Three Mile Island accident

From the late 1950s to the Three Mile Island accident 

Since the early 1960s and even before, in the West, the criterion of locating power reactors in a leak-proof and pressure resistant containment vessel was established and consolidated. In those cases where a significant release of radioactive products could be possible, the design pressure of the containment was 

Indeed, since the 1950s, the US Reactor Safeguards Committee , set up by the Atomic Energy Commission with the task of defining the guidelines for nuclear safety, had indicated that, for a non-contained reactor, an exclusion distance (without resident population) should be provided. This distance, R, had to be equal, at least to that given by Eq. 1.1. 

For a 3000 MW reactor (the usual size today), this exclusion distance is equal to approximately 30 km, which is equal to the distance evacuated after the Chernobyl accident (Bourgeois et al, 1996). Evidently, the reference doses for the short-term evacuation were roughly the same for the two cases. An exclusion distance of this magnitude poses excessive problems to siting, even in a country endowed with abundant land such as the USA, therefore, the decision of adopting a containment is practically a compulsory one. 

The first reactor with leakproof and pressure resistant containment was the SRI reactor (West Milton, NY, built in the 1950s). Built to perform tests for the development of reactors for military ship propulsion this reactor was cooled by sodium and the containment was designed for the pressure corresponding to the combustion of the sodium escaping from a hypothetical leak in the cooling circuit. 

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The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

Several accidents in nuclear facilities have been extensively analyzed and reported. The three most widely publicized accidents were at Windscale (now known as Sellafield), United Kingdom, in 1957 Three Mile Island, Pennsylvania, in 1979 and Chernobyl, Ukraine, in 1986 (UNSCEAR 1988 Severn and Bar 1991 Eisler 1995). From the accident at Windscale about 750 trillion (T)Bq 22 TBq Cs, 3 TBq Sr, and 0.33 TBq °Sr were released and twice the amount of noble gases that were released at Chernobyl, but 2000 times less and Cs. From the Three Mile Island accident, about 2% as much noble gases and 50,000 times less than from the Chernobyl accident were released. The most abundant released radionuclides at Three Mile Island were Xe, Xe, and but the collective dose equivalent to the population during the first post-accident days was <1% of the dose accumulated from natural background radiation in a year. 

The Three Mile Island accident, rated as one of the top media events of the century, was a crowning blow in the battle. The media consistently portrayed the accident as a close call on a public health disaster, and continue to do so to this day, although none of the studies done after the accident gives any reason to believe that to be the case. As demonstrated in those studies, the containment building would have prevented release of large amounts of the radioactivity regardless of what might have hap-... 

In the five years since the first volume was published, there has been increased interest in the chemistry within gas lasers and the chemistry induced by laser radiation, the kinetics and photochemistry within fusion and industrial plasmas, as well as in the normal and perturbed lower and upper atmosphere. And. since the Three Mile Island accident there has been renewed interest in radiation damage to living and nonliving things. This state of affairs has not only precipitated a variety of spectroscopic studies, but has also brought more attention to the nonspectroscopic aspects of excited state production and the interaction of excited species. The latter topic was stressed in the earlier volume and the emphasis is retained here. 

The release of 131I and other fission products in reactor accidents has been considered in the previous chapter. In the Windscale accident, the temperature in the fire zone reached an estimated 1300°C and 8 tonne of uranium metal melted. Over 25% of the 1311 in the melted fuel escaped to atmosphere. In the Chernobyl accident, the fuel was U02, the temperature exceeded 2000°C, and about 25% of the total reactor inventory of 131I was released to atmosphere, as vapour or particulate aerosol. In the Three Mile Island accident, 131I remained almost completely in the reactor coolant. The activities of 131I released in reactor accidents, including that at Chernobyl, have totalled much less than the activities released from weapons tests (Table 2.3). 

As anticipated, there have been occasional equipment failures involving reactors, but the safety systems have been sufficiently redundant so that one or more have always worked. Even in the Three Mile Island accident in 1979, the safety systems worked as designed. Much of the damage resulted from operator actions to override the safety systems. As concluded in the Reactor Safety Report, the limitations of the operator created and then seriously aggravated the Three Mile Island incident. Nevertheless, the features of the containment system prevented significant exposure to the plant personnel or any off-site individual, this despite failure of the barriers provided by the fuel cladding and the primary coolant system. 

It should also be remembered that most evolving technologies, whether boilers during the 19th century, airplanes in this century, or nuclear plants, entail some accidents from which lessons are learned. Both the Three Mile Island accident, from which only limited radioactivity escaped to the environment, and the Chernobyl disaster, have led to the introduction of new safety features in nuclear reactors, in plant operating procedures, and in regulations. 

Only two earlier reactor accidents caused significant releases or radionuclides the one at Windscale (United Kingdom) in October 1957 and the other at Three Mile Island (United States) in March 1979 (UNSCEAR-1982). While it is very difficult to estimate the fraction of the Windscale radionuclide core inventory that was released to the atmosphere, it has been estimated that the accident released twice the amount of noble gases that was released at Chernobyl, but 2,000 times less and Cs (DOE-1987). The Three Mile Island accident released approximately 2% as much noble gases and 0.00002% as much l as the Chernobyl accident. 

Systems analysis techniques are used only to a limited extent in the process industries for two reasons. First, such techniques are not generally effective at predicting human behavior (e.g., WASH 1400 did not anticipate the Three Mile Island accident). Yet human performance is a very important component of safety performance in the process industries. Second, the use of PRA methodologies is generally time-consuming and expensive— particularly when used in the chemical industries where there is so much difference from facility to facility. 

This, the Windscale accident, was for many years the most serious reactor accident it had more radiological consequences than the Three Mile Island accident of 1979. Clearly the accident had the salutary effect of making designers and operators safety conscious in the UK. 

With the Three Mile Island accident, much publicity was given to the potential explosion hazard associated with a hydrogen bubble which had formed above the nuclear core. 

Figure 7-1 gives a first impression of possible effective committed doses. It shows data from the Chernobyl, Windscale and Three Mile Island accidents (collected by G. Santarossa), together with a subjective evaluation of the effects of a maximum... 

The decision to create DID in the plants was taken at the start of nuclear energy development which indicates a remarkable farsightedness, as subsequent history has demonstrated that it has been the best defence against the uncertainties of the technology and the mistakes initially made (see, for example, the Three Mile Island accident). Obviously, in the first period of nuclear energy, many protests were made against this waste of resources which consisted in the construction of costly barriers (e.g. the containment) without, according to some, a real need for them. 

Whereas the dangers of the above example never materialized, the behaviour of the reactor pressure vessel during the Three Mile Island accident is exceptional. It withstood the outpouring of about twenty tons of molten core on its bottom, in conditions of highly deteriorated internal cooling. This behaviour indicated to the technical experts the presence of a powerful and up to then neglected barrier in the Defence in Depth, which is now utilized in a planned way as a potential asset. 

In the following, both experience data (from the Three Mile Island accident) and deterministic considerations justified by the existing knowledge are shown. At least a picture of the important factors for the decrease of the vessel damage probability and indications on the still necessary research will be obtained. In particular, the importance of the prevention of severe accidents will be clearly demonstrated. 

The fact that in a small LOCA with the rupture on the top of the pressurizer it is possible to have the pressurizer almost full of water and the vessel almost empty was demonstrated in a couple of transients in Belgium and in the USA, but in a more dramatic way in the Three Mile Island Accident. 

As it can be seen, both the risk objectives and the risk analyses on existing plants are reassuring, but, it is frequently asked, how reliable these analyses are How much the inevitable uncertainties on data and methods can influence the results Is it possible that some accident sequence has been forgotten in performing a probabilistic analysis All the available information, including the analyses made before the Three Mile Island accident and the sequence of events in the accident itself, indicate that a corrective... 

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