John C. Luxat

CANDU reactor kinetics benchmark activity

A description of reactor kinetics benchmark activity in support of the Canadian natural-uranium heavy water (CANDU) reactor program is presented. Two benchmark problems with neutronic characteristics resembling current generation CANDU reactors have been defined, and accurate solutions to both problems are summarized. In addition, the performance of an improved quasi-static code has been tested and compared with a finite difference method. A brief description of the reactor kinetics experimental program related to reactor transient studies is also given.

Space-Time Neutronic Analysis of Postulated Loss-of-Coolant Accidents in CANDU Reactors

Space-time neutronic behavior of CANDU reactors is of importance in the analysis and design of reactor safety systems. A methodology has been developed for simulating CANDU space-time neutronics with application to the analysis of postulated loss-of-coolant accidents. The approach involves the efficient use of a set of computer codes that provides a capability to perform simulations ranging from detailed, accurate three-dimensional space-time to low-cost survey calculations using point kinetics with some effective spatial content. A new space-time kinetics code based on a modal expansion approach is described. This code provides an inexpensive and relatively accurate scoping tool for detailed three-dimensional space-time simulations.

Stability of Peak-Holding Control Systems

The adaptive response of peak-holding control systems is investigated using Liapunovs second stability method. The control loop is simplified by neglecting the dynamic elements associated with performance index measurement and a Liapunov function based on the plant and controller nonlinearities is derived. Stability criteria are obtained for the situation where control parameter bounds exist and for two different cases of variation of plant operating conditions (viz., drift and periodic disturbance of optimum operating point). Expeximental results are given that indicate the validity of the technique used, even in situations where the dynamic measurement elements present in the control loop cannot be neglected. The aim of the paper is to improve the understanding of the performance of peak-holding control systems, of the optimalizing type under time varying operating conditions and to present simple stability criteria that are directly applicable to the practical case.

Thermal-Hydraulic Aspects of Progression to Severe Accidents in CANDU Reactors

Abstract The progression of events that develop into an accident with severe fuel or core damage in the Canada deuterium uranium (CANDU) reactor is discussed. Such events involve a number of broadly common stages in which the thermal-hydraulic behavior of the reactor fuel, fuel channels, heat transport system, and a number of key process systems governs both the rate at which severely degraded cooling conditions develop and the extent of resultant damage to the reactor core. The quantification of core damage states requires the modeling of the physical phenomena that are active in these accidents, which is a focus of this paper. As discussed in this paper, unique passive features of the CANDU reactor design have a beneficial effect in that they delay the progression of severe accidents, thereby providing ample opportunity for operator actions to stabilize the plant and mitigate the consequences. It is shown that large CANDU reactors are inherently tolerant of a prolonged loss of engineered heat sinks at decay power levels. This is because two large volumes of water (the moderator and shield water) surround the reactor core and act as in situ passive heat sinks in severe accidents. This has significant impacts on severe accident management. The pressure tube reactor design precludes melting of the core at high system pressures; that is, high-pressure melt ejection is physically impossible. In the event that severe undercooling of fuel occurs at high system pressure, a pressure tube will fail well before any significant molten fuel material can accumulate.

Global warming and sustainable energy supply with CANDU nuclear power systems

Abstract The United Nations Intergovernmental Panel on Climate Change (IPCC) review of global warming issues suggests mans activities have resulted in a discemible influence on global climate. The panel identifies options which could be employed to ameliorate the climate influencing greenhouse effect which is attributed primarily to carbon dioxide and other gaseous emissions from fossil fuel energy sources. One option identified is nuclear power, as an alternative energy source which would reduce these emissions. The panel observes that, although nuclear power is a relatively greenhouse gas free energy source, there are a number of issues related to its use which are slowing its deployment. This paper enumerates the issues raised by the IPCC and addresses each in turn in the context of CANDU reactors and sustainable development. It is concluded that the issues are not fundamental barriers to expanded installation of nuclear fission energy systems. Nuclear reactors, and CANDU reactors in particular, can meet the energy needs of current generations while enhancing the technological base which will allow future generations to meet their energy needs. The essential requirements of a sustainable system are thus met.

Suboptimal adaptive control of a class of non-linear systems†

The problem of controlling a class of nonlinear systems with uncertain parameters and imprecisely defined system and measurement noise terms is considered. A sub-optimal adaptive control algorithm is developed as a solution to coupled linear modelling and control optimization problems. Features of the algorithm are the forward-recursive generation of the modelling and control processes and necessity for past and present values of states and parameters alone in order to implement the algorithm (i.e. no precomputed nominal trajectory is required). These features make the algorithm ideally suited to real-time control applications Real-time control of a non-trivial fifth-order system, (a model of a plasma torch furnace) is considered and the results of an experimental study are presented.

A strategy for intensive production of molybdenum-99 isotopes for nuclear medicine using CANDU reactors.

Technetium-99m is an important medical isotope utilized worldwide in nuclear medicine and is produced from the decay of its parent isotope, molybdenum-99. The online fueling capability and compact fuel of the CANDU(®)(1) reactor allows for the potential production of large quantities of (99)Mo. This paper proposes (99)Mo production strategies using modified target fuel bundles loaded into CANDU fuel channels. Using a small group of channels a yield of 89-113% of the weekly world demand for (99)Mo can be obtained.

Developments Toward Mechanistic Modeling of Critical Heat Flux on a CANDU Calandria Tube Following Pressure Tube Contact

Abstract Heavy water moderator surrounding each fuel channel is one of the important safety features in CANDU reactors since it provides an in situ passive heat sink for the fuel in situations where other engineered means of heat removal from fuel channels have failed. In a critical-break loss-of-coolant-accident scenario, fuel cooling becomes severely degraded because of rapid flow reduction in the affected flow pass of the heat transport system. This can result in pressure tubes (PTs) experiencing significant heatup during early stages of the accident when coolant pressure is still high, thereby causing uniform thermal creep strain (ballooning) of the PT in contact with its calandria tube (CT). The contact of the hot PT with the CT causes rapid redistribution of stored heat from the PT to the CT and a large heat flux spike from the CT to the moderator fluid. For conditions where subcooling of the moderator fluid is low, this heat flux spike can cause dryout of the CT. This can detrimentally affect channel integrity if the CT postdryout temperature becomes sufficiently high to result in continued thermal creep strain deformation of both the PT and the CT. The focus of this work is to develop a mechanistic model to predict critical heat flux (CHF) on the CT surface following a contact with its PT. A COMSOL multiphysics model using a two-dimensional transient fluid-thermal analysis of the CT surface undergoing heatup is used to predict the flow and temperature profiles on the CT surface. A mechanistic CHF model is to be proposed based on a concept of wall dry patch formation, prevention of rewetting, and subsequent dry patch spreading.

Design and Analysis of an Electronic Peak-Holding Controller

Peak-holding control offers a simple and inexpensive method whereby operation of a certain class of processes may be kept in the region of optimum performance. The design of an electronic peak-holding controller is described in this paper, and techniques for analyzing its behavior are presented. The possible occurrence of spurious (false) peaks being detected by the controller is demonstrated and a method of both predicting and preventing this peak detection phenomenon is developed. Application of the controller in optimizing the feed rate to a simulated pebble mill grinder is presented.

Energetic Liquid Droplet Boiling on High-Temperature Metal Surfaces

Abstract Experimental studies of thermal interactions of cold liquid droplets impinging on metal surfaces have been performed and the result of these studies are summarized in this paper. In these experiments rapid, energetic (explosive) breakup of the liquid drops were observed using high-speed video camera recordings. These energetic interactions occurred over a range of high temperatures of the metal surfaces and varied with the type of metal employed. Three metals were used in the study, namely, copper, brass, and stainless steel. The test sections included curved-plate (sections machined from metal cylinders) and flat-plate geometries. The choice of metals was determined by the objective of establishing the influence of thermal diffusivity of the hot material on the thermal interaction between the cold liquid droplet and the hot metal surface, and the two metal surface geometries were used to study the influence of droplet spreading behavior after impact with the hot metal surface. Metal surface temperatures ranged from 30°C to 700°C and controlled single water droplets at a temperature of 25°C were released from a specially designed rig employing a small fast–opening/closing solenoid valve. Experimental results are presented in this paper that demonstrate the processes that occur during the interaction of the droplet with the hot metal surface during a time frame of 1 to 20 ms.