David D. Lanning

Automatic controller for steam generator water level during low power operation

Abstract This research proposes a new controller which ensures a satisfactory automatic control for the steam generator water level from low power to full power. It is premised that the current analog control loop is replaced with digital computer control thus expanding the range of possible solutions. The proposed approach is to compensate the level measurement for thermal shrink and swell effects which cause complications in level control during low power operation. A non-linear digital predictor is a part of the controller and is used to estimate shrink and swell effects. The predictor is found to be stable and on-line applicable with micro-processors. The controller is evaluated by calculations in which it controls an existing non-linear digital computer model of a steam generator. For a multi-ramp power increase from low power to full power, the proposed controller shows good performances for the entire range. Water level settles down within 3 min after a single ramp increase (5% power increase in a minute) without any stability problem. Even at very low power, the maximum overshoot is judged to be acceptable.

Design and Experimental Evaluation of an Automatically Reconfigurable controller for Process Plants

A digital controller that reconfigures control strategies as a result of changes in plant operating conditions has been developed and demonstrated. The reconfiguration logic involves the organization of available plant information, the definition and identification of plant operating conditions, the selection of the appropriate control law for the given operating condition, and the verification of the control choice through on-line performance evaluation. Also included is a decision and supervisory logic which interfaces with a knowledge base to insure that plant operating guidelines, procedures, and specifications are not exceeded. This methodology is entirely general and may be used with any process system. It was demonstrated experimentally on the 5 MWt MIT Research Reactor. The reconfigurable controller successfully controlled the reactor power under steady-state, power maneuver, and experimentally-induced anomalies in the control laws.

Digital Control of Power Transients in a Nuclear Reactor

An integrated, closed-loop, control system for online operations in nuclear power plants has been developed and demonstrated with an LSI-ll/23 micro-processor on the 5 MWt fission reactor (NITR-II) that is operated by the Massachusetts Institute of Technology. This control system has inherent capabilities to perform online fault diagnosis, information display, sensor calibration, and measurement estimation. Recently, its scope has been extended to include the direct digital control of power changes ranging from 20-80% of the reactors licensed limit. This controller differs from most of those discussed in theoretical and simulation studies by recognizing the non-linearity of reactor dynamics, calculating reactivity on-line, and controlling the rate of change of power by restricting both period and reactivity. The controller functions accurately using rods of non-linear worth in the presence of nonlinear feedback effects.

Design and Experimental Evaluation of a "Fuzzy" System for the Control of Reactor Power

A rule-based, digital controller that incorporates fuzzy logic has been designed and experimentally demonstrated on the 5 MWt MIT Research Reactor. The potential value of fuzzy logic to the control of reactor power was assessed by comparing the performance of the fuzzy controller to that of one of identical structure that did not rely upon fuzzy logic. The fuzzy system perfomed better under degraded conditions, was less sensitive to noise, and required less movement of the actuator mechanism. Additional tests showed that the fuzzy controller adjusted the reactor power smoothly and without overshoot in response to changes in setpoint. Also, it functioned properly in the presence of externally-induced disturbances. Sensitivity experisents were conducted to identify the structural and design features responsible for the performance of the fuzzy system. Finally, the possible role of fuzzy logic in the control of industrial processes was explored.

CONSIDERATIONS IN THE DESIGN AND IMPLEMENTATION OF CONTROL LAWS FOR THE DIGITAL OPERATION OF RESEARCH REACTORS

In this paper factors relevant to the design and implementation of digital controllers for research reactors are discussed with emphasis on the rationale for incorporating a system model in the control law. For this purpose, proportional-integral-derivative and period-generated control are compared. The latter is a model-based technique that achieves excellent trajectory tracking of nonlinear systems. It does this by combining feedback and feedforward control action in a manner that cancels the effects of the systems dynamics on the controllers performance. Model-based control is also superior in that it permits replication of some of the functions that humans perform when exercising control. In particular, models can be used to predict expected plant response and thereby facilitate diagnosis. Experimental results obtained from trials of digital controllers on both the 5-MW (thermal) Massachusetts Institute of Technology Research Reactor and the Annular Core Research Reactor that is operated by Sandia National Laboratories are included.

Design and Evaluation of Model-Based Compensators for the Control of Steam Generator Level

The design and evaluation of a controller that uses modelbased compensators to offset the inverse response behavior of water level in the steam generators of nuclear power plants is described. Included is a review of steam generator level dynamics, the development of a model that replicates the effects of feedwater and steam flowrate as well as temperature on steam generator level, the design of both the compensators and the overall controller, and the results of simulation studies in which the performance of this modelbased controller and existing analog ones were compared. The compensator-based controller is stable and its use significantly improves the controllability of steam generator level.

Issues in the Closed-Loop Digital Control of Reactor Power: The MIT Experience

The 5 MWt MIT Research Reactors license was recently amended to permit the general use of a closedloop digital control system for the adjustment of reactor power. This regulatory approval is significant because the avoidance of challenges to the reactors safety system now depends on the design of the controller rather than, as was previously the case, on the characteristics of the actuator. Considerations involved in the granting of this license amendment are discussed including information on the technical basis for the controller, the prevention of challenges to the safety system, and the relation between the control and safety systems. The results of the initial use of the controller are presented together with a discussion of the possible applications of this controller.

Flow stability analysis of a particle bed reactor fuel element

This paper describes an investigation of thermal hydraulic flow stability in a particle bed reactor fuel element. The work starts from an adaptation of the stability criterion used by Bussard and DeLauer (1958 and 1965). A one‐dimensional evaluation was then performed, using the Ergun relation (1952) to evaluate numerically the pressure drop. If one considers the entire element from cold frit to hot frit, the analysis would indicate flow stability for all conditions. When one uses the pressure drop in the bed for the criterion, the resultant criterion curve has the same shape as the curve obtained by using the Bussard‐DeLauer methodology, but predicts higher flow rates are required for stability. A two‐dimensional thermal hydraulic study used to verify the one‐dimensional analysis yields consistent results. Finally, results from an evaluation to determine if the heat deposition shape within the bed could impact the stability show that, for a chosen average element operating condition, a flat deposition sh...

Experimental demonstration of automated reactor startup with on‐line reactivity estimation

A generic method for performing automated startups of nuclear reactors described by space‐independent kinetics under conditions of closed‐loop digital control was developed, implemented, and tested on the 5‐MWt MIT Research Reactor. The technique entails first obtaining a reliable estimate of the reactor’s initial degree of subcriticality and then substituting that estimate into a model‐based control law so as to permit a power increase from subcritical on a demanded trajectory. The estimation of subcriticality is accomplished by application of the ‘Perturbed Reactivity Method’ which was developed in the course of this research. The shutdown reactor is perturbed by the insertion of reactivity at a known rate. Observation of the resulting period permits determination of the initial degree of subcriticality. A major advantage to this method is that repeated estimates are obtained of the same quantity. Hence, statistical methods can be applied to improve the quality of the calculation. In addition to describ...

Computer Control of Power in a Nuclear Reactor

A signal validation methodology has been simultaneously applied to the on-line fault diagnosis of neutron flux detectors, primnary coolant flow and temperature sensors, and to the computer control of power in an operating nuclear reactor. A CRT display presents the validated data and sensor diagnostics. The feedback signal to the controller is a digitally processed, weighted average of several valid power sensor signals where the weights are not a priori fixed but dependent on the a posteriori probabilities of failure of individual sensors computed on the basis of past and current observations.