05.08.2020

VVER vs RBMK

What is the difference вetween VVER and RBMK Reactors


The main advantage of VVER reactors over RBMKs is that they are much safer. This is due to three main reasons:

 

  • In principle, a VVER reactor does not have so-called positive feedbacks, i.e. in the event of loss of coolant and loss of core cooling, the chain reaction of nuclear fuel combustion dies out, and does not accelerate, as in RBMK;
  • The VVER core does not contain a combustible substance (graphite), which in the RBMK core contains about 2 thousand tons;
  • The VVER reactor has a containment that prevents radioactivity from leaving the NPP even if the reactor vessel is destroyed; it is impossible to make a single protective cap for RBMK because of the large branching of the reactor circuit pipes.

 

The most important advantage of the VVER reactor is its great safety, the significance of which was fully realized, unfortunately, only after the Chernobyl disaster, although this was known long before it. And the fact that in the power industry of the same Russia RBMK power units have nevertheless become widespread is explained by the fact that before the commissioning of the Atommash plant in the late 70s, which produces VVER-type reactors, the USSR could produce only one vessel of such a reactor per year (at the Izhora plant). Now Russia produces only advanced VVER reactors. The pace of the Atommash plant has grown significantly and now it can produce from 4 to 8 reactors per year.


However, a few more words must be said in defense of RBMK.
The VVER vessel is of gigantic size, and its manufacture is very laborious. Its dimensions are limited by the achievement of the ultimate strength state, since the mechanical stresses that rupture the housing are proportional to its diameter and internal pressure in it (in this case, it is necessary to take into account the embrittlement of the metal under the action of neutron irradiation). In addition, the dimensions of the VVER vessel are limited by the requirements of rail transportation. All this leads to the fact that for VVER there is a certain limiting power due to the size of the vessel. For example, the maximum power of VVER in the USA has reached 1300 MW and the length of its body is 13.42 m with a diameter of 5.6 m.For RBMK, there are no such problems, since its power can be increased by simply increasing the number of parallel technological channels in the graphite stack (with this, of course, complicates the system for distributing and collecting steam from the technological channels). Increasing the unit power of reactors is very important, since the cost of building a nuclear power plant is very high and exceeds $ 1100 / kW. An increase in the unit power always leads to a decrease in the cost of 1 kW of installed power, since at the same time such elements as MCPs, steam generators (or separator drums), a steam turbine with all its complex economics are enlarged, the unit cost of the automation system, water supply, etc. ... A 1500 MW (e) RBMK reactor has been operating at the Ignalina NPP (Lithuania) for a very long time.

 

An important advantage of the RBMK-type reactor is the possibility of continuous refueling (replacing fuel assemblies) with a refueling machine, with the help of which 3-4 fuel assemblies are replaced daily. The VVER reactor must be shut down annually (with the removal of the upper block and the lid) in order to extract 1/3 of the fuel from the central part of the core, where burnup is faster. Then 1/3 of the fuel is transferred from the middle part of the core to the central part and from the peripheral part to the middle one; fuel assemblies with fresh fuel elements are installed in the vacated peripheral part of the core. However, refueling is combined with scheduled preventive repairs (PM) lasting 20-40 days of other equipment of the power unit. But in any case, the need to reload the fuel annually (now the issue of extending the fuel company to 18 months is being gradually resolved) leads to a decrease in the installed capacity utilization factor (ICUF).

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