There is as yet, according to Thorp plant technicians, no set date as to when or if the reprocessing unit will come into operation again. Repairing damage that occurred during an April 18th pipe rupture in what is called the plants fuel clarification cell is set to take months officials at the Thorp plant told Bellona Web on a visit to Thorp this week.
They could not be more specific as the approach to setting the results of the accident right are a first-time event, and plant operators do not wish to be pinned down by a particular deadline, as much of the plants technical re-evaluation has to literally be invented as works progress.
But according to officials from both British Nuclear Group (BNG) —which now operated Sellafield under the authority of the newly formed Nuclear Decommissioning Authority as of April 1st this year—the aim is to get the plant functioning again, contrary to previous reports in the British media.
BNF has meanwhile published a public report on the incident that is unprecedented in its self-criticism and depth, which is available on the and BNG web sites.
British media and Bellona Web conversations with highly placed NDA officials previously indicated that the new decommissioning body had been considered shutting it down after it publicised a highly radioactive leak of 83 cubic meters of plutonium, uranium and nitric acid onto the floor of the plants clarification cell—an incident the construction of the cell is designed to handle. Plant officials say it is designed to hold more than 250 cubic meters of leaked liquor.
A final decision on whether or not the Thorp plant will, in fact, go back on line, is dependent on the decision of the British Government based on BNG calculations as to how much money it will cost to put the plant back into operation. The NDA will weigh these considerations as well, and make its own independent recommendation.
What is the clarification cell?
Thorps fuel clarification cell comprises a stainless steel-lined space 60 metres long, 20 metres wide and 20 metres high and its concrete walls are 2 to 3 metres thick to absorb radiation. BNG Sellafield said the cell was designed to withstand the possibility of a leak and, because stainless steel does not dissolve in nitric acid, the leak has been contained.
There has been no radiation dose to Sellafield workers as a result of the leak and no release of radioactivity into the atmosphere, confirmed a spokesman for UKs Nuclear Installations Inspectorate (NII).
Thorps raw materials are the used fuel rods from nuclear power stations. After receipt at Thorp, they are stored for several months to allow the radioactivity of short-lived fission products to decay to safer levels. The tubular rods are then cut up into small chunks and lowered in baskets into strong nitric acid.
The uranium, plutonium and fission products dissolve and the remnants of the steel rods are removed. But the fluid remaining from the process, called liquor, still contains small shards of steel, or tailings, from burrs created as the rods were chopped up. So the liquor must be centrifuged to get rid of the steel contaminants, a process called clarification. It is at this clarification stage that the leak occurred.
Chronology of the incident
The chronology of the incident is as follows: On April 18th, a camera inspection of the clarification cell was initiated to determine why one of the two so-called accountancy tanks was experiencing a drop in the level of plutonium and uranium. Measurements of how much liquor each of the tanks hold are taken by weight.
During the camera inspection process it was noted that liquor had been leaking onto the floor of the clarification cell for as long as nine months through a single pipe leading into accountancy tank B. It was first suspected that a manufacturers welding error had been the cause of the leak. But further investigations shows that, though the leak in the 40 millimeter wide pipe—one of several dozen running into the accountancy chamber—had occurred near a welding point, it was a matter of metal fatigue that had caused the rupture.
The radioactive liquor has long been drained from the floor of the facility, and the task now, according to Thorp engineers, is no longer determining the cause of the accident and cleaning it up, but looking forward to making the complicated system of pipes and tanks workable again—and how to avoid similar incident in the future.
Specifically, said one Thorp engineer, technicians will be examining the gravimetric approach to measuring the amount of liquor in the accountancy tanks. When Thorp was commissioned in 1994, one of its unique features distinguishing it from other plants was that the amount of liquor held in the accountancy tanks was measured by weight. This means that the maze of pipe-work leading into the tanks has to move horizontally and vertically to accommodate rising levels of liquor in the accountancy tanks
It is in this design scheme that Thorp engineers interviewed by Bellona Web think the fault for the accident may lie, because more motion that is applied to the pipes, the more likely they are to succumb to metal fatigue. As NDA and BNG officials described it, the pipe rupture was roughly akin to taking a standard aluminum soda can and bending it several times in the middle.
Eventually, after doing this for some time, you will crack the can, said one Thorp engineer. He was quick to emphasise, however, that, of the numerous pipes running into the accountancy tank, only one had ever faced such a crisis, and the the tubular walls of the ruptured pipe are certainly several millimeters thicker than the walls of a soda can. Nonetheless, inspections of pipe integrity are on going. One option for ensuring the further safe usage of Thorp is to fix the entire assembly of pipe works and tanks in place rather than relying on gravimetric accountancy procedures. Accountancy would then take place under more standard methods such as regular measurements of the accountancy tanks contents of uranium and plutonium.
|The seven stage scale of ranking nuclear indcidents and accidents.|
BNG officials were regretful that they were not quicker to notify the public and international governments—among them the Norwegian government, which was taken off guard by the Guardian piece—and offered Bellona Web its apologies for not communicating events sooner.
We were not being secretive, said one BNG official. It was simply that our early assessments, given that the entire incident was contained and no plant or public personnel were affect, did not warrant an international alarm.
BNG officials, however, especially under the guidance of the NDA have pledged to be more forthcoming with stakeholders in the future.
Nonetheless, the event was classified as a serious incident, which corresponds to level three on seven level International Nuclear Event Scale (INES) that was developed in the wake of Chernobyl. As a level three event, the Sellefield spillage classified at one step below an accident. A rating of 4 corresponds to an accident without serious off-site risk.
NDA and BNG officials interviewed during Bellona Webs incident expressed surprise that the Thorp incident—which was entirely contained and resulted in no external or employee exposure to radiation—had been ranked so high. But technicalities within the language of the rating scale itself officially pushed it up to a 3 incident.