RBMK design

Some technical data of this reactor type are summarized in Table 2.2. As can be seen from Fig. 2.4., the 1000 MW plants have a cylindrical reactor core of about 7 m in height and about 12 m in diameter. The moderator block is made of graphite bricks which are penetrated by about 1700 vertical cooling channels into which the fuel assemblies are inserted. For transport of the heat generated in the fuel, water 

The fuel assemblies in the reactor core represent a total core fuel mass of about 190 Mg of slightly enriched uranium (about 1.8% The fuel is designed to 

A reactor core of such a large volume has the disadvantage that load and load distribution in the reactor are difTicult to control. Another problem of this reactor type is the positive void coefficient of reactivity which may occur under certain circumstances. With increasing concentration of steam bubbles in the coolant, the absorption of neutrons in the coolant decreases while moderation of the fast neutrons by the graphite moderator remains unchanged. For this reason, an increase in reactor power is not limited by inherent physical properties of the reactor core and, therefore, the reactor load has to be controlled by extensive active measures, i. e. by complicated instrumentation. 

In order to protect the graphite moderator, which has an operating temperature of 500 to 700 °C, against corrosive attack by air, the reactor core is surrounded by a steel liner in which an inert atmosphere under slight overpressure is maintained. 

The RBMK-1000 plants are equipped with safety systems which are designed to cope with loss-of-coolant and station blackout accidents. These are, in particular, an emergency core cooling system, an emergency feedwater system, a system for condensation chamber cooling and the emergency power supply. The emer- 

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However, serious problems have been reported both in organising supplies of equipment and in maintaining the high standards of construction necessary for the safe operation of nuclear plant. A British assessment of RBMK designs in 1976 was critical about a number of design features. 

The RBMK design had a number of recognised weaknesses (the possibility of a rapidly acting positive power coefficient and the slow response of the protection system if the control rods were withdrawn too far) which were not protected against other than by administrative rules. 

In summary, the RBMK design has some economic and strategic advantages, but these are offset by the shortcoming in design which in 1986 destroyed reactor number 4 of the Chernobyl power station. 

For RBMKs designed and licensed before the main safety criteria and requirements came into force in 1993, the radiation criterion is absolute in the sense that the consequences of a DBA (in terms of radioactive releases and discharges to the environment) should never result in such a population exposure as to require emergency protective actions in the early phase of an accident (i.e. for about ten days after the accident). 

In addition to the weaknesses in the RBMK design described above, the detailed implementation of the design also allowed greater freedom of action for the operators than would be normal in a reactor design elsewhere. For example, the operators could override reactor trip systems at the flick of a switch in Western designs, key interlock systems would have prevented this. Also it was essential for the safe operation of the plant that the control rods should never be withdrawn beyond the point at which the control rod reactivity margin became dangerously low, yet this vital aspect was left entirely to the operators, with no automatic trip system. 

The RBMK design violated at least three of the four generic technical safety requirements described previously. The reactor power was unstable, the reactor shutdown systems were slow and inadequate, and the containment systems were insufficiently robust. 

In the case of Chernobyl, the nature of the government in the Soviet Union meant that independent oversight was not really viable the requirements of central planning seemed to take precedence. The RBMK design was unsafe by international standards. 

The Russian Graphite Moderated Channel Tube Reactor, NPC(R) 1275, Nuclear Power Company Ltd, March 1976.(This report does not appear to be available on-line. It gave an early Western view of the RBMK design and noted some of its major weaknesses.). 

Russia continues to operate both the RBMK and VVER type reactors within its borders, and several former members of the Soviet Union also operate these reactor types. Of the over 25 GigaWatts of nuclear power plant capability in Russia in 2015, nearly 50 % was still the RBMK design. The remaining production capacity is dominated by the VVER style. 

All four Chernobyl reactors shared the same Soviet design, the Reaktor Bolshoy Moshchnosty Kanalny (RBMK), high-power channel reactor (Anon., 2010). The RBMK was a uniquely soviet design that evolved from the early Russian graphite moderated reactors used for the Soviet Union s production of plutonium for then-weapons program. As a result, the RBMK design included several features that made it distinctly different from the commercial reactors developed in the West. One major... 

Unfortunately, due to a fundamental design difference, the RBMK design was inherently less safe than Western reactors. All Western reactors are required to have a negative core power feedback coefficient. This means fliat if an unexpected error occurs in core functioning, all automatic feedback mechanisms will tend to reduce core power and deescalate the situation. Thus, any unexpected occurrence will tend to decrease the chances of an accident happening. The RBMK reactor, however, had a positive feedback coefficient under certain circumstances. 

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