A statue of Vladimir Lenin stands in the middle of a small park in the port of Chernobyl near the frozen river of Pripyat on January 29, 2006 in Chernobyl, Ukraine. The Chernobyl Port was abandoned soon after the 1986 Catastrophe.
Design Error
The RBMK reactor core is unstable at low power which is below 700 Megawatts-thermal, about a quarter of full power. In this condition the reactor is difficult to control and any tendency toward a runaway chain reaction is automatically and rapidly amplified. This very dangerous feature is characteristic of the RBMK design. The Chernobyl explosion occurred during a test at low power, that is, at a moment when the reactor was unstable. The Soviet authorities were warned well before the Chernobyl accident, but the warning fell on deaf ears. Deaf ears, what have you done?
A guide holds a Geiger counter showing radiation levels 37 times higher than normal as a woman takes a picture in front of the sarcophagus of the destroyed fourth block of the Chernobyl nuclear power plant on September 16, 2010.
Before the Chernobyl accident happened there were a few minor failures where any of these could just have caused the initiating event for this or an almost identical accident. They included:
- Pump failure- disturbance of the function of coolant pumping or pump cavitations, combined with the effect of the positive void coefficient. Any of these causes could have led to sudden augmentation of the effect of the positive void coefficient.
- Failure of zirconium- alloy fuel channels or of the welds between these and the stainless steel piping, most probably near the core inlet at the bottom of the reactor. Failure of a fuel channel would have been a cause of a sudden local increase in void fraction as the coolant flashed to steam; this would have led to a local reactivity increase which could have triggered a propagating reactivity effect.
Graffiti is seen on a wall of one of the buildings in the ghost city of Pripyat, near Chernobyl nuclear power plant on February 22, 2011.
Control rods and Safety design- The control rods of an RBMK reactor are inserted into the reactor core from above, except for 24 shortened rods which are inserted upwards and which are used for flattening the power distribution. A graphite rod termed a 'displaced is attached to each end of the length of absorber of each rod, except for twelve rods that are used in automatic control.
The lower displacer prevents coolant water from entering the space vacated as the rod is withdrawn, thus augmenting the reactivity worth of the rod. The graphite displacer of each rod of all RBMK reactors was, at the time of the accident, connected to its rod via a 'telescope', with a water filled space of 1.25 m separating the displacer and the absorbing rod. The dimensions of rod and displacer were such that when the rod was fully extracted the displacer sat centrally within the fuelled region of the core with 1.25 m of water at either end.
Part of the collapsed roof at the Chernobyl nuclear plant, damaged in a fire, is pictured in this photo taken, Friday, Oct. 13, 1991 in Chernobyl, Ukraine during a media tour of the facility.
On receipt of a scram signal causing a fully withdrawn rod to fall, the displacement of water from the lower part of the channel as the rod moved downwards from its upper limit stop position caused a local insertion of positive reactivity in the lower part of the core. The magnitude of this 'positive scram' effect depended on the spatial distribution of the power density and the operating regime of the reactor.
The control rods do not have a great emergency system. A fast insertion of control rods is needed in the critical situation. But in the RBMK reactor the control rods are inserted slowly. The reactor takes about 20 seconds for full insertion, while it takes less than 2 seconds in other reactors throughout the world. This is much too slow to prevent runaway of the core while it is operating in the unstable mode.
The damaged Chernobyl unit 4 reactor building
Size of reactor - Owing to the largeness of the reactor core which is height 7 m and the diameter is 11.8 m the chain reaction in one part of the core is only very loosely coupled with that in other, distant, regions. This leads to a requirement to control the spatial power distribution almost as if there were several independent reactors within the core volume.
This situation in extreme conditions can be highly unstable, because small spatial re distributions of reactivity can cause large spatial re distributions of the power. One manifestation of this decoupling of the core is that just prior to the accident the chain reactions in the upper and lower halves of the reactor were preceding almost independently, a situation that was exacerbated by heavy xenon poisoning in the intervening central region.
Lt. Colonel Leonid Telyatnikov, Head of the Pripyat Fire Brigade which fought the Chernobyl blaze, points at a photograph of the power station's damaged fourth reactor following the April 26, 1986 nuclear accident. The reactor has since been entombed in concrete.
A Soviet-designed and built graphite moderated pressure tube type reactor, using slightly enriched (2% U-235) uranium dioxide fuel. It is a boiling light water reactor, with two loops feeding steam directly to the turbines, without an intervening heat exchanger.
Sub - cooling of the inlet water-The RBMK reactors are boiling water reactors. The coolant enters the reactor core from below as water, sub - cooled below the boiling temperature, and boiling begins at some distance along the flow path through the core. Analysis and experiment have shown that the amount of sub - cooling of the entry coolant of a boiling water reactor is important for the stability of the reactor.
If the sub - cooling falls to near zero, boiling begins almost at the core inlet and, because of the void coefficient of reactivity, reactivity effects become very sensitive to the inlet coolant temperature. Furthermore, since there is not much change in fluid temperature between the coolant pumps and the core inlet, the temperature of the water in the pumps and at their intakes is near boiling if the sub - cooling is very small. Pump behavior under these circumstances can become erratic, and pumping action can be reduced substantially or can even stop completely under some conditions.
A Kurchatov Nuclear Institute worker walks in the light streaming into the cement-entombed room of the Chernobyl nuclear power plant's exploded reactor on Friday, Sept. 15, 1989, three years after the nuclear disaster.
The most important characteristic of the RBMK reactor is that it possesses a "positive void coefficient". The steam bubbles are called voids, and that proportion of the coolant volume. The changes of void fraction also affected the changes in reactivity and it must be offset by control rods.
This means that if the power increases or the flow of water decreases, there is increased steam production in the fuel channels, so that the neutrons that would have been absorbed by the denser water will now produce increased fission in the fuel. However, as the power increases, so does the temperature of the fuel and this has the effect of reducing the neutron flux (negative fuel coefficient). The net effect of these two opposing characteristics varies with the power level. At the high power level of normal operation, the temperature effect predominates, so that power excursions leading to excessive overheating of the fuel do not occur.
However, at a lower power output of less than 20% the maximum, the positive void coefficient effect is dominant and the reactor becomes unstable and prone to sudden power surges. This was a major factor in the development of the accident. (Click Here) The increasing of void coefficient reactivity and the less number of fixed absorbers in the core were the other causes of Chernobyl accident. The fuel enrichment in the reactor was set at only 2% and it should be increased by 0.4% in order to have better operating system. The additional absorbers require the use of higher fuel enrichment to compensate for the increased neutron absorption.
A Kurchatov Nuclear Institute worker stands in the operators room of block number four Chernobyl's nuclear power plant inside the sarcophagus on Friday, Sept. 15, 1989, three years after the nuclear disaster.
In the event of coolant loss, water in the pressure tubes turns to steam and steam pockets, or voids, is formed. Steam is less dense than water and has less cooling power, so the fuel gets hotter. However, where the water also provides the moderating function, the neutrons will speed up due to the lack of moderation and the reaction will slow down.
This is known as a negative void coefficient, and ensures that any uncontrolled increase in core temperature will slow down and ultimately stop the reaction. Most reactors are built this way and are thus inherently 'safe.' Conversely, in the RBMK at lower power levels (less than 20% of maximum) the graphite moderator allowed the reaction to proceed in spite of the loss of coolant. The number of free neutrons would increase, as there is no water to absorb them. This positive void coefficient meant that an uncontrolled temperature increase could, in the right circumstances, lead to a runaway reaction.
Equipment- The reactor is equipped with a fuel rod leak detector. A scintillation detector, sensitive to energies of short-lived fission products, is mounted on a special dolly and moved over the outlets of the fuel channels, issuing an alert if increased radioactivity is detected in the steam-water flow.
Reactor Poor Protection and Emergency System- In RBMK reactors have neither a system to filter exhausted gases nor a containment structure. In the worst of scenarios, the latter would at least have reduced and slowed the escape of radioactive material into the environment. Such a containment structure protects reactors all over the world, including the most recent reactors (VVER 1000) installed in the former Soviet Union.
A nurse at a children's health clinic in Warsaw administers an iodine solution to a three-year-old girl held in her mother's arms in Poland, May 1986. Protective measures were taken for possible radiation poisoning from the Soviet nuclear accident in Chernobyl.
The reactor at Three Mile Island was so enclosed and consequently there was no significant release of radioactivity. Lacking a containment structure, the RBMK reactor is like a bus without a body - the containment structure is obviously a major and essential safety requirement, although it is not invulnerable. The lower operating reactor margin (OMR) which is 26-30 rods was the factor that leads to the accident.
This should be increase to 43 – 48 rods. With the increment of the number of fixed absorbers and the ORM it helps to reduce the value of the void coefficient of reactivity to +ß (where ß is the effective delayed neutron fraction). Yet the magnitude of the ORM was not conveniently available to the operator, nor was it incorporated into the reactor's protection system. In the discussion of the scenario, the operators seemed not to be aware of the other reason for the importance of the ORM, which was the extreme effect it could have on the void and power coefficients.
The computer system does not have an emergency mode in calculating and collecting data of reactivity margin. The searching data scale is too large and it takes almost ten to fifteen minutes to cycle through all the measurements and calculates the results.
Error Committed By the Operating Crew
Six human errors were identified. Two permanent operating rules were violated: not to run the reactor for any length of time at reduced power level and never to have fewer than thirty control rods fully inserted into the core. One error consisted of not following the test procedure, and three safety mechanisms were deliberately bypassed - one for emergency water injection, and two others for emergency shut-down.
It is evident that the operators were not adequately trained and did not realize the dangerous nature of their actions. If any one of these six errors had not been committed, the explosion would not have occurred. On the other hand, it would be too easy to lay the blame for the catastrophe on the operating crew because they were doing their job with the training they had received.
That training was insufficient and totally inconsistent with absence of passive safety features in the RBMK reactor design. Not knowing much about the behaviour of the reactor core, they were unable to appreciate the implications of the decisions they were making, and their situation was even more dangerous in that the test was being done at low power and in violation of standing orders. Furthermore, the operating instructions, both the standing orders and the specific instructions for the test, were incomplete and imprecise.
A general view of the Chernobyl nuclear power plant.
The world nuclear power industry is quick to point out that the Chernobyl accident was a unique event that could never be repeated at a Westinghouse, General Electric, Babcock and Wilcox or Combustion Engineering design. The industry claims that Chernobyl was the product of a severely flawed reactor design that could never be licensed to operate in the Malaysia. Industry proponents continue to claim that all nuclear reactors are designed to ensure that radioactive materials would be contained in the event of a serious accident.
We must learn from the past to be smarter for the future.
Regards,
Nuclear Boy.
