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Sizewell C and nuclear accidents

No2Nucear Power March 2019, EDF Energy has been running its third stage of public consultation on the proposals for a new nuclear power station at Sizewell in Suffolk. The consultation closes on 29 March 2019. (1) As with earlier consultations on proposed new nuclear stations, the principle of nuclear power generation is deemed to have been settled during the process of drawing up National Policy Statements. (2) Nevertheless, with the anniversaries of Fukushima, Chernobyl and Three Mile Island all occurring during March, this seems like a good opportunity to re-visit the risk of a nuclear accident at the proposed nuclear stations.

A severe accident scenario was postulated by the Radiological Protection Institute of Ireland in 2013. (3) This would involve a loss of coolant combined with a bypass of the containment. Core damage would be initially delayed by actions of the plant operators, but eventually takes place after 12.75 hours. The release of fission products to the environment starts 12.8 hours after reactor shutdown, and lasts for 35.2 hours eventually stopping 48 hour after reactor shutdown.

Nuclear engineer, the late John Large, expanded on this type of scenario pointing out that the fuel core would completely melt after about 16 hours and the corium mass slumps to the bottom of the Reactor Pressure Bessel (RPV), thereafter burning through the RPV steel shell to fall and slump onto the primary containment floor. At this point in time, the hydrogen gas in the RPV circuit is released into the primary containment whereupon it reacts with the air in the containment, deflagrating and exploding with sufficient might to breach the containment surety and, with this, the first phase release of radioactivity to the atmosphere for dispersion and deposition further afield commences. He said this scenario is very similar to the events at Fukushima. (4)

According to EDF Energy´s Environmental Statement for Hinkley Point C (Appendix 7E “Assessment of Transboundary impacts”), the likely impacts of an accident do not extend beyond the county of Somerset and the Severn Estuary. In contrast a report for the Austrian Environment Agency says severe accidents at HPC with considerable releases of caesium-137 cannot be ruled out, although their probability may be low. There is no convincing rationale why such accidents should not be addressed in the Environmental Statement (ES); quite to the contrary, it would appear rather evident that they should be included in the assessment since their effects can be widespread and long-lasting. (5)

The RPII Severe Accident Scenario suggests a radioactive release of I-131 and Cs-137 amounting to 610,000TBq which is quite a bit larger than Fukushima. Cs-137 has a half-life of 30 years, whereas I-131 only has a half- life of 8 days. So Cs-137 is much more important in the longer term. With its longer half-life Cs-137 is around for much longer. Having said that I-131 distribution after an accident is important when looking at the incidence of thyroid cancer. Austria had the second highest average I-131 deposition density, outside Belarus, Ukraine and Russia, after Chernobyl. (As ever, whether there was an increase in thyroid cancer in Austria after Chernobyl is controversial – see TORCH 2016).

Spent Fuel Storage Unlike spent fuel generated by existing UK nuclear reactors, it is not the intention of future reactor operators to reprocess spent fuel from new nuclear reactors, so spent fuel will almost certainly remain on-site for decades, rather than being transported off-site to Sellafield as it is at the moment at most sites apart from Sizewell B. Although it is possible that spent fuel might start to be transported off site during the 60 year lifetime of new reactors, prospective operators generally take the view that it is prudent to plan to store all of the lifetime arisings of the planned reactors on-site probably in spent fuel storage ponds. At Hinkley Point C, EDF is planning to be able to extend the life of the storage ponds for up to 100 years after the reactors close. (14)

A recent study in the US detailed how a major fire in a spent fuel pond “could dwarf the horrific consequences of the Fukushima accident.” The author Frank von Hippel, a nuclear security expert at Princeton University, who teamed with Princeton’s Michael Schoeppner on the modelling exercise said “We’re talking about trillion-dollar consequences.” (15) This would clearly involve major transboundary radioactive releases much larger than those suggested in the RPII scenario, because the spent fuel store could contain up to 60 years’ worth of spent fuel.

According to the Austrian Analysis PSA 2 results (in the Pres-Construction Safety Reports by EDF and Areva) show that a possible severe accident in the spent fuel pool could result in a release of 1,780,000 TBq of Cs-137. (16) In other words, the greatest risk is one that could remain in place until at least 2130. …………..

March 25, 2019 - Posted by | safety, UK

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