Nuclear Safety

When the building of the IBR-2 reactor was designed and constructed in 1971, all climatic characteristics of the reactor site such as snow cover weight, force of the wind, rated winter temperature and seismicity of the site were taken into account. External factors, which can affect the safety (failure of the dam that is higher up the Volga river, accidents with automobile, railway or water vehicles transporting explosives, combustible gases or liquids) were considered as well.

 

 

  • Is there a risk of destruction of the reactor by an earthquake?

 

There are no seismically hazardous physico-geological processes and phenomena (karst, landslides, land subsidence, underground mining, etc.) in the area of the site.
According to the analysis of the seismotectonic situation and in accordance with the Temporary Scheme of the general tectonic division of the European part of the Russian Federation for the given site, the seismicity can be taken to be equal to 5 points with a recurrence interval of 100 years and to 6 points with a return period of 10000 years.
The assessment of the stability of the IBR-2 building subjected to a maximum credible earthquake of magnitude 6 with a maximum design earthquake of magnitude 5 has demonstrated that the concrete frame of the biological shield around the reactor does not collapse and moves together with the Earth’s surface as a single whole.

 

  • Can the reactor site be flooded due to the overflowing of the Volga river?

 

The IBR-2 site is on the watershed of the Volga river and the Dubna river and has high-water marks of 119,5÷120,5 m. The Volga is 1.8 km to the north of the site, the Dubna (confluent of the Volga) is 4 km to the south, the Moscow Channel flowing into the Ivankovo Reservoir is 3.5 km to the west. The tail-water of the Ivankovo hydroelectric power plant (IHPP) is backed up by the Uglich hydroelectric power plant located on the Volga river 137 km below the IHPP dam.
The calculations show that the maximum annual water levels of the Volga river at the IBR-2 site might correspond to 119.7 m once in 1000 years and 118.6 m once in 100 years, which will not result in the flooding of the reactor site.

 

  • … and in case of the IHPP dam failure?

 

In case of the IHPP dam failure the watermark near the IBR-2 site may reach +121.0 m. The elevation mark of the center of the reactor core is +127.00 m, the bottom elevation mark of the reactor vessel is +123.27 m, which rules out the possibility of flooding the reactor vessel in case of the IHPP dam failure.

 

In the vicinity of Dubna south-west winds prevail during the greater part of the year. The maximum wind speed occurring every 20 years is 24 km/s.

The city of Dubna is located in a high-risk tornado area, therefore the maximum possible impact of tornados on the reactor building has been calculated. The estimated characteristics of a probable tornado are:

 

Tornado probability, reactor/year 26,444 × 10-7
Design class 3,40
Tornado rotational speed, m/s 90,23
Tornado translational velocity, m/s 22,56
Differential pressure, hPa 99,73

 

For the given wind impact parameters the intensity of tornado load on the IBR-2 reactor building is 1.5 orders of magnitude less than the equivalent calculated load from a shock wave of a technogenic explosion and does not affect the nuclear and radiation safety of the IBR-2 reactor.

 

  • Can a strong external explosion affect the safety of the reactor?

 

Security measures for protection of the site exclude the possibility of bringing explosives to the reactor building and the adjacent territory. The assessment of the consequences of external influences on the building of the IBR-2 reactor includes the calculations of the impact of an external air shock wave formed by the explosion of 300 g of trotyl on the railway located at a distance of 1300 m from the reactor building.
The upper volume of the central block including the reactor hall is almost impermeable to air shock waves, and shock waves cannot lead to radiation accidents and affect the safe operation of the reactor.

 

  • Can tornadoes destroy the reactor building?

 

In the vicinity of Dubna south-west winds prevail during the greater part of the year. The maximum wind speed occurring every 20 years is 24 km/s.
The city of Dubna is located in a high-risk tornado area, therefore the maximum possible impact of tornados on the reactor building has been calculated. The estimated characteristics of a probable tornado are:
For the given wind impact parameters the intensity of tornado load on the IBR-2 reactor building is 1.5 orders of magnitude less than the equivalent calculated load from a shock wave of a technogenic explosion and does not affect the nuclear and radiation safety of the IBR-2 reactor.

 

  • Can severe frosts and intense heat affect the nuclear and radiation safety of the IBR-2 reactor?

 

At the calculated (design) winter temperature of minus 26 °C the temperature variations of ambient air from – 57°С to + 44°С were considered in the calculations of the influence of extreme fluctuations of absolute minima and maxima.
The temperature fluctuation of ambient air in the specified limits has no effect on the load-bearing capacity of the walls of the building.

 

  • Can the nuclear material storage and highly radioactive waste storage facilities of the IBR-2 reactor be hazardous to the environment?

 

The floor of the storage facilities is at the elevation marks of + 6.5 m and + 3.2 m, which excludes both the ingress of groundwater and the flooding of the IBR-2 storage facilities in case of the IHPP dam failure.
The water supply lines of the IBR-2 technological systems located in the reactor hall are at a considerable distance from the temporary spent fuel assembly storage facilities, which eliminates the possibility of ingress of water into the temporary storage facilities.
Even the destruction of the floor slabs of the reactor hall by the impact of external shocks or earthquakes will not lead to the damage of the storage facilities because they are buried in the reinforced concrete structure of the central block and have protective reinforced concrete floors of sufficient strength.

 

  • A movable reflector (MR) is used for periodic modulation of reactivity and generation of power pulses. May an emergency stop of blades of the main reactivity modulator (MRM) and auxiliary reactivity modulator (ARM) present a safety problem?

 

None of the possible failures of the movable reflector (axial displacement of the MRM shaft, failure of the ARM bearing, breakage of the MRM shaft, rupture of the MR membrane, disconnection of the MR drive, desynchronization of rotation of MRM and ARM) could result in nuclear or radiation accidents.
The analysis of 20 possible emergency situations for the IBR-2 reactor provides convincing evidence that all design basis accidents will have no consequences due to the reliable operation of the automatic safety system (ASS).
In addition, in case of a complete electrical power failure at IBR-2 all operating devices of the regulation and safety systems will fall down by gravity. The efficiency of any of the ASS units is enough for bringing the reactor to a subcritical state within the time between pulses (0.2 s).