[ RadSafe ] Graphite moderated reactors

[JGinniver at aol.com]

WARNING - long e-mail to follow on the virtues of the UK Gas/Graphite  
Reactor programme.  The only Radiological Protection comment in the post is  that 
the Second generation of UK Nuclear Power Plants have the lowest collective  
doses to operators of any large scale commercial design.  What follows is  not 
directly related to RP :-)
 
Hmmm, it's interesting how little seems to be known on Graphite Moderated  
Reactors in the US.
 
Perhaps the first large scale (for their time) nuclear generating stations  
were Graphite Moderated.  This issue is contentious, because the US claims  to 
have been the first to generate 'nuclear' electricity using EBR 1 at Idaho  
(although I'm hazy and it may have been EBR II). The USSR used to claim to  have 
been the first to generate significant quantities of nuclear electricity 6  
MW electrical capacity at Obninsk?(a closed city in the USSR).  However  Calder 
Hall at Sellafield in Cumbria in the UK was the first large scale Nuclear  
Power Plant, although didn't have a great thermal efficiency or large generating 
 capacity and was really intended to produce plutonium for the UK weapons  
programme.  It was constructed to demonstrate the feasibility of nuclear  
electricity production in the UK, while meeting the need for increased plutonium  
production for the UK weapons programme.  The first of the Calder Hall  reactors 
supplied electricity to the UK national Grid October 1958 and the last  
closed at the end of March 2003.  A remarkable feat for one of the  first large 
scale Nuclear Power plants.
The UK then embarked on the construction of a number of Large scale  plants 
based on the Graphite reactor technology pioneered at Calder  hall.  The 
following summarises the different plants, numbers of reactors  and the thermal and 
electrical capacities of each reactor.
Calder Hall        4 Reactors each  producing     268MW (Th)  and     50 MW 
(elec.)
Chapelcross     4 Reactors each  producing     268 MW (Th)  and     50 MW 
(elec.)
Berkley            2  Reactors                             558MW  (Th)        
    138  MW (elec)
Bradwell            2  Reactors                            531MW  (Th)        
    150  MW (elec)
Hunterston A     2  Reactors                            570MW  (Th)           
 160  MW (elec)
Trawsfynydd      2  Reactors                            860  MW  (Th)         
   250  MW (elec)
Hinkley Point A  2  Reactors                            971MW  (Th)           
 250  MW (elec)
Dungeness A     2  Reactors                            840  MW  (Th)          
  275  MW (elec)
Sizewell A         2  Reactors                            948  MW  (Th)       
     290  MW (elec)
Oldbury            2  Reactors                            892  MW  (Th)       
     300  MW (elec)
Wylfa                2  Reactors                            1875  MW (Th)     
      590  MW (elec)
 
These were the first generation of Nuclear Power Plants in the UK, with the  
last 2 (Oldbury and Wylfa) being an intermediate step towards the second  
generation.  The reactors were Graphite Moderated, used CO2 as a coolant  and the 
natural uranium clad in a special (low neutron absorption cross section)  
magnesium alloy fuel cladding - commonly referred to as MAGnesium Non  OXidisng 
(MAGNOX).
 
These plants didn't have a particularly high thermal efficiency, but did  
improve over time as designs were developed.  The reason for this was that  the 
Fuel Temperature had to be kept below 650 deg.C (or something  similar) as the 
Uranium metal undergoes a phase change (it swells) somewhere  around 700 deg. 
C (apologies for some woolly-ness as I'm relying on my  memory for a lot of 
this).  However due to the comparatively low  temperatures and pressures MAGNOX 
power plants could be refuelled while at full  power.  For early plants the 
life limiting factor was neutron embrittlement  of the steel pressure vessel for 
the last two plants (which will close before  2012) the life limiting factor 
is the reprocessing plant for MAGNOX fuel which  must close by about 2015.  
MAGNOX fuel is not stable and cannot be stored  for decades or hundreds of years 
which is one of the reasons why the UK has a  large commercial reprocessing 
industry.
 
The next phase of nuclear generation in the UK was the Advanced Gas Reactor  
programme.  These AGRs could achieve much higher thermal efficiencies as  they 
used Enriched UO2 ceramic fuel clad in stainless steel which allowed the  
plants to generate superheated steam (something PWRs cannot).  This led to  
another series of even larger Nuclear Power Plants.  (again the generating  
capacity is for each reactor at the power plant)
 Dungeness B        2  Reactors                        508MW  (elec) 
(distinguished by having one of the longest construction times for a  nuclear power 
plant)
Hinkley Point  B      2 reactors                           610MW (elec) 
Hunterston B         2  Reactors                          623MW (elec)
Hartlepool               2  Reactors                        660MW  (elec)
Heysham  1            2  Reactors                        666MW  (elec)
Heysham  2            2  Reactors                         660 MW (elec)
Torness                2  Reactors                          701MW (elec)
 
Currently these plants are expected to operate for approximately 35 years,  
and the life limiting factor is decomposition of the Graphite at the much 
higher  operating temperatures.  This can be mitigated through careful operation  
and injection of methane, but there continues to be a loss of graphite from the 
 cores.  Unlike all of the types of water reactor (PWR,BWR and PHWR) the  
major components are not replaceable. 
 
While the UK may be the only country to embark on almost exclusively  
Graphite Reactor programme, the French constructed 6 large scale power  plants before 
moving to the PWR, These were Chinon-A1,Chinon-A2,Chinon-A3, G-2  (Marcoule), 
G-3 (Marcoule) and Bugey-1.  Spain constructed one (based I  believe on a 
French design) at Vandellos -1, Italy one based on a Magnox station  at Latina 
and Japan one based on a Magnox at Tokai-Mura.
 
The reasons for the selection of Graphite and CO2 for the initial nuclear  
programme in the UK was, the lack of Uranium enrichment capability, lack of  
information on the PWR due to the McMahon? act following the end of the second  
world war, the cost of heavy water production which prevented the development 
at  that time of a PHWR like the CANDU, the cost of helium production.  So  
initially the choice of moderator, coolant and fuel were really dictated by what  
couldn't be used, if circumstances were different, the UK may well have 
chosen a  different reactor design entirely. 
 
The above doesn't cover at all the large numbers of Soviet Union  water 
cooled graphite reactors as pioneered at Obninsk and made infamous at  Chernobyl.  
As far as I can tell at least 27 reactors of this type were  constructed. 
 
Hopefully the above covers reasonably well the issues of electricity  
production, I can't add to much to the second part of your question regarding  
plutonium production except that to produce plutonium suitable for use in  nuclear 
weapons, the fuel had to spend a relatively short time in a Magnox  reactor, 
approximately 9 months (I think) whereas the commercial Magnox reactors  would 
retain the fuel for about 7 years, and the plutonium would become an  important 
part of the fuel helping to increase the useful life of the  fuel.
 
Feel free to contact me if you would further information on the UK  
Gas/Graphite Reactor programme.
 
Regards,
    Julian