Subcritical Facility Research Articles

Physical start-up of the Nuclear Subcritical Facility “Neutron Source” of NSC KIPT

National Science Center “Kharkov Institute of Physics and Technology” jointly with the Argonne National Laboratory developed, designed and constructed the Subcritical Nuclear Facility “Neutron source based on the subcritical assembly driven by electron linear accelerator”. The article presents the main results obtained during the physical start-up of the plant, the program of which involved the staged loading of fuel assemblies into the core with subsequent measurements of its neutron-physical characteristics. In total, 37 fuel assemblies of WWR-M2 type were loaded into the facility core. The values o f the effective neutron multiplication factor keff and reactivity ρ of the subcritical assembly were measured after each loading step using the multiplication and area ratio methods. In addition, the neutron flux to electron current ratio method was tested for on-line reactivity monitoring. The investigations were performed when the accelerator was operated with 100 MeV electron beam energy, 2.7 μs electron pulse duration, 20 Hz pulse repetition rate and ≤ 40 mA beam pulse current. The obtained results were analyzed in terms of ensuring the facility nuclear safety at the next stage of its commissioning which is the pilot operation.

Reactivity Measurement Methods and the First Results of the Physical Start-up for the Nuclear Subcritical Facility “Neutron Source”

The methods of the neutron-physical parameter measurements of the subcritical assembly neutron source driven with an electron linear accelerator during start-up are described in the article. The results of neutron multiplication factor and reactivity measurements by different methods are represented. The measurement results are compared with each other and with simulation results of MCNPX code. The measured results of the facility parameters are analyzed.

On the determination of neutron multiplication by the Rossi-alpha method

Introduction. This work contains the results of determining the prompt neutron multiplication factor in the subcritical state of a one-core BFS facility, obtained by the neutron coincidence method, for which the influence of the error in the βeff in determining the multiplication factor turned out to be insignificant. The core of the facility consisted of rods filled with pellets of metallic depleted uranium, 37% enriched uranium dioxide and 95% enriched plutonium, sodium, stainless steel and Al2O3. Stainless steel served as a reflector. Methods. In contrast to the inverse kinetics equation solving (IKES) method, which is convenient for determining reactor subcritical states, the neutron coincidence method practically does not depend on the error in the value of the effective fraction of delayed neutrons βeff. If in the IKES method the reactivity value is obtained in fractions of βeff, i.e., from the measurement of delayed neutrons, the neutron coincidence method is based on the direct measurement of the value (1 – kσp)2, where is the effective multiplication factor by prompt neutrons. The total multiplication factor is defined as keff = kσp + βeff. If, for example, keff ≈ 0.9 (which is typical for determining the fuel burnup campaign), then it is the error in determining kσp that is the main one in comparison with the error in βeff. Thus, a 10% error in βeff of 0.003–0.004 (typical for plutonium breeders) will make a contribution to the error 1 – keff equal to 1 – kσp + βeff ≈ 0.00035, i.e., approximately 0.35%, but not 10%, as in the IKES method. Rossi-alpha measurements were carried out using two 3He counters and a time analyzer. The measurement channel width Δt was 1.0 μs. From these measurements, the value of the prompt neutron multiplication factor was obtained. In this case, the space-isotope correlation factor for the medium with a source was calculated using the following values: Φ(x) – solutions of the inhomogeneous equation for the neutron flux and Φ+(x) – solutions of the ajoint inhomogeneous equation. Results. The authors also present a comparison of the results of the Rossi-alpha experiment and measurements of the BFS-73 subcritical facility by the standard IKES method in determining the multiplication factor value. The data of the IKES method differ insignificantly from the results of the Rossi-alpha method over the entire range of changes in the subcriticality with an increase in the subcriticality of the BFS-73 one-core facility. Conclusion. It was impossible to apply the neutron coincidence method to fast reactors; however, the method turned out to be quite workable on their models created at the BFS facility, which was successfully demonstrated in this study.

Preliminary design and assessment of a heat pipe residual heat removal system for the reactor driven subcritical facility

A heat pipe residual heat removal system is proposed to be incorporated into the reactor driven subcritical (RDS) facility, which has been proposed by MIT Nuclear Reactor Laboratory for testing and demonstrating the Fluoride-salt-cooled High-temperature Reactor (FHR). It aims to reduce the risk of the system operation after the shutdown of the facility. One of the main components of the system is an air-cooled heat pipe heat exchanger. The alkali-metal high-temperature heat pipe was designed to meet the operation temperature and residual heat removal requirement of the facility. The heat pipe model developed in the previous work was adopted to simulate the designed heat pipe and assess the heat transport capability. 3D numerical simulation of the subcritical facility active zone was performed by the commercial CFD software STAR CCM + to investigate the operation characteristics of this proposed system. The thermal resistance network of the heat pipe was built and incorporated into the CFD model. The nominal condition, partial loss of air flow accident and partial heat pipe failure accident were simulated and analyzed. The results show that the residual heat removal system can provide sufficient cooling of the subcritical facility with a remarkable safety margin. The heat pipe can work under the recommended operation temperature range and the heat flux is below all thermal limits. The facility peak temperature is also lower than the safety limits.

DISTRIBUTION OF THE BEAM DENSITY AT THE TARGET OF SUBCRITICAL FACILITY “NEUTRON SOURCE”

NSC KIPT subcritical facility “Neutron Source” uses rectangular tungsten or uranium target of 6464 mm top cross-section. To generate maximum neutron flux, prevent overheating of the target and reduce thermal stress during the facility power operation one should provide uniform electron beam distribution at the target top surface. During the facility design three different possibilities of electron beam density redistribution above the target surface were considered. These were the fast beam scanning with two dimensional scanning magnets; the method of uniform beam distribution formation with linear focusing elements (dipole and quadrupole magnets) and nonlinear focusing elements (octupole magnets), when final required rectangular beam shape with homogeneous beam density is formed at target; and combined method, when one forms the small rectangular beam with homogeneous beam density distribution and scan it over the target surface with scanning magnets. In the paper the all three methods are considered and discussed taking into account the layout of the transportation channel of NSC KIPT subcritical facility “Neutron Source”. For the first stage of the facility start-up and pilot operation the fast scanning method was chosen, realised and tested. The results of the beam distribution measurements over the surface of the target during the facility adjustment and start up are presented.

Numerical simulation and analysis of the pulsed neutron source method measuring the prompt neutron decay constant of a Godiva-like subcritical facility

The theory and method of the pulsed neutron source (PNS) method measuring the prompt neutron decay constant α0 of a subcritical system was introduced. Using the Monte Carlo code JMCT, the neutron transport process in a Godiva-like subcritical facility was simulated. The α0 value of the facility was obtained with PNS method, and was compared with the α-k power iteration result. To investigate the factors affecting the calculated α0 value of the PNS method, the system response, the fitting residual distributions, and the neutron energy spectra of different time intervals were analyzed. It is observed that the statistical fluctuation is the key factor leading to the inaccuracy of the α0 calculation value of the facility. Furthermore, to obtain an effective α0 value with the PNS method, the reasonable width of fitting time window was discussed by analyzing the relationship of the calculated α0 values and the fitting time width in this paper.

Licensing of the Neutron Source in Ukraine: Challenges and Solutions

A research nuclear facility, the Neutron Source Based on the Subcritical Assembly Driven by a Linear Electron Accelerator (Neutron Source), is under commissioning in the National Scientific Center “Kharkiv Institute of Physics and Technology” (NSC KIPT), Ukraine, as an international collaborative project of NSC KIPT and the Argonne National Laboratory (ANL), USA. The Neutron Source is intended to study subcritical systems and generate neutrons to be used in applied and fundamental research in physics, chemistry, biology, and medicine. The construction of a state-of-art nuclear research facility is a challenge for both the operating organization (NSC KIPT) and the regulatory authority (SNRIU). NSC KIPT faced the technical challenges caused by the unique design features of the facility. SNRIU faced the two major challenges: no regulations to govern nuclear and radiation safety; no experience in licensing such a type of facility. This paper discusses aspects related to the challenges and solutions in licensing of the Neutron Source, including development of the regulatory and legal framework on nuclear and radiation safety, review of the safety justification documents for the new nuclear subcritical facility and verification calculations.

USE OF THE MONTE CARLO SERPENT CODE FOR MODELING THE SECOND SERIES OF EXPERIMENTAL DATA OF KUCA SUBCRITICAL INSTALLATION

Modeling of the neutron-physical characteristics of the Kyoto University KUCA subcritical facility was conducted using the Monte Carlo Serpent code. The effective multiplication factors for the critical experiments of the series II on the KUCA research subcritical facility were calculated. The presented calculation results were compared with the experimental results and the results of the calculations made using the Monte Carlo codes MCNP6 and KENO-VI.

Parametric identification of integrated model of a coal-fired boiler in a thermal power plant

Retrofit or replacement of few units in a subcritical facility may not only improve overall efficiency of conversion of energy in a power plant but also support sustainability issues. The primary objective of this article is to identify model parameters of a coal-fired integrated boiler and to present a comparative study on three different identification methods. This leads to select most suitable models that are applied for the developed model of the boiler of 210 MW coal-fired thermal power plants. The mathematical models of economizer, drum, and super-heater assembly are derived using mass balance and energy balance equations. The derived multi input–multi output model is then validated, and the model parameters are identified using three different identification methods namely nonlinear least square technique, maximum likelihood estimation, and expectation maximization algorithms. Identification of the plant model will essentially help to frame a good controller. In this article, parameter estimation has been carried out from real-time plant as it provides selective tool through quantitative comparative study of the three methods. The expectation maximization method has been found to provide suitable results compared to the other two methods. Model parameters of integrated boiler of a comprehensive structure have been obtained for the first time using expectation maximization method.

Валідація Монте-Карло коду Serpent на основі експериментальних даних на дослідницькій підкритичній установці

Валідація Монте-Карло коду Serpent на основі експериментальних даних на дослідницькій підкритичній установці

Experimental Study of a Differential Beam Intensity Monitoring for the CIADS LINAC

A BPM based beam loss monitoring scheme for the China initiative accelerator driven subcritical (CIADS) facility has been proposed for the MEBT section of its high power Linac. In this scheme, a differential beam monitoring algorithm is utilized that relies on beam intensity measurements using BPM electrodes. Discussions of the experimental results for the scheme are presented. Further experiments have been performed with some promising results. This paper describes the experimental results with some analyses on measurement errors of the system. The proposed physical design of this system is described and further development is presented.

Neutronics feasibility of an MIT Reactor-driven subcritical facility for the Fluoride-salt-cooled High-temperature Reactor

The current study explores an innovative option for demonstrating the Fluoride-salt–cooled High- temperature Reactor (FHR) technologies with a reactor-driven subcritical facility. The FHR uses clean salt coolants, carbon-matrix coated-particle fuel similar to that used in High-temperature Gas-cooled Reactors and can be coupled to a nuclear air-Brayton combined cycle. Recent assessments indicate favorable economics and safety characteristics, but no FHR has been built. The question is what experimental facilities should be constructed to reduce technical uncertainties before a decision to build a test or demonstration reactor? The MIT Reactor design and license would allow the construction and operation of a subcritical facility with 700°C salt circulating through multiple full-width partial-height fuel assemblies operating with a power density up to 60% of a commercial FHR. This option would allow hot systems testing as a major step toward building the test or demonstration reactor. Preliminary system design, power control options, testing capabilities, and key nuclear characteristics of such a reactor-driven subcritical facility are described. A method of deriving subcritical multiplicity using surface source has been proposed and verified in this study. Finally, the neutronic impacts on the driver facility, ie, the MIT Reactor, have been evaluated.

Conceptual Development of a 600–1000 MeV Proton Beam Accelerator-Driver with Average Beam Power >1 MW

The results of a comprehensive analysis of the possibility of developing in our country a powerful proton accelerator-driver as an element of subcritical electro-nuclear facility are presented. The investigations are performed taking account of the latest advances of accelerator technology and the real capabilities of industry. The design layout developed for the accelerator is discussed. It is shown that in principle a prototype of a ~1 GeV accelerator-driver with current 1–5 mA, which can subsequently be increased to 10–15 mA, can be built in Russia.

Experimental subcritical facility driven by D-D/D-T neutron generator at BARC, India

The paper presents design of an experimental subcritical assembly driven by D-D/D-T neutron and preliminary experimental measurements. The system has been developed for investigating the static and dynamic neutronic properties of accelerator driven sub-critical systems. This system is modular in design and it is first in the series of subcritical assemblies being designed. The subcritical core consists of natural uranium fuel with high density polyethylene as moderator and beryllium oxide as reflector. The fuel is embedded in high density polyethylene moderator matrix. Estimated keff of the system is ∼0.89. One of the unique features of subcritical core is the use of Beryllium oxide (BeO) as reflector and HDPE as moderator making the assembly a compact modular system. The subcritical core is coupled to Purnima Neutron Generator which works in D-D and D-T mode with both DC and pulsed operation. It has facility for online source strength monitoring using neutron tagging and programmable source modulation. Preliminary experiments have been carried out for spatial flux measurement and reactivity estimation using pulsed neutron source (PNS) techniques with D-D neutrons. Further experiments are being planned to measure the reactivity and other kinetic parameters using noise methods. This facility would also be used for carrying out studies on effect of source importance and measurement of source multiplication factor ks and external neutron source efficiency φ∗ in great details. Experiments with D-T neutrons are also underway.

Про спорудження ядерної підкритичної установки «Джерело нейтронів»

Про спорудження ядерної підкритичної установки «Джерело нейтронів»

BRAHMMA: A compact experimental accelerator driven subcritical facility using D-T/D-D neutron source

A zero-power, sub-critical assembly BRAHMMA driven by a D-T/D-D neutron generator has been designed and commissioned at Bhabha Atomic Research Centre, India. This facility has been conceived for investigating the static and dynamic neutronics properties of accelerator driven sub-critical systems. This paper describes the design details of the system. Preliminary results of flux measurements and reactivity measurements using pulsed neutron source techniques have also been presented. This system has the advantage of being modular in design which enables its keff values to be changed depending upon type of fuel being used and it is first in the series of subcritical assemblies being designed.

Spatial and Source Multiplication Effects on the Area Ratio Reactivity Determination Method in a Strongly Heterogeneous Subcritical System

The area ratio method of Sjöstrand is generally considered one of the most reliable reactivity determination methods and thus is a major candidate for off-line calibration purposes in future accelerator-driven systems for high-level waste incineration. In this work, the Sjöstrand area ratio method has been evaluated experimentally under thorough conditions in the strongly heterogeneous subcritical facility YALINA-Booster. Both strengths and weaknesses of the method have been identified. Most surprisingly, it has been found that the area ratio reactivity estimates may differ a factor of 2 depending on detector position. It is also shown that this strong spatial dependence can be explained based on a simple two-region point-kinetics model and corrected by means of correction factors obtained through Monte Carlo simulations. A new Monte Carlo correction method is proposed that includes, at the same time, the spatial disturbance and the effective delayed neutron fraction. In that way, the value of the effective multiplication factor is obtained from the measured dollar reactivity without the need of calculating the effective delayed neutron fraction explicitly, and thereby, the delayed neutron transport is performed only once. Further, it has been found that the Sjöstrand area ratio method is not sensitive to perturbations of the source multiplication factor.

Shielding design and analyses of KIPT neutron source facility

Argonne National Laboratory (ANL) of USA and Kharkov Institute of Physics and Technology (KIPT) of Ukraine have been collaborating on the conceptual design development of a neutron source facility. An electron accelerator drives a sub-critical facility (ADS) is used for generating the neutron source. The facility will be utilized for performing basic and applied nuclear researches, producing medical isotopes, and training young nuclear specialists. Monte Carlo code MCNPX has been utilized as the major design tool for the design, due to its capability to transport electrons, photons, and neutrons at high energies. However the ADS shielding calculations with MCNPX need enormous computational resources and the small neutron yield per electron makes sampling difficulty for the Monte Carlo calculations. The high energy electrons ( E > 100 MeV) generate very high energy neutrons and these neutrons dominant the total radiation dose outside the shield. The radiation dose caused by high energy neutrons is ∼3–4 orders of magnitude higher than that of the photons. However, the high energy neutron fraction within the total generated neutrons is very small, which increases the sampling difficulty and the required computational time. To solve these difficulties, the user subroutines of MCNPX are utilized to generate a neutron source file, which record the generated neutrons from the photonuclear reactions caused by electrons. This neutron source file is utilized many times in the following MCNPX calculations for weight windows (importance function) generation and radiation dose calculations. In addition, the neutron source file can be sampled multiple times to improve the statistics of the calculated results. In this way the expensive electron transport calculations can be performed once with good statistics for the different ADS shielding problems. This paper presents the method of generating and utilizing the neutron source file by MCNPX for the ADS shielding calculation and similar accelerator facilities, and the accurate radiation dose analyses outside the shield using modest computational resources.

Biological Shield Design and Analysis of KIPT Accelerator-Driven Subcritical Facility

Argonne National Laboratory of the United States and Kharkov Institute of Physics and Technology of Ukraine have been collaborating on the conceptual design development of an electron accelerator-driven subcritical facility. The facility will be utilized for performing basic and applied nuclear research, producing medical isotopes, and training young nuclear specialists. This paper presents the design and analyses of the biological shield performed for the top section of the facility. The neutron source driving the subcritical assembly is generated from the interaction of a 100-kW electron beam with a natural uranium target. The electron energy is in the range of 100 to 200 MeV, and it has a uniform spatial distribution. The shield design and the associated analyses are presented including different parametric studies. In the analyses, a significant effort was dedicated to the accurate prediction of the radiation dose outside the shield boundary as a function of the shield thickness without geometrical approximations or material homogenization. The MCNPX Monte Carlo code was utilized for the transport calculation of electrons, photons, and neutrons. Weight window variance-reduction techniques were introduced, and the dose equivalent outside the shield can be calculated with reasonably good statistics.

Comments on the Kinetic Parameters for Source-Driven Systems

The idea of introducing a new subcriticality parameter has been suggested in connection with Accelerator Driven Subcritical (ADS) facilities. In an International Atomic Energy Agency (IAEA) meeting, some of the participants addressed the possibility that the traditional integral parameters like k eff and neutron life time might be meaningless in a deeply subcritical state. The present work mentions two instances proving the opposite. The first one is a two decade long experimental project to study the physics of Water-Water Energetic Reactor (WWERs). The project served as an experimental basis of validation of WWER calculational models. The second one is connected to an INES-3 level incident at Paks NPP. As a result of the incident, in a cleaning container, there remained some 30 heavily damaged fuel assemblies, with geometry unknown. In both cases, we calculated and measured reactivity of a subcritical facility. We intend to point out that the asymptotic eigenfunctions obtained from a source problem are k-type eigenfunctions, whereas the eigenfunctions obtained from a time dependent, source driven problem are of ω-type eigenfunctions in nearly critical states. That experience is shared in the present work.