Did Graphite in the Chernobyl Reactor Burn?

In two previous posts, " Does Nuclear Grade Graphite Burn?," and "Did the Graphite in the Windscale Reactor Burn?" I reviewed a number of reports and other information sources on Nuclear Graphite Flamibility. Although I did not come to a firm conclusion, i did find strong evidence that Nuclear Graphite does not burn under many conditions in which one would expect fire. There is also startling evidence that at least one of the the two reactor fires which are attributed to graphite, the Windscale accident, appears to have not involved a graphite fire. I concluded my Windscale review with the statement,

Given these facts, the assertion that there was a core graphite fire at Chernobyl ought also to be revisited.

This post considers several reports that are relivant to an evaluation of the role of graphite in te Chernobyl fire.

In the wake of the Chernobyl Reactor fire, the United States Department of Energy had a serious concern. The DoE operated a reactor that was similar to the Chernobyl reactor, the N reactor at Hanford, Washington. The N reactor, like the Soviet RBMK-1000, had graphite in its core. The DoE wanted to know if a Chernobyl type accident would be possible at Hanford. The DoE commissioned a review of N Reactor safety in light of the Chernobyl accident. The researchers asked

What is the potential for obtaining conditions conducive to a graphite fire in N Reactor?

And answered,

The graphite stack is protected by a helium cover gas contained within the shield structure. Combustion cannot occur unless the shield structure is sufficiently damaged to leak inert gas faster than available makeup supply. Should that occur, the rate of oxidation would be very slow because graphite temperatures would remain below the threshod for rapid oxidation because of heat removal from the stack by the ECCS [Emergency Core Cooling System] or the GSCS [Graphite and Shield Cooling System], The GSCS alone is capable of removing both decay heat and any heat load from graphite oxidation, stabilizing temperatures in a range which ensures control.

In the Chernobyl accident sequence, the plant was effectively destroyed and conditions for exothermic chemical reactions involving a number of core materials were present before graphite fire made any contribution. It is likely that the major contribution from graphite was to serve as a refractory container for decay heat buildup, zirconium oxidation along with carbothermic reduction of the UO2, and complex gas producing redox reactions. For any N Reactor accident where the GSCS and biological shield are intact, there is no way to achieve ignition of the graphite. It has been demonstrated experimentally that oxidation nuclear grade graphite takes very high temperatures to initiate, and the contribution to total heat load is only a small fraction of the decay heat.

They also reported finding that

Detailed reaction rate models have been developed to analyze graphite oxidation. These models tend to show that graphite oxidation in N Reactor would be limited both by available oxygen and the requirement that a high-temperature source (>1100°C) be available to drive a significant reaction. The analyses have effectively shown that graphite will not con- tribute significant accident heat loads.

Why then did the Chernobyl reactor graphite burn? According to the N Reactor review,

The Chernobyl release must be viewed as resulting from both very high temperatures in the core rubble, extensive mechanical disruption and dispersal of core material and the large draft "chimney effect" that followed the total disruption of that particular reactor configuration. There is no accident sequence that could produce an equivalent disruption of N Reactor; there would be some confinement even in the lowest probability event sequences. Because of the horizontal arrangement of pressure tubes, Chernobyl fission product release rates and magnitude are not pertinent to N Reactor accident scenarios with mechanistic initiators.

In 1987 the NRC did its own safety assessment of the Graphite Reactors it licensed. The NRC report described the limitations of graphite fires,

For reasons that are well understood, graphite is considerably more difficult to burn than is coal, coke, or charcoal. Graphite has a much higher thermal conductivity than have coals, cokes or charcoals, making it easier to dissipate the heat produced by the burning and consequently making it more difficult to keep the graphite hot. Concomitantly, coals, cokes and charcoals develop a porous white ash on the burning surfaces which greatly reduces radiation heat losses while simultaneously allowing air to reach the carbon surfaces and maintain the burning. In addition, coals, cokes and charcoals are heavily loaded with impurities which catalyze the oxidation processes. Nuclear graphite is one of the purest substances produced In massive quantities.

The literature on the oxidation of graphite under a very wide range of conditions is extensive. Effects of temperature, radiation, impurities, porosity, etc., have been studied in great detail for many different types of graphites and carbons [Nightingale, 1962]. This information served as a foundation for the full scale detailed studies on graphite burning accidents In air-cooled reactors initiated and completed at Brookhaven National Laboratory [Schweitzer, 1962a-f]. After British experimenters at Harwell confirmed the results obtained at BNL [Lewis, 1963] there appeared to be no new conclusions from additional work in this field. The aspects of the work pertinent to evaluating the potential for graphite burning accidents are described here In some detail.

Burning, as used here, is defined as self-sustained combustion of graphite. Combustion is defined as rapid oxidation of graphite at high temperatures. Self-sustained combustion produces enough heat to maintain the react- ing species at a fixed temperature or is sufficient to increase the temperature under actual conditions where heat can be lost by conduction, convection, and radiation. In the case where the temperature of the reaction Increases, the temperature will continue to rise until the rate of heat loss Is just equal to the rate of heat production. Sustained combustion is distinguished from self-sustained combustion when, in the first case, the combustion is sustained by a heat source other than the graphite oxygen reactions (e.g., decay heat from reactor fuel).

Early attempts to model the events at Windscale [Robinson, 1961; Nairn, 1961] were followed by the BNL work described here.

Some 50 experiments on graphite burning and oxidation were carried out in 10-foot long graphite channels at temperatures from 600°C to above 800°C. To obtain a lower bound on the minimum temperature at which burning could occur, the experiments were specifically designed to minimize heat losses from radiation, conduction, and convection.

The objectives of the full scale channel experiments were to determine under what conditions burning might initiate in the Brookhaven Graphite Research Reactor (BGRR) and how it could be controlled if it did start. Channels 10-feet long were machined from the standard 4 in. x 4 in, blocks of AGOT graphite used in the original construction. The internal diameter of the BGRR channel was 2.63 Inches. Experiments were also carried out on channel diameters of one to three Inches on 10-foot long test channels In order to obtain generic Information. The full length of the channels was heated by a temperature controlled furnace and was Insulated from conductive heat losses. At intervals along the length there were penetrations in the furnace through which thermocouples used to read the temperature of the graphite and air were introduced, and from which air and air combustion products were sampled. A preheater at the inlet of the graphite channel was used to adjust the air to the desired temperature. The volume of air was controlled and monitored by flow meters to allow flow measurements in both laminar and turbulent flow conditions.

In a typical experimental run the graphite was first heated to a preselected temperature. The external heaters were kept on to minimize heat losses by conduction and radiation. The temperature changes along the graphite channel were then measured for each flow rate as a function of time with the heaters kept on. It was observed that below 675°C it was not possible to obtain temperature rises along the channel if the heat transfer coefficient (h) was greater than 10~ cal/cm-sec-°C. Below 650°C it was not possible to get large temperature rises along the channel with 30°C inlet air temperatures at any flow rate. For h values lower than 10~ cal/cm-sec-''C maximum temperature rises were 0-50"C and remained essentially constant for long periods of time (five hours). For h values greater than 10~ cal/cm-sec-°C the full length of the channel was cooled rapidly.

There were two chemical reactions occurring along channels. At low temperatures the reaction C + O2 to form CO2 predominated. As the temperature Increased along the channel CO formed either directly at the surface of the channel or by the reaction CO2 + C. At temperatures above 700"C, CO reacts in the gaseous phase to form CO2 with accompaniment of a visible flame. It was observed that the unstable conditions which were accompanied by large and rapid Increases in temperature Involved the gas phase reaction CO + O2 and occurred only for h values below 10~ cal/cm-sec-°C below 750"C. Temperature rises associated with the formation of CO2 from C + O2 were smaller than those due to CO + O2 and decreased with time. They too occurred at h values below 10" cal/cm-sec-°C.

In a channel which was held above 650°C there was an entrance region running some distance down the channel which was always cooled. A position was reached where the heat lost to the flowing gas and the heat lost by radial conduction through the graphite was exactly equal to the heat generated by the oxidation of the graphite and of the CO. This position remained essentially constant with time. Beyond this point rapid oxidation of graphite occurred with the accompaniment of a flame (due to the CO-0 gas phase reaction). Under conditions of burning, the phenomena were essentially Independent of the bulk graphite chemical reactivity. Rate controlling reactions during burning were determined by surface mass transport of reactants and products.

The experiments were used to develop an equation which expressed the length of channel that can be cooled as a function of temperature, flow rate (heat transfer coefficient), diameter and reactivity of the graphite. It was found that the maximum temperature at which thermal equilibrium (between heat generated by graphite oxidation and heat removed by the air stream) will occur in a channel can be predicted from the heat transfer coefficient, the energy of activation and a single value of the graphite reactivity at any temperature. Above this maximum temperature the total length of channel Is unstable and graphite will burn. The studies show that the bounding conditions needed to initiate burning are:
1. Graphite must be heated to at least 650°C.
2. This temperature must be maintained either by the heat of combustion or some outside energy source.
3. There imist be an adequate supply of oxidant (air or oxygen).
4. The gaseous source of oxidant must flow at a rate capable of removing gaseous reaction products without excessive cooling of the graphite surface.
5. In the case of a channel cooled by air these conditions can be met. However, where such a configuration is not built into the structure it is necessary for a geometry to develop to maintain an adequate flow of oxidant and removal of the combustion products from the reacting surface. Otherwise, the reaction ceases.

The report went on to discuss the potential contribution of Wignarian energy to a graphite reactor fire, and found that if a reactor operated at a high enough temperature to preform Wignerian annealing its graphite would not accumulate Wignerian energy. The report also stated that,

The factors needed to determine whether or not graphite can burn in air are the graphite temperature, the air temperature, the air flow rates, and the ratio of heat lost by all possible mechanisms to the heat produced by the burning reactions [Schweitzer, 1962a-f]. In the absence of adequate air flow, graphite will not burn at any temperature. Rapid graphite oxidation in air removes oxygen and produces CO2 and CO which, along with the residual nitrogen, suffocate the reaction causing the graphite to cool through unavoidable heat loss mechanisms. Self-sustained rapid graphite oxidation cannot occur unless a geometry is maintained that allows the gaseous reaction products to be removed from the surface of the graphite and be replaced by fresh reactant. This necessary gas flow of Incoming reactant and outgoing products is Intrinsically associated with a heat transfer mechanism. When the incoming air is lower in temperature than the reacting graphite, the flow rate is a deciding factor in determining whether the graphite cools or continues to heat. Experimental studies on graphite burning have shown that for all the geometries tested which Involved the conditions of small radiation and conduction heat losses, it was not possible to develop self-sustained rapid oxidation for graphite temperatures below about 650*'C when the air temperatures were below the graphite temperature. At both high and low flow rates, the graphite was cooled by heat losses to the gas stream even under conditions where other heat loss mechanisms such as radiation and conduction were negligible.

At temperatures above about 650°C, in realistic geometries where radiation is a major heat loss mechanism, graphite will burn only in a limited range of flow rates of air and only when the air temperatures are high. At low flow rates, inadequate ingress of air restricts burning. At high flow rates, the rate of cooling by the flowing gas can exceed the rate of heat produced by oxidation.

Studies have shown that burning will not occur when there is no mechanism to raise the graphite temperature to about 650°C [Schweitzer, 1962a-f]. If the temperature is raised above 650°C, burning will not occur unless a flow pattern is maintained that provides enough air to sustain combustion but not enough to cause cooling. Since the experiments were designed to minimize all heat losses other than those associated with the air flow, 650°C can be considered a lower bound for burning.

Thus the NRC's answer to the original question which I asked at the beginning of this series is "yes, graphite does burn" but only under a very limited set of conditions.

The NRC report simply assumed that those conditions had been meet at Windscale and Chernobyl. We now know what the NRC did not know in 1987, that the Windscale fire was not a graphite fire. Neither report reviewed here offers conclusive evidence that the Chernobyl fire was a graphite fire. A major conclusion of the report draws a big question mark over the Chernobyl graphite fire hypothesis,

in order to have self-sustained rapid graphite oxidation in any of these reactors certain necessary conditions of geometry, temperature, oxygen supply, reaction product removal and favorable heat balance must exist.

Yet the Soviets claimed and American nuclear safety experts like H.J.C Kouts accepted the notion that Graphite could burn like charcoal.

The emission of radionuclide continued for about nine days, aided by burning of the graphite. It is estimated that upwards of ten percent of the graphite in the core burned, in a manner similar to the rapid oxidation of charcoal.

We know that Kouts view cannot be correct, nuclear graphite does not burn like charcoal, and the assertion that only 10% of the Chernobyl core graphite burned does not suggest graphite was the major source of the Chernobyl fire. The question is were the conditions conditions that are conducive to a graphite fire present at Chernobyl, and if so how? In answers to these questions, and without other evidence we must consider the claim of a graphite fire at Chernobyl to be unconfirmed.

As we have seen, the use of graphite in a reactor core is consistent with safe reactor operations. The danger of a core fire due to graphite burning is quite limited. The time has now arrived to ask the question, is it dangerous to use graphite in the core of a Molten Salt Reactor.

We have already noted that the possibility of graphite fires in a reactor core can be eliminated by core design. In the case of Molten Salt Reactors, the possibility of a core fire is eliminated by the two modes of MSR operation. A MSR is only active if liquid salt is present in the core of the reactor. But if liquid salt is present then air cannot be. In the case of the presence of molten salt in the core, the presence of salt would prevent air from reaching the graphite. If the salt is drained, either deliberately, by accident or by operation of the freeze valve safety system, then the heat producing fission products will be drained from the core as well. The absence of fission products in the core would mean that a high enough temperature required to trigger a graphite fire would not be possible. Thus the use of graphite in a Molten Salt Reactor core would be inherently safe.

Is "Chernobyl: Consequences of the Catastrophe for People and the Environment" reliable?

Monday was the 24th anniversary of the Chernobyl disaster and the opponents of nuclear power have taken the opportunity to go on the attack. A previously obscure book, Chernobyl: Consequences of the Catastrophe for People and the Environment (Annals of the New York Academy of Sciences) has drawn significant attention. "Chernobyl Consequences" was written by Alexey V. Yablokov (Center for Russian Environmental Policy, Moscow, Russia), Vassily B. Nesterenko, and Alexey V. Nesterenko (Institute of Radiation Safety, Minsk, Belarus). The study is in fact inaccessible to most people. It was published in an edition of the Annals of the New York Academy of Sciences, although copies can be obtained by forking over considerably more that one hundred dollars to Amazon.com. No scholarly reviews of this study have yet been published,

Yablokov has for some time argued what other researchers take to be extreme views on Chernobyl casualties, for example arguing that

Chernobyl is associated with the worst technical catastrophe in human history.

Yablokov has accepted without question, the debated linear no threshold (LNT) radiation safety hypothesis. He argued in 1999 that such theories are

tragically confirmed in the territories affected by small doses of radioactive emissions from Chernobyl. Official forecasts of merely a few additional cases of cancer by the end of this century have been disproved by a hundredfold increase in the incidence of thyroid cancer.

Thus one potential problem objective reviewers need to be alert for is that Yablokov and his associates may have a bias toward the confirmation of hypotheses that he believes to be true. A second danger is that of circular arguments. That is that Yablokov simply assumes what they were trying to prove, namely that the LNT hypothesis.

Now in order to test the LNT hypothesis given Chernobyl data, researchers must eliminate test illness and death data for non radiation causal sources. Only after all other potential causes are eliminated, can cause be attributed to Chernobyl related radiation. Yablokov seems to jump to conclusions, for example in 1999 he stated

Another unexpected consequence of human exposure to small radiation doses was a significant increase in the number of spontaneous miscarriages. Establishing the exact number is extremely difficult because of incomplete medical statistics, but individual observations seem to tell the story. Consider, for instance, that in Sweden the number of successful conceptions decreased by 600 in June and July of 1986. In Greece, the number of live births from January through March of 1987 was 2,500 lower than expected.6 The same decline was observed in Italy, Germany, Belorussia, and even in the United States.

This argument is an example of the post hoc proper hoc fallacy. A drop in the number of live birth occurred in a number of countries after the Chernobyl accident, but that drop might have been due to other causes, not related to Chernobyl. We would have to subject the birth rate data to a number of tests before we can decide if Chernobyl had any relationship at all to the decline. It is troubling for the argument that the decline of the American birth rate is mentioned, even though the United States received very little Chernobyl related fallout. Unless Yablokov applies far more rigorous tests to his argument, it would not appear to be sound.

Other Yablokok arguments seem to be equally problematic. in his 1999 essay he argued, he argues,

Another terrible consequence of Chernobyl’s low-dose pollution is the sharp increase in the number of retarded children. A comparison of 2,213 newborns in the polluted territories of Belorussia, Russia, and the Ukraine with 2,120 children born in nearby unpolluted territories has shown that more than half of the children born in the former regions display signs of retarded mental development.

Such tests should have been performed with children who were demographically matched to children in the polluted area, and further tests to children from the polluted area who were born prior to the Chernobyl accident. Thus test conditions as stated in 1999 by Yablokov would not have yielded valid conclusions.

From these appearantly invalid arguments in 1999 Yablokov concluded

These data confirm observations on fetal development made in Hiroshima and Nagasaki in the years following detonation of atomic bombs. Clearly, small radiation doses do disturb the normal formation of the central nervous system of the fetus in certain sensitive stages of development.

in fact the rules of science and of logic have been violated in the arguments, so nothing has been demonstrated about the health effects of Chernobyl related radiation. Yabolkov adds,

During the decade following Chernobyl, the danger that small doses of radiation can harm humans has turned from an assumption to a scientific fact backed by dozens—or even hundreds—of scientific studies. These results also highlight the narrow focus of past research, which was mainly oriented toward increased incidence of leukemia. While leukemia is one untoward consequence of radiation exposure, other illnesses and abnormalities are equally as telling and should be emphasized in future studies.

Acceptance of the LNT hypothesis by the scientific community is har from unamous, and Health Physicists, the scientist who by training and experience are best qualified to judge the issue, have stated professionally, that

In accordance with the current knowledge of radiation health risks, the Health Physics Society recommends against quantitative estimation of health risks below an individual dose of 5 rem in one year or a lifetime dose of 10 rem in addition to background radiation. Risk estimation in this dose range should be strictly qualitative accentuating a range of hypothetical health outcomes with an emphasis on the likely possibility of zero adverse health effects. The current philosophy of radiation protection is based on the assumption that any radiation dose, no matter how small, may result in human effects, such as cancer and hereditary genetic damage. There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent.

Health Physicist, Bernard Cohen who studied the health consequences of exposure to high levels of natural background radiation concluded that even scientifically and logically valid studies of the health effects of radiation yielded at best circumstantial evidence, and that conclusions drawn from such studies are not true with mathematical certainty. Cohen included a discussion of common errors found in radiation related epidemiological studies.

Yablokov was also a general editor of the 2006 Greenpeace report, The Chernobyl Catastrophe Consequences on Human Health.

Thus Yablokov is certainly in the orbit if not also the pay of Greenpeace and perhaps other like minded anti-nuclear organizations. This does not mean that what he says is untrue, but it does mean that before his arguments are credited, they should be be carefully vetted. And as I have noted Yablokov has in the past used argument which are on their face invalid and unscientific.

A New York State University professor Karl Grossman has published an account of some of the books findings in Counterpunch. How well Grossman is qualified to write anything like an objective review of a book on Chernocyl causalities is open to question. Mr. Grossman has written two previous books, both of which focused on some nuclear related threat. In Wrong Stuff: The Space Program's Nuclear Threat to Our Planet, Grossman charged that a nuclear related conspiracy lay at the heart of the space program. Amazon reviews were decidedly mixed. One described "wrong stuff" as "Ignorance At Its Utmost." The reviewer added,

The Wrong Stuff clearly shows that uninformed Americans are apt to believe a gross distortion of the truth, so long as it sounds scary and apocalyptic. This book sounds good for a science fiction, conspiracy theory. The book definitely is a thriller, fictitious all the way, designed to scare and outrage the reader. Sadly, the book is poorly researched and has a very faulty logic

Another reviewer titled his review "Dumb," and added,

If you like raving paranoid conspiracy theories, this one might be for you. It seemed poorly researched with big gaps in logic to me. Not enough plot to be science fiction, not enough facts to be journalism.

Anothe Grossman book, "Cover Up: What You Are Not Supposed to Know About Nuclear Power," also received decidedly mixed Amazon reviews. One reviewer wrote,

I can't remember the last time I read a book with so little content.

While another reviewer added,

This book does not make any reasonable arguements. Nor does it support it claims.

Quite obviously the writings of Mr. Groosman belong on Counterpunch. But given the comments of the Amazon readers, it is not clear that we can count of him to offer us an accurate representation of any book on Chernobyl, or to offer an objective review of any book that cast nuclear power in a negative light.

Mr. Grossman makes the following statements:

* The book is solidly based—on health data, radiological surveys and scientific reports—some 5,000 in all.

* It concludes that based on records now available, some 985,000 people died of cancer caused by the Chernobyl accident. That’s between when the accident occurred in 1986 and 2004.

* the International Atomic Energy Agency . . . the new book shows, is under-estimating, to the extreme, the casualties of Chernobyl.

* That the IAEA is corrupt

* That an agreement between the IAEA and the World Health Agency "has muzzled the WHO, providing for the “hiding” from the “public of any information…unwanted” by the nuclear industry."

* that “hundreds of millions of curies, a quantity hundreds of times larger than the fallout from the atomic bombs dropped on Hiroshima and Nagasaki” were released by the chernobyl accident.

* That before the accident, more than 80% of the children in the territories of Ukraine, Belarus and Russia extensively contaminated by Chernobyl “were healthy,” the book reports, based on health data. But “today fewer than 20% are well.”

And so on.

If Mr. Grossman's arguments accurately reflect both the book and the facts, they offer a shocking picture of the consequences of the Chernobyl accident. But even given this picture, these facts do not enhance an objective case against nuclear power. The Chernobyl reactor used a core design that was know to be unsafe right from the dawn of the nuclear era. No current reactor project uses this unsafe design, and it is very unlikely, given the Chernobyl accident that any reactor using a similar design will ever be built again. Secondly, in addition to the dangerous core design, soviet reactor designers omitted two important levels of containment from the Chernobyl reactor. Had those levels been present, it is doubtful that the Chernobyl accident would have had such serious consequences. All reactors built today include these added containment levels. Thus given the safer core design of modern reactors, the addition of numerous additional safety features in new reactor designs, and the presence of the added levels of containment, the likelihood of an accident that would cause a single radiation related casualty would be a once in the life of the universe event. Thus even if the worse case argument about Chernobyl consequences proves true, it does not in the slightest support the case against nuclear power.

In addition to Grossman, an account of the Chernobyl book was recently published by The Environmental News Service. Groosman's account was largely detaile free, but the It added more details about the Environmental News Service account is more detailed. Among the claims reported was the claim that between 1986 and 2005 of the 830,000 people who had participated in the Chernobyl clean up. between 112,000 and 125,000 had died. Although this seems like quite a lot, it should be noted that between 1986 and 2005 the life expectancy of men in the former Soviet Union dropped dramatically. The life expectancy of all Ukrainian men was 62.6 years, while the life expectancy of Russian men, most of whom had not been heavily exposed to Chernobyl related fallout, was 58.8 years.

A United Nations Development Program report stated that the reason for the increased deaths included

rise in self-destructive behaviour, especially among men.

Among self destructive behaviors listed were alcoholism which basically doubled after the Soviet collapse, drug misuse and suicides. In addition the transition to a market economy in the former Soviet Union had produced a great deal of economic displacement, an a significant increase increase in poverty, lowering even further the already low standard of living in the former Soviet Union. Thus the reported deaths of people, mainly men, who died alleged consequence of the Chernobyl accident must be view within the context of the actual death rate of men in the former Soviet Union between 1986 and 2005. Thus a report that by 2005 15% of any group of men working in the former Soviet Union were dead, would probably be considered typical, and perhaps unexpectedly low.

I do not regard this post as a comprehensive review of the study Chernobyl: Consequences of the Catastrophe for People and the Environment, But I am suggesting, that before its arguments and conclusions can be relied on, they need to vetted by well informed, critical reviewers, who are not inclined to accept invalid and unscientific arguments.

The History of Reactor Safety: Chernobyl

Nuclear safety has been an important concern of reactor designers since the first reactors were designed. The earliest reactor designers realized that some reactors designs were safer than others. Safety flaws were known to exist in the reactors built in the Hanford, Washington complex. For that reason the complex was situated in a relatively remote lightly inhabited area.

The safety problem with the Hanford reactor stemmed from the use of water cooling in a graphite reactor. If the reactor heated up to much, the water inside the reactor could could boil within its pipes. Water inside the graphite moderated core, tends to act as a break on the chain reaction. When water is removed from the water cooled graphite reactor the break is released, and the chain reaction speeds up. That adds more heat to the reactor core, and the added heat boils off more water. This process can build up very rapidly until there is a steam explosion that could potentially destroy the reactor core.

Why would anyone ever build such an unsafe reactor? The answer in the case of the United States, was that the Hanford Reactors were built because the American Government believed that it had a military necessity to do so. In 1942, the Germans were known to be developing nuclear technology, and that technology was assumed to have a military purpose. Few of the people who were involved at the time believed that nuclear safety was the most important issue. The danger to national survival that would emerge if Germany was the first to develop a nuclear weapon was the primary concern. Thus the decision was made to build unsafe reactors at Hanford because the reactors would be an important source of weapons grade fissionable Plutonium. The Hanford reactor design was never copied elsewhere in the United States, and when the United States Government around 1950 made the cold war decision to expand its plutonium output, a different, safer reactor design was chosen.

The political leadership of the Soviet Union dictated that nuclear safety was not an issue, When Yuri Andreyev took an examination to become a Soviet nuclear plant operator, he was asked to describe how a reactor could explode. He answered the question by describing three different scenario. The examiner criticized Andreyev 's answer,

"Keep it in your mind, man -- Soviet reactors cannot explode,"

the examiner told him.

With this attitude the Soviet Union chose to copy the Hanford design for plutonium production reactors. Later these military heated coolant water from these reactors was used to produce steam that drove electrical generators. This experiment was considered successful, and the Soviet leadership chose to develop a new class of power generating civilian reactors that was based on the old, unsafe Hanford design, During the 1950's the Soviets began producing a slightly modified Plutonium production reactor, that could also generate steam for electrical generation. These reactors were designed with little regard to the nuclear safety concerns, but they were cheap and easy to manufacture, so the Soviets adopted the design for a single purpose power reactor, the RBMK.

All of the safety problems of the original Hanford design were still present in the RBMK. While thinking about nuclear accident containment had advanced in the West, the Soviets failed to see the point. Western researchers had investigated how radioisotopes could escape into the environment in a nuclear accident, and how that escape could be prevented. Western power reactors reactor designs included a system of multiple barriers to the release of radioactive materials into the environment in case of an accident. The Western nuclear containment system was tested at Three Mile Island when a an error by a reactor operator turned a minor nuclear incident into a core meltdown. The TMI accident was contained without and known illnesses or deaths as a consequence. Some radiation was released in the form of radio active gases, but radio active gasses are also released by burning natural gas in the home. Despite the evidence that the Western containment system prevented radiation related deaths and illness, the Soviets slightly modified RBMK reactor building design to improve containment, but this modification still left RBMK containment far short of Western standards.

In addition to its known safety problems, the RBMK design included a serious hidden safety flaw. Sonja Schmid, in a study of Soviet nuclear safety practices notes,

Soviet design choices display a similar circular pattern: while the “Sovietness” of the graphite-water reactor (the “Chernobyl’ type” reactor) was invoked to legitimize its development, mass production and implementation, the reactor design itself then served as proof of Soviet technological prowess.

In addition to the already noted RBMK safety flaws, there was a defect in the design of its control rods. There was a graphite extension of the control rod, that entered the reactor prior to the neutron absorbing metal that formed the main body of the control rod. The control rod channels were normally filled with water, which as I have already noted acted as a break on the chain reaction. The presence of more graphite actually accelerated the chain reaction, as did the displacement of water in the control channels. As a result, as the control rods entered the reactor, there was a massive spike in core reactivity. Power output rose quickly to an estimated 30 Billion thermal watts, ten times what the RBMK was designed to handle. As this process unfolded, the heat of the reactor core increased dramatically, fuel elements and full channels began to rupture and pressure built up in the coolant water tubes As steam began to escape the rupturing tubes, steam pressure inside the reactor case increased, until the case explosively ruptured. A two thousand ton plate that covered the top of the reactor was blown off by the force of the explosion, opening the upper surface of the reactor to the sky. Three seconds later, an even more violent explosion occurred ripped the core apart, flinging chunks of burning radioactive matter high into the air.

Under ordinary circumstances graphite does not burn, but the graphite in the Chernobyl was heated white hot, and the mass of superheated graphite was exposed to an airflow, that ignited it. Then all of the Soviet nuclear mistakes, all of the Soviet arrogance that vainly assumed that nothing bad could happen to Soviet reactors came home with horrific force.

The Chernobyl reactor disaster was not simply a result of poor reactor design, but it was the result of an ideological system that believed that Soviet reactors could not fail. Mistakes made by the Chernobyl operators played a very large role in the accident. Those mistakes were the result of a test requirement imposed on the reactor staff, by the authoritarian Soviet system. Reactor safety issues were simply ignored in the performance of the test, and the reactor staff drove the reactor to and past its breaking point. It is unlikely that there would have been a Chernobyl, had there not been a Soviet Union.

A Primer on Nuclear Safety: 2.2 Defense in Depth

A Primer on Nuclear Safety:
2.2 Defense in Depth
Controlling Nuclear Reactions in Light Water Reactors

The Enrico Fermi was the first nuclear scientist to find a solution for controlling chain reactions in a nuclear reactor. Fermi said that his Chicago Pile was "a crude pile of black bricks and wooden timbers." Of course natural uranium fuel was added. There was, Fermi realized, something else needed in order to make his reactor safe. That was a means of soaking up the neutrons created by the chain reaction in order to control it. The way Fermi chose to control the Chicago Pile was to insert a number of cadmium-coated control rods into the reactor. Inserting the rods would slow the chain reaction and eventually stop it. In fact history reports that the chain reaction in Fremi's first pile was initiated by lifting control rods that were initially embedded in the pile.

Fermi and his associates were none to confident in the mechanical reliability of the control rods. Thus a back up system was devised for an emergency shut down of the reactor in case the control rods failed to operated properly during a shut down. A history of the Chicago Pile experiment states:

Since this demonstration was new and different from anything ever done before, complete reliance was not placed on mechanically operated control rods. Therefore, a “liquid-control squad,” composed of Harold Lichtenberger, W. Nyer, and A. C. Graves, stood on a platform above the pile. They were prepared to flood the pile with cadmium-salt solution in case of mechanical failure of the control rods.

In many respects the Fermi's CP-1 was the evolutionary ancestor of the Light Water Reactor, and the control scheme for Light Water Reactors is basically the same as for the CP-1 although a few twists have been added.   The control rods now use Hafnium rather than cadmium for neutron absorption.  And rather than flooding the core with a Cadmium salt solution, boron in the form of boric acid is injected directly into the cooling water. Because the dilution of the boric acid in the cooling water can be easily altered, the use of boron is by no means limited to reactor shutdown. By altering the boric acid content of cooling water reactor operators can actually control the chain reaction, thus providing a simple but effective throttle for a chain reaction.

It is possible to completely shut dow a LWR by increasing the boron content of the coolant water, or by a high concentration of boric acid in the emergency coolant water. However reactor shut downs and start ups are normally controlled by the control rods. Control rods can also be used to control chain reactions in parts of the reactor, and the play a major role in counteracting Xenon poisoning.

Xenon-135 is a nobel gas that is produced in the fusion process. It is highly radioactive, but in addition it has a very powerful neutron absorbing property. Because of this property, even a relatively small amount of Xenon, produced during a chain reaction has the capacity to slow down and even stop the chain reaction. Hence reactor controls must posses a means of balancing reactor power output as the amount of Xenon in the reactor increases due to Xenon production by nuclear fission.

Controlling reactor power in the face of Xenon poisoning is not simple. Xenon builds up with the fission process, and decreases as it undergoes nuclear decay. There can be a lag between the positioning of a control rod and its effect on the power level and heat generation of a reactor. Running a light water reactor at low power increases difficulties related to Xenon poisoning. Xenon may not be evenly spread through the core. Thus hot and cool spots may develop in the core. since coolant flow is based on average power output, coolant flow to reactor hot spots may not be sufficient, and as fuel pellets begin to overheat, and their cladding begin to overheat, their integrity may begin to break down.

Control rods played a far more critical role in the Chernobyl accident than would be possible in a light water reactor accident. In the Chernobyl RBMK reactors the control rods were divided into three segments. The upper and lower segments were made of graphite, while the middle segment was made of graphite a nuclear moderator, while the middle segment was made of a of a material that served as a neutron poison. As the control rod is lifted coolant water fills the bottom of the control rod channel. Under normal operations the control rod is lifted to the position in which its lower graphite segment fills the channel inside the reactor core. But because a highly dangerous test was being conducted on the Chernobyl reactor, the lower graphite segments of control rods were also partially withdrawn from the reactor. Thus more water entered the reactor core through the control rods channels. As I have already observed coolant water served as a break on the chain reaction within the Chernobyl reactor. As we observed poor management of the Chernobyl reactor during a test lead to the boiling of the coolant water inside the reactor and the voids created by the steam bubbles began to removed the break placed on the chain reaction by the presence of coolant water in the core. As the overly withdrawn control rods began to descend into the core of the Chernobyl reactor they first displaced water that had served as a break on the chain reaction, and replace it with graphite, a moderator that greatly increased the chain reaction.

The insertion of the graphite tips of the control rods into the core of the Chernobyl RBMK was sort of like attempting to hit the breaks of car that is running out of control, and hitting the accelerator instead. The power level of the Chernobyl reactor went off the charts, and as reactor heat increased dramatically the remaining coolant water in the reactor core flashed to steam. There was a large steam explosion, which as we have seen destroyed the top of the reactor and the surrounding radiation shield.

A similar accident could not occur in a light water reactor because (a) water in the control rod channels moderates rather than slows the nuclear reaction, (b) the water in the control rod channels is immediately displaced by a neutron poisoning material in the control rod, and (c) voids created by heat related bubbles in the reactor coolant work in concert with the control rods rather than against them.

Control rod insertion during an accident can be accomplished by gravity rather than mechanical means. Control rods can be attached to the lifting mechanism by electro-magnets. The termination of power output during an accident would automatically produce control rod insertion and shut down. The shutdown can also be triggered by operator control.

There are redundancies in the control rod system and of course reactor operators always have the options of shutting the reactor down by adding more boric acid to the coolant water. The loss of reactor coolant does cause a termination of the chain reaction, because the water serves as a moderator for the chain reaction in Light Water Reactors. At the same time the loss of coolant water is highly undesirable, because coolant water is required to remove the residual heat from fission product decay from the reactor core.

The control system of Light Water Reactors thus provides for operational redundancies and safety backups. Emergency shutdowns can be accomplished by passive safety features that use the law of nature to insure that a chain reaction stops as soon as the reactor ceases to produce electrical power. Different control systems insure that operators have more than one emergency shutdown system available. Finally automatically operating passive shut down systems, insure that the reactor will automatically shut down before serious safety problems emerge, thus interrupting chains of events that could lead to serious reactor accidents.