RBMK What Happened?

RBMK:What Happened?


ReaktorBolshoy Moshchnosti Kanalnyy (RBMK) is one of Russia’s earlygeneration nuclear reactors designs that used water for cooling thecore and using graphite as its moderator. Other designs that rely onwater for cooling have employed different moderators such as othersubstances water, lithium or beryllium. The RBMK has been criticisedfor having basic design flaws in using graphite as a moderator whichcreated a positive loop and resulted in one of the world’s worstdisasters in Chernobyl. The disaster however, points not onlyengineering competence but also to ethical and professional issuesconcerning engineering as a profession.

Thedesign flaws can be traced to the positive output coefficient. Thispertains to the ratio of feedback when the output in MW is added by aunit during lower power operation. It occurs to generate the voidsfrom cooling water when in contact with fuel to raise the reactivitylevel which further adds up reactivity through additional voidgeneration. The fuel temperature coefficient under normal operationswas negative but would be positive at lower level of power output.The minimum power output permitted was 22% or 700mw. Therefore in thecase of emergency shutdown, the reactors would continue to emit 7% oftheir thermal input hence continue to be cooled. At 7% output, thepositive output coefficient ratio would have increased reactivity andthus the need for cooling. The water cooling system relied onexternal power and would also be affected in case of external powerfailure.

Forthe Chernobyl reactors, of the four reactors constructed, each wasfitted with 1600 individual fuel channels served by a coolant flow of28000 per hour. There were installed backup generators to run thewater pumps pumping cooling water but there was an unacceptable 65-75seconds lapse before the generators could start pumping water.Engineers were interested to learn whether the residual rotationalenergy from the steam turbine could be used to generate power forabout 45 seconds to bridge the gap between external power failure andemergency generators. The experiment was slated for April 25th, 1986during a maintenance shutdown of reactor 4. It basically sought toidentify whether the reactor would sustain itself n terms of coolingin case of a combination of emergency shutdown and external powerfailure.

Threeengineers planned for the experiment that was to start on the 25thafternoon but were delayed to the night shift. The experiment whichhad not been approved by the country’s nuclear regulatory body ofthe chief designer of the reactor was headed by the Leonid Toptunov,Alexander Akimov and Deputy Chief engineer Dyatlov. At around 1:00am morning of 26thApril, reactor power was reduced to about 3200 MW followed byswitching of one turbine. By 3:47 am, thermal power stood at 1600MW. By 2:00 am, the emergency core cooling system was disconnected aspart of the test with continued power reduction planned forthroughout the day. An interruption occurred with power restored tofull until the night shift.

Continuedreduction of power targeted 700-1000 MW starting around 11:10 as thetest was resumed. However, power unexpectedly dropped to 30 MW. By1:00 26thApril, power was increased to 200 MW but there was already a build-upof xenon. To compensate for this, control rods were retracted morethat it was allowed. Water flow was reduced and boiling increased. By1:23 am, an attempt for emergency shutdown by inserting the controlsrods triggers an opposite effect with power jumping up rapidly. By1:24 am, a steam explosion followed a chemical explosion destroys thereactors and the building followed by a fire. To contain thesituation, boron carbide, limestone, lead clay and sand are dumped onthe remainder of the site to inhibit further chain reactions.

Thedisaster can be attributed both to the design flaws and human error.The researchers, electrical engineers with not enough experience innuclear physics conducted the test with several safety systems turnedoff. For of all the operators did not know when to stop when thingswere not going as planned. Furthermore, the operators were unaware ofthe effect of xenon poisoning would decrease speedily in case powerlevel increased. In terms of design, the positive coefficient rationmade the reactor unstable and thus unsafe at low output levels. Thecontrol rods made of boron carbide with the lower parts of each rodmade of graphite were partially hollow. Retracting the rods thus didnot serve the intended purpose as the empty space left behind wasfilled with water which acted as a poison and thus defeating thepurpose of retracting the rods. Inserting the rods thus increasedpower instead of reducing. Additionally, the reactor was not wellhoused as it standard. Unit 4 reactor did not have strong concretewalls to contain radiation in case of leakage.

Thislevel of negligence n part of the engineers and their lack ofknowledge the fact that xenon would increase rapidly in case ofincreased power levels contravenes key provisions of the ProfessionalEngineers Ontario Code of Ethics, Section 77 of the O. Reg 941-77. Inpart, it calls for “devotion to high ideals of personal honour andprofessional integrity, knowledge of developments in the area ofprofessional engineering relevant to any services that areundertaken, and competence in the performance of any professionalengineering services that are undertaken” (Ethics, 2014). From thecase, the engineers were basically not well updated on thedevelopments in industry pertinent to their profession hence leadingto negligent actions.

Afterthe accident, the RBMK at Chernobyl continued with operations untilshutdown in December 2000 despite the risks and contamination of thesite. The site continued to pose risks to the engineers and employeesworking at the site and the general public. It is the role ofengineers to protect the public and the environment from any harmfuleffect of engineering activities. For those engineers who continuedworking at the site, their actions not only exposed others to harmbut themselves. By all accounts, this amounts to unethical andunprofessional behaviour.

Currentlythere are about ten RBMK design reactors in operation all located inRussia. The reactors, though having initial design flaws have beenmodified but their safety remains a huge concern for s a number ofexperts who remain worried about the use of graphite as a moderatorand about the positive coefficient loop. However, Russia hascontinued to use these RBMK designs for economic and politicalreasons. For one, the overhauling of the design would be a sign ofweaknesses on Russia as a country. Economically, the country hasinvested billions of money in these facilities and it thus lookingforward to utilize them fully.


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