Implementation of a dynamic Monte Carlo method for transients analysis with thermal-hydraulic feedbacks using MCNPX code

Characterization neutron and gamma beams at the tangential beam port of TRIGA 2000 reactor was done in order to prepare the development of Prompt Gamma Neutron Activation Analysis (PGNAA) facility. Characterization was performed by simulation using Monte Carlo method with MCNPX and PHITS computer codes. The MCNPX code was used to simulate the neutron-gamma fluence and energy spectra in the reactor core from fission reactions. The PHITS code was used to simulate the distribution of the neutron-gamma fluence and energy spectra in the tangential beam port. The simulation was done by modeling the geometry and elemental composition of reactor material and radiation source model in the form of fission reaction in the reactor core. The results indicate that the tangential beam port provides better neutron spectrum and neutron-gamma fluence ratio for PGNAA. These results will be used to simulate the colimator design and neutron interaction with samples in PGNAA facility.

Neutron and gamma beams characterization at the beam ports of TRIGA 2000 reactor was done in order to prepare the development of Prompt Gamma Neutron Activation Analysis (PGNAA) facility. Characterization was performed by simulation using Monte Carlo method with MCNPX and PHITS computer codes. The MCNPX code is used to simulate the neutron-gamma fluence and spectra of the reactor core from fission reactions. The PHITS code is used to simulate the distribution of the neutron-gamma fluence in the beam ports. The simulation was done by modeling the geometry and elemental composition of reactor material and radiation source model in the form of fission reaction in the reactor core. This results will be used to select the most suitable one of the four beam ports for the PGNAA use, based on the prescribed requirements, such as the neutron spectral characteristics and neutron-gamma fluence ratio at the beam port’s outlet. The results indicate that the tangential beam port provides better characteristics of neutron spectrum and neutron-gamma fluence ratio so become most suitable for PGNAA facility.

In this thesis a new method for the analysis of power transients in a nuclear reactor is developed, which is more accurate than the present state-of-the-art methods. Transient analysis is important tool when designing nuclear reactors, since they predict the behaviour of a reactor during changing conditions, such as a control-rod movement, induced by an operator, or an accident scenario. The current methods for transient analysis apply deterministic solvers to calculate the neutronic response, but not only do these deterministic solvers always need to discretise the problem, they also often apply more fundamental approximations such as homogenization or diffusion. Therefore a stochastic method is needed, which only has a statistical uncertainty. A challenge for applying the Monte Carlo method on transient analysis, is the different types of particles which must be simulated in a single simulation. It is common for a neutronics calculation to simulate the prompt neutrons, however the simulation of precursors is new. The main challenge when simulating precursors lies in their long lifetime, which is seconds, whereas the prompt neutrons have a lifetime of microseconds. This has been solved by dividing the transient problem into time intervals and forcing the precursors to produce a delayed neutron is all time intervals. This will ensure prompt-neutron chains in all intervals. The prompt neutrons will form prompt-neutron chains, which poses another challenge for the Monte Carlo method. The chain length of these prompt neutrons varies a lot and the can also split into many branches, which averages out in a real power reactor, but is difficult to simulate on a computer. To improve the chain-length statistics, a new variance reduction technique is developed, which dictates that a single neutron will emerge from each interaction. This emerging neutron can be the result of a scattering event or a fission event. Finally, a dynamic simulation scheme, which can run in parallel, has been devised, simulating all time intervals consecutively. Also, a method for sampling the initial conditions is created and this scheme is implemented in a purpose-built Monte Carlo code and in a general-purpose Monte Carlo code. The two codes have been tested in several cases and they behave as expected, agreeing with deterministic method where expected, but deviating when the deterministic methods are no longer valid. In order to simulate realistic transients in a power reactor, feedback has to be taken into account. To achieve this, the dynamic Monte Carlo method has been coupled to a thermal-hydraulics code, using an explicit scheme. The results of this coupled simulation are compared to a state-of-the-art coupled diffusion calculation in a NURISP-benchmark calculation and the results agree well, except for a small deviation towards the end. The exact source of this deviation should be further investigated, but the difference is small, when realising that there is already a deviation in the steady-state results. The results demonstrate the feasibility of performing a fully dynamic transient analysis, using only Monte Carlo for the neutronics part of the calculation. Transients can now be calculated, with detailed modelling of any complex geometry and with continuous energy, which is especially useful for newly developed reactor types and one-of-a-kind research reactors.

Maloney, Emily D. (University of Florida College of Medicine, Gainesville), and Herbert E. Kaufman. Multiplication and therapy of Toxoplasma gondii in tissue culture. J. Bacteriol. 88: 319-321. 1964.-The effect of temperature on the rate of multiplication of the RH strain of Toxoplasma gondii and the results of treatment with pyrimethamine, an antifolic, at various temperatures were studied in tissue culture. At 37 C incubation, the organism was found to have a lag time of 10.5 hr and a mean generation time thereafter of 4.85 hr for binary multiplication during the exponential phase of growth. At 34 C, the lag time was 12 hr and generation time was 5.26 hr. When the organisms were incubated at 31 C, the lag time was 12.5 hr and generation time was 11.1 hr. Treatment of infected monkey kidney tissue cultures with 0.05 mg per 100 ml of pyrimethamine at 37 C for 4 days killed all organisms and permitted survival of the mice which were injected with the cultures. When cultures were incubated at 34 C, a dose of 0.6 mg per 100 ml was required to spare any mice, and with a 31 C incubation even this dose was only slightly effective when therapy was continued for 4 days. If the treatment was prolonged for 7 days before injection of the cultures into mice, however, no difference in therapy with different rates of multiplication was found. Since the rates of multiplication at 34 and 31 C are similar to the rates of multiplication of some naturally occurring strains, these data confirm the fact that prolonged therapy, as used by many, is rational.

Purpose: High-energy protons are generally used for neutron production by Pb, W, Li, Be, and Ta targets that are used for the Born Neutron Capture Therapy (BNCT) technique. Neutron production targets are destroyed by proton spallation (evaporation of nuclei). The purpose of this study is the investigation of neutron activation and proton spallation damage of converter targets using the MCNPX code, which is based on the Monte Carlo method. Materials and Methods: The MCNPX code was used to extract the activation and spallation information of secondary particle production in Pb, W, Li, Be, and Ta targets. The neutron activation and proton spallation damage, including radioactive elements production in converter targets, was extracted from data in the MCNPX output file. Results: Results showed that the highest probability of radioactive elements production by proton with low-level energy in the Ta target are 180Hf, 179Hf, and 178Hf, and in the Li target is 7Be, respectively. In addition, the most probable radioactive elements produced by 200, 800, and 1200 MeV proton spallation in lead target are 118Tl and 78Pt, and in tungsten target are 98Hf, 110Ta, and 111Ta, respectively. The calculations showed that the production of radioisotopes in reactions with neutrons is lower than the production in reactions with a proton beam, and with increases in the energy of the proton beam, production of the radioactive elements was increased. Conclusion: The results illustrated that the radioactive elements are produced in W, Pb, Li, Be, and Te targets in the BNCT method, which should be avoided as radiation hazards.

Comparison of carbon deposition on tungsten between molecular dynamics and dynamic Monte Carlo simulation

Life cycle of A. exsertus female differed according to temperature degree; the shortest period was at 30 °C, 8.23 days, while the longest at 20°C, 12.02 days. The developmental rate ranged from 68.14% at 20 °C to 73.04% at 30 °C for female. Survival female's immatures %was also affected with temperature being 73.45, 80.24 and 83.61% at 20, 25 and 30 °C, respectively. The total amount of consumed preys/predator different developmental stages and sex increased with increased temperature from 20 to 30 °C. The total average number of T. urticae fed by predator immatures was 17.23, 14.76 and 10.71 preys at 30. 25 and 20 °C. Also, female predator consumed greater number of prey individuals at high temperature, 78.34 preys at 30 °C, decreased to 65.25 preys at 25 °C but the rate of consumption was less at low temperature 50.00 preys at 20°C. The effect of temperature on life table parameters was determined. Net reproductive rate (Ro) differed according to temperature as this value increased with temperature increase. Thus, these values averaged 11.21, 15.04 and 21.57 times for 20, 25 and 30 oc. The intrinsic rate of increase (rm) was 0.10, 0.13 and 0.15 individuals/female/day for the above same order. The mean generation time (T) decreased when the temperature increased.

Non-linear, time dependent target accuracy assessment algorithm for multi-physics, high dimensional nuclear reactor calculations

Evaluating the characteristics of emerging SARS-CoV-2 variants of concern is essential to inform pandemic risk assessment. A variant may grow faster if it produces a larger number of secondary infections (“R advantage”) or if the timing of secondary infections (generation time) is better. So far, assessments have largely focused on deriving the R advantage assuming the generation time was unchanged. Yet, knowledge of both is needed to anticipate the impact. Here, we develop an analytical framework to investigate the contribution of both the R advantage and generation time to the growth advantage of a variant. It is known that selection on a variant with larger R increases with levels of transmission in the community. We additionally show that variants conferring earlier transmission are more strongly favored when the historical strains have fast epidemic growth, while variants conferring later transmission are more strongly favored when historical strains have slow or negative growth. We develop these conceptual insights into a new statistical framework to infer both the R advantage and generation time of a variant. On simulated data, our framework correctly estimates both parameters when it covers time periods characterized by different epidemiological contexts. Applied to data for the Alpha and Delta variants in England and in Europe, we find that Alpha confers a+54% [95% CI, 45–63%] R advantage compared to previous strains, and Delta +140% [98–182%] compared to Alpha, and mean generation times are similar to historical strains for both variants. This work helps interpret variant frequency dynamics and will strengthen risk assessment for future variants of concern.

A TLD-based dosimeter of polyethylene-lead-polyethylene, was developed and characterized with Monte Carlo simulations, using the MCNPX code. This passive system for the determination of the ambient dose equivalent (H*(10)) for neutrons over a wide energy range can be used for the dosimetry of neutrons from atmospheric cosmic radiation, on the ground, and onboard aircraft. A method assisted by Monte Carlo simulations that improves the calibration of fast neutron dosimeters based on moderation and thermalization of the incident fast flux and the measurement of the thermal flux by a sensor, which respond mainly to thermal neutrons, is presented in this work. The H*(10) energy response of this dosimeter was obtained from simulations for monoenergetic neutrons from 10-10 to 104 MeV. The validation of the modeling was done with irradiations for ISO standard neutron fields of 241Am-Be, 252Cf and 252Cf(D2O) at Instituto de Radioproteção e Dosimetria (IRD, Brazil) and at CERN-EU high-energy reference field (CERF).

Introduction: Cancer is, in essence, a genetic disease and is the second leading cause of death globally. Fortunately, many common types of cancer are treatable if detected early and of course there are many medications and treatments available today. Among the new methods to treat cancer, radiotherapy seems to be hopeful in patients with malignancies. This work investigates the strontium beta irradiation (90Sr) dose response on the clonogenicity of Animal blood cancer cell row K562. Scope of this work is the survey of cells survival for three time periods of irradiations and calculating absorbed doses of cancer cells by Monte Carlo simulation. Materials and Methods: The K562 cells were cultured and irradiated with β-particle emitted from 90Sr, daily 30 minutes, 1 hour and 2 hours for 3 days. Cells survival was evaluated by three methods of MTT assay, cell visual survey and DNA electrophoresis after irradiation. To compare the results of cells survival after irradiation, determination of absorbed dose of cancer cells is necessary. But, the short ranges of the emitted beta particles and the associated large dose gradients make experimental measurements particularly difficult. In this circumstance, we have motivated by using of Monte Carlo simulation techniques. To calculate absorbed doses of cancer cells via Monte Carlo, the problem was simulated with MCNP code. Of course, to verify results of simulation with results experimental, we used a Geiger Muller counter and estimated a Scaling Factor. Results: cell visual survey presents photomicrographs of blood cancer cells sections indicating the number of the survive cells after irradiations. Survival percent were obtained for three time periods of irradiations in MTT assay. The survival of cells was described as a function of dose and these absorbed doses were determined using MCNPX code. Finally, to recognize that The cells death was been only due to the destruction of the cell membrane, or the irradiation affect DNA and break it, we did DNA electrophoresis assay. Conclusion: Cell visual survey indicates the positive effectiveness of beta particles on cancer cells. Also, Comparing the survival curves, that hypersensitivity component and dose rate effects are small or absent in high LET, we estimated α component. Eventually, checking the DNA electrophoresis, we understood that in very low-dose- rate up to 2.213±0.0015cGy, cells death Arises from destructing the cell membrane but form 2.213±0.0015 cGy to top, ray begin to effect on DNA.

The life-cycle of Rhabditis marina Bastian 1865 was studied at 25°C and 20‰ salinity. The following demographic parameters were computed from life-table data: the intrinsic rate of natural increase (r m ) = 0.914 day; net reproductivity (R o ) = 400; minimum generation time (T min ) = 4.5 days; cohort generation time (T c ) = 7.2 days; mean generation time (T) = 6.6 days; and the age of an adult female when a median egg is deposited (T) = 6.1 days. Several approximate equations used to estimate r m were compared with these life-table calculations. Some give erroneous estimations and should be used with extreme caution. For iteroparous organisms (including most free-living nematodes), T min only gives an indication of the development time and is therefore unsuitable for describing 'mean generation time'.

Light-tissue interactions are commonly studied for near infra-red (NIR) spectroscopy for imaging and diagnosis. While imaging is bound to the surface due to the high tissue scattering, many diagnosis methods, such as the photoplethysmograph (PPG) and pulse oximeter, focus on sensing instead of imaging. There are two approaches for the investigation of light-tissue interactions: one numerical and the other analytical. The most common numerical method is the Monte Carlo (MC) simulation, which is a statistical study of photon migration from the optical properties of the different tissue regions. However, the yield of such statistical approaches is limited. Analytical methods are commonly based on the diffusion theory, yet they are inaccurate near the light source. There are several solution methods, where extrapolated boundary conditions lead to a more accurate solution. Previously, we proposed measuring the full scattering profile (FSP), which is the angular intensity distribution, of cylindrical tissues. MC simulations revealed that the FSP has a fixed intensity point, named the iso-pathlength (IPL) point, which does not depend on the reduced scattering coefficient. The location of this point depends solely on the geometry. In this study, we derived the FSP from the steady state diffusion theory based on the extrapolated zeroboundary condition. A comparison of different reduced scattering coefficients revealed the IPL point, where the intensity remains constant. Furthermore, a MC simulation was performed under the same geometry and optical properties. We show that the position of the IPL according to the diffusion theory is in agreement with the MC simulation.

As energy demand increases and global warming progresses, expectations for fuel cells, which generate electricity through chemical reactions between hydrogen and oxygen, are rising. There are various types of fuel cells, which are classified according to the electrolyte. Polymer electrolyte fuel cells (PEFCs) are expected to be used in stationary power sources for home use and fuel cell vehicles because of their low operating temperature, short start-up time, and ease of miniaturization. The cost has been an issue in the widespread use of fuel cells. The amount of platinum used in the catalyst layer (CL) must be reduced to reduce costs. However, it increases the current density per unit area of platinum. There are three factors that contribute to voltage drop: ohmic polarization, activation polarization, and diffusion polarization. At high current densities, the voltage drop due to diffusion polarization is dominant. The main cause of diffusion polarization is the transport resistance of oxygen molecules in the CL, and improving the oxygen transport properties in the CL will lead to lower cost PEFCs. In this study, we analyzed the transport properties of oxygen molecules in the CLs of PEFCs using the Monte Carlo (MC) method. The effect of oxygen scattering on the overall oxygen transport properties was investigated by considering the behavior on the surface of the ionomer film (surface diffusion) into the MC method, which simulates the overall transport in the PEFC CL. The objective of this study is to reproduce accurate oxygen transport by comparing the results of MC and molecular dynamics (MD) methods. First, we used the MC method to analyze oxygen scattering phenomena. In this study, we used the 3D structure data of CL in the MC simulations. The three-dimensional structure of the sample was reconstructed from sequential slice images. Molecules were assumed to reflect specularly on the boundaries of the simulation domain. Oxygen molecules were randomly placed in the simulation system. The time step was set at 5 ps and the simulation time was set at 4 μs. In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using various conditions of surface diffusion. We calculated the effective diffusion coefficient of oxygen in the CL as the transport property of oxygen molecules in the MC method. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the effective diffusion coefficient of oxygen increases due to a decrease in the time constant, which determines the probability distribution of the surface residence time. However, when the surface diffusion coefficient is large, a different trend emerges. As the time constant increases, the effective diffusion coefficient has a peak value and then decreases. Next, to verify the model of surface diffusion in the MC method, we analyzed the scattering phenomenon of oxygen molecules in a simulation system that simulates the pores in the CL using the MD method. We modeled the nano pores in the CL as a slit between the ionomer walls in the MD method. Nafion with the equivalent weight (EW) of 1146 was used as polymer model. The number of Nafion chains is set at 4, and the water content λ is set at 7. Three-dimensional periodic boundary conditions were applied to the simulation domain. After the system was equilibrated, 1 ns of NVT simulation was performed for production. The temperature was set at 297 K. The behavior of oxygen molecules impacting on the ionomer wall was analyzed. We analyzed the behavior of oxygen molecules colliding with the ionomer membrane using the MD method. The average residence time of surface diffusion on the ionomer thin film was of the order of 10-10s. When the calculation results were compared with the MC results, it was found that the average residence time obtained by the MD method was the value where the surface diffusion coefficient affects much on the effective diffusion coefficient. Comparing the results of the MD method with the surface diffusion model used in the MC method, there are some discrepancies between the results and the model. For example, the MC method used a model in which oxygen molecules reflect diffusively on the ionomer surface. However, in the MD method, it was found that oxygen molecules tend to reflect in the direction of travel (Fig. 1). Therefore, a more accurate model of the MC method needs to be developed in the future. Figure 1

Evaluation of neutron radiation damage in zircaloy fuel clad of nuclear power plants: a study based on PKA and dpa calculations