Table of contents

The scientific problems in the development of nuclear power divide conveniently into the physical, chemical and metallurgical problems. The physicist is concerned with prediction of the nuclear performance of reactors. He has to be able to predict the critical size and the amounts of fuel and the degree of enrichment required to enable it to operate at the designed heat output. He has further to be able to predict the course of the reactivity - how much heat can be extracted per ton of fuel before the fuel has to be changed. In order to do this he must know a great deal about the interaction of neutrons with the important heavy element isotopes inducing plutonium 240 and 241. He must know how the number of neutrons per fission changes with neutron energy and the relative chances of a neutron being captured with and without producing fission. He must know the chances of neutrons being absorbed by the fission products formed in the fuel elements, and any non-fissile materials used by the metallurgists and engineers. Different data are required for fast and thermal reactors. The physicist must also keep a watchful eye on the safety of reactors - he must be wary lest changes in temperature or operating conditions induce additional reactivity and cause power increases. He must also study the effects of high speed, high intensity neutron bombardment on the properties of important structural materials in the reactor such as graphite and steel.

The chemists and metallurgists have the problem of providing all the nuclear materials - fissile and non-fissile - in a high degree of purity. The chemist must devise processes for separation of fertile materials, fissile materials and fission products. The metallurgist has to understand the effect of radiation on uranium and thorium metal at the operating temperature and has to devise methods of counteracting deleterious effects. To do this he has to study the various possible alloys or cermets bearing in mind the prohibitions of the physicist who dislikes neutron absorbing materials and the chemist who dislikes the job of processing highly alloyed metals.

The chemists and metallurgists have also to study the compatibility of all the materials used with the coolants specified by the engineer. Thus the reaction of graphite with carbon-dioxide gas at 400° C in the presence of reactor radiation must be measured; the compatibility of liquid sodium with zirconium and stainless steel at 550° C must be determined for a reactor of the sodium-graphite type; solutions of uranyl sulphate in water must be compatible with structural materials used in homogeneous reactors. All these problems require prolonged study in test rigs or reactor experiments.

The scientist must also plan ahead, for any new type of reactor is likely to take at least 10 years to come into full scale use from the commencement of work. During this period he has to carry out research and development on any new materials which may be required such as heavy water, thorium, zirconium and beryllium. Work has to be started many years before full scale use can be certain.

The crystal ball is never more required than in the development of atomic energy.

This lecture is an attempt to characterize the various promising types of solid fuel power reactors in terms of the neutron-physics characteristics of their moderators, coolants, and fuel cycles.

The neutron-physics design objectives which have the greatest effect on power cost are:

(i) use of a fertile isotope (uranium 238 or thorium 232) as a major source of energy; (ii) internal conversion ratio high enough to allow long fuel element life; (iii) safety characteristics favourable enough to avoid remote location and high containment cost.

These objectives are not restrictive enough to define the "best" reactor type, but they are the major factors in determining the composition and geometric arrangement of specific reactor designs once the type has been chosen.

The thermal reactors which are at present being designed or constructed utilize uranium 235 as the initial fissile isotope, and there is a strong incentive to utilize ratios of fissile to fertile material which depart as little as possible from the "natural" value. With this additional design restriction, the lattice characteristics appropriate to the various moderator-coolant combinations are still more sharply defined. Examples are drawn from the current group of reactor designs to illustrate these characteristics. The possible consequences of a greater latitude in the choice of fissile-fertile ratio are discussed, and the relative characteristics of the possible fuel cycles are considered.

The characteristics of fast reactors fit into a quite different pattern. The possible conversion ratio is high on either fuel cycle, and the reactivity changes associated with fuel burn-up are relatively low simply because of the very large fissile-material content of the reactor. The requirement that the average neutron energy be kept high is a rather restrictive one. It appears now that the range of variation in fast reactor design will be less than that in the thermal reactor field. The characteristics of typical fast reactors are presented, their special problems are discussed, and some recent experimental results are quoted.

The successful design, construction and operation of nuclear reactors requires detailed information concerning the interaction of various nuclear radiations with matter. In principle this can be obtained from a knowledge of fundamental atomic and nuclear properties combined with mathematical analysis. In practice a considerable number of integral measurements are required. These experiments measure combinations of various fundamental atomic and nuclear properties and the predictions of the mathematical analysis.

The detailed information required is outlined and various integral measurements are discussed together with their correlation with fundamental data and with various mathematical models. Consideration is given to the use of sub-critical, and low-power and high-power critical reactor assemblies in this field.

ZEPHYR is a small low-power fast reactor with a plutonium core surrounded by an envelope of natural uranium. No cooling is required since the maximum operating power is about 2 W. This reactor first diverged in February, 1954, and has been used to make a detailed study of the neutron physics of fast reactors. Of the many aspects investigated, particular attention has been given to a determination of the breeding characteristics of the system and the measurement of the nuclear parameters required for a theoretical interpretation of fast reactors.

Ionizing radiations dissipate most of their energy in solids by exciting electrons. Severe radiation damage is produced in organic substances by this process, but in metals the free electron structure is immune to ionization damage. Atoms are displaced by collisions with fast particles and the point defects so created alter the electrical and thermal properties of solids. These point defects interact strongly with dislocations so that the mechanical properties of metals are sensitive to heavy-particle bombardment. In semi-conductors the displaced atoms can trap electrons and holes, thereby producing large changes in electrical properties. Prolonged bombardments produce drastic effects; some minerals become non-crystalline, uranium crystals change shape, some metals become brittle, and rare gas atoms created by transmutations may produce swelling and disintegration at high temperatures.

A brief introductory discussion is given of the design of shielding for nuclear reactors. It includes consideration of design requirements, choice of materials and dimensions, nuclear reactions that affect the penetration, heating of the shielding and production of radioactivity in the coolant. Because the basic data available are insufficient for high accuracy in design calculation, the use of very complex theory and very laborious calculation has little advantage. Very approximate methods are used, and allowance made for possible error in calculating the thickness of shielding required. The problem is simplified by neglecting radiation processes that have relatively little effect on the overall penetration, but the importance of these effects depends on the design of the shielding.

The concepts of reactivity, neutron lifetime and delayed neutrons are described and their relationships with the rate of change of neutron density and reactor power are discussed. The amounts of reactivity to be controlled in a reactor may be large and varying with time. The various components of this reactivity are noted and discussed. The physical methods of control are surveyed and the safety of reactors is briefly considered.

The paper presents a survey of the instruments needed for the normal operation of reactors. All reactors produce power: the instruments used for its measurement and control and its effect on the temperature of the reactors components are described. These include instruments operated from detectors sensitive to neutron flux and those concerned with physical measurements such as temperature and flow of coolant. The principles of operation of the various instruments are described and their relative importance under various operating conditions indicated.

In power reactors instruments are also necessary for the determination of the change of the neutron flux pattern with time and for the detection of faults in protective fuel containers. The basic principles of such instruments are described. An assessment of the essential instruments for a large power producing reactor is made.

The development of atomic energy has been unique from a chemical point of view, in that a whole series of new elements, the actinide series, has been produced and some of these, such as neptunium, plutonium and americium, play a major role in the technology. Furthermore, many elements which were previously little known and of academic interest only, have become of technical importance in the project, as for example, ruthenium, zirconium and protactinium.

Atomic energy can be divided into the following broad fields, from the chemical point of view, viz., raw materials; production of fuel elements and other special reactor materials; reactor chemistry; chemical processing of irradiated materials; disposal and utilization of radioactive waste materials.

Although the provision of materials and the processing of irradiated fuel elements are of great importance, the nuclear reactor itself has always been the centre of interest. The earlier forms of reactor did not in themselves present any very serious chemical problems. Newer forms of reactor, however, operate at high temperatures and frequently bear more resemblance to chemical or metallurgical plants than they do to mechanical apparatus.

The development of nuclear energy both for weapons and more particularly for peaceful applications is essentially a new branch of applied science. Like many new engineering projects some of the limitations to future progress depend on metallurgical difficulties. Problems arising in the construction of the large chemical plants for any nuclear power project are essentially the same in kind, although perhaps rather more acute in character, as those in normal chemical industry where dangerous and toxic chemicals are involved. On the other hand, the manufacture of fuels for nuclear reactors has required a new industry to be set up for the extraction of uranium, and other new metals are becoming increasingly important if progress is not to be retarded. Apart from new metals, new alloys and higher quality material in conventional metals have been required for the protection of uranium from attack by the coolants used. From the earliest days this has been one of the acute problems of nuclear energy. Finally the metallurgical problem peculiar to atomic energy is the development of materials which will withstand the effects of neutron irradiation. The effects on non-fissile materials are usually minor changes in physical properties, but on fissile material the effects are much more severe. At lower temperatures the anisotropic nature of uranium results in large dimensional changes which are being overcome by grain refinement techniques involving alloying and heat treatment. At high temperatures the gaseous fission products tend to expand and disrupt the material by mechanisms which are not yet fully understood, and for which cures are still being developed.

The heart of the programme at Chalk River is the high-flux, heavy-water-moderated NRX reactor. The primary aim justifying this and the even more costly NRU reactor is to explore the possibilities of deriving low-cost electricity from nuclear energy. This aim is not, however, pursued narrowly and the reactors are used to foster basic research in nuclear physics, nuclear chemistry, radiation physics, chemistry and biology. Since separated fissile material has high intrinsic value in atomic energy research, emphasis has been and still is given to the production of plutonium and uranium-233 as well as to certain radioisotopes, tritium and cobalt among others.

Nuclear power will become important within Canada only if it is cheaper than power from the present-day alternatives. The more costly of these are coal in Ontario, coal and oil in the Maritime Provinces, and oil in remote industrial installations such as mines and wood-pulp mills. Coal in Ontario at $8/ton and contributing 3 mills/kWh to the cost of electricity, sets the major target. Results obtained in the NRX reactor indicate that once the large development costs have been worked off nuclear power costs will be definitely lower than this target.

Canada is relatively rich in uranium and must immediately consider the particular requirements of foreign as well as domestic consumers. Consequently the research remains broad, and very full programmes lie ahead, not only for the NRX reactor, but also for ZEEP which is mostly used for measurements on fuel arrangements, and a proposed PTR ("Swimming-Pool" Test Reactor) which will take over the function of ZEEP for measuring reactivity effects of materials, particularly after irradiation. The most powerful and versatile research reactor is, however, the NRU, a heavy-water-cooled and moderated, high-flux reactor, which should be in operation in 1956.

A brief description is given of the U.K. research reactors. Five are in operation at present and four more are in process of being constructed. They are:

1. GLEEP, a low flux natural uranium graphite-moderated reactor;

2. BEPO, a medium flux natural uranium graphite-moderated reactor;

3. DIMPLE, a low flux heavy-water-moderated reactor, designed to take a variety of core arrangements.

   The remaining two reactors already operating are fast reactors. These are:

4. ZEPHYR, a plutonium fuelled reactor and

5. ZEUS, a ²³⁵U fuelled reactor.

The four reactors being built are all thermal reactors. Three use heavy-water moderation, and are designed to give a flux of 10¹⁴ n cm^-2 s^-1. The fourth is natural-water moderated of the "swimming-pool" type, and should give a flux of 10¹² n cm^-2 s^-1. These four reactors will all be fuelled with uranium 235.

The uses of these reactors are listed and a description of some of the more important uses is presented. In particular the effect of radiation on chemical reactions is receiving considerable attention.

In the reactor technology field, work in high-flux reactors is essential and this is taking the form of so-called "loop tests." These are small sections of proposed reactor cores built into research reactors and cooled by a separate circuit.