RBMK reactors

Other RBMK reactors are still in operation in the former USSR. Some changes improving the prospects of safe operation have been made. 

All RBMK reactors have positive void coefficients which means that increasing the boiling rate increases the steam fraction in the core which increases reactivity causing more steam void which causes more reactivity and so on. Competing factors provide stability, but startup, shutdown and maneuvering below about 600 MWt are unstable, hence, there is a rule prohibiting extended operation below 700 Mwt. 

The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

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. 

In the 1990s there was much concern in Western Europe regarding the presence of Chemobyl-type (RBMK) reactors in the EU and the status of the Ignalina plant became one of the main issues in the Lithuanian accession negotiations (Euro.Lt, 2008)). The fourth protocol of Lithuania s treaty of accession to the European Union states in Article 1 ... 

Within its various specialties, the TRACTEBEL Group is active in about one hundred countries. The presence of TRACTEBEL in the CIS is well known, more specifically in the Russian Federation in the engineering studies and backfitting for safety and reliability (EU-TACIS-BERD) for VVER-1000 and RMBK and a reactor simulator for the Beloyark reactor. In Ukraine, TRACTEBEL made feasibility studies for a nuclear power plant. The company developed a simulator for an RBMK reactor in Lithuania and is very active in the construction and operation of electrical power and heat generation plants in Kazakhstan. 

The first RBMK-type reactor started up at Obninsk in 1954—two years before Calder Hall. It is still operating. There are plans for 70000 MW of new nuclear plant to be commissioned between 1984 and 1994 but how that target will be affected by the decision to build no more RBMK reactors is not clear. 

RBMK reactors of the same physical size as Chernobyl, but with improved heat transfer surface on the fuel cans, are operating at 1500 MW(e) at Ignalis in Lithuania. This was the standard for new RBMK plants. The RBMK type of reactor supplied 70% of the nuclear electricity in 1985, the balance being supplied by the Soviet type of PWR (WER). There have been problems in meeting the target production of heavy pressure vessels for these PWR reactors. 

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 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... 

The RBMK reactor is peculiar compared to UK thermal reactors in being built with a wider than optimum spacing of the fuel elements, the lattice pitch . This leads to a degree of over-moderation and a consequent effect upon loss of moderator or coolant, described in more detail in the following sub-section. 

A second IAEA visit to the Soviet Union, including Chernobyl, was made in January 1987. This followed up agreements reached in the immediate aftermath of the Chernobyl accident and reviewed the upgrading of safety provisions at the 13 operational and 8 RBMK reactors under construction. Proposals for implementing long term studies of radiation-affected individuals were also considered. 

The RBMK system was developed from this. It has the basic advantages of large scale manufacture without the need for advanced fabrication facilities and, without major development, expansion to large outputs. Currently 1500 MW(e) RBMK reactors are in operation and under construction using a reactor the same size as the Chernobyl reactors which have outputs of 1000 MW(e). 

A minimum steady operating load of a quarter of full power was laid down for RBMK reactors. The operators of the reactor setting up the plant for the turbo-alternator test operated the reactor at a thermal power as low as 30 MW(th), i.e. 1% of full power. By withdrawing more control rods, the operators managed to increase power to 200 MW(th), about 7% of full power but still only 30% of the specified minimum. 

Among other decisions taken in the Soviet Union, several appear important. Improvements will be made in respect of the safety of existing RBMK reactors. 

In addition, operator training is being overhauled and simulators will be more widely used. There will be increased security of the reactor automatic protection against unauthorised interference by operators and improved administrative arrangements to ensure that rules and regulations will be complied with. Research and development of the reactor physics and fuel element cooling of RBMK reactors will be increased. 

Finally, the Working Group discussed the question could such an accident occur in the United Kingdom At a trivial level perhaps the answer is no , because the UK has no RBMK reactors. But at a deeper level the Group asks ... 

Chernobyl. Site of the Russian RBMK reactor which exploded in April 1986. 

Obninsk. Site of the first Russian RBMK reactor. 

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