Failure Knowledge Database / 100 Selected Cases

Chernobyl Accident
April 26, 1986 at the Chernobyl nuclear power plant, Pripyat, Ukraine (then part of the SovietUnion)
Masayuki Nakao (Institute of Engineering Innovation, School of Engineering, The University of Tokyo)
The Cher nobyl station was testing the Uni t 4 reactor’s turbine g enerator to det ermine the t urbine p ower
generation in the rundown phase (Figure 1). Flaws in the reactor design that it could easily cause nuclear burst
and in adequate oper ators’ j udgment caused t he p ower of th e r eactor to in crease r apidly dur ing the t est. Thefollowing e xplosion b lew u p the r eactor bu ilding. T he death to ll reached 31 as o f the end of July, 1987, an d
135,000 people living within a 3 0 km radius of the plant were evacuated. The accident released a significant
amount of radioact ivity t hroughout th e world conta minating a gricultural produ ce i ncluding milk, meat an dvegetables. Hu ndreds of thyroid can cer ca ses an d deaths hav e been repo rted, however, th e situat ion h as n ot
been very clear.

(New York Times, April 30, 1986)
Figure 1. Location of the Chernobyl Nuclear Power Plant [2]1. EventThe Chernobyl station was testing the Unit 4 turb ine generator to d etermine the turbine power generation
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Failure Knowledge Database / 100 Selected Casesin the rundown phase. The power of the reactor increased rapidly during the test. A large steam explosion
destroyed the reactor building. The death toll reached 31 as of the end of July in 1987, and 135,000 people
living wi thin a 30 km radius of th e plan t were e vacuated. The accident r eleased a significant a mount o fradioactivity worldwide, contaminating agricultural produce inc luding milk , meat a nd ve getables. Theaccident r eleased a significa nt am ount of ra dioactivity t hroughout th e wo rld contaminating ag ricultural
produce i ncluding milk, meat an d veg etables. Hu ndreds o f thyroid cancer cases a nd deaths h ave beenreported, however, the situation has not been very clear.
2. Course
The r eactor at Cher nobyl is a RB MK ( reaktor bolshoy moshchnosti kanalniy, w hich means in En glish“reactor of l arge power of the c hannel t ype”) r eactor th at uses l ight wa ter for co oling a nd gr aphite formoderation. As Figure 2 shows, an RBMK employs long (7 meter) vertical pressure tubes running throughgraphite m oderator. The se 1,6 93 pressure tubes (80 mm ca liber, 88 mm ou tside diameter) are made ofzirconium alloy and called channels. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5
meters l ong. The water pumps i nject coolant water ( 270 degrees C, 70 millibars) into the t ube from its
bottom at a r ate of 1.2 m/ sec. The c oolant water, e vaporating partially, r emoves he at from the f uel
assemblies as it passes t hrough th e core. The steam-water mixture t hen continues to the lar ge separator
drums in w hich the wat er s ettles. Th e st eam proceeds to t he turbines, ge nerating power. Temperature ofgraphite reaches 600 decrees C during normal operation.
The aim of the t est was to determine t he ab ility of th e r eactor’s t urbine gen erator to generate su fficientelectricity to power the reactor’s Emergency Core Cooling System (in particular, the water pumps) in theevent of a loss of external electric power.

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Failure Knowledge Database / 100 Selected Cases

Figure 2. The RBMK Graphite-Moderated Reactor and Heat Cycle [1]The experiment started after connecting the main circulation pumps and the turbines (1) and dea ctivating
ECCS so that it would not respond to the reactor’s low-power output during the experiment (2). The reactor
operators reduced the power level too rapidly, close to the maximum scale of the power drop allowed bysafety regulations. Then, the con trol r ods were p ulled out of the reactor somewhat farther than normallyallowed by safety regulations to raise the power output (3). Then there was hardly any margin left to safely
control the reaction in the r eactor. Nevertheless, the operators shutdown the steam supply to the turbine tocontinue the test(4), and the turbines s lowed down to coast ( 5). The r ecirculation pu mps that w ere
receiving power from the generator then slowed down and the cooling water flow dropped (6). The reactor
then gained reactivity and the power started to rise (7). Automatic control rods were inserted in an attempt
to lower the power, however, their slow spe ed fai led to su ppress t he reactivity rise ( 8). The operat ors
further ordered full insertion of emergency control rods (9), however, inserting these r ods chased out thecooling wat er and the fir st 6 seconds r ather boos ted t he r eactivity. Soon a nuc lear bur st st arted f urther
raising th e fuel te mperature and an they r uptured ( 10). I t was only 40 se conds a fter the experiment had
started. Sudden steam generation stopped the co olant water c irculation and the fuel t emperature reached3,000 – 4,000 degrees C. By that time the enti re core coolant was boiling and the steam pressure rapidlyincreased ( 11). The pressure tub es r uptured to let the coolant water to gush ou t (12) which then came incontact with th e graphite moderators and a s team explosion occurred. The explosive fo rce blew of f t hereactor lid and the upper part of the reactor and the reactor building was destroyed (13). A second explosion
threw o ut frag ments of burning fu el an d graphite fr om the core and all owed ai r to r ush in, caus ing th e
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Failure Knowledge Database / 100 Selected Cases
graphite moderator to burst into flames (14).

1. Preparation of the experiment

2. Experiment started
Figure 3. Chernobyl Accident: Sequence of Events 1 [1]4Failure Knowledge Database / 100 Selected Cases

3. Experiment: 20 seconds

4. Experiment: 36 seconds
Figure 4. Chernobyl Accident: Sequence of Events 2 [1]5Failure Knowledge Database / 100 Selected Cases

5. Experiment: 40 seconds

6. Experiment: 43 seconds
Figure 5. Chernobyl Accident: Sequence of Events 3 [1]6Failure Knowledge Database / 100 Selected Cases

7. Experiment: 44 seconds

8. Experiment: 60 seconds
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Figure 6. Chernobyl Accident: Sequence of Events 4 [1]3. Cause(1) Reactor without self controlability
This nuclear reactor was designed to have a positive output coefficient (ratio of the output to feedback
when the output is added by a unit) during lower power operation. A positive output coefficient works
to generate voids from the cooling water when it touches the fuel to raise the reactivity which thenfurther increases the void generation to add reactivity. This was a basic design that can easily cause areactivity accident (a runaway reaction). They avoided reactivity accidents with an operation rule todisallow operations at low power.
(2) Control rod design flaw
In a nuclear reactor, control rods are inserted into the reactor to slow down the reaction. However, inthe RBMK reactor design, inserting the emergency control rods increased the reactor’s power output
because the partially hollow control rod extenders displaced coolant water.
(3) Reactor operators uninformed of design flaws
The Ministry of Machine Building, responsible for designing and building the reactors, did not inform
details on the reactor to the Ministry of Atomic Energy that was responsible for operating reactors. The
management and the operators were not informed about the reactor’s unstableness and danger duringlow-power operation.
(4) Un reliable instrumentation
The pressure tubes wer e made o f 4mm thin pipes. They wer e not stron g enough to hold again stpressure in crease in abnormal circumstances. The reactor sch eme had p otential danger o f a stea m
explosion c aused by the e xtremely high temperature of graphite mo derator and coolant water le aked
from ruptured pressure tubes. The reactor core had neither a steel pr essure vessel nor containment forpressure-proof. The reactor core had many pressure tubes running through vertically. The containment
structure w as relatively di fficult to des ign b ecause it must allow fuelling. A lso, i n or der to allow
fuelling, the reactor building was tall. The reactor building did not have a robust structure – th e walls
had only concrete slabs on trussed structure.
4. Immediate Action
To manage the immediate crisis, from April 28 to May 2, helicopters dumped boron, lead, clay, sand, anddolomite ( 5,000 to ns in t otal) to smother th e f ire and co ver the reactor. The government se nt in 30 0,000
workers to close up the reactor. Despite their efforts, a large amount of radioactivity spread over the world.
The worst contamination was reported in the area 300k m no rth of Ch ernobyl station. The area within a
30km radius from the reactor was evacuated where it is still closed. There is evidence of leukemia due toradiation exposure from Chernobyl among people evacuated from the contaminated area and the workers
involved in the recovery and cleanup after t he accident. Some illness-related suicides are also reported. A8

Failure Knowledge Database / 100 Selected Cases
risk of genetic disorder due to radiation exposure from Chernobyl may become evident in future.
5. Countermeasure
The accident investigation committee of the Soviet Union reported that the accident was attributed to flaws
in the RBM K reacto r desig n and negligence of the power plant o perators. Ho wever, anticipating publicconfusion and distrust, Politburo a placed the blame solely on the power plant operators and the Ministry ofAtomic E nergy, w hich w as r esponsible fo r op erating r eactors. Being a fraid of s tirring up international
opinion against nuclear power generation in the Western countries at that time, the Soviet Union concealed
the reactor flaws from The International Atomic Energy Agency (IAEA) at its General Conference.
In 1991, 5 years after the accident, the accident evaluation committee published its report and attributed theaccident to flaws in the RBMK

reactor design. Th e IAEA also ackn owledged in 1992 that th e RB MK

reactor had design flaws. There is an ongoing investigation and study at the site about the actual sequenceof events and circumstances.
6. Summary
A flawed reactor design op erated by inadequately trained p ersonnel and without proper regard for s afety
resulted in catastrophic steam explosions. Governmental organizations concealed crucial information about
the r eactor t o evad e their responsibility and pl aced the b lame s olely on the p ower plan t o perators. It
delayed determination of the true cause including the reactor’s design flaws. Since the Chernobyl accident,
remaining RBMKs have been operated with some improvements, considerably enhancing their safety.
7. Knowledge(1) Unstable mechanism increases a risk of a devastating incident in which a small incident often leads toa deadly one. In particular, flaws in a large-scale system may result in a serious accident.
(2) Designers must inform operators the reason for the given operational restrictions and the possible
consequences of disobeying the rules so that the operators fully understand them.
(3) Mechanical flaws and facts may be concealed to the for political reasons. For the technologies thathave great impact on the society, the manufacturer should disclose its known issues and make
improvement that the public approves.
8. BackgroundIn the eastern part of Belorussia and the marsh-rich area, the Chernobyl station sits at the waterfront where
the Prypyats’ meets the Dnieper.
At the t ime of the accident, the Chernobyl station h ad 4 r eactors in operation and 2 under con struction.
They were th e RBMK, th e boiling-li ghtwater-cooled po wer r eactors ba sed on th e Soviet Un ion’sgraphite-moderated plutonium production reactors designed to be able to produce plutonium for weapons
as well as energy. The R BMK was built only in the Soviet Union, and it has b enefits of refuel capability
9Failure Knowledge Database / 100 Selected Cases
while generating po wer, large gross capacity, and easier inlan d con struction w ithout need o f heav ymachinery. Its drawback was complicated control due to a great number of control rod channels.
The Unit 4 reactor at Chernobyl was an RBMK-1000, capable of producing 1,000KW of electric power.
The pressurized water reactor (PWR) and the boiling water reactor (BWR), typically used in nuclear power
stations in Japan, use ordinary light water for both coolant and for neutron moderator. In the reactor core,
the primary cooling circuit water is also the mod erator, and if any of it turned to steam bubbles (void) thefission reaction would slow down (negative void coefficient). If the moderator and coolant are in s eparate
circuits, or are of different materials, excess boiling of the coolant simply reduces the cooling and neutronabsorption with out inhibiting the fi ssion reactio n. Although the Un it 4 reactor had a lar ge positive void
coefficient, it wa s compensated by the negative f eedback characteristic o f t he f uel u nder hi gher power
conditions. Howev er, at p ower condition s 20% lower than i ts n ormal capacity of 3.2 GW , the fuel’s
negative feedback fails to counter the positive feedback as the void form in the co olant water, making thereactor unstable and dangerous.
There was also a fl aw in the safety system. The control rod insertion mechanism was too s low and it took
more than 1 8 second s to complete ins ertion o f t he contro l rods from th eir withdr awn po sition after t heoperators pressed t he e mergency butto n tha t or dered a full inser tion of al l con trol r ods. The con trol r od
insertion in the light-water reactor usually takes 2 – 4 seconds to complete.
References[1] Yotaro Hatamura (Editor), Jissai-no Sekkei (Practical Design) Research Foundation (1996) Zoku-Zoku
Jissai-no Sekkei (Practical Design III), The Nikkan Kogyo Shimbun, LTD.
[2] Genshiryoku Hyakka Jiten (Nuclear Energy Encyclopedia),
http://sta-atm.jst.go.jp/atomica/02070411_1.html[3] Koichi Hirokawa (1994) Chernobyl Report, Iwanami Shoten.

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