Safety Aspects and Analysis of Accidents

Abstract

The large inventory of radioactive substances in a nuclear reactor or in an intermediate storage system as example represents the real problem of use of nuclear energy. Just very small shares of this material are allowed to be released during normal operation and accidents. There are several requirements which have to be fulfilled to meet this requirement: safe removal of the decay heat, maintaining an admissible neutron balance, protection of the barriers for fission product retention in the plant, and radiation protection. This chapter discusses some accidents, which could restrict these requirements. A major aspect is decay heat production and removal. For a modular HTR, the self-acting principle of decay heat removal just uses conduction, radiation, and natural convection. No supply of electricity and water are necessary. Core meltdown or overheating above 1600 °C, corresponding to today available TRISO fuel, are not possible in a suitable designed reactor, if all active decay heat removal has failed. The release of fission products then is limited to small values. An important step of heat transport inside the core occurs mainly by radiation and conduction in case of a depressurized system. If the reactor stays under pressure, natural convection becomes relevant too and the heating up of internal components has to be considered. The outer heat sink consists of a surface cooler, and finally, the decay heat can be stored in concrete. Even if all building structures would be destroyed, the self-acting heat removal works and the maximal fuel temperature stays limited near 1600 °C. Modular HTR has a strong negative temperature and power coefficient, and this characteristic causes a stabilization of the chain reaction even in case of extreme reactivity accidents. As an example, if the total first shutdown system would be lost, the maximal fuel temperature would stay below 1600 °C. Air ingress accidents could cause corrosion of fuel elements and structures, and explosive gas mixture could be formed. These effects are governed by a suitable design of the inner concrete cell, by a filter system, and by the reactor building. The amount of air, which is ingressing into the primary circuit and which is responsible for graphite corrosion, will be limited to tolerable values. The ingress of water into the reactor is limited by a special design of the steam generator and by arranging this component geodetically under the core. In extreme accidents of water ingress, the water can be removed from the primary system in short time. Nuclear power plants today have to be designed against strong impacts from outside. Some important cases are earthquake, tsunami, explosion of gas clouds, tornados, and airplane crash. Some conditions of these events and measures against it are explained in this chapter. Further developments are possible; especially, underground siting could principally improve the safety standards. The failure of large components as example on the secondary side could initiate subsequent damages of the core; therefore, their consequences have to be considered in the licensing process, and measures have to be realized to avoid damages. The release of fission products from the primary system can be possible because of two source terms. On the one hand, radioactive substance, which has been deposited on graphite or metallic surface during operation, can be remobilized and transported out of the primary circuit. The second source term is caused by heating up the fuel elements after a loss of coolant accident. The further release of radioactivity from the plant depends on the velocity of depressurization of the primary system and an effective filtering behind the inner concrete cell. For modular HTR, solutions can be realized with very small release to the environment. The risk caused by the plants normally is expressed by the number of early and late fatalities, by the loss of land due to contamination, and by monetary damage. For a well-designed modular HTR, no serious consequences must be expected.