The main thing in ensuring NPP operational safety is to provide conditions that prevent the release of fission products during a nuclear chain reaction. For this purpose, all the fuel at the NPP shall be loaded into the reactor in the fuel rods - hermetically welded tubes made of zirconium alloy. The main part of the uranium fission products formed during the operation of the reactor remains in these tubes, which are combined in assemblies. When a fuel rod has served its term, it shall be removed from the reactor, and sent to a special plant for processing and extraction of valuable elements. No significant leakage of radioactive substances occurs. This is one of the significant advantages of nuclear energy over other types of power plants, the emissions (waste) of which in the form of ashes, slag and gases in many cases are released into the environment without proper purification.
There are the NPP physical safety barriers provided for prevention of a potential release of radioactive substances - uranium fission products:
- Reactor fuel pellet: holds back the movement of practically all the fission fragments. (The radioactivity under the fuel cladding is 10 000 times less than the radioactivity in the fuel pellet).
- Fuel cladding walls: are made of zirconium alloy, nuclear fuel is placed inside; prevent the release of radioactive fragments of nuclear fission from the fuel pellets into the primary coolant. (The radioactivity of primary coolant is 1000 times less than the radioactivity under the fuel rod).
- Primary circuit equipment: the reactor plant design, reactor vessel, pipelines, steam generators, pumps, primary circuit filters, as well as pressurizers and emergency core coolant system accumulators made of alloy steel.
Containment: is an hermetically sealed, reinforced concrete, pre-stressed shell of a power unit reactor compartment, which hides the reactor and the equipment adjacent to it; it can localize radioactive substances in the event of a maximum design basis accident. The thickness of the reinforced concrete wall of the containment is 1.2 m. On the inner side it has a sealed metal lining with a thickness of 6 mm.
The containment is made of reinforced concrete and is able to withstand:
(a) 7 magnitude earthquake;
(b) explosion shock wave impact.
Additionally, the principle of system redundancy is applied at NPPs, i.e. when one system fails for any reason, a backup system will be switched on immediately, for example, one pump is enough to perform its functions, but two pumps (or more) are installed in case of failure or taking the first one out of service to repair. As well as the equipment diversification, which involves the use of various unstandard systems that perform the same functions, for example, a pump can have an electric or turbo actuator.
The militarized fire departments for the nuclear power plant protection have been established at the NPP. There are also systems that provide internal and external fire extinguishing, as well as a system of stationary installations of water-based fire suppression. In order to maintain permanent readiness, fire training is held to practice the actions of personnel in case of fire.
There are special measures provided for handling fresh and spent fuel, radwaste at the NPP:
Fresh fuel storage and transportation
Fresh fuel is delivered in containers transported in special rail cars or on special platforms covered with a cap. Upon arrival, the fuel enters the fresh fuel unit, which is intended for storage and mandatory inspection of fuel assemblies (FA) before their transportation to the reactor compartment.
Radioactive water treatment
At the NPP, the discharge of sewage water contaminated with radioactive substances is excluded. Such a water shall be treated in special treatment facilities. After passing through the facilities, the treated water shall be sent to the power units for reuse.
Primary circuit gas treatment
Radioactive gases and aerosols are also subjected to special treatment and aging before their discharge into the vent pipe.
The radwaste generated during the NPP operation, according to the content of radionuclides in them, are classified as low-level and high-level radwaste.
The main task in the radwaste treatment is the maximum reduction of their volume. Various technologies have been developed for this purpose:
- solid radwaste shall be compressed;
- liquid radwaste shall be evaporated;
- combustible radwaste shall be incinerated.
Spent fuel treatment
After unloading from the reactor, spent fuel shall be stored for 5 years in the cooling pond, which serves for residual radioactivity reduction and cooling. Then, if there is a spent fuel storage at the NPP, it shall be sent their for temporary storage. If there is no such a storage, spent fuel is either transported to a reprocessing plant or for disposal.
Spent fuel transportation
Spent fuel is transported as part of a separate freight train with a buffer car and an escort car on specially equipped platforms in the containers made of stainless steel with an autonomous protection system.