RBMK Reactor Design

A total of four reactors were built at the Chernobyl site, in what is now the northern edge of Ukraine, a mere 20 km south of the Belarus border. These plants were located on the banks of a man-made lake that had been created by blocking the Pripyat river. The lake provided cooling water for all four of the power plant reactors, and provided a convenient media in which to inject the reactor s waste heat. 

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 

Filburn, S. Bullard, Three Mile Island, Chernobyl and Fukushima, 

The pressure tubes had yet another advantage. They allowed the reactor to use lower enrichment fuel to achieve criticality. The RBMKs operated with fuel that was only enriched to 2% considerably lower than the 3 % enrichment found in the PWR design (Anon., 2016). This resulted in lower fuel costs and significantly reduced fuel production times. 

The pressure tubes were divided into two distinct regions in the core (Fig. 5.1). Each half received its cooling water from a dedicated set of pumps and supply manifolds. Each side also supplied a separate steam drum that connected to the main steam system and turbine. The overall effect of all of this equipment was that it produced a complex myriad of pipes housed within the reactor building. 

---

It may be noted that the RBMK reactor design was chosen by the USSR (despite warnings from the Soviet Academy of Sciences) because it was better suited to available production facilities than the VVER types (which required the manufacture of large pressure vessels). The rapid introduction of RBMK reactors in the USSR made previously used energy resources (oil and gas) available for export to the West, giving a very needed hard currency income. 

The RBMK reactor design had a complex containment system. Varions parts of the plant had containment structures around them designed to prevent the release of steam to the environment. If a part of the plant failed, the steam would be contained and diverted to the suppression pools where the steam would condense and any radioactivity would be trapped in the water. 

The worst sort of organizational failure occurred at Chernobyl (Chapter 15) because of political interference in the former USSR. In a centrally planned state, with top-down direction and little upwards feedback, an unsafe RBMK reactor design was built. The operators perhaps did not know that weaknesses in the reactor design could lead to its instability so, because of time pressures, they over-rode protection systems and disobeyed operating procedures until the reactor was in a dangerous state. Perhaps some sort of need to know secretive attitude meant that the operators did not actually even understand the full extent of the safety problems with the reactor design. 

Figure 5.1 shows a simplified schematic of the RBMK reactor design. An added benefit of the pressure tubes was that they allowed additional flexibiUty in operation that was unavailable with other reactor types. One of the main RBMK advantages was the ability to use onhne refueling. Individual fuel elements could be removed and new elements installed while the reactor remained in use. This allowed the fuel inside flie core to be shuffled about (somewhat similar to the way logs are turned in a fire) and thus provide for greater fuel utilization. Even with this unique capabUity, an RBMK still required a complete shutdown every 12-24 months to complete regular system and component maintenance and to conduct needed repairs (Schmid, 2015). 

Anon., 1993. Russian RBMK Reactor Design Information, Richland, WA Pacific Northwest Lab. Anon., 2010. RBMK Reactors. [Online]. Available at http //www.world-nuclear.org/infonnation-librarv/nuclear-fuel-cvcle/nuclear-power-reactors/aPDendices/rbink-ieactors.aspx. [Accessed... 

Pressurized-tube boiling-water reactor (RBMK) (Russian design) 4 uo. ( 2%) Zr(l% Nb) Graphite H2O 280 300 7 8 1000 18- 19... 

The purpose of this section is to compare the features of the RBMK reactor operated at Chernobyl with reactor types pertinent to the UK. It will be recollected that the RBMK covers a large number of reactors and the comparisons made are indeed with Chernobyl No. 4. The UK reactors covered are in three classes the commercial reactors now built and operated or in commission (Magnox and Advanced Gas-cooled Reactor (AGR)) the prototype Steam Generating Heavy Water Reactor (SGHWR) and Prototype Fast Reactor (PFR) that have comparable performance to commercial reactors and the proposed Pressurised Water Reactor (PWR) or Sizewell B design which, it... 

RBMK. A Russian reactor design which uses a graphite moderator and natural water coolant. 

A confidential safety analysis of an RBMK reactor, similar to the Chernobyl one, was performed some years before the accident by a European design company. It concluded that this reactor, in many respects, did not meet the safety standards in use in the Western world. Copies of this safety analysis were... 

Chernobyl, on the contrary, is an example of what can happen if a completely opposite principle is applied, that to do only what is necessary for safety. In RBMK reactors, like the Chernobyl reactor, the safety margins were not stringent enough. For example, the plant had a containment system for the primary circuit but it was only partial the reactor itself, and in particular the fuel channel heads, were not included in it. The designers thought that it was sufficient only to install protective monitoring instrumentation. Figure 3-4 shows the containment for a typical 900 MWt PWR and the Chernobyl reactor containment. 

Operational limits should be observed during normal operation of RBMKs. The most significant Umits related to the RBMK-1000 and RBMK-1500 reactor designs are listed in Table 3. 

In assessing the source term, it is necessary to take into account the operational experience and specific design features of RBMKs such as on-line refuelling. Long operational experience shows that the reference (initial) activity of the radionuclide 1 in the coolant is normally well below the operational limit. It is obvious, however, that there is no direct relation between the number of leaking fuel rods and the operational limit for the radioactivity. On the basis of operational experience with RBMK reactors, it can be stated that ... 

Transient and accident analyses for RBMK reactors should reflect the peculiarities of the design, such as the use of adequate correlations for critical heat flux, and consideration should be given to hydrodynamic instabilities. 

Russia has produced two main reactor designs, known as VVERs and RBMKs. The former is a type of LWR, whereas the latter are light water-cooled graphite reactors. These reactors predominately exist in Russia, other countries that were part of the former Soviet Union, and Eastern Europe. Both designs use enriched uranium and therefore require UFg conversion. 

By the end of 1994, 92 BWR nuclear power plants with a total electrical capacity of about 79 GWe were in operation in the Western countries and Japan an additional 5 plants with about 5.6 GWe were under construction at this time. Within the borders of the former Soviet Union a particular type of BWR had been built, the so-called RBMK reactor 16 plants of this type with about 17 GWe were operating by the middle of 1993. The characteristic feature of the BWR design - in contrast to the closed, one-phase PWR design - is heat removal from the reactor core by boiling water, i. e. by a mixture of water and steam. As a consequence of this difference in design, the behavior of many radionuclides in the BWR primary system during plant operation differs considerably from that in the primary circuit of a pressurized water reactor. 

Unlike the BWRs of Western design, in which boiling water is both moderator and coolant, the RBMK reactors use graphite as a moderator while the heat is removed from the reactor core by boiling water in separate cooling channels. Be-... 

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. 

---