Nuclear fission.

The process.

Nuclear fission is the process where a nucleus of a large mass is split into two nuclei of smaller mass. This action can also lead to the release of energy, and neutrons or gamma rays, or both. Neutron bombardment is the usual cause of fission and this case is called induced fission, although spontaneous fission is possible especially in the case of the heavier elements.

The process of nuclear fission can be modelled on a liquid drop, as was proposed by George Gamow, a Russian who worked in America. The diagram below shows how a neutron penetrating the 'nuclear liquid' of uranium-238 forms the isotope uranium-239, which then oscillates due to the extra energy. If this energy is sufficient then the drop becomes elongated and splits into two separate parts but if the amount of energy is insufficient then a neutron is eventually lost and the uranium returns to its original form.

The energy released by the reaction can be determined from Einstein's equation of mass energy equivalence:

E = mc² Where E = energy released in joules

m = mass deficit, i.e. loss in mass after the reaction in kg

c = velocity of light (m s^-1)

Chain reactions.

The isotope uranium-235 is more fissile than uranium-238 as it more readily captures an extra neutron. This neutron capture leads to a release of two or sometimes three neutrons and these can then go on to take part in further reactions under the right conditions. This process is known as a chain reaction as it provides its own conditions for continuation due to the additional neutron release. This reaction is also the source of enormous amounts of energy and is used in a controlled state in nuclear reactors.

Nuclear reactors.

As the nuclear reactor allows control of the fission process, useful heat energy can be used for power generation. The fission process occurs under the following conditions:

1. There are enough heavy nuclei of the fuel material packed together to capture released neutrons. There must be enough fission to start the reaction.

2. These neutrons must have the correct energy to cause fission in other nuclei before they escape from the material.

The reactor is made of three essential parts:

a) the rods of fuel that provide the heavy nuclei and neutrons

b) control rods which absorb some of the neutrons thus regulating the number of neutrons in the system

c) a moderator that slows down the neutrons

In the reactor the fuel isotope-235, captures the neutrons and releases a large amount of energy. The fast neutrons released by the fission are slowed down by the graphite moderator as this increases the chance of capture. This slowing effect occurs, as the graphite does not absorb the neutrons as they collide with it, it only removes kinetic energy through the successive collisions. The speed of the neutrons must be quite precise to ensure a good rate of capturing. Control rods are also used in the reactor to limit the number of neutrons. These rods are made of a neutron absorbing material, such as boron and as the boron captures the neutrons it forms stable elements helium and lithium, thus removing these neutrons from the chain. The number of neutrons is steadily monitored and when the number rises above a predetermined level, the control rods are pushed further into the core to absorb more neutrons therefore slowing the reaction rate. Inversely removing the rods increases the reaction rate. The energy released from these reactions in the form of heat, is transferred to a liquid or gas that circulates the core which is then used to generate electricity.

RBMK reactors, design and flaws.

The RBMK 1000 reactor at Chernobyl was of a Soviet design. The RBMK is an acronym for graphite moderated, water-cooled channel type reactor of which this was an example. This design differed considerably from most other power reactor designs, although it was favoured by the Soviets because it produced both power and plutonium, which could then be utilised in atomic weapon production.

The Chernobyl accident showed that flaws in the RBMK's design created instability when operating at a low power output, the major ones being the design of the control rods and a positive void coefficient. Since the accident a number of changes have made to RBMK's to eliminate the dangers that arise from these problems. The following description of the RBMK is based on the original design, as was in use at the time of the accident and also gives a definition of the positive void coefficient and when it occurs.

RBMK design.

' Fuel rods: these are made of zircaloy tubes 3.65 m long which are filled with pellets of enriched uranium oxide. A fuel assembly is made of 2 sets of 18 rods arranged cylindrically in a carriage. These assemblies are approximately 10 m in length and can be lifted into and out of the reactor by mechanical means. This allows refuelling of the reactor while it is still in operation.

' Pressure tubes: each fuel assembly is contained within its own pressure tube or channel inside the reactor. Each channel is then separately cooled by pressurised water.

' Graphite: graphite blocks surround and therefore separate the pressure tubes. The graphite then acts as a moderator that slows down the neutrons that are a product of the fission reaction, allowing a state of continuous fission to be maintained. A helium/nitrogen gas mixture increases heat transfer between the graphite.

' Control rods: boron carbide control rods that absorb neutrons are used to control the rate of fission. These are used, in the form of a few short rods inserted upwards into the core, to ensure an even power distribution throughout the reactor. The main control rods are inserted from the top of the core and allow automatic, manual and emergency control. In-core detectors facilitate automatic regulation of the rods. Any deviation from normal operational parameters, for example, an increase in reactor power, allows the rods to be dropped into the core to reduce or stop reactor activity. During normal operation several rods remain in the core.

' Coolant and pumps: four pumps operate on each of two separate water cooling systems, with ninety-five percent of the heat from the fission being transferred to the coolant. In the event of either of the cooling circuits being interrupted, an emergency core cooling system comes into operation.

' Turbines, generator and condenser: using the steam from the heated coolant to turn the turbine, electricity is produced by the generator. The steam is then fed to the condenser and then back into the circulating coolant.

' Reactor vessel: the core itself located in a concrete lined cavity that acts as a radiation shield. The upper part of this vessel is called the pile cap that is made of steel and supports the fuel assemblies.

Positive void coefficient.

One of the flaws that were implicated in the Chernobyl disaster was that the reactor had a positive void coefficient. Although other types of reactor have positive void coefficients, they have suitable built in safety features that the RMBK reactor sadly lacked. The problem with a positive void coefficient is that when in a low power situation the reactor can become unstable, which may result in a rapid and uncontrollable power increase. The safety features in other reactors prevent this instability from occurring.

The instability occurs because in a water-cooled reactor, steam can collect to form pockets that are known as voids. If excess steam is produced, more voids than normal are created which in turn disturbs the normal operation of the reactor. Reactor operation is affected by two factors, these being that water is more efficient at cooling than steam and that water also acts as a moderator and neutron absorber where steam does not. The difference between positive and negative void coefficients is that, in the case of a positive coefficient, the excess steam leads to an increase in power generation whereas in the negative case the voids lead to a decrease in power generation. The coefficient is simply a measure of the rate of change in the state of the reactor. The problem in the case of the positive coefficient is that an increase in power generation results in an increase in the level of steam, which thus leads to a further increase in the power generation. This 'runaway' effect is very difficult to control. No safety problem occurs in the case of the negative coefficient because excess steam tends to lead to a reactor shut down.

The majority of the world's power reactors have negative coefficients, with the same water circuit acting as both moderator and coolant. In this design a drop in power occurs when excess steam is produced, due to a reduction in the slowing of the neutrons necessary to sustain the fission reaction. In reactors with separate circuits for both moderator and coolant, or where these are of different materials, excess steam reduces the cooling of the reactor without affecting the fission reaction. In the case of the RMBK reactor, and some others, the fact that water can absorb neutrons is significant in its operation. In the case of reduced neutron absorption due to excessive steam production there are extra free neutrons available to increase the rate of reaction. This causes excess power production which then in turn leads to more additional heating of the cooling system resulting in more steam production. This greater quantity of steam results in less cooling and neutron absorption. This cycle keeps repeating itself very rapidly and is most difficult to stop, as it is a self-propagating process. This is the type of situation that occurred at Chernobyl-4.

Timetable of disaster

at

Chernobyl 4.

Background.

At the time of the accident at Chernobyl, the reactor was not operating under normal circumstances. The reactor was to undergo routine maintenance that required the reactor to be shutdown. To take advantage of this shutdown a test was devised to assess the safety margin of the reactor in a given situation. The fact that the test had to be carried out at less than full power made this window of opportunity a prudent time for the operation.

The test.

As can be imagined nuclear power plants use electricity as well as produce it. One of the many places this power is used is in the pumps used to circulate the coolant in the reactor. In most situations this power is taken from the power grid, but if this was to fail, most reactors can supply their own electricity. In the case of a reactor that is operating but is not producing power, for example in the shutdown cycle, other sources of power are required. In the case of Chernobyl, diesel generators were on standby to take over energy supply to the pumps but these took a period of fifty seconds to reach full power. This time delay was too long for the reactor to be without fully operational cooling pumps.

The test to be carried out at unit 4 was to prove that a coasting turbine could produce enough power to allow sufficient coolant to be circulated around the core while waiting for the diesel generators to reach full power. It was believed that the coolant circulation in the test would be sufficient as to give an adequate safety margin.

Although a number of reports of the events at Chernobyl at the time of the accident have been produced, there have been numerous discrepancies between them. As the single most important piece of evidence, the reactor, was destroyed the reports had to be based on other evidence that was open to different interpretation by different people and as time has passed more information has become available. The following sequence represents only what may be considered a likely course of events, as no one will ever know for sure what happened.

Test schedule.

April 25th 1986: Prelude

01:06 Scheduled shutdown of the reactor is started with a gradual decreasing of the power level.

03:47 Power lowering halted at 1600 Mw(t).

14:00 Emergency Core Cooling System (ECCS) is isolated as part of the test. This is to stop it coming into play later in the procedure. The fact that this system was isolated did not in itself contribute to the accident although it may have lessened the impact slightly.

At this stage the power was due to be reduced further but the electricity grid controller in Kiev asked for the reactor to be kept on-line to allow electricity demand to be met. Thus the reactor was kept at the 1600 Mw(t) power level and the test was delayed. In hindsight this delay was a blessing as without it the test would have been carried out during the 'day shift'.

23:10 Power reduction is restarted.

24:00 Shift change.

April 26th 1986: Test preparation

00:05 The power level was down to 720 Mw(t) and was still being decreased. Because of the positive void coefficient the recognised safe operating level for a pre-1986 RBMK is now recognised to be 700 Mw(t).

00:28 Power level is now down to 500Mw(t). At this point the control was transferred from the local to the automatic regulating system. This resulted in an unexpected fall in power to 30 Mw(t), either because the operator failed to give the 'hold power at required level' or there was a failure in the regulating system.

00:32 (approximately) The operator, having noted this power fall, retracts a number of the rods in an attempt to restore the power level. It is thought that this action contravened station safety procedures, as these stated that the chief engineer was required to approve operation of the reactor with less than the equivalent of 26 control rods. At this point it is believed that there was less than this number in the reactor.

01:00 The reactor power had now risen to 200 Mw(t).

01:03 As part of the test procedure, an additional pump was switched into the left hand cooling circuit to increase the water flow in the core.

01:07 Continuing with the test an additional pump was switched into the right hand cooling circuit. The use of these additional pumps removed heat from the core quicker. This also reduced the water level in the condenser.

01:15 The condenser automatic trip systems were deactivated to allow the reactor to continue to operate.

01:18 The feed water flow was now increased by the operator in an attempt to solve the problems in the cooling system.

01:19 At this time more control rods were withdrawn to increase power and temperature and the pressure in the condenser. The station operating policy required that 15 manual control rods be inserted in the reactor at all times. The number of manual rods inserted was now thought to be eight. The total number was greater though due to the automatic rods that were in place.

01:21:40 Feed water flow rate was now reduced to below normal by the operator to stabilise condenser water level, which in turn reduced the heat removal rate from the core.

01:22:10 Steam is beginning to be produced spontaneously in the core.

01:22:45 The reactor is still apparently stable in the eyes of the operator although some strange indications were being observed.

The test

01:23:04 The actual test procedure begins with the turbine feed valves being closed to start the turbine coasting.

01:23:10 The automatic control rods were now withdrawn from the core. This was a normal occurrence and was designed to compensate for the drop in reactivity after the turbine feed valves were closed. The average withdrawal time was 10 seconds. This decrease is usually precipitated by a decrease in the quantity of steam in the core caused by an increase in cooling system pressure. This time the amount of steam did not decrease as there was a reduced feed water flow rate to the core.

01:23:21 The quantity of steam in the core was at a point that due to the positive void coefficient a further increase in steam would create a rapid increase in power.

01:23:35 There was now an uncontrolled increase in the quantity of steam in the core.

01:23:40 The operator now pressed the emergency button (AZ-5). The control rods now started to enter the core from the top. This had the effect of concentrating the reactivity in the bottom of the core.

01:23:44 The reactor power had now peaked at approximately 100 times the reactors rated value.

01:23:45 Fuel pellets had now started to shatter and as they then reacted with the coolant, this produced a burst of high pressure in the fuel channels.

01:23:49 The fuel channels ruptured.

01:24:00 At this time there was thought to be two explosions, one a steam explosion and the other being a product of the fuel vapour expansion. These explosions lifted the pile cap of the reactor vessel which allowed air to enter. This resulted in the ignition of flammable gas and a reactor fire ensued.

Among the various the various things that were ejected from the core at this time were:

' Approximately 8 tonnes of fuel consisting of plutonium and other radioactive materials.

' A portion of the radioactive graphite blocks.

' Caesium and iodine vapours.

The aftermath.

Immediate reaction.

Almost immediately after the accident, the staff at the plant attempted to assess the extent of the damage to unit 4 and to limit the spread of the fire to the other reactors. In doing so many of these brave people averted what may have been a far greater catastrophe but also many lost their lives as a result of lethal radiation doses. Fire-fighters also risked their lives pouring water into the burning unit 4 reactor and over a period of roughly 12 days, the Soviet air force dropped some 10 000 tons of material into the reactor core in an attempt to smother the fire. These materials consisted of sand, clay, boron, lead, dolomite, trinatriumphosphate and polymerising liquids. The pilots who flew on these dumping missions died from the massive radiation doses received and a dozen giant helicopters became so radioactive that they had to be dumped along with many trucks, cars and other items of plant in the area around Chernobyl.

Once the fire had been extinguished it had to be decided what to do with the rubble and debris that had escaped from the core. It was decided to gather as much as possible and push it back into the reactor. This dangerous task was at first undertaken by robots but these were soon found to be unable to cope with the terrain that was involved and kept getting stuck, this led to the use of 'biorobots' to remove the rubble. These men, who were volunteers from the army, were only able to work in the area for a maximum of one minute even with heavy lead protective clothing on, as the radiation levels were dangerously high. This was obviously still too long as many of these 'biorobots' eventually succumbed to illness which may well have resulted from this radiation exposure.

The overall response to the accident was conducted by a large number of personnel these being volunteers from the military and the fire service, as well as a large number of non-professional people. This emergency response team became known as the 'liquidators'. In total approximately 600 000 to 800 000 people took part in the cleanup after Chernobyl, with 200 000 working in the region in the period 1986-87 when radiation exposure levels were at their highest. These figures include persons who were involved in cleaning up around the reactor, construction of the sarcophagus, building of roads, decontamination, destruction and burial of contaminated buildings, forests and equipment, as well as people who received on average low doses such as interpreters, cooks and physicians who worked in the contaminated areas.

Sarcophagus.

Within seven months of the accident the reactor was contained within the purpose built 'sarcophagus' which is approximately 60 metres high by 60 metres long and is supported on the remains of the old reactor building. Although this building was supposed to last for 30 years, it is already showing serious signs of deterioration because of poor standards maintained during construction. Another problem is the fact that the concrete is suffering from constant irradiation and a considerable temperature differential between the inner and outer faces. There is in certain circles a belief that the 'sarcophagus' may be in danger of collapse that would cause another release of radioactive particles into the atmosphere.

The zone.

After the accident an exclusion zone was created around the Chernobyl power station. This fenced off area is 2 827 square kilometres in size and 30 kilometres (18.6 miles) across. About 135 000 people lived in this area including 45 000 that lived in nearby Pripyat and they were all evacuated during April and May 1986. The power station which has been kept operating due to the need for its electrical power is staffed by workers who commute daily from the purpose built town of Slavutych, which is on the edge of the zone.

Scattered around the zone are around 800 dumps of radioactive waste. These dumps usually consist of an open pit which has a 10 centimetre lining of clay, and contain everything from soil, timber and vehicles to domestic items such as fridges and clothing. Some of the pits contain the remnants of the 'red forest' which surrounded the power station and absorbed so much radiation that the trees had to be destroyed in the same manner as radioactive waste. These pits still represent a possible environmental threat due to their close proximity to the main water table.

This is a problem because the Prypiat River, the main water feature of the area, flows into the Dnieper River which supplies the water needs of 35 million people, including the residents of Kiev.

Police and military personnel guard the zone, but a few people mostly the elderly have returned to live in the zone, and in spite of the radioactivity have been there for five years. They say that they can't see, taste, smell or touch the deadly radiation so it doesn't bother them.

Environmental and

health implications.

Ecosystem.

In the period just after the accident many radiosensitive areas of the local ecosystem received lethal doses, this was most prominent in coniferous trees and small mammals present within 10 km of the reactor. In the autumn of 1986 dose rates had fallen by a factor of 100, and by 1989 these areas had begun to recover and it may now be said that there have been no obvious long term effects to either animal populations or ecosystems. There is a possibility of long-term genetic effects, as these have still to be studied.

Key foodstuffs, such as milk and green vegetables, were subject to contamination by radioactive materials early after the accident, but due to the extremely short half-life of one of the main radionuclides, iodine-131, and long-term application of agricultural countermeasures, internal exposure to radiation has been limited. However, doubts remain over how effective the controls implemented by the Soviet government directly after the accident were in reducing dangers to the general populous.

Health.

The Chernobyl accident can be described as having acute health affects such as death and severe health impairment, and late health effects for example cancers. Plant personnel, fire fighters, medical staff and cleanup workers suffered acute health effects. Overall 237 individuals were suspected of suffering from acute radiation sickness (ARS), with this being confirmed in 134 cases. Out of the original 237, 28 died from radiation exposure, 2 died at the time of the accident from non-radiation linked causes and 1 died at this time from what is now thought to be a coronary thrombosis.

Of the 134 confirmed cases, 11 received doses so great that they suffered early and lethal changes in intestinal function. A further 26 out of the 28 deaths in the 3 months after exposure were attributed to skin lesions that affected over 50% of body surface area. After that another 14 died in the following 10 years but their deaths could not be definitely linked to the initial ARS.

One of the main long term effects was observed in children and in 1996 a joint report from the European Commission (EC), the International Atomic Energy Authority (IAEA) and the World Health Organisation (WHO) stated that, a highly significant increase in the rate of thyroid cancer in children in the three most affected countries, Russia, Belarus and the Ukraine was the only evidence at that time of a public health impact due to radiation exposure as a result of the accident. This has been confirmed by most international experts, and amounts to approximately 800 cases reported in the age group 0-15 years old at the end of 1995.

Apart from the thyroid cancer increase in young people there have been possible increases in certain cancers and a possible increase in the rate at which cancers are appearing in the people who where involved in the liquidation process and who live in contaminated areas. The reason that no definite conclusions can be drawn is that reports vary in the numbers affected. For example, incidences of leukaemia were expected to rise as a result of the Chernobyl accident in the order of 200 in the 3.7 million residents of the contaminated areas and 200 in the liquidators who worked in the period 1986-87. To see if this is the case more specific studies will need to be carried out. There has also been a slight increase in non-specific ailments reported amongst the liquidators.

Other reports give details of such problems as 50% drop in birth rates in Belarus with a steady rise in miscarriages and birth defects. Some estimate that over 3 million Russians suffered radiation exposure with 370 000 having a significant risk of developing a radiation linked illness. With so many varying reports it may be that the true death toll from Chernobyl will

never be known, although there can be no doubt it will be substantial.

The future.

Nuclear safety.

The future for the Chernobyl plant is uncertain with the Ukrainian authorities seeking financial assistance from the G-7 countries to shut the plant down by the year 2000. There are also existing sarcophagus faults that will require repair and/or ultimately replacement.

As to whether the accident could happen again, this is uncertain as the main causes of the Chernobyl accident was the combination of major deficiencies in the reactors physical design and in the design of the shut-down system and the violation of procedures. Although considerable back fitting and remedial measures have removed some of the design weaknesses in the original RBMK plant, they still have deficiencies such as the partial containment concept that require further attention. One of the most feared is the Ignalina RBMK reactor in Lithuania. It is the largest reactor of its type in the world and many experts believe it to be one of the unsafe, it still has many manual safety features and these are prone to malfunction. Just like Chernobyl though, this plant seems to have a secure future as it supplies 80% of the country's electrical needs and according to some estimates would cost a billion dollars to shut down but also a billion dollars to keep running safely.

Either way it is unlikely that the problems surrounding the Chernobyl Accident will be solved any time soon.

Bibliography.

Books.

The Truth about Chernobyl: Grigori Medvedev: I.B.Tauris & Co. Ltd.

Chernobyl and the safety of nuclear reactors in OECD countries: Nuclear Energy

Agency.

Sarcophagus safety '94: Nuclear Energy Agency.

Higher core physics: Geoff Cackett, Jim Lowrie, Alastair Steven: Oxford University Press

Journals.

New Scientist: No1765: 20 April 1991:

An ill wind from Chernobyl: Vera Rich

The legacy of Chernobyl: Marko Bojcun

World Wide Web.

International Conference: One decade after Chernobyl: Summing up the consequences of the accident: World Health Organisation.