Yoichiro Shimazu

Development of a Compact Digital Reactivity Meter and a Reactor Physics Data Processor

Reactor physics tests at initial startup and after refuelings are performed to verify the nuclear design and to assure safe operation. Analog computers and instruments are widely used for the acquisition of data, and these data are reduced by hand. These conventional procedures, however, require much time and labor. Since there has been great progress in the development of digital computers and devices, these procedures are digitalized, which successfully reduces the time and labor required for reactor physics tests.

Reactivity-Initiated-Accident Analysis without Scram of a Molten Salt Reactor

Recently, a conceptual design of a small MSR, named FUJI-12 has been proposed. FUJI-12 operates with the same fuel salt as the Molten Salt Breeder Reactor (MSBR) designed by ORNL. The authors are interested in the MSR concept due to its high potential in the areas of safety, proliferation resistance, resource sustainability and waste reduction, all necessary requirements for the generation IV nuclear power systems. The authors believe that additional investigations are necessary for future study. From this point of view, the authors have analyzed various reactivity insertion accidents due to control rod malfunctions in FUJI-12. The MSR can be operated with a small excess reactivity. However, at the same time, the delayed neutron fraction is quite small due to the usage of U-233 as fissile material and the circulation of the fuel salt. Therefore, the reactivity insertion accident should be qualitatively evaluated. The reactor transients were analyzed without scram in order to evaluate the severity of such accidents against the safety. Although the total primary system design of FUJI-12 is not completed, and thus, the accident analyses include some crude assumptions, it can be expected that the reactivity insertion accident in FUJI-12 would not result in severe plant conditions.

Subcriticality Measurement by Neutron Source Multiplication Method with a Fundamental Mode Extraction

A new neutron multiplication method has been proposed for an accurate measurement of subcriticality. The proposed method consists of two correction processes for (1) extraction of the fundamental mode from measuring data of a neutron detector that contains higher modes as well as the fundamental mode feeding from an external neutron source and (2) spatial corrections for perturbations induced by a reactivity addition in the distributions of the fundamental mode and a neutron importance field. Feasibility of the proposed method has been verified from a numerical study, although under some limitations such that the neutron multiplying system to be analyzed is small-sized and a reactivity change takes place homogeneously in a fuel loaded region. With extraction of the fundamental mode and the spatial corrections, the subcriticality can be estimated accurately even with measuring data highly contaminated with higher modes due to a detector position near to an external point neutron source. For a future application to measurement of control rod bank worth of a nuclear power plant from measuring data during a reactor physical testing, some useful guidelines have been obtained.

Preliminary Safety Analysis on Depressurization Accident without Scram of a Molten Salt Reactor

The Molten Salt Reactor (MSR) concept has recently been considered as one of the candidates for the generation IV nuclear power systems. MSRs have many advantages such as improved safety, proliferation resistance, resource sustainability and waste reduction. But MSR developmental activities have lagged and there are few data available to support detailed analyses. However, the authors believe that additional investigations are merited for future study. From this point of view, the authors have analyzed the depressurization accident of the MSR “Fuji-12” using a newly developed MSR transient analysis code. In Fuji-12, a small amount of helium gas bubbles are circulated in the primary loop for stripping out gaseous fission products. A depressurization of the primary system would cause these bubbles to expand, resulting in a positive reactivity insertion. We have attempted to determine the severity of such an accident. Although the analysis is still preliminary and the assumptions are crude, it can be expected that the depressurization would not result in a severe accident in Fuji-12.

Some Technical Issues on Continuous Subcriticality Monitoring by a Digital Reactivity Meter during Criticality Approach

It has been proposed that a digital reactivity meter can be used for not only subcriticality measurement but also continuous subcriticality monitoring during criticality approach. In this study, a mock up numerical simulation was carried out to investigate some technical issues that still exist. These are sampling interval, time lag of estimated subcriticality due to filtering effect as well as due to the error contribution to the initial reference subcriticality and the way to avoid time lag of estimated subcriticality from safety point of view. It was found that there is influence of sampling interval on the reactivity fluctuation. Filter can cause time lag of estimated subcriticality when a large filter time constant is used. In subcriticality monitoring, the error contribution to the initial reference subcriticality obtained by some design calculation should be investigated to detect accurately an unexpected criticality as earlier as possible. It was known that the negative error contribution to the initial reference subcriticality should be used from safety point of view. We have analytically obtained the relation between the error of the initial reference subcriticality, ramp rate of reactivity addition, warning subcriticality and the margin of the time interval for the counter action.

Optimal burnable poison-loading in a PWR with carbon coated particle fuel

An innovative PWR concept that uses carbon-coated particle fuels moderated by graphite as that of HTGR but cooled by pressurized light water has been studied. The aim of this concept is to take both the best advantages of fuel integrity against fission products release and the reliability of PWR technology based on the long operational experience. The purpose of the study is to optimize loading pattern of burnable poison in the proposed core in order to suppress excess reactivity during a cycle. Although there are many parameters to be determined for optimization of the usage of burnable poison, the emphasis is put here on loading patterns of Gadolinia in an assembly and in the core. We investigated the burnup characteristics of the core varying the concentration of burnable poison in a fuel rod, the number of burnable poison-rods in an assembly, and the number of burnable poison-assemblies in the core. The result suggested that Gadolinia was more suitable for this reactor than boron as burnable poison, and it was possible to make the reactivity swing negligible by combining at least three kinds of burnable poison-assemblies in which the amount of Gadolinia was different. Therefore the requirement for the number of control rods was reduced and it meant that Control Rod Programming would become easier.

Continuous Guidance Procedure for Xenon Oscillation Control

A new concept for the control of axial xenon oscillations in large PWRs has been proved to be effective. The concept is based on two additional newly defined axial offsets, AOi and AOx together with the conventional axial offset of power distribution AOp. AOi and AOx are the axial offsets of power distributions which would give the current iodine and xenon distributions under equilibrium conditions, respectively. This method can give the optimum timing for control rod operations and the target AOp. However this method cannot give continuous guidance to the operator for xenon oscillation control such as the direction and the amount of control rod movement. In order to overcome this disadvantage, we have developed a new control procedure which can provided information on the xenon oscillation at a glance by displaying the relation of the three AOs as a trajectory of (AOp-AOx, AOi-AOx) in the X-Y plane. This trajectory shows a very simple characteristic behavior. This trajectory can give continuous and clear...

Subcriticality Monitoring with a Digital Reactivity Meter

Feasibility of subcriticality monitoring with a digital reactivity meter was experimentally investigated using a sub-critical assembly. In the experiment, neutron detectors were placed in the reflector region where the neutron flux multiplied by the difference of the multiplication factor from unity is almost constant. The significant fluctuation of the neutron signal was filtered with a smoothing and a low-path filter of first order delay. In the subcritical system above with a constant neutron source, it was confirmed through the experiment that a digital reactivity meter with noise filtering module could give real time subcriticality when the detector was placed at appropriate positions. Based on the experiment, we have proved that the reactivity meter can be used for monitoring the subcriticality on a real time basis.

Subcriticality Measurement of Pressurized Water Reactors during Criticality Approach using a Digital Reactivity Meter

A mockup experiment of subcriticality measurement with a digital reactivity meter during criticality approach in a PWR was carried out using plant data. There are some difficulties in the direct application of a digital reactivity meter to the subcriticality measurement. Some treatments were needed in using the count rate of Source Range (SR) detector as input signal to the digital reactivity meter. To overcome these difficulties, we proposed a digital reactivity meter combined with a methodology of the modified Neutron Source Multiplication (NSM) method with the fundamental mode extraction. In this study, the neutron source strength was defined as a constant in terms of a known initial stable subcriticality and the neutron signal from a steady state condition. Even though initial stable subcriticality might be contained some errors, the errors of estimated subcriticalities do not depend on the error contribution of initial stable subcriticality when we normalize the estimated subcriticalities with exactly known reference subcriticality. It was proved that the digital reactivity meter could be used not only for the subcriticality measurement or control rod worth measurement but also for the continuous monitoring of the subcriticality during criticality approach.

Xenon Oscillation Control in Large PWRs Using a Characteristic Ellipse Trajectory Drawn by Three Axial Offsets

We have proposed a very simple xenon oscillation control procedure for axial xenon oscillation control based on a characteristic trajectory. The trajectory is drawn by three offsets of power distributions, namely, AOp, AOi, and AOx, which are defined as the axial offset (AO) of power distribution, AO of power distribution under which the current iodine distribution is obtained as the equilibrium, and that for xenon distribution, respectively. When these offsets are plotted on the X-Y plane for (AOp − AOx, AOi − AOx) the trajectory shows a quite characteristic ellipse (or an elliptic spiral). When the plot is at the origin of the X-Y plane, no xenon oscillation exists. Using the characteristics of the ellipse, the xenon oscillation can be eliminated by guiding the plot to the origin with control rod operation. In this paper, we will present an innovative and very simple procedure to generate the trajectory for the prediction of the oscillation behavior. Using the prediction capability of the xenon oscillation, the optimal control search calculation can be done. Since it is calculated by simple trigonometric functions and an exponential function, the prediction takes less than 1 s with a PC. It is especially effective for radial xenon oscillation control when the control rod motion is not flexible as for axial xenon oscillation control.