It has promised to present firm proposals to ministers by the end of this year. One option is to freeze the 200ft depth of the water-filled, vertical shaft. But, whatever solution is embraced, the highest priorities will be to eliminate the risk of a chemical explosion or a ``criticality'' - a runaway nuclear chain-reaction.
For 20 years, starting in 1957, radioactive waste from Dounreay, on the northernmost coastline of mainline Britain, was dropped into the shaft; a volume equivalent to a medium-sized house took the plunge. It has left Dounreay under a pall of intense criticism and would never be contemplated today.
In 1977, a hydrogen explosion blew the concrete lid off the shaft, scattering small quantities of radioactive material. Dumping ceased and ever since the shaft has been monitored for any build-up of explosive gas. Were that to happen, nitrogen would be pumped in to prevent a blast.
But this is not a permanent solution; for the next few tens of thousands of years, any escape of waste from the shaft would be highly dangerous. Unless a breakwater is built, and that too is under consideration, the sea will breach the shaft in about 200 years. Only UKAEA's sealed-off Number One pile at Windscale, Cumbria, site of the 1957 reactor fire which was Britain's worst nuclear accident, presents greater clean-up difficulties.
The UKAEA Dounreay director, Roy Nelson, said: ``Clearly, the shaft is the most challenging task we face here.'' A solution will cost hundreds of millions of pounds. Dounreay, 20 miles from John O'Groats, was where Britain spent 40 years trying to perfect the fast-breeder reactor, which turns uranium into plutonium - ``breeding'' its own fuel.
Three reactors were built and all have shut. The programme was killed because of costs, but the site will employ hundreds of people and absorb billions of pounds into the next century. The 15ft-diameter shaft was used to haul away rock carved out when a tunnel was bored out to sea, taking Dounreay's liquid low-level radioactive waste 600 yards offshore.
In the 1950s, permission was obtained to use the shaft as a dump for low and intermediate-level radioactive waste. A concrete plug was placed at the bottom to seal the shaft from the tunnel and the sea. Some 10,000 items were taken there in flasks from plants and laboratories around Dounreay. These would open and the waste plunge into the fresh water which had seeped in. UKAEA has been combing old logbooks to find out what was dumped and has interviewed retired Dounreay workers.
Items as big as lathes went down the shaft. So did glove boxes, used to shield workers as they manipulated highly radioactive materials. The 1977 explosion was caused by a mixture of sodium and potassium, the volatile coolant in fast-breeder reactors, inadvertently dumped in the shaft. The mixture reacted with water to produce hydrogen. All that was needed was a spark to ignite it and because sodium burns in air, that was readily available.
It will take thousands of years for radioactivity in the waste to decay to negligible levels. Making it safe in the long term may require pulling it out of the shaft and placing it in a permanent repository. It is too dangerous a task for humans, so remote-controlled arms and grabs combined with closed-circuit television will be used. The worst nightmare is a nuclear chain-reaction beginning in a sludge of uranium and plutonium particles which may build up at the bottom of the shaft.
Doug Graham, a scientist at Dounreay, said such a "criticality" was inconceivable in the undisturbed shaft. But any technique devised for removing the waste would have to provide absolute assurance that no chain-reaction could happen.
While the ultra long-term solution will probably involve emptying the shaft, UKAEA is also considering interim solutions. One option is to pump in refrigerants through bore holes to freeze the water and waste.