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About the safety of Soviet-designed nuclear reactor types

Nuclear Monitor Issue: 
#493-494
Special: Agenda 2000: Will it increase nuclear safety in Eastern Europe?
19/06/1998
Article

(June 19, 1998) All Soviet-designed reactors present a serious and imminent threat to environmental stability as well as to human health. After more than a decade of talking about upgrading the unsafe Soviet-designed reactors, most agreements and money are spent in "upgrading the safety culture", and not in improving the design deficiencies. Improving design faults is, of course, almost impossible, or at least extremely expensive. The decade-long discussion concentrate, however, mainly on the issue of technical improvement. And more important is that the problem of adequate safety culture in Eastern countries cannot be solved by technical measures: safety culture cannot be imported or purchased.

 

RBMK


The RBMK is a graphite-moderated, water-cooled, boiling water reactor. The nominal pressure and temperature operating conditions are similar to those of most BWRs operated in Western countries. Although a number of improvements have been introduced since the Chernobyl accident, "many nuclear experts think", according to the NSA/EBRD, "that these reactors cannot be improved to standards acceptable for long-term operation".

Major design flaws of the RBMK are:

  • Positive reactivity feedback: as the reactor operates at lower power levels, the fission reaction accelerates. Therefore, in the event of low-power operation, even the act of shutting down increases the risk of a serious accident;
  • The flammability of the graphite moderating agents intensifies the risk of a catastrophe from either a common-cause failure or an emergency;
  • The reactors exhibit positive void coefficient, this means that if the coolant evaporates, the reactivity will increase. This can lead to a run-away chain reaction;
  • The reactors have no containment vessels to lessen the impact of radiation leaks, or protect the surroundings in the event of a core meltdown;
  • The fuel channels critical to the safety of the reactor have a history of rupture;
  • In general, significant differences between the different generations of RBMK reactors and even significant differences among reactors within the same generation have been identified. Therefore, it is essential that plant-specific safety studies be performed for each station in order to get an accurate assessment of the safety level and also the effectiveness of the modifications.

VVER


There are three generations of VVER reactors. VVERs are Pressurised Water Reactors (PWRs), similar in their basic design to the Western PWRs. The oldest type (developed in the 1960s) is the VVER440/230 which, according to NSA/EBRD, "should not be kept in operation in the long term".

Main safety deficiencies of the 440/230:

  • No containment, this increases radiation-release risks. Adding to this deficiency, the absence of a pressure relief system impairs the plant's ability to respond to an emergency, while the reactor vessel's embrittlement, present at almost every site (East and West), increases the risk of a radiation release.
  • In fact, at some of the older reactors at Kozloduy and Bohunice, the reactor vessels have a high copper content, which accelerates their embrittlement further;
  • Insufficient emergency core cooling capability limits the reactor's ability to head off a core meltdown;
  • Almost no redundancy and separation of safety equipment. This hampers the reactor's ability to respond to day-to-day abnormalities and increase the chance of common-cause failures. The very design of safety systems ignores the possibility of common-cause failures. These oversights are particularly dangerous because components of the 230 have been historically unreliable;
  • Deficient instrumentation and control system;
  • Serious deficiencies in fire protection

Safety deficiencies of the VVER440/213:
The next generation VVERs (the 440/213, designed between 1970 and 1980) corrects some safety deficiencies of the 230, but still has some imported flaws left:

  • Although an improvement over the 230, the core-cooling system is still between 10 to 50 times less reliable than contemporary Western systems;
  • No containment system. Therefore, there is a much higher risk of radiation release;
  • Same problems as the 230 with embrittlement, redundancy, fire protection and substandard construction techniques and materials.

The third generation (developed between 1975 and 1985) is the VVER 1000:

  • The reactor pressure vessel is too small in relation to the power output of the reactor. This leads to an increase in neutron bombardment of the reactor wall and consequent embrittlement;
  • The layout of the plant reduces the ability to inspect and replace key components and increases the fire risk;
  • The instrument and control technology is not of a sufficient standard.

Sources

  • Chernobyl and the safety of nuclear reactors; OECD/NEA 1987
  • Russian roulette; Friends of the Earth, 1992
  • The Nuclear Safety Account; EBRD, December 1994
  • International Assistance to Upgrade the Safety of Soviet-designed Nuclear Power Plants; IAEA, December 1993
  • Shutdown!; Greenpeace International, June 1993
  • RBMK-report 1996; Koller/Donderer, Greenpeace, March 1996.