Applicability of neutron noise technique for diagnostics of in-core components for heavy water reactors

Neutron flux signal is composed of a steady or mean component resulting from the flux produced by power operation of the reactor and a very small fluctuating component called ‘noise’ component. Analysis of neutron noise from suitably located sensors is a proven technique to monitor the in-core components of light water reactors (LWRs). However, the use of neutron noise has been rare, if any, for heavy water reactors (HWRs) as it was generally felt that the unfavourable transfer function characteristics of the reactors would limit its applicability. To assess the applicability of technique in pressurised heavy water reactors (PHWRs), experiments were carried out using in-core and out-of-core neutron sensors in a research reactor. This paper discusses the measurement details and results of the experiment. This paper concludes that the neutron noise technique can be effectively utilised for diagnostics/characterisation of the in-core components of heavy water reactors.     

An on-line fuel management method using in-core detector readings for CANDU reactors

An on-line fuel management method for a CANDU reactor has been developed. In the method, the in-core detector readings are used for channel power generation for refueling channel selection. The in-core detector readings are converted to measured mesh readings, and the Kalman filtering technique is applied to reduce calculation and measurement errors of the mesh readings. Then, the estimated channel powers are fed into the refueling channel selection process, in which the channels are refueled so that the difference of zone power from the reference one is minimized. The performance of the method has been demonstrated against the operating data of CANDU 6 reactor. Also, it is found that the core tracking fuel management could be implemented, so that the proposed method would contribute to economic and safe operation of the reactor.     

Simplified method for the seismic qualification using measured modal data

The seismic response estimation by the response spectrum method using only the experimental modal data are presented here. The modal participation factors (MPFs) used for the response estimation are calculated using the experimental mode shapes only. This response is compared with the response estimated using the conventional MPFs but the experimentally extracted mode shapes are used along with the mass matrix estimated corresponding to the measured degree of freedoms (dofs) using physical dimensions of the structure. The presented study eliminates the uncertainty associated with analytical modelling for evaluating mass matrix.     

Study of ultimate seismic response and fragility evaluation of nuclear power building using nonlinear three-dimensional finite element model

The probabilistic safety assessment (PSA) is important for nuclear power buildings in Japan because the risk of the occurrence of seismic ground motions beyond the design assumption cannot be denied. In this paper, the building fragility of the seismic PSA was evaluated using a high accuracy analysis model (three-dimensional nonlinear FEM building model considering soil-structure interaction and basemat uplift behavior). First, the response analyses were conducted increasing the input acceleration up to 3500 Gal, until the damage of the building reached the ultimate condition. The damage of the building was estimated from the shear strain, the axial stress, and the consumed strain energy of the shear walls. Then, the influence on the response given by the vertical ground motion and the basemat uplift was evaluated. In addition, considering the shear destruction of the web wall and compressive crash of the flange wall as the fracture modes, the building fragility was evaluated. As a result, it was shown that the investigated method is efficient for more accurate seismic PSA estimation.     

Electromagnetic load calculation of the ITER machine using a single finite element model including narrow slits of the in-vessel components

We evaluate electromagnetic (EM) loads on the main systems of the ITER machine using a single finite element model. The 20° sector of the full ITER machine includes the main in-vessel components as well as the vacuum vessel. Narrow slits of the in-vessel components are effectively modeled by using the element splitting method without significant increase of computation memory and time as well as without sacrificing the accuracy. Furthermore, the halo current is taken into account at the same time together with the plasma current. To apply both currents concurrently, dedicated conversion codes are utilized to transfer the plasma simulation results by DINA to the electromagnetic analysis by ANSYS-EMAG used here. The electromagnetic loads on the ITER machine are calculated for various disruption scenarios. Investigation on the analysis results is made to find the worst plasma disruption case and the design-driving load component for each system as well as to compare load contribution from eddy and halo currents. The effect of the narrow slits on load reduction is also examined.     

Experience in using the finite element method for reactor calculations

The finite element method can be used to solve the stationary and the transient neutron diffusion equation. Formulations and discretizations of both equations are given. Experience derived from applying the finite element method to practical reactor physics problems is summarized.     

New developments for in-core equipment in the SM-2 and RBT-6 and −10 reactors

Translated from Atomnaya Énergiya, Vol. 69, No. 6, pp. 378–381, December, 1990.     

核电厂大型组合结构的有限元抗震分析方法研究

在现代核电站抗震设计中,有限元法是各类相关设备抗震分析与评价的重要数值仿真工具。对于形状复杂、部件众多的大型组合结构,采用整体三维建模的有限元模型通常需要很大的存储和计算规模,超出现有的计算条件。因此需要首先研究组合结构各个部件的动力学特性,从而建立合理的三维简化力学模型,并以该模型为基础进行有限元数值仿真。本文以某地车-吊车组合结构为例,给出此类大型组合结构的抗震分析方法,并将等效静力法与反应谱法相结合,对该结构进行分析,最后根据相关法规对各子结构进行评价,以确保总体组合结构在极限安全地震条件下能够保持结构完整性。     

Use of an unconventional technique for seismic qualification of equipments

There is a great deal of equipment in nuclear power stations which is required to withstand predefined levels of earthquakes. Such equipment is generally qualified analytically or experimentally by shake-table tests. However, some equipment is so complicated that an analytical simulation is very difficult. This equipment could also be so large and heavy physically that shake-table testing may not be possible in many cases. One typical example of such equipment is the Diesel Generator (DG) sets of Nuclear Power Plants (NPP's). For functional qualification of such equipment, the use of railway track unevenness to induce stationary random vibrations is being put forward as an economical and conservative alternative. This article also brings out the feasibility of using such a technique for all difficult to model and/or test equipment both in a passive and an active state.     

Simplified seismic analysis methods used by AECL for the seismic qualification of CANDU nuclear power plants

This paper presents some of the simplified procedures and methods used by AECL for the seismic qualification of CANDU Nuclear Power Plants (NPPs). The approaches described herein are well tested and have been used in Canada and elsewhere for a number of years. Most of these simplified seismic analysis, testing and inspection procedures, and their underlying principals, have been accepted by the Atomic Energy Control Board of Canada for licensing purposes. In this respect, a comprehensive inspection of completed NPPs, to determine their ability to safely survive a design basis earthquake (DBE), is a prerequisite for licensing of CANDU NPP's in Canada. Many of the methods and recommendations given in the following tie in closely with [1].     

The use of model updating for reliable finite element modelling and fault diagnosis of structural components used in nuclear plants

Reliable finite element (FE) modelling in structural dynamics is very important for studies related to the safety of structural components used in the nuclear power industry. FE model updating is a tool to produce these reliable models. The method uses an initial FE model and experimental modal data of the structural components to modify physical parameters of the initial FE model, and a number of approaches have been developed to perform this task. This paper presents an overview of model updating and its use in fault diagnosis, using typical examples. The paper concentrates on the usefulness of the updating method, rather than describing the different updating methods in detail.     

Seismic analysis of RCS with finite element model for advanced PWR

Reactor Coolant Pumps (RCPs) are very important to the safe operation of Nuclear Power Plants (NPPs), especially during the earthquake, which needs detailed seismic analysis of individual RCPs and the boundary conditions, for example, at the nozzles. In this paper, three-dimensional finite element model of Reactor Coolant System (RCS) is constructed from a systematic perspective to perform dynamic evaluation, in which the boundary conditions could be given. The seismic spectrum analysis with three orthotropic directions is performed to obtain the stress and displacement response, which shows that the maximum Tresca stress locates in the connection part of SG with RCP and the maximum displacement occurs at the surge line. Sensitivity analysis of spectrum input angle and stiffness of supports is performed, which may be useful to further design and analysis. Furthermore, direct integration method is used to perform time-history analysis, and the boundary conditions of RCP, the loads, acceleration and displacement at nozzles are obtained, which could support the detailed analysis of RCP components. Besides, the lumped mass model of RCS is also constructed to compare with three-dimensional finite element model, which means that for the complicated geometry the 3-D model is better than the lumped mass model.     

Inspection qualification programme for VVER reactors and review of round robin test results

Experience obtained, especially from in-service inspections of VVER 440-type reactor pressure vessels and from the Czech round test trials with international participation of ultrasonic teams, has highlighted the need for an in-service inspection qualification programme in the Czech Republic focused on NDT procedures, equipment and personnel. Recently, several national and international regional projects included in the PHARE programme (projects 4.1.2/93 and 1.02/94), briefly described, have been initiated. These projects are to cover step by step the programme of the in-service inspection qualification in view of technical justification as well as of practical assessment—performance demonstration—for all the main VVER-type primary circuit components.     

Development and qualification of materials for structural components for the high-temperature gas-cooled reactor

This paper summarizes the materials evaluation work which has been done for the advanced HTR projects. Essentials of the materials information needed for the design of prestressed concrete pressure vessels (PCPV), steel pressure vessels, graphite incore structures, and metallic heat components inside the pressure vessel are discussed. The main emphasis is given to the metallic high-temperature components which are exposed in the temperature regime in which all properties are sincerely temperature and time dependent. For those components and for graphitic side reflectors the methods of analysis and proof of integrity of the components during the total operation time are provided.     

Practical considerations for thermal transient finite element analysis using isoparametric elements

The performance of 4- and 8-node isoparametric elements in the solution of linear thermal transient problems is compared by spectral analysis. In particular, the reason why 4-node elements often give results which are more acurate and exhibit less oscillations in the time domain than an identical mesh of 8-node elements is discussed. The time integration algorithms examined include the Crank-Nicolson, Galerkin and Euler-Backward schemes. It is shown how the Modified Galerkin Scheme can provide computationally economical solutions which are both accurate and free from temporal oscillations.The problem of spatial oscillations which can arise with real material properties (e.g. concrete) when the diffusivity parameter is small and convection boundary conditions are prescribed is examined. The concept of a ‘penetration depth’ in determining the element mesh size which accurately models the temperature distribution in the initial stages of the transient response is discussed.     

Discontinuity stress analysis of pressure vessels using the finite element method

In pressure vessels the centre lines of the cylinder and dome portions often do not coincide thereby leading to a discontinuity at their junction. Structural analysis of such a structure assuming the centrelines of the cylinder and dome portions to be coincident leads to an incorrect estimation of stress and displacement distributions around the discontinuity. To predict accurately the stress and displacement distributions around the discontinuity, an iterative finite element scheme is developed in this paper using a conical shell finite element. The method is applied to two typical pressure vessels, one with hemispherical end domes and the other with ellipsoidal end domes. It is found that the solution converges in a few iterations.     

Combined experimental and analytical method for a realistic seismic qualification of equipment

The seismic qualification of equipment/structures are, in general, carried out either exclusively by analysis or exclusively by testing using a shake table. The analytical methods have the risk of the model not being a true reflection of the structure unless very elaborate modelling techniques are used. Even with an elaborate model there are many idealisations made which may not actually be realised in practice. The shake-table testing, avoids the modelling deviations to a large extent, but is also not without drawbacks. The important ones are the cost and the availability of a shake table of the required size and capacity. The shake-table testing is also carried out on the isolated equipment without the piping/structural connections from other components. The present paper suggests a combined experimental and analytical method on the ‘as installed’ equipment as an attractive alternative which overcomes the above drawbacks. In contrast to the existing practice of using the experimental results just to validate the analytical model, the suggested method uses the experimentally obtained dynamic characteristics of the ‘as installed’ equipment to obtain the response to the design seismic load. The paper brings out through an example of a simple storage tank which is too heavy for a shake table, the large deviations in its actual behaviour vis-à-vis an idealised analytical model.