RBMK, nuclear reactor

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. 

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. 

The fuel used in most nuclear reactors consists of uranium dioxide (UO2), a ceramic material. This is in the form of small pellets (less than an inch in diameter) contained within metal tubes, usually of an alloy of zirconium (as in the RBMK) or of stainless steel. These tubes are collected into bundles (fuel elements) and, in the RBMK, are inserted into the pressure tubes to form the core. The vast majority of the radioactive material produced by the fission process is held by the ceramic material itself and what little does escape from the ceramic matrix is retained by the metal tube surrounding the pellets. Fission products can only be released from the fuel elements if these overheat. This is a three-stage process ... 

A very low power condition might appear trivial in a normal machine if the power decreases too much, it is made to rise again by the dedicated controls but in a nuclear reactor and especially in a RBMK, this is not so. Besides the reluctance of any reactor to increase power after a reduction, due to some isotopes which slow the chain reaction down and which are produced precisely in these transients, in an RBMK at low power the steam production in the channels stops and they fill up with water. As described earlier, the nuclear power level tends to decrease even more (the typical instabUity of RBMKs). 

Abstract The chapter is devoted to the practical application of the fission process, mainly in nuclear reactors. After a historical discussion covering the natural reactors at Oklo and the first attempts to build artificial reactors, the fimdamental principles of chain reactions are discussed. In this context chain reactions with fast and thermal neutrons are covered as well as the process of neutron moderation. Criticality concepts (fission factor 77, criticality factor k) are discussed as well as reactor kinetics and the role of delayed neutrons. Examples of specific nuclear reactor types are presented briefly research reactors (TRIGA and ILL High Flux Reactor), and some reactor types used to drive nuclear power stations (pressurized water reactor [PWR], boiling water reactor [BWR], Reaktor Bolshoi Moshchnosti Kanalny [RBMK], fast breeder reactor [FBR]). The new concept of the accelerator-driven systems (ADS) is presented. The principle of fission weapons is outlined. Finally, the nuclear fuel cycle is briefly covered from mining, chemical isolation of the fuel and preparation of the fuel elements to reprocessing the spent fuel and conditioning for deposit in a final repository. 

Although relevant exercises have been conducted, and Cold War nuclear weapons programs provide validated analytic platforms, there have been no actual post-det terrorist incidents involving an IND or RDD to date. Consequently, no technical investigations in the contemporary embodiment of nuclear forensic analysis exist for an actual post-det situation, and all discussed case studies necessarily focus on interdicted, pre-det materials. (However, a nuclear accident that is perhaps exemplary of maximum-credible consequences of successful terrorist activities was the uncontrolled criticality and resultant explosion of the Soviet RBMK power reactor at Chernobyl in 1986.)... 

All three designs (BWR, PWR, and RBMK) rely on control rods to provide a safe shutdown of the nuclear reactor during emergency situations. Both the PWR and RBMK use control rods inserted from the top of the reactor, which also allows for gravity to help the lowering of the rods in the event of a power loss. This is not possible in BWR reactors, however, because the top areas of the reactor vessels are filled with steam separation equipment. Instead, BWRs use cruciform-shaped control rods energized by hydraulic pressure, which are pushed between the fuel assemblies and up into the reactor from the bottom (Fig. 1.6). 

Ukraine has five nuclear power stations with fifteen reactors with a total power output of 13.6 thousand MW (13 reactors of WWR type and 2 reactors of RBMK type in the Chernobyl NFS). In addition there are 47 thermal power stations with a total power output of 32.4 thousand MW, 6 large hydraulic power stations on the Dnieper and 55 small stations on other rivers. 

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. 

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. 

At the time of the accident, the nuclear power station at Chernobyl comprised four operating units and two under construction. Each unit is made up of one reactor of the RBMK-1000 type and two turbine-generators. The two units 3 and 4 are accommodated in one block as shown in Fig. 2.1. The two reactors are separated by a compartment housing common services. Alongside is the turbine hall with the four turbines in line. The blocks accommodating units 1, 2, 3 and 4 are adjacent (so that all eight turbines are in line). The block for units 5 and 6, not now to be completed, is sited 1.5 km to the south-east. 

Nuclear power plants were also cancelled or postponed in countries such as Yugoslavia and the Philippines. However, in countries outside Europe, in particular the USA and Japan, Chernobyl had comparatively little impact on the context of nuclear power planning. The design differences between the Soviet RBMK and the reactor types deployed and planned in these countries seem to have been sufficient to allay public opinion to the extent that governments did not feel under pressure to review their existing programmes. A full analysis of the possible impact of... 

The water inventory decreases when more steam is produced in the reactor. In fact, the steam bubbles produced expel the liquid water from the reactor. This is what happens in a boiling kettle which, if initially overfilled, as boiling starts, causes the water to be spilled out. If this kettle is heated on a gas cooker, the water spilled extinguishes the flame and, if the cooker is provided with an automatic gas supply stop, everything terminates without consequences. But this is not so in a RBMK because, as we have just mentioned, when the production of steam bubbles increases, the nuclear power (the heat produced by the cooker in the example) tends to increase instead of decrease. 

This Safety Report on safety analysis for nuclear power plants with RBMK reactors has been developed taking into account Russian national regulations [4-6], experience gained with safety analysis reports for RBMKs and international reviews of these reports. 

FEDERAL NUCLEAR AND RADIATION SAFETY AUTHORITY OF THE RUSSIAN FEDERATION, Recommendations on In-depth Safety Assessment of Operating Nuclear Power Units with WWER and RBMK Reactors (OUOB AS), Rep. RB-001-97 (RB G-12-42-97), Moscow (1997) (in Russian). 

Accident Analysis for Nuclear Power Plants with Graphite Moderated Boiiing Water RBMK Reactors... 

ACCIDENT ANALYSIS FOR NUCLEAR POWER PLANTS WITH GRAPHITE MODERATED BOILING WATER RBMK REACTORS... 

Accident analysis for nuclear power plants with graphite moderated boiling water RBMK reactors. — Vienna International Atomic Energy Agency, 2005. 

Consistent with these publications, the IAEA in 2002 issued a detailed report on Accident Analysis for Nuclear Power Plants (Safety Reports Series No. 23) that provides practical guidance for performing accident analysis. That report covers the steps required for accident analyses, i.e. selection of initiating events and acceptance criteria, selection of computer codes and modelling assumptions, preparation of input data and presentation of the calculation results. It also discusses aspects that need to be considered to ensure that the final accident analysis is of acceptable quality. Separate IAEA Safety Reports deal with specific features of individual reactor types, such as pressurized water reactors, boiling water reactors, pressurized heavy water reactors and RBMKs. 

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