The Chernobyl disaster is a nuclear accident in the Chernobyl Nuclear Power plant, on the 26 April, 1986, near Prypriat, in Ukraine. It is an explosion of the unit 4 of the RMBK-1000 nuclear plant. It is rated as the worst accident in the history of the nuclear industry. The accident is an explosion of a nuclear reactor which led to the release of highly radioactive materials into the air, this lead to the evacuation and resettlement of about 200,000 people from a 30km radius of contaminated area. Thirty two deaths were attributed directly to the accident, 38 died in the following months due to radiation sickness and WHO (World Health Organisation) suggests that death could be up to 4,000 civilian deaths excluding the military workers (estimated to be above 500,000) who cleaned up due to radiation poisoning. This caused the humans, plants, animals and environment far from the Chernobyl Plant to be poisoned by radiation. Large area of Belarus, part of Ukraine and Russia have been rendered inhabitable. In safety engineering, losses can be classified into human consequences, commercial losses, environmental damage, reputational damage, etc. It is difficult to compute the value and worth of damage done by the Chernobyl accident due to the fact that the radiation has both short and long term effects. The long term effects has been the increase in cancer cases, large expanse of land wasted and poisoned by radiation which is estimated to be 100,000km2, it is absolutely difficult to give monetary value to this. However the Belarus is estimated to have spent US$235 billion over the last 30 years. 5% to 7% of the Ukrainian government spending is related to Chernobyl. The Chernobyl accident has caused fears and mass protest against the use of nuclear reactor for generation of power . The Chernobyl accident is an eye opener on the destructive nature of a nuclear accident. The repercussions of the Chernobyl accident is still being felt today.
1.1 RMBK Reactor
RMBK is Russian acronym for “Reaktor Bolshoy Moshchnosti Kanalniy” which is translated in English as “High Channel-type Power Reactor”. It is a graphite moderated nuclear power reactor and uses light water for cooling, however it acts primarily as the neutron absorber. The reaction chamber is cylindrical which housed by reinforced concrete, the top and bottom are made of metallic plates. The reactor has a graphite stack and helium-nitrogen mixture for inert atmosphere and aids in the transfer of heat .
The fuel used is Uranium Oxide (UO2) ceramic, the enrichment level is 1.8percent. The Uranium Oxide is clad in Zirconium cylindrical fuel elements which is in two sets of 18 fuel rods that forms an assembly. There are 1659 fuel assemblies in the core and weighs about 114.7Kg in each assembly. Each Zircoly fuel tube is about 3.65m and the total length of the fuel assembly is 10m. It is assembled in such a way that allows for refuelling while the reactor is in operation. The cylindrical core has a 12m diameter and 7m height. The total mass of Uranium in the core is 190.2 metric tonnes. The control rods are put in to the core from the top. The reactor has a capacity to generate 1GW of power. The various components of the system are:
- On-line refuelling machine
- Gas tight containment steel vessel
- Steam drum for steam and water separation
- Steam line to turbine
- Refuelling channels through top plate
- Reactor core
- Main coolant pump
- Returning cooling water from steam drum
- Concrete biological shield and structure
Fig 1. Vertical Sectioning of RMBK. 1: On-line refuelling machine 2. Reactor core enclosure 3. Concrete shield 4. Steam drums 5. Steam headers 6. Reactor upper plate shield 7. Reactor core 8. Circulation pumps 9.Feedwater return pipe
Each fuel assembly is positioned in its own pressure tube in the reactor and is cooled by pressurised boiling water. Graphite blocks separate the pressure tubes and slows down neutrons released during fission.
1.2 Safety Features of RMBK
The RMBK is fitted with various types of safety systems for normal and emergency operations. The system is fitted with in-core sensors that measure the amount of activity in the core. If they detect an increase in power they lower control rods to reduce power and if they detect a decrease in power they raise the control rods to increase power. If there is a sharp increase in power the sensors can insert all boron rods to stop the reaction.
The reactor has the Reactor Protection System which is activated automatically and also manually when needed, this can be used to stop reaction. The system also has radiation monitoring equipments that measure radiation from the plant and the nearby environment. The system has an inert atmosphere of helium-nitrogen which prevent the oxidation of graphite. Another safety system is that which condenses water and collects it under the reactor, this is accumulated in the pressure suppressing pool below the reactor. The primary circuit piping was enclosed in concrete that can stand up to 4bars explosion. The system has two separate water coolant circuit with four pumps.
The reactor core is located in a concrete cavity that acts as a biological radiation shield. The steam drums are housed in their own concrete shields. The reactor is surrounded by a biological shield in the form of a cylindrical coaxial tank. It is covered on top and below by steel, it has opening to allow the passage of multiple communication pipes.
The reactor also has the Accident Localized system which is another safety system that serves as a containment system. There is also an Emergency cooling system (EMS) that would come into operation when the coolant circuit is interrupted.
1.2 The Safety Test
The Chernobyl accident occurred in the fourth reactor of the nuclear power station. The metal cover of the cylinder was blown open and radioactive substances were released into the atmosphere. The accident occurred while testing was being carried out for one the safety systems. The test was aimed at knowing if one of the turbines can power the feed-water pumps till the backup turbo generator would come up in case of a local power failure . The test was to be carried out steps:
- Lower the power of the reactor
- All control rods were taken out of the core, this step is to ensure the activation of the backup.
- The automatic emergency core cooling system was disconnected to prevent the core from shutting down automatically
- Trip the turbine to initiate the test.
1.3 The Accident Chronology
00:05:00; Power level reduced to 720MWth and it continued to be reduced
00:28:00; Power level dropped drastically to 30MWth and the operator retracted some control rods to
increase the power output.
01:00:00; The reactor power rose to 200MWth
01:03:00; the left hand cooling circuit was switched on to allow flow into the core (part of the test
01:07:00; The right hand cooling system was also switched to allow more flow into the core (part of
the test procedure)
01:15:00; Automatic trip systems were disabled not to obstruct experiment
01:18:00; Operator increased feed water flow to address cooling system
01:19:00; Manual rods were also retracted to increase power
01:21:40; Feed water rate reduced in order decrease heat removal from the core
01:22:10; Steam generation in the core began
01:22:45; The reactor is “stabilized”.
01:23:04; The safety test started with the turbine feed valve closed
01:23:10; Automatic control rods is withdrawn from the core since there was a decrease in reactivity
01:23:21; Increase in steam generation
01:23:35; Steam generation becomes uncontrollable
01:23:40; The operator pressed the emergency reactor trip button. The rods were being inserted from
from the top and the reaction was concentrated at the bottom of the core. However at this
point, the power was increasing exponentially.
01:23:44; Power rose to 100 times design value
01:23:45; Fuel pellets began to scatter and react with the cooling water
01:23:49; Fuel channels ruptured
01:24:00; Explosion of the reactor. The explosion caused the lifting off of the lid cover.
Fig 2. Showing the interior of the RMBK reactor after the explosion
This led to the release of 100Mega Curies (MCi) or 3.7 × 1018 Becquerel. The release of Caesium which has long term effects due to high half life. All the noble gases (Xenon133 and Krypton85), about half of volatile elements (iodine131, cesium134 and cesium137). About 3-5% of core inventory elements such as strontium, plutonium and ruthenium were released .
The Chernobyl disaster is going to be analysed from two perspectives:
(a) The Design of the Chernobyl Plant
(b) The operatives
1.4.1: The Design of the Chernobyl Plant
From the data acquired, it presents strong evidence that there are faults in the design of the RMBK reactor. The pressure suppression pool below the reactor is a poor design because the designers probably never took into account that the core could ever melt because the meltdown core touching the water would cause a steam explosion. The top metal cover was not designed to withstand very high pressure as the cylindrical chamber which remain practically undamaged after the accident. If the metal was designed to withstand such high pressure, probably the top would not just blow open.
The design has a weak shutdown system. The rods came from the top while the bottom of the reactor continued reacting. In Engineering, machines are designed for both use and a degree of misuse this happens not to be the case in the RMBK reactor. Taking the reactor to such a low power state was a degree of misuse by the operatives but this should have been taken care of in the design. It should have been one of the questions that should have been answered in the “what if session”. The reactor became very unstable at very low power state, this should have been taken care of in the design.
The reactor has so many safety systems to prevent accident but the design did not inculcate containment in case of an accident. The cylindrical chamber remained practically undamaged from data acquired. This means the top metal covering is weak for containment in case of an accident.
The design team should have let the operators know that the system is very unstable at low power. If the operators had known that the system would become highly unstable at low power, they would have never taken it to such a low power state. Even a little caution note in a conspicuous location probably in the operating room would convey the message.
Steam can react with graphite which is carbon to release producer gas with the release of energy. This energy is the explosion that took off the metal cover of the reactor. This further buttress the point that the RMBK was not designed for accident.
Having a nuclear plant working on a commercial scale, with all safety systems not fully tested is wrong with very high consequences no matter how low the probability of occurrence is.
1.4.2 Plant Operatives
The design operatives should not have carried out the operation without the presence of a member of the design team. This is a very vital and should have not been violated. The operatives pushed the reactor way beyond the design limits without knowing.
The design operative should never have disabled the Emergency Cooling System. In any industry, safety comes first. Disabling the Emergency Cooling system by the operative is wrong.
All RMBK Nuclear plant should be shut down because the Chernobyl accident is primarily a design fault accident with blame also on the operators. The RMBK reactor should be redesigned, the best way to solve a design based problem is to redesign. However if it is not possible to shut down RMBK plants then the safety systems of the RMBK should be improved and monitored closely.
Kofi Anan said “At least 3million children in Belarus, Ukraine and the Russian require physical treatment (due to the Chernobyl accident)…”, Louisa Vinton said “two decades after the Chernobyl accident, the residents in the affect areas still lack the information they need to lead the healthy and productive lives that are possible…”. The Chernobyl accident remains the worst nuclear accident in history. The value of damage done is difficult to quantify because the effects are still being felt today. Billions of dollars are still being spent to curb the effect of the Chernobyl accident. After analysing the Chernobyl accident, it is evident that certain known knowns where ignored or not taken into consideration in the design of the reactor.
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