Editing Magnox (section)

==Safety==
{{more citations needed section|date=October 2010}}[[File:Bradwell nuclear power station, from entrance road.jpg|thumb|left|The reactor buildings of [[Bradwell nuclear power station|Bradwell]] magnox nuclear power station]]
The magnox reactors were considered at the time to have a considerable degree of inherent safety because of their simple design, low power density, and gas coolant.  Because of this they were not provided with [[Containment building|secondary containment]] features. A safety design principle at the time was that of the "maximum credible accident", and the assumption was made that if the plant were designed to withstand that, then all other lesser but similar events would be encompassed. [[Loss of coolant]] accidents (at least those considered in the design) would not cause large-scale fuel failure as the Magnox cladding would retain the bulk of the radioactive material, assuming the reactor was rapidly shutdown (a [[SCRAM]]), because the decay heat could be removed by natural circulation of air. As the coolant is already a gas, explosive pressure buildup from boiling is not a risk, as happened in the catastrophic [[steam explosion]] at the [[Chernobyl accident]]. Failure of the reactor shutdown system to rapidly shut down the reactor, or failure of natural circulation, was not considered in the design. In 1967 [[Chapelcross nuclear power station|Chapelcross]] experienced a fuel melt due to restricted gas flow in an individual channel and, although this was dealt with by the station crew without major incident, this event had not been designed or planned for, and the radioactivity released was greater than anticipated during the station design.

Despite the belief in their inherently safe design, it was decided that the magnox stations would not be built in heavily populated areas. The positioning constraint decided upon was that any 10-degree sector would have a population less than 500 within {{convert|1.5|mi}}, 10,000 within {{convert|5|mi}} and 100,000 within {{convert|10|mi}}. In addition population around the site in all directions would be less than six times the 10-degree limits. Planning permission constraints would be used to prevent any large growth of population within five miles.<ref name=grimston-201310>{{cite report |url=http://www.econ.cam.ac.uk/research-files/repec/cam/pdf/cwpe1344.pdf |title=The Siting of UK Nuclear Power Installations |author1=M.C. Grimston |author2=W.J. Nuttall |publisher=University of Cambridge |date=October 2013 |id=CWPE 1344 & EPRG 1321 |access-date=16 September 2018}}</ref>

In the older steel pressure vessel design, boilers and gas ducting are outside the concrete biological shield. Consequently, this design emits a significant amount of direct [[gamma radiation|gamma]] and [[neutron radiation]], termed ''direct shine'', from the reactors.<ref name=no2nuclearpower->{{cite journal |url=http://www.no2nuclearpower.org.uk/articles/se95-Fairlie.pdf |title=Magnox gamma shine |last=Fairlie |first=Ian |journal=Safe Energy |volume=95 |date=July 1993 |access-date=18 June 2018}}</ref> For example, the most exposed members of the public living near [[Dungeness nuclear power station|Dungeness]] magnox reactor in 2002 received 0.56{{nbsp}}[[Sievert|mSv]], over half the [[International Commission on Radiological Protection]] recommended maximum radiation dose limit for the public, from ''direct shine'' alone.<ref name=discharges-2002>{{cite web |url=http://www.bnfl.com/dischargesreport2002/BNFL_2002.pdf |title=Discharges and Monitoring of the Environment in the UK – Annual Report 2002 |author=Director, Environment Health Safety and Quality |publisher=BNFL |pages=7–8,87–88,119–121 |archive-url=https://web.archive.org/web/20041116175519/http://www.bnfl.com/dischargesreport2002/BNFL_2002.pdf |archive-date=16 November 2004}}</ref> The doses from the [[Oldbury nuclear power station|Oldbury]] and [[Wylfa]] reactors, which have concrete pressure vessels which encapsulate the complete gas circuit, are much lower.