The total release of radioactive iodine-131 was calculated to be about 750 000 GBq. This resulted in concentrations of radioactive iodine in milk produced locally and a subsequent ban on the use of milk produced in the coastal area around the site for a short period.
This is the only major reactor accident which has ever occurred in the United Kingdom. The reactor was designed for plutonium production, no electricity was generated. The cooling system was a once through system, and there was effectively no containment. This reactor therefore bore little similarity to any commercial nuclear power station.
4.7 Three Mile Island 1979
Three Mile Island is a nuclear power station using PWRS, which was operated by Metropolitan Edison. It is situated on the Susquehanna River near Harrisburg, Pennsylvania, USA. In March 1979 an accident happened at the plant's Unit 2 reactor. Analysis of the accident is fairly complex but in brief the events were as follows.
The main feed pumps, providing water to the secondary, steam side, of the steam generators, tripped out. The loss of this heat removal system caused the temperature and pressure in the primary circuit to rise. This was not an unexpected series of events, the automatic systems came into operation, the control rods were driven into the core and the fission process stopped.
Subsequently, however, a series of relatively minor faults resulted in the operators in the control room being faced with a highly confusing set of instrument readings which they failed to interpret correctly. Most importantly, a system for providing water under just such fault conditions had been closed off some time before the accident for routine testing and had inadvertently been left in that condition, moreover there was no adequate information of that fact in the control room. Loss of coolant via a relief valve that failed to close, and failure to adequately make up the coolant to the core, led to overheating of the fuel pins and subsequent damage. Primary circuit water together with fission products escaped into the containment building leading to a high water level in the containment building sump. This caused a sump pump to start automatically, pumping some of the sump contents to an auxiliary building. The containment building was not at this point isolated from the auxiliary building. Fission products in the auxiliary building were vented to atmosphere via a stack. The subsequent release to atmosphere consisted of about 500 GBq of I-131 together with noble gases.
The radiological consequences of this major accident were small. The maximum dose to a member of the local population was less than 0.5 mSv. Of the 600 milk samples taken only 49 were found to contain any measurable amount of iodine-131 and all were well below the derived emergency reference level for iodine in milk, (see Chapter 10).
There is no doubt that the Three Mile Island accident was serious. It was caused by a catalogue of failures, misinterpretations and wrong decisions which resulted in the wrecking of a very valuable piece of capital plant. Notwithstanding this, the radiological hazard to the local population was small, principally because the PWR containment functioned well. If a similar major accident were to occur at Daya Bay its containment would also ensure that the environmental radiological consequences would be small. Extensive reviews of the details of the TMI accident have resulted in significant improvements in design and operating procedures in PWRS, including the Framatome designed PWR at Daya Bay. In particular alarm displays have been redesigned to ensure that the operators are presented with clearer information.
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