Nuclear safety increased to an even higher level

Nuclear safety increased to an even higher level
Slovenské elektrárne started working on a new project for coping with severe accidents back in 2005 – 2006. It was completed in 2015.

Safety, and especially nuclear safety comes first at Slovenské elektrárne and, without exception, takes precedence over achieving electricity production targets. The largest power generating company in Slovakia takes a very conservative approach in nuclear power plant management. Constantly looking for improvements is the approach that moves it forward. Based on these factors, as well as best international practice and experience, Slovenské elektrárne at the turn of the millennium began seriously considering measures that would ensure even better protection against potential incidents previously disregarded when designing power plants many years earlier.

Although the likelihood of a severe accident is very small, it most certainly must not be underestimated. A severe accident at a nuclear power plant is the state when there is a meltdown of the nuclear fuel in the reactor due to overheating of the fuel elements because of insufficient reactor cooling. In the history of commercial nuclear units there have only been three such cases, with each of these events having then been investigated down to the smallest detail, and the results transmitted directly to the nuclear industry. The industry immediately responded with new measures to prevent any such similar event occurring elsewhere in the world.

The Three-Mile Island incident in Pennsylvania (USA) in 1979 is considered to have been the first severe accident in history. It occurred following a leakage of coolant through an open valve, resulting in a partial meltdown of fuel in the reactor. The accident triggered a national debate regarding the safety of nuclear power, and led to the introduction of symptom-oriented operating rules, and a new perspective on emergency planning.

The second severe accident occurred at Chernobyl on 26 April 1986 – the worst in the history of nuclear power, evaluated at level 7 of the INES scale. During an experiment by operators at the power plant, the reactor overheated and exploded, releasing a large amount of radioactive material into the air. It raised fears about the safety of the Soviet nuclear industry, slowing its expansion for many years. In practice it meant a new set of instructions from the International Atomic Energy Agency for the management of operational safety at nuclear power plants, and the improvement of reactor protection systems, so that they are resistant also to operator errors.

The third event was the Fukushima power plant in Japan, devastated by tsunamis. The accident led to the largest ever series of stress tests at nuclear power plants, testing against extreme combinations of adverse conditions.

The International Nuclear and Radiological Event Scale

Slovenské elektrárne began preparing a project for coping with severe accidents back in 2005 – 2006. The first units where severe accident projects began to be introduced were the V2 blocks at Bohunice NPP, and all measures were completed two years ago. Design preparation for projects on units 1 and 2 of Mochovce NPP began in 2009, with their implementation beginning a year later. These measures were already incorporated into the initial design for units 3 and 4 at Mochovce NPP, and are being implemented during the course of the new units’ construction.

  • The first project in the framework of the measures is the cooling of the reactor’s pressure vessel from the outside of the shell, i.e. the supply of the necessary amount of coolant (water) into the area under the reactor, where coolant circulates by gravity, and thus cooling the reactor’s vessel from the outside.
  • The second one is the system for rapidly depressurising the primary circuit. Its purpose is to depressurise, with the aid of two valves, in a short period the primary circuit, from the nominal operating pressure of 12.26 MPa (121 atmospheres) to less than 2 MPa (20 atm.).
  • The third project is the hydrogen management in the containment, being a direct reaction to the Fukushima event. The system consists of passive hydrogen recombiners, which combine hydrogen with oxygen to water, or water vapour.
  • The fourth project is a system for preventing the creation of negative pressure in the containment due to steam condensation.

New electric engine of the shower system pump, with parameters meeting the severe accident requirements.

New electric engine of the shower system pump, with parameters meeting the severe accident management requirements.

The long-term heat removal from the containment vessel consists in a project serving for removing heat accumulated in the facilities, primarily in the walls of the containment vessel. Its other task is also to add coolant to the spent fuel storage pool. The emergency source of coolant consists in a project identical for both the units and is connectable with the same system for the 3rd and 4th unit. Its role in the case of a severe accident is to deliver enough coolant into the reactor, the spent fuel storage pool and the shower system. Another system that can power all the systems of both the units is the emergency source of electricity – a separate seismic-proof diesel-generator station.

The last within this programme is the management and information system for a severe accident. It is separate for each unit and has offices in each unit control room and both have their "twin" of the control workplace at the Emergency Control Center. The system consists of a number of new measurements (hydrogen concentration, temperature, radiation measurement in the containment vessel, etc.).

All the projects, from the technological aspect, were completed for the Mochovce units 1 and 2 at the end 2015. The project for severe accidents is the largest completed project at the Mochovce units since their commissioning in 1998 and 2000. The completion has fulfilled a condition set by the Slovak Nuclear Regulatory Authority.

Source: E_atom | February 2016