I've just had a really amazing experience: a guided tour of the nuclear reactor complex at Torness on the Scottish coast. What made this tour unusual is that the tour guide in question, Les, happens to be one of the reactor engineers (as well as a friend) -- and he showed me (and a couple of other friends) right around the plant over a period of several hours. This wasn't the usual cheery public relations junket: it was the real thing. I got to crawl on top of, over, under, and around, one of the wonders of the modern engineering world: an operational AGR reactor. I got to look around the control room, be deafened in the turbine hall and steam-baked in the secondary shutdown test facility, gawp at the shiny bright zirconium tubes full of enriched uranium in the fuel rod assembly room, be subjected to the whole-body contamination detectors at the checkpoints, and boggle at the baroque masses of sensors and control racks that trigger a reactor trip if any of its operational parameters go out of bounds.

(Note: there are no photographs; nor did I take a notepad, so I'm writing this from memory. Cameras were verboten -- not because of security, but as an operational precaution. For starters, some embedded controllers in racks in the auxilliary deisel generator control rooms have EPROMs which have been known to be erased by camera flashes in the past, triggering a generator trip; for seconds, we had to wear protective clothing -- try explaining to a visitor that their expensive Nikon has been contaminated and needs to be left behind!)

Torness is located on the coast, about thirty miles south-east of Edinburgh. It's a huge white slab of a building, not unlike NASA's vertical assembly building in appearance, visible as a landmark from miles away. We reported to the gate where Les was waiting for us: we were signed in, issued with visitor ID tags, and ushered into the front of the building. Once you get past the double barbed wire fences, cameras, concrete anti-vehicle obstacles, and security post, it's surprisingly like any other big corporate office (with a reception desk downstairs, open-plan staircase, executive offices and boardrooms, and the usual racks of glossy documentation). In the boardroom we were given coats, boots, gloves, goggles, helmets and dosimeters -- and then it was on with the guided tour.

Les started by taking us through the machine shop, then via an elevator up to the 37.5 metre level in the main reactor hall. Here we were led out onto a huge circular ampitheatre, paved with steel hatches -- the lid of one of the two Advanced Gas-cooled Reactors.

AGRs are an unusual, British reactor design; only half a dozen have been built. Like the more familiar light water reactors, there's a pressure vessel with fuel rods containing enriched uranium at their core. Unlike a BWR or PWR, the core of an AGR is filled with carbon dioxide, circulating at a temperature of 700-800 degrees celsius. This heats the secondary steam circuit to about 580 degrees and 70 bar pressure -- this steam in turn drives the huge turbines in the adjacent turbine house. The two reactors at Torness have a combined electricity output of 1200 MW, and a thermal output of close to 3000MW, but at the time of our visit one was shut down for maintenance.

I can report that, standing on top of an operational 600 MW nuclear reactor weighing several thousand tons, all you can feel is a slight rumbling vibration like distant traffic felt through a road surface -- there's no indication that metres below your feet, hundreds of tons of gas compressed to conditions more normally associated with the surface of Venus are being blasted through the guts of a radioactive inferno.

The reactor vessel itself is immensely thick, held under constant tension by masses of steel cables: the only thing remotely similar to it that I can point to is a suspension bridge's supports. It's literally woven into a cocoon of steel wire strands, bundled into thousands of inch-thick cables. Crash a fully laden 747 into it, and the plane would simply smear itself across its surface. I've been around a heavy cruiser, and that's about the only thing I've ever seen that came close to giving the same impression of engineering solidity. Whoever designed these things didn't believing in using half-inch steel plate where two-inch plate would do.

I can also report that there's an almost eerie workplace cultural emphasis on safety. Posters plastered on almost every available surface exhort you to know what you're doing ahead of time, understand and avoid the risks, don't be careless, wear your helmet, check -- are your shoelaces tied? -- there was even (near the exit) a poster urging everybody to "take safety mindedness away -- most accidents happen at home!" Everything, and I mean everything , was tagged, bagged, festooned with padlocks, controlled with keys, labelled, numbered, and itemised. It was like being in the middle of a wartime scare orchestrated by a committee who were terrified of forgetting where they'd left their screwdriver.

So, after we'd stood on top of nearly a million horsepower and gone "wow" a lot, Les led us down towards the basement, by way of numerous nooks and crannies ...

And plumbing.

There is nothing on earth like the plumbing in a big nuclear reactor complex. Around the two reactor cores, the building is divided into quadrants. Each of them has its own steam circulation system for feeding live steam at 580 degrees from the reactor and out to the big GEC turbines in the adjacent hall. Below the 37.5 metre deck, extending down for roughly forty metres -- 125 feet -- there's a maze of pipes, ranging from thin quarter- inch ducts leading to valves and dials for manual control, to enormous three foot armoured pipes that rumble with the flow of gas. Live steam at this kind of pressure is explosive, corrosive stuff -- each quadrant is sealed off from the others, with blast ducts and other safety mechanisms to vent steam in event of an explosion. (Not that there's ever been a spontaneous failure of that kind at an AGR, but current disaster planning tends to emphasize scenarios such as a fully-fuelled 747 flying straight into the plant at full speed -- can't think why.) Virtually every control board, valve wheel, and instrument is padlocked shut or accessible only using a special key; and every automatic, motorized valve has a manual backup, right down to the huge cast-iron handwheel for closing off the main steam circulation pipes in event of a power failure. (Which isn't likely; in addition to the grid connection, there are four twelve megawatt diesel generator stations spaced around each corner of the plant -- each with two generators, any one of which is able to provide operating power to keep the reactor's safety systems working.)

Down below the reactor vessel, nine metres underground, there's a big cork gasket. I mean big. I'm running out of adjectives for scale here, but it's not every day you see a cork heatproof mat sized to sit underneath a forty-five metre tall nuclear reactor. Below that, there's a concrete plinth and the other end of the cable bundles; the entire mess of reactors and turbines and steam pipes are suspended in a cats' cradle of cables that are designed to damp out or absorb the forces of an earthquake or a major impact.

There's a lot of other stuff spaced around the reactors. On one side, there's the fuel rod assembly room. Graphite tubes about 30 centimetres in diameter and a metre high are filled with spacer grids and gleaming screw-corrugated tubes of fuel a centimetre in diameter and a metre long; the fuel is uranium enriched to 2.8% U235. A stack of these tubes, bolted together and held under tension by internal rods, can be assembled into a fuel rod and lowered into the reactor to join the hundreds already inside. Controling the reaction kinetics are a total of 89 control rods, neutron absorbers that can be raised or lowered into the reactor to damp down the fission rate. In event of a failure there are other control mechanisms, housed deep in the plant under the big kettle; the reactor can be flooded with nitrogen gas under pressure -- a strong neutron absorber -- and if that fails, as a last-ditch measure the operators can blow clouds of tiny perspex glass beads into the reactor.

Before you can get in or out of the reactor building itself, there are turnstile barriers to go through: access using card badges and past a security checkpoint, egress using the same card badges and through a full-body sized cubicle full of scintillation detectors. Outside the controlled area it might as well be any other big power station, but inside it, the level of attention to detail is mind-numbing.

The reactor control room itself was obviously designed with guided tours in mind: a carpeted corridor in the non-controlled zone leads past windows giving a full view of the room from above -- but to get access you need to go through a separate set of card-controlled (and guarded) security turnstiles. Inside the control room, the most interesting feature not visible from the windows is the wall of lever-arch binders that contain the manuals; the reactors are usually controlled by a gang of elderly Ferranti mainframes (due for replacement next year), but everything can be operated by remote control from the control room, or by hand (with the right keys and a lot of patience!) from the access levels surrounding the reactor itself. Other than that, it was familiar from a thousand press photographs: subdued lighting, big boards with illuminated lights showing the status of the transformers and switches, horseshoe shaped consoles full of buttons to control each reactor (with a prominent red LED display showing output in megawatts), and so on. The white heat of technology at work.

What you don't see in the control room is the huge air-conditioned equipment room below it and to one side, full of cabinets where the cables from the thermocouples, neutron sensors, and other monitoring gear terminates. There are four sets of control cabinets, each of which contains triply-redundant sensors configured to trigger a reactor trip if any operational parameter goes out of bounds. Everything's locked down, with no changes permitted until they've been passed by a full review process and signed off. The big business of running an AGR seems to consist of shuffling paper and handing out keys to padlocks.

The turbine hall is enormous, as you'd expect for a plant designed to hold two 660 MW turbines: they're loud, but there's nothing here that's radically different from what you'd expect at any other power station. Two gigantic GEC turbines and their associated generators sit at either side of the hall, sucking the huge steam pipes from each reactor; a maze of plumbing surrounds them, extending down to sub-basement level, to provide the cooling side of the thermal cycle. On the far side of a wall from the generators are the equally brobdingnagian circuit breakers and then the cinderblock bunker containing the grid transformer that boosts the output up to 440 kV. Big stacks on insulators, poles for switching the output on and off, a pervasive droning 50 cycle hum, and the smell of hot oil from the eight pumps that feed oil through the transformer core and then past a cooling system.

(The turbine hall and transformers, and the buildings containing the diesel standby generators, are where the main hazard for the AGR complex lies: the turbines are spinning at more than 3000 rpm and are just barely cooler than red hot, while only metres away the generators spin in a sealed atmosphere of hydrogen gas. Everything's lubricated with oil, and there're big tanks of diesel fuel not far away. The combination of lubricating oil, hydrogen, fuel oil, and hot metal -- not to mention high-tension power supplies -- is much more dangerous than a reactor, which is designed to be shut down nearly instantly if anything goes wrong. So, if the reactor building is peculiar for its emphasis on radiological safety, the turbine hall has fire-fighting systems just about everywhere.)

I'm not going to bother describing some of the other aspects of our visit -- the standby diesel generators, the additional ranks of control cabinets that keep the generators running, or the methane, liquid nitrogen, hydrogen electrolysis, or other gas plants dotted around the complex. (This is already way over-long for a blog report!) I'm not even going to dwell on the more bizarre aspects of the site: the anti-rabbit defenses, the anti-truck-bomb obstacles (on the entrance only -- no self-respecting truck bomber would ever think of driving in through the exit, would they?), or the weirdly victorian-looking plumbing around the 12 metre level (where the manual last-ditch controls are available, all brass dials and hand-wheels). What I think I should end with is an explanation of the title of this piece ...

As Les explained, "nothing like this will be built again". The AGRs at Torness are not ordinary civil power reactors. Designed in the 1970's, they were the UK's bid to build an export-earning civil nuclear power system. They're sensitive thoroughbreds, able to reach a peak conversion efficiency of 43% -- that is, able to turn up to 43% of their energy output into electricity. By comparison, a PWR peaks at 31-32%. However, the PWRs have won the race for commercial success: they're much, much, simpler. AGRs are like Concorde -- technological marvels, extremely sophisticated and efficient, and just too damned expensive and complex for their own good. (You want complexity? Torness was opened in 1989. For many years thereafter, its roughly fifty thousand kilometres of aluminium plumbing made it the most complex and demanding piece of pipework in Europe. You want size? The multi-thousand ton reactor core of an AGR is bigger than the entire plant at some PWR installations.)

It's a weird experience, crawling over the guts of one of the marvels of the atomic age, smelling the thing (mostly machine oil and steam, and a hint of ozone near the transformers), all the while knowing that although it's one of the safest and most energy-efficient civilian power reactors ever built it's a a technological dead-end, that there won't be any more of them, and that when it shuts down in thirty or forty years' time this colossal collision between space age physics and victorian plumbing will be relegated to a footnote in the history books. "Energy too cheap to meter" it ain't, but as a symbol of what we can achieve through engineering it's hard to beat.