The Long Term Safety Review's of these  older reactors  revealed startling evidence  of  degeneration. 

 John Large (an independant nuclear  engineer) brought out a key report  following the NII's findings on duct weld  embrittlement.In that report he drew up a  Chernobyl-like scenario for Magnox:
 
 1) top gas duct weld snaps (due to  embrittlement).

 2) coolant rushes out through broken  weld leading to loss of pressure at top  of  the reactor vessel (RV).

 3) bottom of RV does not lose  pressure  so fast as coolant is  restricted by fuel  channels

 4) differential pressure causes  instability  which causes restraint  garter on core to  fail.

 5) core is displaced and bilological  shield  collapses.

 6) control rods can no longer enter  core  because of core displacement.

 7) reactor overheats and catches fire.

 8) meltdown.

 NB top gas ducts open to atmosphere  therefore no secondary containment.