NRC short history The Problem of Core Meltdown The regulatory staff sought to gain as much experimental data as possible to enrich its knowledge and inform its collective engineering judgment. This was especially vital in light of the many unanswered questions about reactor behavior. The AEC had sponsored hundreds of small-scale experiments since the early 1950s that had yielded key information about a variety of reactor safety problems. But they provided little guidance on the issue of greatest concern to the AEC and the ACRS by the late 1960s--a core meltdown caused by a loss-of-coolant accident. Reactor experts had long recognized that a core melt was a plausible, if unlikely, occurrence. A massive loss of coolant could happen, for example, if a large pipe that fed cooling water to the core broke. If the plants emergency cooling system also failed, the build-up of decay heat (which resulted from continuing radioactive decay after the reactor shut down) could cause the core to melt. In older and smaller reactors, the experts were confident that even under the worst conditions--an accident in which the loss of coolant melted the core and it, in turn, melted through the pressure vessel that held the core--the containment structure would prevent a massive release of radioactivity to the environment. As proposed plants increased significantly in size, however, they began to worry that a core melt could lead to a breach of containment. This became their primary focus partly because of the greater decay heat the larger plants would produce and partly because nuclear vendors did not add to the size of containment buildings in corresponding proportions to the size of reactors. The greatest source of concern about a loss-of-coolant accident in large reactors was that the molten fuel would melt through not only the pressure vessel but also through the thick layer of concrete at the foundation of the containment building. The intensely radioactive fuel would then continue on its downward path into the ground. This scenario became known as the China syndrome, because the melted core would presumably be heading through the earth toward China. Other possible dangers of a core meltdown were that the molten fuel would breach containment by reacting with water to cause a steam explosion or by releasing elements that could combine to cause a chemical explosion. The precise effects of a large core melt were uncertain, but it was clear that the results of spewing radioactivity into the atmosphere could be disastrous. The ACRS and the regulatory staff regarded the chances of such an accident as low; they believed that it would occur only if the emergency core cooling system (ECCS), made up of redundant equipment that would rapidly feed water into the core, failed to function properly. But they acknowledged the possibility that the ECCS might not work as designed. Without containment as a fail-safe final line of defense against any conceivable accident, they sought other means to provide safeguards against the China syndrome. The Emergency Core Cooling Controversy At the prodding of the ACRS, which first sounded the alarm about the China syndrome, the AEC established a special task force to look into the problem of core melting in 1966. The committee, chaired by William K. Ergen, a reactor safety expert and former ACRS member from Oak Ridge National Laboratory, submitted its findings to the AEC in October 1967. The report offered assurances about the improbability of a core meltdown and the reliability of emergency core cooling designs, but it also acknowledged that a loss-of-coolant accident could cause a breach of containment if ECCS failed to perform. Therefore, containment could no longer be regarded as an inviolable barrier to the escape of radioactivity. This represented a milestone in the evolution of reactor regulation. In effect, it imposed a modified approach to reactor safety. Previously, the AEC had viewed the containment building as the final independent line of defense against the release of radiation; even if a serious accident took place the damage it caused would be restricted to the plant. Once it became apparent that under some circumstances the containment building might not hold, however, the key to protecting the public from a large release of radiation was to prevent accidents severe enough to threaten containment. And this depended heavily on a properly designed and functioning ECCS. The problem facing the AEC regulatory staff was that experimental work and experience with emergency cooling was very limited. Finding a way to test and to provide empirical support for the reliability of emergency cooling became the central concern of the AECs safety research program. Plans had been underway since the early 1960s to build an experimental reactor, known as the Loss-of-Fluid-Tests (LOFT) facility, at the AECs reactor testing station in Idaho. Its purpose was to provide data about the effects of a loss of coolant accident. For a variety of reasons, including weak management of the test program, a change of design, and reduced funding, progress on the LOFT reactor and the preliminary tests that were essential for its success were chronically delayed. Despite the complaints of the ACRS and the regulatory staff, the AEC diverted money from LOFT and other safety research projects on existing light-water reactor design to work in the development of fast-breeder reactors. A proven fast breeder was an urgent objective for the AEC and the Joint Committee; Seaborg described it as a priority national goal that could assure an essentially unlimited energy supply, free from problems of fuel resources and atmospheric contamination. To the consternation of the AEC, experiments run at the Idaho test site in late 1970 and early 1971 suggested that the ECCS in light-water reactors might not work as designed. As a part of the preliminary experiments that were used to design the LOFT reactor, researchers ran a series of semiscale tests on a core that was only nine inches long (compared with l44 inches on a power reactor). The experiments were run by heating a simulated core electrically, allowing the cooling water to escape, and then injecting the emergency coolant. To the surprise of the investigators, the high steam pressure that was created in the vessel by the loss of coolant blocked the flow of water from the ECCS. Without even reaching the core, about 90 percent of the emergency coolant flowed out of the same break that had caused the loss of coolant in the first place. In many ways the semiscale experiments were not accurate simulations of designs or conditions in power reactors. Not only the size, scale, and design but also the channels that directed the flow of coolant in the test model were markedly different than those in an actual reactor. Nevertheless, the results of the tests were disquieting. They introduced a new element of uncertainty into assessing the performance of ECCS. The outcome of the tests had not been anticipated and called into question the analytical methods used to predict what would happen in a loss-of-coolant accident. The results were hardly conclusive but their implications for the effectiveness of ECCS were troubling. The semiscale tests caught the AEC unprepared and uncertain of how to respond. Harold Price, the director of regulation, directed a special task force he had recently formed to focus on the ECCS question and to draft a white paper within a month. Seaborg, for the first time, called the Office of Management and Budget to plead for more funds for safety research on light-water reactors. While waiting for the task force to finish its work, the AEC tried to keep information about the semiscale tests from getting out to the public, even to the extent of withholding information about them from the Joint Committee. The results of the tests came at a very awkward time for the AEC. It was under renewed pressure from utilities facing power shortages and from the Joint Committee to streamline the licensing process and eliminate excessive delays. At the same time, Seaborg was appealing--successfully--to President Nixon for support of the breeder reactor, and controversy over the semiscale tests and reactor safety could undermine White House backing for the program. By the spring of 1971, nuclear critics were expressing opposition to the licensing of several proposed reactors, and news of the semiscale experiments seemed likely to spur their efforts. For those reasons, the AEC sought to resolve the ECCS issue as promptly and quietly as possible. It wanted to settle the uncertainties about safety without arousing a public debate that could place hurdles in the way of the bandwagon market. Even before the task force that Price established completed its study of the ECCS problem, the Commission decided to publish interim acceptance criteria for emergency cooling systems that licensees would have to meet. It imposed a series of requirements that it believed would ensure that the ECCS in a plant would prevent a core melt after a loss-of-coolant accident. The AEC did not prescribe methods of meeting the interim criteria, but in effect, it mandated that manufacturers and utilities set an upper limit on the amount of heat generated by reactors. In some cases, this would force utilities to reduce the peak operating temperatures (and hence, the power) of their plants. Price told a press conference on June 19, 1971 that although the AEC thought it impossible to guarantee absolute safety, he was confident that these criteria will assure that the emergency core cooling systems will perform adequately to protect the temperature of the core from getting out of hand. The interim ECCS criteria failed to achieve the AECs objectives. News about the semiscale experiments triggered complaints about the AECs handling of the issue even from friendly observers. It also prompted calls from nuclear critics for a licensing moratorium and a shutdown of the eleven plants then operating. Criticism expressed by the Union of Concerned Scientists (UCS), an organization established in 1969 to protest misuse of technology that had recently turned its attention to nuclear power, received wide publicity. The UCS took a considerably less sanguine view of ECCS reliability than that of the AEC. It sharply questioned the adequacy of the interim criteria, charging, among other things, that they were operationally vague and meaningless. Scientists at the AECs national laboratories, without endorsing the alarmist language that the UCS used, shared some of the same reservations. As a result of the uncertainties about ECCS and the interim criteria, the AEC decided to hold public hearings that it hoped would help resolve the technical issues. It wanted to prevent the ECCS question from becoming a major impediment to the licensing of individual plants. The AEC insisted that its critics had exaggerated the severity of the ECCS problem. The regulatory staff viewed the results of the failed semiscale tests as serious but believed that the technical issues the experiments raised would be resolved within a short time. It did not regard the tests as indications that existing designs were fundamentally flawed and it emphasized the conservative engineering judgment it applied in evaluating plant applications. But the ECCS controversy damaged the AECs credibility and played into the hands of its critics. Instead of frankly acknowledging the potential significance of the ECCS problem and taking time to fully evaluate the technical uncertainties, the AEC acted hastily to prevent the issue from undermining public confidence in reactor safety or causing licensing delays. This gave credence to the allegations of its critics that it was so determined to promote nuclear power and develop the breeder reactor that it was inattentive to safety concerns. end quote
Posted on: Mon, 22 Sep 2014 01:06:55 +0000
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