Nuclear power stations use controlled chain reactions, where the uranium nucleus absorbs slow-moving neutrons and thus fissures into fast-moving lighter elements and additional neutrons. The energy from the reaction heats the water, the water transforms into steam, which then goes into a turbine generator and thus electricity is produced (you can see the info-graphics here, or read it on wikipedia). The fission material used in reactors comes in form of pellets, that are put in the so-called rods. Those are then assembled into bundles. In case of a reload, the processed fuel is taken from the reactor, and new bundles are loaded.
As of now, two types of reactor systems are used in Russia: High Power Channel Reactor (RBMK) and Pressurized Water Reactor (PWR). The technological process of fuel change in these reactors differs considerably. In PWR reactors, the bundles go from the center to the sides of the reactor, and the used bundles are then removed, put into protective containers and taken for storage. Then, new bundles are loaded. This process takes up to several days, the reactor stops for it, so it's much safer than the one used in RBMK reactors, where the so-called hot fuel change is used — the fuel change is conducted while the reactor is working, using a special refueling machine. To make the process safe, all the control, supervision and safety systems have to work effectively. As there are a lot of them, it is hard to define which of them work well and which — not, so safety can become an issue.
Schematics of a reactor's operation Source: depositphotos.com
"A popular method of safety analysis is the logical-and-probabilistic method. It is really good for systems with a complex structure, also, it is widely used when it's hard to get the statistics of failures — as in nuclear power plants. Still, this method is relatively subjective, so we've decided to combine it with the HAZOP method that's more precise at defining the accident initiators", comments Valeria Popova.
For her graduation thesis, she has developed a new method.
As of now, accident initiators are usually defined by a group of experts. Thus, the subjective approach that may lead to risks is used since the very start of risk assessment. Yet, the HAZOP method allows to define the accident initiators using a formalized procedure,
and is really effective in working with large amounts of technical properties: all operations are split into "sub-operations", and for each of those critical conditions are defined. For instance, the pressure level — it can be high, low, normal, etc. Then, all such data is assembled into charts, which makes analyzing different situations easier. Thus, it becomes clear which of them can cause critical changes of this or that parameter.
ITMO University. Valeria Popova
As a result, this approach allows to define the accident initiators without using the subjective analysis by an expert group. Also, the method is multipurpose, and can be used for any systems with complex structure.
"After conducting the calculations, we get the quantitative probability of an accident initiator happening. If we aren't fine with the result, we take the necessary measures for improving the safety system. The risks that are related to nuclear power plants can be of any kind, but as we don't have the necessary statistics for failures, we have to formalize the procedure of defining the accident initiators — and that would allow us to considerably increase their objectivity", concludes Valeria Popova.
