Modified Neutron Source Multiplication Method Research Articles

The first application of modified neutron source multiplication method in subcriticality monitoring based on Monte Carlo

Abstract The control rod drive mechanism needs to be debugged after reactor fresh fuel loading. It is of great importance to monitor the subcriticality of this process accurately. A modified method was applied to the subcriticality monitoring process, in which only a single control rod cluster was fully withdrawn from the core. In order to correct the error in the results obtained by Neutron Source Multiplication Method, which is based on one point reactor model, Monte Carlo neutron transport code was employed to calculate the fission neutron distribution, the iterated fission probability and the neutron flux in the neutron detector. This article analyzed the effect of a coarse mesh and a fine mesh to tally fission neutron distributions, the iterated fission probability distributions and to calculate correction factors. The subcriticality before and after modification is compared with the subcriticality calculated by MCNP code. The modified results turn out to be closer to calculation. It's feasible to implement the modified NSM method in large local reactivity addition process using Monte Carlo code based on 3D model.

Application of the modified neutron source multiplication method to the prototype FBR Monju

The Modified Neutron Source Method (MNSM) is applied to the prototype FBR Monju. This static method to estimate the reactivity in sub-critical conditions has already given good results on commercial pressurized water reactors. The MNSM consists both in the extraction of the fundamental mode seen by a detector to avoid the effect of higher modes near sources, and the correction of flux distortion effects due to control rod movement between different reactor states. Among Monju’s particularities that have a big influence on the MNSM factors are: the presence of two californium sources near the core and the position of the detector, which is located far from the core outside of the reactor vessel. The importance of spontaneous fission and (α, n) reactions, which have increased during the shutdown period of 15years, will also be discussed. In order to evaluate the detector count rate, a propagation calculation has been conducted from the reactor vessel to the external detector. For two sub-critical states, an estimation of the reactivity has been made and compared to experimental data obtained in the restart experiments at Monju (2010). Results indicate a good agreement between the MNSM reactivity and the reactivity measured with other methods. The reactivity dependence of the correction to apply to the point kinetic equation is discussed.

Experimental determination of subcriticality at subcritical PWRs in Korea

A direct subcriticality measurement system (SMS) based on the Feynman-α method has recently been developed by KEPRI. It was applied to six commercial pressurized water reactors in Korea. However, the obtained Feynman curves failed to give proper multiplication factors. The objective of these tests was to investigate the performance of the Feynman method to predict directly the subcriticality of a given subcritical reactor by using the neutron pulse counts only without any reactor perturbation in the large commercial reactors. Recently, two methods developed by Hokkaido University and Westinghouse Electric Corporation. These methods have a defect due to being based on the modified neutron source multiplication method. To overcome this defect and derive operational benefits is necessary to estimate the subcriticality of a subcritical core directly from the neutron pulse counts only. The performance of the developed SMS was verified in the Kyoto University Critical Assembly and applied to eight 1000 MWe Optimized Pressurized Water Reactors (OPR1000) in Korea. The obtained results show that the SMS based on the Feynman method can be a useful tool to estimate the reactivity of a subcritical power reactor. Although the discrimination level of the signal-processing unit in OPR1000 suffers from noise and gamma ray effects, SMS can provide good Feynman curves and effective multiplication factors. However, the SMS has failed to give the reactivity for the entire measured data set. Improving the SMS and investigating the effects of different discriminator level settings at SPU in OPR1000 will be topics for further study.

A New Experimental Correction Method for the First-Order Perturbation Approximation in the Steady Subcritical Reactor

In the subcritical nuclear reactor, the subcriticality is closely related with the neutron flux distribution. Since the relationship between them is complex, the subcriticality measurement by conventional methods involves some difficulties, particularly when the subcriticality is large. A new subcriticality measurement method is proposed for the steady subcritical state. By this method, the relative distribution of the change in neutron flux between before and after the perturbation is numerically calculated. On the other hand, the amplitude of the perturbation is estimated from the count rates of neutron detectors. By this combination of the calculation and experiment, the subcriticality can be accurately estimated, to the extent that the perturbation is qualitatively known beforehand. One of the advantages of this method over the conventional modified neutron source multiplication method is that numerical calculations are only executed on unperturbed states. In addition, the completeness of the eigenfunctions is not assumed. The validity of this method is confirmed through the numerical calculation.

A New Experimental Correction Method for the First-Order Perturbation Approximation in the Steady Subcritical Reactor

In the subcritical nuclear reactor, the subcriticality is closely related with the neutron flux distribution. Since the relationship between them is complex, the subcriticality measurement by conventional methods involves some difficulties, particularly when the subcriticality is large. A new subcriticality measurement method is proposed for the steady subcritical state. By this method, the relative distribution of the change in neutron flux between before and after the perturbation is numerically calculated. On the other hand, the amplitude of the perturbation is estimated from the count rates of neutron detectors. By this combination of the calculation and experiment, the subcriticality can be accurately estimated, to the extent that the perturbation is qualitatively known beforehand. One of the advantages of this method over the conventional modified neutron source multiplication method is that numerical calculations are only executed on unperturbed states. In addition, the completeness of the eigenfunctions is not assumed. The validity of this method is confirmed through the numerical calculation.

Analysis of Reactivity Measurements in Core Physics Experiments on Full-MOX BWR

A core physics experimental program FUBILA has been performed to study core physics characteristics of full-MOX BWR cores consisting of high Pu-enriched MOX assemblies for high burnups. The program includes the measurement of reactivity worth, which is essential in operating BWR cores. The reactivity worth is due to the reactivity caused by (1) changes in the in-channel void fraction of assemblies, (2) the insertion of a B4C control blade, (3) Gd2O3-UO2 rods and UO2 rods in assemblies, and (4) the mixing of boron in a moderator related to a stand-by liquid control system. The reactivity worth was measured by the modified neutron source multiplication method using the reactivity worth of a pilot rod as a reference worth. The measured reactivity worth was determined by processing count rates of neutron detectors taking into account the detector efficiency and effective neutron source intensity analyzed by three-dimensional transport calculations. Comparing the measured reactivity worth with the results obtained by deterministic calculation methods and a Monte Carlo calculation method, the accuracy of the calculations was evaluated. The calculated results generally well reproduce the measurements, except for the boron reactivity worth in the moderator, for which the calculations overestimate the reactivity worth.

Analysis of Reactivity Measurements in Core Physics Experiments on Full-MOX BWR

A core physics experimental program FUBILA has been performed to study core physics characteristics of full-MOX BWR cores consisting of high Pu-enriched MOX assemblies for high burnups. The program includes the measurement of reactivity worth, which is essential in operating BWR cores. The reactivity worth is due to the reactivity caused by (1) changes in the in-channel void fraction of assemblies, (2) the insertion of a B4C control blade, (3) Gd2O3-UO2 rods and UO2 rods in assemblies, and (4) the mixing of boron in a moderator related to a stand-by liquid control system. The reactivity worth was measured by the modified neutron source multiplication method using the reactivity worth of a pilot rod as a reference worth. The measured reactivity worth was determined by processing count rates of neutron detectors taking into account the detector efficiency and effective neutron source intensity analyzed by three-dimensional transport calculations. Comparing the measured reactivity worth with the results obtained by deterministic calculation methods and a Monte Carlo calculation method, the accuracy of the calculations was evaluated. The calculated results generally well reproduce the measurements, except for the boron reactivity worth in the moderator, for which the calculations overestimate the reactivity worth.

Feasibility Study for Evaluation of Control Rod Worth in Pressurized Water Reactors using Neutron Count Rate during a Control Rod Drop Testing

Applicability of the modified Neutron Source Multiplication (NSM) method with extraction of the fundamental mode to subcriticality measurement has been proposed. Following the feasibility verification in the previous study based on numerical analyses, its applicability has been proven in a more realistic situation; in a withdrawal sequence of control rod banks during the PWR startup. Subcriticalities with various control rod insertion configurations were estimated based on the modified NSM method. The subcriticality could be evaluated with a good accuracy even with the mockup experiment where any special treatments for accurate measurement were not taken into account and furthermore the insensitivity of measured signals by reactivity changes and their large fluctuations were seen. Based on this fact, we further investigated a feasibility to use neutron count rate data obtained during the control rod drop testing, which is carried out before the reactor physics tests at hot zero power condition. When it is proven that these data could be used for the estimation of each control rod worth, the following reactor physics tests could be performed with the advanced knowledge of each control rod worth and procedures for detailed control rod worth measurement could be simplified or eliminated from the reactor physics tests.

Subcriticality Measurement of Pressurized Water Reactors by the Modified Neutron Source Multiplication Method

Applicability of the modified Neutron Source Multiplication (NSM) method with extraction of the fundamental mode to subcriticality measurement has been investigated. This concept can be applied to the control rod worth measurement that is performed during reactor physics tests at periodical inspection of commercial PWRs. Following the feasibility verification in the previous study based on numerical analyses, its applicability has been proven in a more realistic situation; Regarding a withdrawal sequence of control rod banks during the PWR startup as a mockup experiment, subcriticalities with various control rod insertion patterns were estimated based on the modified NSM method. The subcriticality could be evaluated with good accuracy even with the mockup experiment where any special treatments for accurate measurement were not taken into account and furthermore the insensitivity of measuring signals by reactivity changes and their large fluctuations were seen. Some vital treatments for accurate estimation were identified: The reference subcritical state must be as closer to zero as possible. It is necessary to wait long enough for the delayed neutron effect to attenuate after a reactivity change by control rod bank withdrawal. Gamma-ray contamination contained in the detector signals should be corrected.

The Effect of Neutron Source Distribution on Subcriticality Measurement of Pressurized Water Reactors Using the Modified Neutron Source Multiplication Method

A modified neutron source multiplication method has been investigated for application to subcriticality measurement in commercial Pressurized Water Reactors (PWRs). This method is based on the extraction of the fundamental mode from measured neutron signals and corrections for spatial effects. In commercial reactors, mainly two types of neutron sources are expected, one from burnt fuels and the other from in-core fixed neutron sources. To calculate the extraction and spatial correction factors, the fixed source problem was solved by the Finite Difference Method (FDM) to explicitly deal with spatial intensities of neutron sources. Sensitivity analysis was carried out to investigate how the accuracy of estimated subcriticality depends on approximation levels of neutron source distributions. It was found that both of the neutron sources should be taken into account. For a pin rod type of the in-core fixed neutron source, a node-wise distribution for which the localized source intensity is averaged over the radial direction in the relevant nodes is applicable, since it can estimate the subcriticality with almost the same accuracy as in the case with a rigid spatial treatment. This ensures that nodal calculations can be used in the evaluation of correction factors instead of burdensome FDM calculations.

The Effect of Neutron Source Distribution on Subcriticality Measurement of Pressurized Water Reactors Using the Modified Neutron Source Multiplication Method

The Effect of Neutron Source Distribution on Subcriticality Measurement of Pressurized Water Reactors Using the Modified Neutron Source Multiplication Method

Subcriticality Measurement of Pressurized Water Reactors by the Modified Neutron Source Multiplication Method

Applicability of the modified Neutron Source Multiplication (NSM) method with extraction of the fundamental mode to subcriticality measurement has been investigated. This concept can be applied to the control rod worth measurement that is performed during reactor physics tests at periodical inspection of commercial PWRs. Following the feasibility verification in the previous study based on numerical analyses, its applicability has been proven in a more realistic situation; Regarding a withdrawal sequence of control rod banks during the PWR startup as a mockup experiment, subcriticalities with various control rod insertion patterns were estimated based on the modified NSM method. The subcriticality could be evaluated with good accuracy even with the mockup experiment where any special treatments for accurate measurement were not taken into account and furthermore the insensitivity of measuring signals by reactivity changes and their large fluctuations were seen. Some vital treatments for accurate estimatio...

Estimation of fuel concentration distribution and subcriticality by using “Pattern matching” method

Development of critically safety monitoring is an important issue in nuclear fuel reprocessing plants. In this paper two subcriticality estimation methods are compared. One method is based on conventional artificial neural network method. But its performance was no good in the sense that the accuracy of estimated subcriticality was the same order as the results estimated by the conventional Modified Neutron Source Multiplication method. The other method considered is referred to as the “Pattern matching” method. It can be used as the critical safety monitor for non-uniform fuel solution systems in reprocessing plants. Numerical experiments were carried out to examine the method above mentioned; the estimated subcriticalities agreed well with the true values. The “Pattern matching” method can also be applied to the fuel concentration estimation problem. The estimated fuel distribution agreed reasonably with the true fuel distribution in all cases. It is concluded that the “Pattern matching” method is available critical safety monitor for non-uniform fuel systems in reprocessing plants.

Numerical validation of a modified neutron source multiplication method using a calculated eigenvalue

To overcome the difficulties of the conventional neutron source multiplication method, the authors have developed the Indirect Bias Estimation Method. This method obtains the bias in calculated K eff using the difference between measured and calculated neutron count rates. This bias in calculated K eff is used for an adjustment of the calculated K eff, deriving a truer K eff. Using neutron diffusion calculations, numerical experiments simulating the neutron source multiplication method were performed to validate this method. It could be shown how closely a true K eff can be reproduced using this method. It was found that an accurate estimation of subcriticality requires neutron count rate measurements at more than three locations.