To search for additional amounts of radiation in the scrap from a nuclear power station you first need to screen out the normal level of radioactive background. To do this you must build a ‘chamber’ using special, non-radioactive bricks.
I find myself in the Hotel Opera, Prague this rainy Thursday evening, tired after having spent a fascinating day at the Czech Centre for Nuclear Research UJV Rez.
There I saw one outcome of a European collaboration (called MetroRWM) designed to answer just one of the difficult questions that arises when one needs to take apart an old nuclear power station. This is something Europe will need to become good at in the near future.
This didn’t concern the highly-radioactive parts of the power station: that’s another story.
This concerned the 99% of a nuclear power station which is no more radioactive than a normal power station.
What should happen is that this material should join the normal scrap system and be re-used.
However, the understandable surplus of precaution that surrounds nuclear matters will prevent this, unless every single bucket load of concrete or scrap metal can be verified to have a level of activity less than a specified standard.
The collaboration based at UJV Rez have built an apparatus to do just that. And most importantly, they have proved that it works i.e. that tiny hot-spots on the inside of pipes can be detected quickly and reliably.
Here is how it works.
To detect the tiny levels of radiation potentially coming from hidden radioactive particles, the apparatus uses ultra-sensitive radiation detectors.
However these detectors are useless if they are not shielded because our normal environment contains too much radioactive material. So the first step is to shield the detectors.
The low-background chamber at UJV Rez At the far end you can see a fork lift truck has just loaded a pallet which will travel through the chamber and emerge at this end. The doors at this end are currently closed.
The UJV team did this by building a ‘room’ using a special type of brick which is almost as good as lead at keeping out radiation, but much cheaper, much lighter, and much easier to work with. Using this they lowered the level of radiation inside to just 1% of the background radiation.
The two ultra-sensitive radiation detectors can be seen inside the shielded room as the doors open to allow the entry of test pallet.
They then built a system for loading pallets of material on a conveyor at one end, and drawing it through the shielded room to check the radioactivity in all parts of the pallet. The measurement took about 5 minutes, and after this the pallet emerged from the other end (Video below).
The key questions are:
- How do you ensure that ‘not detecting something’ means that there is none there?
- Could some activity slip through if it were shielded by some gravel, or steel piping?
- Could it slip through if it was in the bottom corner of the pallet?
To answer these questions the UJV team, in collaboration with scientists across Europe, created samples that simulated many of these possible scenarios.
Pallets of ‘radioactive’ waste. These pallets are a standard size, but there thickness is determined by the need to be sure any radioactivity trapped inside can be detected. The pallets above have been made very slightly more radioactive than the background.
One of their clever ways of testing the machine was to create samples of known radioactivity and place them inside hollow steel balls (actually petanque balls!).
A colleague showing a very low level sample of known activity which can be placed inside a hollow steel ball,simulating radiation trapped inside steel pipes.
The machine could then search for the activity when the balls were arranged in many different ways.
A pallet filled with steel balls, some of which have radioactive samples of known activity concealed inside.
The aim of all this effort is that at the end of the day, scrap material like that in the picture below can be rapidly screened on-site and sent to be recycled in the confidence that no hazard will ensue at any time in the future no matter how this material is treated.
The aim of the system is to screen very diverse scrap such these old pipes and ducts.
These measurements are not easy – but this work really impressed me.