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Dubna’s IBR-2 periodic pulsed research reactors are the most effective source of slow neutrons in extracted beams for studying various structures by using diffraction, small-angle scattering, reflectometry, inelastic scattering, and neutronography, thanks to their short-pulse neutrons and high average flux (reaching 1014 cm–2 s–1). At the same time, due to the specificity of the kinetics, fluctuations in the power energy of pulses in such a reactor are tens of times higher than in stationary reactors and create problems when trying to control the apparatus. This paper proposes and substantiates a method to significantly reduce the level of fluctuations in the power pulses of such reactors using the NEPTUNE pulsed reactor project with the Np‑237 threshold isotope as a nuclear fuel as an example.

Due to the approaching deadline of the reactor IBR-2M to exit from service, the urgent need to design a new source of neutrons has emerged. This new source will not only be used to study the fundamental interactions, structure and properties of nuclei but also will be able to investigate the condensed mater, solid state, liquids, biological systems, polymers, colloids, chemical reactions in real time, engineering systems, etc. This paper presents a brief overview of the new source, and solution of pulse power fluctuation in the new-pulsed reactor.

The IBR-2 (JINR) pulsed reactor is used in the field of solid state physics for experiments on extracted beams. It is predicted that the IBR-2 reactor will reach resource limits in 2032–2037. To keep and develop beam research at JINR, the development of a new pulsed neutron source has begun. According to estimates, the reactor will have average thermal power 12–15 MW, extracted by sodium coolant. At this power, the average flux density of thermal neutrons on the surface of the water moderator will reach 1014 sec–1·cm–2 and the peak density 5·1016 sec–1·cm–2, which is more than 10 times higher than the analogous parameters of the operating IBR-2 reactor. The new reactor is supposed to use neptunium-based fuel. The main characteristics and concept of the new reactor are presented.

A high-intensity pulsed neutron source can be constructed according to the principle of a superbooster, i.e., a neutron multiplying system of the neutron-generating target of the proton accelerator with proton energy of about 1 GeV in the subcritical reactivity modulation system. Usually, superboosters are considered to be safe and reliable nuclear facilities. However, as indicated in this paper, a powerful superbooster may prove to be less stable in operation than a pulsed reactor. Therefore, the problems of its safety call for close study.

The design principles and parameters of a promising neutron source, based on the fission reaction of the isotope 237Np, for use in research in extracted beams are presented. Such a source can operate as a pulsing reactor or super-booster or as a super-booster multiplying the neutrons from the target of a proton accelerator by using a reactivity modulator. Owing to the threshold character of 237Np fission such a neutron source will possess significantly higher characteristics than a similar setup with plutonium, and in terms of utilization efficiency it will be the leading source in the world for research employing neutron spectroscopy.

The current trend in neutron-aided scientific research presupposes the development of new high-intensity pulsed sources. One variant of such an installation could be a pulsed reactor with a fissile-isotope core – 237Np. The present work is devoted to a study of the dynamics of power pulses of a neptunium reactor taking account of fast temperature feedback during pulse buildup. The problem is reduced to a simple model of a single frequency oscillator combined with the equation of single point kinetics of a reactor neglecting delayed neutrons. It is shown that on the basis of such an approximation, owing to feedback on the temperature of a fuel element the estimated energy of accidental power pulses decreases by almost a factor of 10 compared with a calculation neglecting fast feedback.

The principles of designing a subcritical neutron-multiplying system with reactivity modulation, which can be used as the target in a superbooster with a linear proton accelerator, are described. Calculations demonstrate that the output neutron-flux density of a pulse can be expected to reach 1017 cm–2 s–1 at maximum and 2 × 1014 cm–2 s–1 on average. The pulse duration of thermal neutrons can be 200–300 and 20–30 μs depending on the moderator and the corresponding pulse duration of the proton accelerator.

The upper limit of the density of the thermal neutron flux from pulsed sources based on the fission reaction is established. Three types of sources for research on ejected beams are considered: a multiplying target of the proton accelerator (a booster), a booster with the reactivity modulation (a superbooster), and a pulsing reactor. Comparison with other high-flux sources is carried out. The investigation has been performed at the Frank Laboratory of Neutron Physics of JINR.

The paper presents some aspects of pneumotransport theory of solid ball motion along a pipeline of complex geometry having application for the pelletized cold moderator of the IBR-2 reactor. Motion equations for a ball carrying by gas flow along a round tube both smooth and rough, are obtained. Aerodynamics forces and momentum of forces, acting on the ball depending on its velocity are defined on the base of numerical modeling of ball movement, and analytical simplified model of pneumotransport of balls along round tube is created. This model meets the demand for practical application as it shows satisfactory agreement with experimental modelling of ball movement in a glass tube. One of important results of the work is giving proofs of irregular movement of ball along a tube stipulated by roughness of the tube side.

The unique pelletized cold neutron moderator CM-202 at the IBR-2 reactor was put into test operation and have already worked more than 2000 hours. Normal, fast and trouble-free operation of the cryogenic moderator requires strict adherence to technological conditions (fast charging and discharging of moderator chamber, maintenance of required temperature and pressure at different parts of cryogenic system). The system of control and measuring equipment, designed for cryogenic moderator of the IBR-2 reactor, satisfies all the requirements and is simple to use. Access to the system of measuring instruments is organized via network. The working cycles of moderator confirmed the reliability and stable operation of the whole control system.

A description of the irradiation facility and available parameters of neutron and gamma exposures, including the maximum integrated doses, are presented in the paper. The research capabilities for radiation hardness tests of materials in a high-intensity beam of fast neutrons at the IBR-2 reactor of the Joint Institute for Nuclear Research in Dubna (Russia) are outlined.

Methodology and results of measuring the differential density of the neutron flux in irradiation beam line no. 3 of the IBR-2 reactor using neutron activation analysis (NAA) are presented in the paper. The results are compared to the calculation performed on the basis of the 3D MCNP model. The data that are obtained are required to determine the integrated radiation dose of the studied samples at various distances from the reactor.

A description of the irradiation facility and available parameters of neutron and gamma exposures, including the maximum integrated doses, are presented in the paper. The research capabilities for radiation hardness tests of materials in a high-intensity beam of fast neutrons at the IBR-2 reactor of the Joint Institute for Nuclear Research in Dubna (Russia) are outlined.

In July 10, 2012 cold neutrons were generated for the first time with the unique pelletized cold neutron moderator CM-202 at the IBR-2M reactor. This new moderator system uses small spherical beads of a solid mixture of aromatic hydrocarbons (benzene derivatives) as the moderating material. Aromatic hydrocarbons are known as the most radiation-resistant hydrogenous substances and have properties to moderate slow neutrons effectively. Since the new moderator was put into routine operation in September 2013, the IBR-2 research reactor of the Frank Laboratory of Neutron Physics has consolidated its position among the world’s leading pulsed neutron sources for investigation of matter with neutron scattering methods.

The first criticality of a new KZ-202 neutron moderator on the IBR-2M reactor is achieved. The moderator consists of thermal and cold units. The former is a room-temperature comb water moderator; the latter, a moderator using a mixture of aromatic hydrocarbons (mesitylene and m-xylene). The cold moderator is filled with granules of this mixture, which are supplied by a cold helium flow, and operates at 30 K. The combination of two units in one moderator makes it possible to simultaneously take the thermal and cold neutron spectra for extracted-beam spectrometers. The arrangement of the thermal and cold moderators is numerically optimized by the Monte Carlo method. The use of the cold moderator allows a 13-fold increase in the cold neutron intensity from its surface.

The yield of cold neutrons from the cold moderator in the IBR-2M reactor is calculated for several hydrogenous materials at cryogenic temperature. The intramolecular bonds during the interaction with

The world’s first advanced pelletized cold neutron moderator is prepared to be put into operation at the IBR-2M pulsed research reactor. It provides long-wavelength neutrons to the most of neutron spectrometers at the beams of the IBR-2M reactor. Aromatic hydrocarbons are used as a material for cold moderators. It is a very attractive material because of its high radiation resistance, good moderating properties, incombustibility, etc. It is shown that the idea of beads transport by a helium flow at cryogenic temperatures is successful. The recent progress and plans for moderator development at the IBR-2M reactor as well as the experimental results of beads transport are discussed in the paper.

The structural design of the IBR-2 periodic-pulse reactor is described. The principal stages of the project in modernizing the reactor, the technical characteristics before and after modernization, and the results of startup measures are presented.

The unitarization of the amplitude for the decay process K 1 0 → π+π− and allowance for the rescattering of final-state pions in the decay process K + → π+π+π− make it possible to evaluate, by using the parameters extracted from data on K → 2π decays, the K + → π+π+π− decay width. The result agrees with the experimental width value at a level of a few percent. Allowance for corrections for higher order terms of the momentum expansion of the amplitude for the decay process K + → π+π+π− leads to the slope-parameter value of g ++− th = 0.2182, which agrees with its experimental counterpart, g ++− exp = 0.2154 ± 0.0035.

The method and results of an experiment to determine the cold neutron spectrum from solid mesitylene at moderator temperatures of 10–50 K are presented. This study was performed at the DIN-2PI spectrometer of the IBR-2 reactor. The objective of the study was to verify the system of constants used in the Monte Carlo simulation of cryogenic neutron moderators of the IBR-2M reactor and to obtain the cold neutron yield as a function of the moderator temperature. Satisfactory agreement between the experimental and calculated neutron spectra at a mesitylene temperature of 20 K has been obtained; the ratio of cold neutron intensities at 10 and 50 K is ∼1.8.

The feasibility studies for a new ultra cold neutron (UCN) source to be installed at the Institut Laue-Langevin (ILL) are underway. One of these studies concerns a source using solid deuterium (SD2) as a neutron moderator. During source operation the SD2 would be exposed to intense γ-ray and neutron fluxes. These may result in radiolysis in SD2. Our work has shown that this effect—if it is produced—is not incompatible with a normal operation of the source.

Solid methane is still widely in use at pulsed neutron sources due to its excellent neutronic performance (IPNS, KENS, Second Target Station at ISIS), notwithstanding poor radiation properties. One of the specific problems is radiolytic hydrogen gas pressure on the walls of a methane chamber during annealing of methane. In this paper results of an experimental study of this phenomenon under fast neutron irradiation with the help of a specially made low temperature irradiation rig at the IBR-2 pulsed reactor are presented. The peak pressure on the wall of the experimental capsule during heating of a sample irradiated at 23–35 K appears to have a maximum of 2.7 MPa at an absorbed dose 20 MGy and then falls down with higher doses. The pressure always reached its peak value at the temperature range 72–79 K. Generally, three phases of methane swelling during heating can be distinguished, each characterized by a proper rate and intensity.

A neutron albedo from arbitrary homogeneous and finely grained substances is examined on the basis of a new, algebraic, method. In the approximation of an isotropic distribution of incident and reflected neutrons, it is shown that, in the case of thermal neutrons, coherent scattering on individual particles of finely grained media increases only slightly the transport cross section, but, at a given wall thickness, it reduces the albedo because of a decrease in the density of the substance. A significant increase in the albedo is possible only for neutrons of wavelength on the order of dimensions of a powder grain. The angular distribution of reflected neutrons is discussed, and it is proven that a deviation of this distribution from an isotropic one does not lead to a change in the magnitude of the albedo.

Use of the effective Lagrangian incorporating both the scalar and pseudoscalar mesons gives a possibility to calculate the $\pi \pi$-scattering lengths without attraction of the ChPT theory., Comment: 7 pages, 0 figures

Experiments on production of long-lived Hf 178 m 2 isomer in reactor irradiations are described. Properties of this nuclide are promising for its potential application as a relatively safe power source characterized by high density of accumulated energy. Metal natHf samples were activated in the Dubna IBR-2 reactor at positions corresponding to different neutron fluxes. Samples were bare or shielded by Cd and B4C layers. The gamma activity of the samples was analyzed with Ge gamma spectrometers during a two-year period following their irradiation. In the presence of dominant activation products 175Hf and 181Hf, the high-spin isomers Hf 178 m 2 and Hf 179 m 2 were also detected despite relatively low levels. The isomer-to-ground state ratios and cross-sections were determined from the measured yields. For Hf 178 m 2 , the cross-section for burnup (destruction) by neutron capture after its production was also estimated, clarifying the results from earlier experiments. In the context of suggestions for use of Hf 178 m 2 for applications, the results confirm that large-scale production of this isomer by reactor irradiations is not feasible. In contrast, the efficiency of production of Hf 179 m 2 is much higher and an amount of about 1016 atoms may be produced in standard reactor irradiations. For Hf 178 m 2 , more productive methods are known, in particular fast neutron irradiations at En ≥ 14 MeV and spallation reactions at intermediate energies. Neutron cross-sections for isomers may also be significant in astrophysics.

The amplitudes of the K± → 3π and K→ 2π decays are expressed in terms of different combinations of one and the same set of CP-conserving and CP-odd parameters. Extracting the magnitudes of these parameters from the data on K → 2π decays, we estimate an expected CP-odd difference between the values of the slope parameters g+ and g− of the energy distributions of “odd” pions in K+ → π+π+π− and K− → π−π−π+ decays.

Study of radiation resistance of hydrogenous materials at low temperatures is a first priority task in the design of advanced cold neutron moderators. At temperatures 20–100 K the most essential radiation effects in solid hydrogenous substances are: • Formation of radiolytic hydrogen. • Accumulation of “frozen” radicals, which results in a rise of a self-sustaining reaction of their recombination followed with unexpected fast heating of the moderator. • Formation of high-molecular, high-boiling products of radiolysis. • Decrease of thermal conductivity. In the paper, the recently obtained results of the study of the accumulation of chemical energy and the conditions of its release performed with the URAM-2 cryogenic irradiation facility at the IBR-2 research reactor, are presented. Spontaneous releases of stored energy were detected in solid methane, water ice, hydrates of methane and tetrahydrofuran [Particles and Nuclei, Lett. 5 (2002) 82; Radiat. Phys. Chem. 67 (2003) 315] and in frozen mixtures of water ice with atomic hydrogen scavengers. A negligible amount of energy is accumulated in aromatic hydrocarbons which demonstrate no spontaneous self-heating under irradiation. All irradiation runs were performed at up to 20 MGy in the temperature range of 15–50 K.

A phenomenon of spontaneous release of energy accumulated in some hydrogenous materials under fast neutron irradiation at low temperature was studied at a cryogenic irradiation facility of the IBR-2 reactor in Dubna for the purpose of cold neutron moderator development. Spontaneous release of energy occurred in water ice after 5–11 h of fast neutron irradiation at temperature

The scattering of a pion on a pion in the energy region \(\sqrt s \) ≤1 GeV is successfully described on the basis of pole diagrams featuring spinless, spin-1, and spin-2 intermediate particles, provided that the properties of these particles and of their interactions are deduced from the basic principles of QCD and from the requirements of chiral theory.

Current status and future development of the pelletized cold moderator of the IBR-2 reactor in Neutron Physics Laboratory of JINR are represented. Nowadays cold moderator works for physical experiments and allows conducting experiments in the region of wavelengths more than 4 A up to 10-13 times faster in comparison with the warm water moderator. Future development of the pelletized cold moderator is aimed at increasing the time of its operation for experiments and is based on three components: creation of a system of continuous charging and discharging of beads, supplementation of various additives, and use of new materials, such as triphenylmethane.

Neutron guides are widely used to transport the neutrons from the moderator to the sample. Due to the constructive features of the ring corridor of the fast pulsed reactor IBR-2, the minimal distance between the moderator and the guide entrance is around 6 m. The main goal of the paper is to optimize the neutron optical system between the moderator and the entrance of the new neutron guides. Using Monte Carlo simulations we calculate the possible best gain of the neutron flux density at the guide exit. After the described optimization process, the optimal system is obtained. The recommendations for construction of the new beam line are provided too. Similar technique and the proposed system could be easily adapted for another similar beam line at the neutron sources.

The irradiation facility at the beam line no.3 of the IBR-2 reactor of the Frank Laboratory for Neutron Physics is described. The facility is aimed at irradiation studies of various objects with area up to 800 cm$^2$ both at cryogenic and ambient temperatures. The energy spectra of neutrons are reconstructed by the method of threshold detector activation. The neutron fluence and $\gamma$ dose rates are measured by means of alanine and thermoluminescent dosimeters. The boron carbide and lead filters or $(n/\gamma)$ converter provide beams of different ratio of doses induced by neutrons and photons. For the lead filter, the flux of fast neutrons with energy more than 0.1 MeV is $1.4 \cdot 10^{10}$ \fln and the neutron dose is about 96\% of the total radiation dose. For the $(n/\gamma)$ converter, the $\gamma$ dose rate is $\sim$500 Gy h$^{-1}$ which is about 85\% of the total dose. The radiation hardness tests of GaAs electronics and materials for the ATLAS detector to be put into operation at the Large Hadron Collider (LHC) have been performed successfully at this facility.

In January 2013, the start-up and research of the world's first pelletized cold neutron moderator on the basis of solid beads composed of a mixture of aromatic hydrocarbons (mesitylene + metaxylene...

1. With a negative temperature coefficient of reactivity, power oscillations in a pulsed periodic-action reactor grow with increase in power if the oscillations in external reactivity occur at a frequency several times less than the frequency of the power pulsations (by a factor of 2–4). 2. In contrast to steady-action reactor, the region of linear stability of a pulsed reactor with a negative temperature coefficient of reactivity is finite. There is a region of stability with a positive temperature coefficient of reactivity (in the absence of delayed neutrons). The interval of linear stability is determined by Eq. (10). 3. Because of the nonlinearity of the relation between the power-pulse energy and the introduced reactivity, the power oscillations in the region of linear instability (“free” oscillations) are finite. The period and amplitude of the free oscillations depend on the reactor parameters in a complex manner, and may be determined analytically for a region close to the limit of linear stability.

When a reactivity modulator of heterogeneous structure is used [1], by an appropriate choice of modulator parameters we can appreciably enhance the self-quenching effect, thus expanding the range of allowable deviations of reactivity of a periodically acting pulsed fast reactor while, at the same time, increasing the nuclear safety of the plant. In the case of a reactor with the parameters of the IBR-2, when the harmonic component of the reactivity has an amplitude of 3·10−3 and the coefficient of the reactivity parabola is 4·10−5 cm−2 the maximum deviation of reactivity which does not yet result in damage to the reactor core is ∼2·10−3, which is double that for a modulator of heterogeneous structure without a self-quenching effect and triple that when an ordinary movable reflector is used.

Our results show that the MR-3 heterogeneous reactivity modulator, made of EI-437 nickel alloy, makes it possible to reduce the power pulse length of the IBR-2 reactor from the present 215 μsec to 115±5 μsec without changing the rotational speed of the primary reflector (1500 rpm), but with an increase in the rotational speed of the secondary reflector to 1200 rpm. At a rotational speed of 300 rpm for the secondary reflector, the pulse length will be 130±5 μsec. For the steel version of the MR-3 these values of the pulse half-width increase to 170 and 190 μsec, respectively.

The paper is devoted to the experimental investigation of fluctuations in intensity of power excursions in the IBR pulsed reactor in the Joint Institute of Nuclear Research. The measurements were made, using a scintillation detector, at six reactor power levels from 30 to 1200 hr. The relationships obtained can be recommended for studying problems concerning the physics of reactors of this type.

This article is devoted to an experimental study of the fluctuations in burst intensity of the IBR pulsed reactor at the Joint Institute of Nuclear Research. The measurements were carried out using a scintillation detector at six reactor power levels from 30 to 1200 W. Relations were obtained which may be recommended in studying questions of the physics of reactors of this type.