Miroslav Vins

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.

Study of suitability of Fricke-gel-layer dosimeters for in-air measurements to characterise epithermal/thermal neutron beams for NCT

The reliability of Fricke gel dosimeters in form of layers for measurements aimed at the characterization of epithermal neutron beams has been studied. By means of dosimeters of different isotopic composition (standard, containing (10)B or prepared with heavy water) placed against the collimator exit, the spatial distribution of gamma and fast neutron doses and of thermal neutron fluence are attained. In order to investigate the accuracy of the results obtained with in-air measurements, suitable MC simulations have been developed and experimental measurements have been performed utilizing Fricke gel dosimeters, thermoluminescence detectors and activation foils. The studies were related to the epithermal beam designed for BNCT irradiations at the research reactor LVR-15 (Řež). The results of calculation and measurements have revealed good consistency of gamma dose and fast neutron 2D distributions obtained with gel dosimeters in form of layers. In contrast, noticeable modification of thermal neutron fluence is caused by the neutron moderation produced by the dosimeter material. Fricke gel dosimeters in thin cylinders, with diameter not greater than 3mm, have proved to give good results for thermal neutron profiling. For greater accuracy of all results, a better knowledge of the dependence of gel dosimeter sensitivity on radiation LET is needed.

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.

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.

MEASUREMENT OF NEUTRON SPATIAL DISTRIBUTION OF THE BNCT EPITHERMAL BEAM AT THE REACTOR LVR-15

In this study, a measurements of neutron field using a special positioning device with a 6 Li + Si detector and image plate is described. The measurements were provided for Boron Neutron Capture Therapy (BNCT) channel of the LVR-15 reactor in the Research Centre Rez Ltd., Czech Republic. Mapping of neutron field represents an essential and crucial part of planning BNCT treatment (especially for patients suffering from brain tumor Glioblastoma Multiforme).

Dose measurements at epithermal beams of research reactors with fricke gel and thermoluminescence detectors

Suitable dosimeter methods have to be developed to measure the different dose contributions in phantoms exposed to epithermal/thermal neutron beams of a research reactor. The method based on Fricke xylenol orange gel dosimeter in form of layers has shown to be very effective for achieving images of the various dose components in air or in phantoms exposed to epithermal/thermal neutron beams with very high fluence rate. Another useful method is based on the use of TLD-700 chips, from whose response the gamma dose and the thermal neutron fluence can be obtained by means of appropriate parameters of the glow curve.

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.