Basic Design Aspects of a Fission Reactor (Contd.)
| Shielding and radiation protection (S9) | ||
| One of the important aspects of the design of nuclear reactor and its associated equipments is the proper shielding for radiation protection, which will be discussed later. Shielding is done by attenuating nuclear radiation, mainly primary neutrons and gamma rays originating within the core itself and the secondary gamma rays produced by neutron interactions with material external to the core, e.g., reflector, coolant shield etc. The choice of shielding material and its proper thickness to bring down the radiation level to acceptable value is the important aspect of its design. Another aspect, which has to be taken care, is the temperature distribution in the shield material, as absorption of radiation energy will cause heating of the shield material. Therefore, knowledge of the distribution of neutrons and gamma rays throughout the thickness of the shield is very important. In order to get this knowledge, one approach is to determine the interaction cross sections and attenuation coefficients of neutrons and gamma rays in the shield material in analytical manner. However, due to the complex nature of the radiation source and its distribution in the reactor core and shield, and many interactions over a considerable range of energies, this approach is not practical. A semi-analytic procedure is possible, utilizing relatively simple but somewhat approximate mathematical expressions combined with experimental data of a statistical nature. The design of the reactor shield can be decided based on the following stages: first, the maximum permissible radiation dose rates at various locations must be established. This implies the knowledge of the physical layout of the reactor and its associated equipment. With the knowledge of the reactor system, a preliminary choice can be made of reactor materials and shield layout, so that the attenuation can be carried out. If for this the system data are available the comparison procedure can be used, otherwise a simple form of semi-analytic method is applied. The design process is thus iterative in nature, with the calculations becoming more and more refined as the final stages are approached. | ||
| Control System: | ||
| The basic purpose of a reactor control system is to provide smooth and steady operation. The control operation includes starting of the reactor, bringing power output to the desired level, maintain it at that level, and then shut down the reactor when necessary. A nuclear reactor cannot be used to release energy continuously unless the critical mass for the particular fuel, shape etc. is exceeded. For this reason, a power reactor must include extra fuel and so obviously have excess reactivity to an appreciable extent at the time it commences operation, which may lead to a rapid increase in neutron flux. Therefore, an essential aspect of reactor control lies in the precautions to be taken to prevent an excessive rate of increase in neutron flux when the power level is being raised. Since the power level of a reactor is proportional to the neutron flux, the obvious basis for reactor control is to vary the reproduction or multiplication factor or reactivity. If this factor is greater than unity, the reactor is supercritical and the power level will increase continuously. Upon decreasing the factor to unity, so that the reactor is just critical, the power output will remain constant (apart from transient changes) at the level attained at that time. Finally, by making the reactor subcritical, i.e., by reducing the effective multiplication factor to unity, the power level will be decreased. Multiplication factor can be varied by the addition or removal of fuel, moderator, reflector or a neutron absorber. Either individually or combining these methods has been used to control the operation of nuclear reactor. In thermal reactors, generally the insertion or withdrawal of materials (boron, steel, cadmium etc.) having large capture cross section of neutrons is being used in the control process. The control rods may be located within the core or in the reflector close to the core where the thermal neutron flux is high. Because of its high melting point and other useful properties, the element hafnium is employed in water-cooled reactors. This process is useful in controlling the reactors of low power output. However, this is not efficient for reactors having considerable reactivity with high neutron flux. An improvement in neutron economy can be achieved by combining motion of the core material, i.e., fuel and moderator, with that of an absorber. The lower portion of the coarse control rod may be constructed of the same material as the reactor core, whereas upper portion contains cadmium. When such a rod is lowered, so that neutron absorber is inserted, some of the core is removed at the same time, thereby bringing about a further decrease in reactivity. In a fast-neutron reactor, control by means of a neutron absorber is not generally satisfactory because of the low capture cross sections for neutrons of high energy. The reactivity can, however, be changed by the removal of fuel material from (or addition to) the core or by the movement of a part of the reflector. For the reactors of very low output powers, generally manual control is done, whereas otherwise the manual control is not feasible. Emergency safety action, for example, must be made automatic. If an instrument indicates that the neutron flux is rising at a dangerously rapid rate, or if the power level is appreciably higher than that for which the reactor was designed, an automatic system must be devised to tackle such situation. The control system consists of three loops, namely, operator loop, automatic loop and load loop. In ‘operator loop’, any information from any instrumental part of the reactor is received by the operator, who in turn, exerts appropriate action or conveys a signal to the control block. It is then the function of the control block to exert the desired control action on the reactor, usually with the expenditure of power from an external source. In the ‘automatic loop’, the information received from the reactor by the instruments is fed directly into the control block, the operator being by-passed. The ‘load loop’ is intended to represent the interaction of the nuclear aspects of the reactor, i.e., the fuel, moderator, and reflector, with the external conditions, or non-nuclear aspects. The external conditions may be regarded as constituting, in general sense, the ‘load’ on the reactor. The closed cycle in the block diagram (Fig. m3.5) implies that the load can affect the reactor, which in turn influences the load. The changes in the external conditions are indicated on suitable instruments, and information is fed back to the control block via operator and automatic loops so that the necessary action may be taken.1,3 | ||
/m3.7.png) | ||
FIGURE m3.5 Block diagram of the reactor control system (Minor modification required) | ||