators, testing of materials and chemical regimes of water and gas in irradiation rigs and experimental loops (in-pile and out of pile), beam experiments and neutron transmutation doping. The current capacity for neutron transmutation doping was one irradiation facility dedicated to maximum 3 inch ingots. Recently, this capacity was increased by a new irradiation device. Although the current claim for neutron transmutation doping is for ingots with diameter up to 6 inch or even 10 inch, the new device was designed for smaller ingots. The main reason for having a 4 inch diameter maximum for this new device was the limited space in the reactor core. Commissioning purposes of the irradiation channel and for repetitive verification of irradiation parameters special dummy ingots were developed. These dummies can be equipped with three sets of activation detectors, so that the neutron fluence across the ingot can be determined. Within the commissioning, two sets of measurements using the dummy ingots were performed. The primary purpose of these measurements was to verify axial positioning of the irradiation device. The first measurement revealed some issues with accurate positioning of the irradiation device. After these issues were fixed, a new set of measurements using dummy ingots was performed. As a result of these measurements, an estimation of optimal irradiation position was refined. For this refined position a set of silicon ingots was irradiated. From the irradiation results of these ingots, a calibration of the online monitoring system could be done. The successful irradiation of this set of silicon ingots was the last test of the device commissioning; the positioning of the irradiation device and its performance was verified.

Comparison of the neutron energy spectrum and neutron fluence rate in the LVR-15 research reactor with fuel with different enrichment

This contribution compares measured neutron energy spectra and neutron fluence rates in the LVR-15 reactor core fully equipped with IRT2M nuclear fuel (enrichment 36% of 235U in the form of UO2) and then with a partially replaced core equipped with three IRT4M nuclear fuel assemblies (enrichment 19.7%). The measurements were performed in the LVR-15 reactor at Research Center Rez Ltd. in the Czech Republic, and were related to a planned transition to low-enriched nuclear fuel within the scope of the RERTR programme. An activation method was chosen for the neutron spectrum measurement. Iron, cobalt, nickel, copper, titanium, iridium and niobium foils were irradiated at four positions near the replaced fuel assemblies. Reaction rates for observed reaction channels were determined using gamma spectroscopy. Reaction rates along the height of the reactor core at the same positions were determined using iron, nickel, and cobalt foils. The SAND-II and STAYNL computer programs were used for neutron spectrum adjustment, and input approximation for both programs was calculated using MCNPX (v2.6). The results include a comparison of theoretical and measured data. Differences were found between thermal neutron fluence rates inside IRT2M fuel assemblies and IRT4M fuel. This difference was predicted by preliminary calculations, but it becomes less significant as distance from fuel assemblies increases.