UNSCEAR web site re Chernobyl, for the most authoritative material available on the matter.

SOURCES
Nuclear Energy Institute 1996, Info Bank briefing sheets and Source Book, 4th edn.
OECD NEA report Chernobyl Ten Years On, radiological and health impact, Nov 1995;
IAEA 1996, Ten years after Chernobyl: what do we really know? (from April 1996 conference);
UNSCEAR 2000 report, Annex J.
NucNet Chernobyl background papers & news.

World Nuclear Association, Bowater House, 114 Knightsbridge, London SW1X 7LJ, UK

Chernobyl and Soviet Reactors
	Appendices
	UNSCEAR Annex J
Chernobyl - Post Accident Changes to the RBMK
Chernobyl - Positive Void Coefficient
Nuclear power in Ukraine
RBMK light-water graphite reactor

The health effeccts of the Chernobyl accident have been the subject of unprecedented study by health professionals and unprecedented speculation and exaggeration by parts of the media. Three of the appendices here to Information Paper # 7 present authoritative and experrt assessments of the situation.

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From: chapter 1 of NEA 2003 publication: Chernobyl - an assessment of radiological and health impact.

The Chernobyl site and accident sequence

The Chernobyl Power Complex, lying about 130 km north of Kiev, Ukraine, and about 20 km south of the border with Belarus, consisted of four nuclear reactors of the RBMK-1000 design, Units 1 and 2 being constructed between 1970 and 1977, while Units 3 and 4 of the same design were completed in 1983. Two more RBMK reactors were under construction at the site at the time of the accident.

To the South-east of the plant, an artificial lake of some 22 square kilometres, situated beside the river Pripyat, a tributary of the Dniepr, was constructed to provide cooling water for the reactors.

This area of Ukraine is described as Belarussian-type woodland with a low population density. About 3 km away from the reactor, in the new city, Pripyat, there were 49 000 inhabitants. The old town of Chernobyl, which had a population of 12 500, is about 15 km to the South-east of the complex. Within a 30-km radius of the power plant, the total population was between 115 000 and 135 000.

The RBMK-1000 is 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 direct steam feed to the turbines, without an intervening heat-exchanger. Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two 500 MWe [megawatt electrical] turbines. The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium-alloy clad uranium-dioxide fuel around which the cooling water flows. A specially designed refuelling machine allows fuel bundles to be changed without shutting down the reactor.

The moderator, whose function is to slow down neutrons to make them more efficient in producing fission in the fuel, is constructed of graphite. A mixture of nitrogen and helium is circulated between the graphite blocks largely to prevent oxidation of the graphite and to improve the transmission of the heat produced by neutron interactions in the graphite, from the moderator to the fuel channel. The core itself is about 7 m high and about 12 m in diameter. There are four main coolant circulating pumps, one of which is always on standby. The reactivity or power of the reactor is controlled by raising or lowering 211 control rods, which, when lowered, absorb neutrons and reduce the fission rate. The power output of this reactor is 3 200 MWt (megawatt thermal) or 1 000 MWe, although there is a larger version producing 1 500 MWe. Various safety systems, such as an emergency core cooling system and the requirement for an absolute minimal insertion of 30 control rods, were incorporated into the reactor design and operation.

The most important characteristic of the RBMK reactor is that it possesses a "positive void coefficient". 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.

Events leading to the accident

The Unit 4 reactor was to be shutdown for routine maintenance on 25 April 1986. It was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the emergency equipment and the core cooling water circulating pumps, until the diesel emergency power supply became operative. The aim of this test was to determine whether cooling of the core could continue to be ensured in the event of a loss of power.

This type of test had been run during a previous shut-down period, but the results had been inconclusive, so it was decided to repeat it. Unfortunately, this test, which was considered essentially to concern the non-nuclear part of the power plant, was carried out without a proper exchange of information and co-ordination between the team in charge of the test and the personnel in charge of the operation and safety of the nuclear reactor. Therefore, inadequate safety precautions were included in the test programme and the operating personnel were not alerted to the nuclear safety implications of the electrical test and its potential danger.

The planned programme called for shutting off the reactor's emergency core cooling system (ECCS), which provides water for cooling the core in an emergency. Although subsequent events were not greatly affected by this, the exclusion of this system for the whole duration of the test reflected a lax attitude towards the implementation of safety procedures.

As the shutdown proceeded, the reactor was operating at about half power when the electrical load dispatcher refused to allow further shutdown, as the power was needed for the grid. In accordance with the planned test programme, about an hour later the ECCS was switched off while the reactor continued to operate at half power. It was not until about 23:00 hr on 25 April that the grid controller agreed to a further reduction in power.

For this test, the reactor should have been stabilised at about 1 000 MWt prior to shut down, but due to operational error the power fell to about 30 MWt, where the positive void coefficient became dominant. The operators then tried to raise the power to 700-1 000 MWt by switching off the automatic regulators and freeing all the control rods manually. It was only at about 01:00 hr on 26 April that the reactor was stabilised at about 200 MWt.

Although there was a standard operating order that a minimum of 30 control rods was necessary to retain reactor control, in the test only 6-8 control rods were actually used. Many of the control rods were withdrawn to compensate for the build up of xenon which acted as an absorber of neutrons and reduced power. This meant that if there were a power surge, about 20 seconds would be required to lower the control rods and shut the reactor down. In spite of this, it was decided to continue the test programme.

There was an increase in coolant flow and a resulting drop in steam pressure. The automatic trip which would have shut down the reactor when the steam pressure was low, had been circumvented. In order to maintain power the operators had to withdraw nearly all the remaining control rods. The reactor became very unstable and the operators had to make adjustments every few seconds trying to maintain constant power.

At about this time, the operators reduced the flow of feedwater, presumably to maintain the steam pressure. Simultaneously, the pumps that were powered by the slowing turbine were providing less cooling water to the reactor. The loss of cooling water exaggerated the unstable condition of the reactor by increasing steam production in the cooling channels (positive void coefficient), and the operators could not prevent an overwhelming power surge, estimated to be 100 times the nominal power output.

The sudden increase in heat production ruptured part of the fuel and small hot fuel particles, reacting with water, caused a steam explosion, which destroyed the reactor core. A second explosion added to the destruction two to three seconds later. While it is not known for certain what caused the explosions, it is postulated that the first was a steam/hot fuel explosion, and that hydrogen may have played a role in the second.

Some media had reported a seismic origin of the accident, however the scientific credibility of the paper at the origin of this rumour has been discarded.

The accident

The accident occurred at 01:23 hr on Saturday, 26 April 1986, when the two explosions destroyed the core of Unit 4 and the roof of the reactor building.

In the IAEA Post-Accident Assessment Meeting in August 1986, much was made of the operators' responsibility for the accident, and not much emphasis was placed on the design faults of the reactor. Later assessments suggest that the event was due to a combination of the two, with a little more emphasis on the design deficiencies and a little less on the operator actions.

The two explosions sent fuel, core components and structural items and produced a shower of hot and highly radioactive debris, including fuel, core components, structural items and graphite into the air and exposed the destroyed core to the atmosphere. The plume of smoke, radioactive fission products and debris from the core and the building rose up to about 1 km into the air. The heavier debris in the plume was deposited close to the site, but lighter components, including fission products and virtually all of the noble gas inventory were blown by the prevailing wind to the North-west of the plant.

Fires started in what remained of the Unit 4 building, giving rise to clouds of steam and dust, and fires also broke out on the adjacent turbine hall roof and in various stores of diesel fuel and inflammable materials. Over 100 fire-fighters from the site and called in from Pripyat were needed, and it was this group that received the highest radiation exposures and suffered the greatest losses in personnel. A first group of 14 firemen arrived on the scene of the accident at 1.28 a.m. Reinforcements were brought in until about 4 a.m., when 250 firemen were available and 69 firemen participated in fire control activities. By 2.10 a.m., the largest fires on the roof of the machine hall had been put out, while by 2.30 a.m., the largest fires on the roof of the reactor hall were under control. These fires were put out by 05:00 hr of the same day, but by then the graphite fire had started. Many firemen added to their considerable doses by staying on call on site. The intense graphite fire was responsible for the dispersion of radionuclides and fission fragments high into the atmosphere. The emissions continued for about twenty days, but were much lower after the tenth day when the graphite fire was finally extinguished.

The graphite fire

While the conventional fires at the site posed no special firefighting problems, very high radiation doses were incurred by the firemen, resulting in 31 deaths. However, the graphite moderator fire was a special problem. Very little national or international expertise on fighting graphite fires existed, and there was a very real fear that any attempt to put it out might well result in further dispersion of radionuclides, perhaps by steam production, or it might even provoke a criticality excursion in the nuclear fuel.

A decision was made to layer the graphite fire with large amounts of different materials, each one designed to combat a different feature of the fire and the radioactive release. The first measures taken to control fire and the radionuclides releases consisted of dumping neutron-absorbing compounds and fire-control material into the crater that resulted from the destruction of the reactor. The total amount of materials dumped on the reactor was about 5000t including about 40t of borons compounds, 2400t of lead, 1800t of sand and clay, and 600t of dolomite, as well as sodium phosphate and polymer liquids (Bu93). About 150 t of material were dumped on 27 April, followed by 300t on 28 April, 750t on 29 April, 1 500t on 30 April, 1900t on 1 May and 400t on 2 May. About 1800 helicopter flights were carried out to dump materials onto the reactor.

During the first flights, the helicopter remained stationary over the reactor while dumping materials. As the dose rates received by the helicopter pilots during this procedure were too high, it was decide that the materials should be dumped while the helicopters travelled over the reactor. This procedure caused additional destruction of the standing structures and spread the contamination. Boron carbide was dumped in large quantities from helicopters to act as a neutron absorber and prevent any renewed chain reaction. Dolomite was also added to act as heat sink and a source of carbon dioxide to smother the fire. Lead was included as a radiation absorber, as well as sand and clay which it was hoped would prevent the release of particulates. While it was later discovered that many of these compounds were not actually dropped on the target, they may have acted as thermal insulators and precipitated an increase in the temperature of the damaged core leading to a further release of radionuclides a week later.

The further sequence of events is still speculative, although elucidated with the observation of residual damage to the reactor. It is suggested that the melted core materials settled to the bottom of the core shaft, with the fuel forming a metallic layer below the graphite. The graphite layer had a filtering effect on the release of volatile compounds. But after burning without the filtering effect of an upper graphite layer, the release of volatile fissions products from the fuel may have increased, except for non-volatile fission products and actinides, because of reduced particulate emission. On day 8 after the accident, the corium melted through the lower biological shield and flowed onto the floor. This redistribution of corium would have enhanced the radionuclide releases, and on contact with water corium produced steam, causing an increase of radionuclieds at the last stage of the active period.

By May 9, the graphite fire had been extinguished, and work began on a massive reinforced concrete slab with a built-in cooling system beneath the reactor. This involved digging a tunnel from underneath Unit 3. About four hundred people worked on this tunnel which was completed in 15 days,allowing the installation of the concrete slab. This slab would not only be of use to cool the core if necessary, it would also act as a barrier to prevent penetration of melted radioactive material into the groundwater.

In summary

The Chernobyl accident was the product of a lack of "safety culture". The reactor design was poor from the point of view of safety and unforgiving for the operators, both of which provoked a dangerous operating state. The operators were not informed of this and were not aware that the test performed could have brought the reactor into an explosive condition. In addition, they did not comply with established operational procedures. The combination of these factors provoked a nuclear accident of maximum severity in which the reactor was totally destroyed within a few seconds.

 

Chernobyl Accident: Simplified sequence of events

The sequence of events which follows has been compiled following a review of a large number of reports and it represents what is considered the most likely sequence of events, but there remain some uncertainties.

April 25: Prelude
01:06	The scheduled shutdown of the reactor started. Gradual lowering of the power level began
03:47	Lowering of reactor power halted at 1600 MW(thermal).
14:00	The emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later.
	The fact that the ECCS was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly.
14:00	The power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to enable demand to be met. Consequently, the reactor power level was maintained at 1600 MW(t) and the experiment was delayed.
	Without this delay, the test would have been conducted during `day shift'.
23:10	Power reduction recommenced.
24:00	Shift change.
April 26: Preparation for the test
00:05	Power level had been decreased to 720 MW(t) and continued to be reduced.
	It is now recognised that the safe operating level for a pre-accident configuration RBMK was about 700 Mwt because of the positive void coefficient.
00:28	Power level was now 500 MW(t).
	Control was transferred from the local to the automatic regulating system. Either the operator failed to give the `hold power at required level' signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MW(t).
00:32	(approximate time). In response, the operator retracted a number of control rods in an attempt to restore the power level.
	Station safety procedures required that approval of the chief engineer be obtained to operate the reactor with fewer than the effective equivalent of 26 control rods. It is estimated that there were less than this number remaining in the reactor at this time.
01:00	The reactor power had risen to 200 MW(t).
01:03	An additional pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure).
01:07	An additional pump was switched into the right hand cooling circuit (part of the test procedure).
	Operation of additional pumps removed heat from the core more quickly. This reduced the water level in the steam separator.
01:15	Automatic trip systems to the steam separator were deactivated by the operator to permit continued operation of the reactor.
01:18	Operator increased feed water flow in an attempt to address the problems in the cooling system.
01:19	Some manual control rods withdrawn to increase power and raise the temperature and pressure in the steam separator.
	Operating policy required that a minimum effective equivalent of 15 manual control rods be inserted in the reactor at all times. At this point it is likely that the number of manual rods was reduced to less than this (probably eight). However, automatic control rods were in place, thereby increasing the total number.
01:21:40	Feed water flow rate reduced to below normal by the operator to stabilise steam separator water level, decreasing heat removal from the core.
01:22:10	Spontaneous generation of steam in the core began.
01:22:45	Indications received by the operator, although abnormal, gave the appearance that the reactor was stable.
The test
01:23:04	Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test.
01:23:10	Automatic control rods withdrawn from the core. An approximately 10 second withdrawal was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feed valves.
	Usually this decrease is caused by an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. The expected decrease in steam quantity did not occurdue to reduced feedwater to the core.
01:23:21	Steam generation increased to a point where, owing to the reactor's positive void coefficient, a further increase of steam generation would lead to a rapid increase in power.
01:23:35	Steam in the core begins to increase uncontrollably.
01:23:40	The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core.
	The insertion of the rods from the top concentrated all of the reactivity in the bottom of the core.
01:23:44	Reactor power rose to a peak of about 100 times the design value.
01:23:45	Fuel pellets started to shatter, reacting with the cooling water to produce a pulse of high pressure in the fuel channels.
01:23:49	Fuel channels ruptured.
01:24	Two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapour.
	The explosions lifted the pile cap, allowing the entry of air. The air reacted with the graphite moderator blocks to form carbon monoxide. This flammable gas ignited and a reactor fire resulted.
Thereafter, over nine days:
Some 8 of the 140 tonnes of fuel, which contained plutonium and other highly radioactive materials (fission products), were ejected from the reactor along with a portion of the graphite moderator, which was also radioactive. These materials were scattered around the site. In addition, caesium and iodine vapours were released both by the explosion and during the subsequent fire.

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ESTIMATED LONG TERM HEALTH EFFECTS OF THE CHERNOBYL ACCIDENT

April 1996

Principal author:
Elizabeth CARDIS
International Agency for Research on Cancer, Lyon, France

Abstract

Apart from the dramatic increase in thyroid cancer in those exposed as children, there is no evidence to date (1996) of a major public health impact as a result of radiation exposure due to the Chernobyl accident in the three most affected countries (Belarus, Russia and Ukraine).

Although some increases in the frequency of cancer in exposed populations have been reported, these results are difficult to interpret, mainly because of differences in the intensity and method of follow-up between exposed populations and the general population with which they are compared. If the experience of the survivors of the atomic bombing of Japan and of other exposed populations is applicable, the major radiological impact of the accident will be cases of cancer. The total lifetime numbers of excess cancers will be greatest among the 'liquidators' (emergency and recovery workers) and among the residents of 'contaminated' territories, of the order of 2000 to 4600 among each group (the size of the exposed populations is 200,000 liquidators and 6,800,000 residents of 'contaminated' areas). These increases would be difficult to detect epidemiologically against an expected background number of 41,500 and 800,000 cases of cancer respectively among the two groups.

However, the exposures for populations due to the Chernobyl accident are different (in type and pattern) from those of the survivors of the atomic bombing of Japan (and doses received early after the accident are not well known). Predictions derived from studies of these populations are therefore uncertain. Indeed, although an increase in the incidence of thyroid cancer in persons exposed as children as a result of the Chernobyl accident was envisaged, the extent of the increase was not foreseen.

Only ten years have passed since the accident. It is essential, therefore, that monitoring of the health of the population be continued in order to assess the public health impact of the accident, even if any increase in the incidence of cancers as a result of radiation exposure due to the Chernobyl accident, except for leukaemia among liquidators and thyroid cancer, is expected to be difficult to detect. Studies of selected populations and diseases are also needed in order to study observed or predicted effects; careful studies may in particular provide important information on the effect of exposure rate and exposure type in the low to medium dose range and on factors which may modify radiation effects. As such, they may have important consequences for the radiation protection of patients and the general population in the event of any future accidental exposure.

Background paper # 3 from:
One Decade After Chernobyl - Summing up the consequences of the accident, Proceedings of international conference, Vienna, April 1996, sponsored by EU, IAEA & WHO. The summary of the results of this joint conference is available on the web.

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Chernobyl Accident Health Effects - WHO Facts & Figures

November 1995

The World Health Organisation (WHO) issued a summary of the main findings of the November 1995 international conference in Geneva on the health consequences of the Chernobyl disaster, and called for more research in the future.

Speaking after the conference ended, Dr Wilfried Kreisel, the WHO's Executive Director in charge of Health and Environment, said: "The legacy will stay with us for a long time in the shape of radiation-induced diseases and psycho-somatic disorders. We shall be doing a disservice if we fail to extract benefits for mankind out of this monumental human tragedy. If history is not to repeat itself, we should learn very well the lessons of Chernobyl."

Dr Kreisel said one of the conference's main achievements had been to establish scientific consensus on the known health consequences of the accident. He dismissed as "fiction" claims by Ukrainian officials earlier in 1995 that more than 100,000 people had died as a result of the accident, saying the proven death toll so far was about 40. He said 30 of those deaths were from direct exposure at the time, and there had been some 10 fatal cases to date of radiation-induced thyroid cancer.

As regards the conference's main findings, the WHO statement reads:
"Scientists attending the conference have identified three main areas of concern: the large increase in psychological disorders, especially among accident recovery workers and people living in the highly contaminated areas; the health impact of the thyroid cancer incidence among children; and future cancers which could occur in people, particularly leukaemia, breast cancer, bladder cancer and kidney diseases.

"There is a definite increase in thyroid cancer, mostly in children but also in adolescents, following the Chernobyl accident, with nearly 400 cases in Belarus, 220 cases in the Ukraine and 62 cases in the Russian Federation. These cancers are extremely aggressive and locally invasive. There is strong circumstantial evidence that an increase in thyroid cancers in these three countries is due to radioactive fall-out which followed the Chernobyl accident. This is demonstrated by:

• The geographical distribution of cases, which overlaps with that of the radioactive plume
• The time trends: rates of thyroid cancer in children and adolescents were low and stable until around 1990, but have increased sharply since. The increase is apparent in the populations of children born before the accident. Among those conceived later, the rates are similar to those observed before the accident
• The results of a case control study carried out in Belarus."

"Although, so far, there has been no statistically measurable increase in leukaemia and other similar blood disorders, scientists are warning that the peak may occur within the next few years, accompanied by greater incidence in breast cancer, cancer of the bladder and kidney diseases. Preliminary reports on the health status of accident recovery workers indicate that the increase is beginning to show.

"The participants at the conference concluded that psycho-social effects is a priority area, which should be addressed in a much more serious way both by the three affected states and by the international community. Similarities in behaviour patterns on the part of the victims and unaffected populations were observed among the Japanese bomb survivors and those affected by the Chernobyl fallout. In Japan, bomb survivors were discriminated against by prospective employers because they might contract cancer at a later time. Similarly, people evacuated from contaminated areas to clear areas after the Chernobyl accident were shunned by local residents because the evacuees were provided with new houses and pensions.

"Evacuation and relocation of large groups of people after the Chernobyl accident, as well as constant concern and fear of radiation exposure have resulted in a rising number of health disorders being reported to local outpatient clinics. Studies show that headaches, chest pains, intestinal disorders, sleep disturbance, loss of concentration and alcohol abuse are common. This is clearly a priority area not only for the governments and public health services of the three affected states, but for the international community as a whole. Similar psycho-social impact can be observed in the wake of earthquakes, fires, floods and other natural and man-made disasters."

See also WHO booklet Health Consequences of the Chernobyl Accident: Summary Report, issued to coincide with the conference. (order from WHO)

Source: NucNet Background #18/95.

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Chernobyl - Post Accident Changes to the RBMK

March 2001

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Chernobyl - Positive Void Coefficient

March 2001

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Chernobyl - RBMK

March 2001

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The Soviet designed RBMK is a pressurised water reactor with individual fuel channels and using ordinary water as its coolant and graphite as its moderator. It is very different from most other power reactor designs as it was intended and used for both plutonium and power production. The combination of graphite moderator and water coolant is found in no other power reactors. The design characteristics of the reactorwere shown, in the Chernobyl accident, to cause instability when low power. This was due primarily to control rod design and a positive void coefficient. A number of significant design changes have now been made to address these problems.

click to enlarge.

Fuel rods
Pellets of enriched uranium oxide are enclosed in a zircaloy tube 3.65m long, forming a fuel rod. Two sets of 18 fuel rods are arranged cylindrically in a carriage to form a fuel assembly of about 10 m length. These fuel assemblies can be lifted into and out of the reactor mechanically, allowing fuel replenishment while the reactor is in operation.

1. Pressure tubes
   Within the reactor each fuel assembly is positioned in its own pressure tube or channel. Each channel is individually cooled by pressurised water.
2. Graphite moderator
   A series of graphite blocks surround, and hence separate, the pressure tubes. They act as a moderator to slow down the neutrons released during fission. This is necessary for continuous fission to be maintained. Conductance of heat between the blocks is enhanced by a mixture of helium and nitrogen gas.
3. Control rods
   Boron carbide control rods absorb neutrons to control the rate of fission. A few short rods, inserted upwards from the bottom of the core, even the distribution of power across the reactor. The main control rods are inserted from the top down and provide automatic, manual or emergency control. The automatic rods are regulated by feedback from in-core detectors. If there is a deviation from normal operating parameters (e.g. increased reactor power level), the rods can be dropped into the core to reduce or stop reactor activity. A number of rods normally remain in the core during operation.
4. Coolant
   Two separate water coolant systems each with four pumps circulate water through the pressure tubes. Ninety-five percent of the heat from fission is transferred to the coolant. There is also an emergency core cooling system which will come into operation if either coolant circuit is interrupted.
5. Steam separator
   Steam from the heated coolant is fed to turbines to produce electricity in the generator. The steam is then condensed and fed back into the circulating coolant.
6. Containment
   The reactor core is located in a concrete lined cavity that acts as a radiation shield. The upper shield or pile cap above the core, is made of steel and supports the fuel assemblies. The steam separators of the coolant systems are housed in their own concrete shields.

click to enlarge.

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