The Royal Observer Corp Bunker 'OXF41'
Around 1955, an ROC underground monitoring post was installed due to the mounting tensions between the west and the USSR.
The Royal Observer Corp bunker in situated near to the site of the former air despatch area on the way to the Oak and Furrows Wildlife Centre and may be reached by the general public with gated access from the perry track. The bunker is SEALED SHUT (welded and padlocked) and cannot be entered. The bunker is a perfect roosting location for bats.
The bunker would have been manned by 3 ROC personnel - their role being to report a nuclear attack on the UK.
It is chilling to think that in the days before the Internet, mobile phones, and satellite communications, the only way to report back to the government of the day that Wiltshire had been hit by a nuclear attack was to rely on a physical network of bunkers that could relay the messages back via UHF radio.
We know that one of the UK targets was USAF Fairford which is only 13 miles away from RAF Blakehill. Using estimates based on the yield from weapons aimed at us during the 70's/80's, the nearby town of Cricklade would have been destroyed and the resulting fireball would have killed many thousands across Swindon and Cirencester. The radioactive fall-out would have traveled north east.
Once an attack had taken place, the crew of the bunker had a number of instruments available to gather information on the nature of the attack, location, direction, fallout, size of device or whether is was a ground or air-burst detonation.
The Ground Zero Indicator (GZI)
The GZI consisted of four horizontally mounted cardinal compass point pinhole cameras within a white enamelled metal drum, each 'camera' contained a sheet of photosensitive paper mounted within a clear plastic cassette on which were printed horizontal and vertical calibration lines delineating compass bearing and elevation above the horizon. The bright flash from a nuclear explosion would burn a mark on one or two of the papers within the drum. The position of the burn spot enabled the bearing and height of the burst to be estimated. With triangulation between neighbouring posts these readings would give an accurate height and position.
The altitude of the explosion was important because a ground or near ground burst would produce radioactive fallout, whereas an air burst would produce only short distance and short lived initial radiations (but no fallout). Once combined with the peak-overpressure readings from post Bomb Power Indicator readings the power of the burst in megatons could also be calculated by the Triangulation Team in the group control building, using a hand held plastic calculator device.
The mount for this instrument survives.;
The Bomb Power Indicator
Above ground a pair of circular plated baffles would be affected by the passing of the blast wave. The detector was connected by a steel pipe to the indicator dial below ground in the protected monitoring post. The dial was wall mounted and measured readings from 0.1 to 5 pounds per square inch peak overpressure. Readings below 0.3 pounds per square inch were noted but not reported.
The baffles were normally stored below ground and only screwed onto the top of the pipe at the start of exercises or at Transition To War. Outside of operations the BPI pipe was protected by a screw on cap and there was a drain valve at the base of the instrument to remove any excess rainwater.
If the BPI registered a reading of 0.3 or higher the operator would wait ten seconds before pressing the reset button and making a report to the group control. One minute after a BPI reading an observer is sent above ground to change the photographic papers in the Ground Zero Indicator.
You read that correctly - the bunker would be opened and an observer would go above ground following the blast. Remember that when you have a bad day at work......
The mount for this instrument survives and is easy to trip over.;
The Fixed Survey Meter - Radiac Meter Head
The Radiac Survey Meter No 2 or RSM was a 1955-meter which used an ionisation chamber to measure gamma radiation, it could measure beta, by removing the base-plate and opening the beta shield. This meter suffered from a number of disadvantages: it required three different types of batteries, of which two were obsolete and had to be manufactured to special order, the circuit included a single electrometer valve or tube. These were favored as they had been tested on fallout in Australia after the Operation Buffalo nuclear tests, and remained in use until 1982 by commissioning a manufacturer to regularly produce special production runs of the obsolete batteries. Within the ROC the RSM was only used at post sites for three years when it was superseded in 1958 by the FSM and the RSM retained only for post attack mobile monitoring missions.
The Fixed Survey Meter or FSM introduced in 1958, could be operated from within the post with a cable leading to the detector mounted externally and protected by a polycarbonate dome. The FSM used the same obsolete high voltage batteries as the RSM. In 1985 this instrument was replaced by the PDRM 82(F).
The PDRM82 (F) was the fixed desktop version of the new Portable Dose Rate Meter, that were manufactured by Plessey and introduced during the 1980s, gave more accurate readings and used standard 'C' cell torch batteries that lasted many times longer, up to 400 hours of operation. The compact and robust instruments were housed in sturdy orange coloured polycarbonate cases and had clear liquid crystal displays. The PDRM82 (F) had a remote sensor, mounted above ground under a polycarbonate dome and connected down a steel probe-pipe to the instrument by a coaxial cable.
The mount for this instrument survives;
Photos from within the Blakehill Bunker
Very grateful to Stuart Latham who has kindly provided photos from within the bunker during its operation, showing some of the instruments taken during his time as an observer serving at 'bunker OXF41 Cricklade Post' No. 3 Corp. Tom Dunford was the Chief Observer and the leading observer was Stuart Moulding.