|
|
European Laboratory for Particle Physics |
As you probably know by now: France does not have any clearance levels for radioactivity. This fact means that in principle all machine elements in the LEP collider are considered to be active and must be treated as very weak radioactive material (TFA = très faiblement radioactif) in the dismantling phase of this machine. Luckily enough our French colleagues from Saclay found help in a document from the CEA (Commission de l'Energie Atomique) when they wanted to decommission an experimental area of the long shut-down Saclay Linear Accelerator. Here is what they did and what we probably will copy:
First you must divide the area you would like to decommission into three distinct zones (zonage):
The first one is where you can show beyond any doubt that it contains no radioactivity like e.g. a generator building in your accelerator complex. The second zone comprises all areas where you are sure that radioactivity was induced and where specific activities are definitively above 1 Bq/g for the usual beta/gamma emitters found around accelerators. The material in these areas is definitely TFA (below 100 Bq/g) and possibly sometimes even FA.
The third zone is the most interesting one because you cannot exclude some beam interaction but you know that activity levels are well below 1 Bq/g. What do you do? Showing by measurements that your assumption is true is not sufficient. The French approach requires that you prove the "non radioactivity" by "reasoning". This means that you collect all information on beam energies, intensities and possible loss pattern in your accelerator or beam lines and make the appropriate calculations of the possible radionuclides that could have been created with a correction for their half-lives. If the results of your reasoning turn out as they should to be well below the "exemption limits" in the European Directive of 1996 you only add to your official decommissioning report for the French authorities a few measurements where the results in term of specific activity should be even lower than your pessimistic reasoned estimations. Under these conditions the material in question is considered to be inactive with no restriction on its further use.
As the time for approval of the report is known to be rather long, my group has started with some Monte Carlo calculations to predict the induced radioactivity in the LEP beam dumps for which the number of electrons or positrons dumped and their energies are well known. With the help of the machine people, we will then develop some other typical beam loss scenarios and also get the total number of leptons used in LEP since the start-up of the machine.
The result of all this: I am convinced that we can by using the French approach "reason" that 95 % of the LEP machine is non radioactive. Finally, is it more tedious to write an elaborate report for the authorities with a few spot checks like measurements of induced radioactivity or is it preferred to make many individual measurements that are not trivial (let us say determine 0.01 Bq/g with 30% precision)?
As an additional benefit the French authorities claim that their method of dealing with TFA-material for each decommissioning case individually is more acceptable to the public than simply applying universal clearance levels for radioactivity. Let us hope that this is true and we can decommission LEP without any disturbance.
Marco Silari <mailto:marco.silari@cern.ch> wrote:
The existing CERN accelerator complex currently injecting electrons and
positrons into the Large Electron Positron collider (LEP) will be used for
injecting protons into the future Large Hadron Collider (LHC). Two new
transfer lines will be built for transporting 450 GeV protons from the
Super Proton Synchrotron (SPS) to the LHC, in both the clockwise and
anti-clockwise direction. The first (TI2) will connect the target station
TCC6 of the SPS with point 2 of the present LEP tunnel; the second (TI8)
will link straight-section 4 (LSS4) of the SPS with LEP point 8. Before
starting the excavation work, the level of the induced activity in the
walls of the SPS tunnel in the two areas involved had to be determined, to
decide what part of the initial layers of the concrete and rock will have
to be treated as radioactive waste.
The TCC6 area was subjected to activation since the beginning of the SPS experimental program. This area houses targets and collimators and is situated near the beam extraction elements towards the neutrino facility. LSS4 housed the internal beam dumps of the SPS until 1980, when the accelerator was modified for p-pbar operation and the most active elements were removed. This area was submitted to important excavation work for the construction of the experimental area ECX4; the present walls date to that period and are therefore comparatively "young". Since then LSS4 is a straight section with very low proton beam losses.
Core borings of concrete and the underlying rock were taken from both areas and analyzed by gamma-spectrometry. A non-irradiated rock sample, taken in the French region nearby, was also measured and used as reference. One of the core borings from TCC6 showed little induced activity only in the first 60 cm from the surface, whilst for another (taken in front of a collimator) it was concluded that the whole concrete part is activated. Nothing above background was detected in the rock. A decrease in the specific activity in concrete can be clearly seen with increasing distance from the surface for both samples. A fit results in an apparent attenuation length of 60 g/cm2 and 95 g/cm2, respectively, which corresponds to the value for the forward cascade produced by 100 MeV and 150 MeV protons on an iron target.
Overall, the measurements showed that the levels of induced specific activity in the two areas of the SPS complex are generally very low when compared with the natural background in the region. For LSS4 the measured values are also very low when compared with the Swiss exemption limits, those proposed by the EU and the clearance levels of the IAEA. On the other hand, some of the samples taken in TCC6 present an induced specific activity that exceeds some of these limits in the case of 22Na, 54Mn, 60Co and 152Eu. These results suggest that there will be neither any radiological hazard during the excavation of the LHC injection tunnels nor in the disposal of the concrete and rock removed. However, the rock and the concrete from the break-through between TI2 and TCC6 will be considered as radioactive and treated accordingly. Therefore all work in this zone will require that people are classified as persons exposed in the exercise of their profession wearing film badges (and other personal dosimeters if considered necessary) as well as protective clothing.
Further reading:
M. Silari and L. Ulrici, Induced activity in the tunnel walls of the CERN
Super Proton Synchrotron, CERN Divisional Report, CERN/TIS-RP/98-05 (1998).
[Next Section]
Problems? Contact page owner
[Previous Section]
[Back to Top]