Shielding gamma rays with thick water shielding - what if you fall in?
Published: Mar 31, 2024
Source: Design & implementation by Dr Chris Robbins (Grallator) / Facilitated by Ionactive radiation protection resource
Water is a pretty good gamma radiation shield in pools / ponds. You need plenty of it (i.e. thickness) and it offers other advantages such as cooling and allows the manipulation of sources via remote means from the surface. High activity beta / gamma emitters in water will produce Cherenkov radiation - the classic blue glow.
The water is of the highest purity - cleaned, filtered and put through ion exchange columns to meet strict quality assurance requirements to inhibit corrosion. Contrary to what some might believe, modern facilities do not contain radioactive contaminated water, and the water is well sterilised too and quite safe to drink (not recommended mind you!).
Ionactive works with these facilities and we are often asked - what would happen if someone were to fall into a storage pool / pond, would they become irradiated? We often reply with a question and statement: can they swim? If they can then they will be fine, if they cannot then they might drown. This is why falling in is avoided by use of barriers and fall arrest systems - deep water (5-8m depth) is a significant occupational hazard, the highly radioactive sources at the bottom are a secondary concern. This is illustrated by this radiation protection wizard which calculates dose / dose rate based on water depth and resident time in the water at maximum depth.
We have worked with Dr Chris Robbins (Grallator) to create this resource exclusively for Ionactive. Chris is a whizz at mathematics, physics, nuclear physics and coding and it's a pleasure to be working with him on this widget.
Water radiation shielding - what if you fall in to a pool / pond
Additional widget information
In this section we provide some additional information which the reader may find helpful.
Cobalt-60 (Co-60) was chosen over spent nuclear fuel as it is a single radionuclide and so the radiological signature is known exactly. The TVT (10th value thickness) for Co-60 in water is about 50cm - this is rounded to an easy to use number (0.5 m), just right for the pool widget. We don't think the TVT for spent nuclear fuel in water will be that far off (probably less).
The dose rate specified at 1m, unshielded, has been set at 1.135 x 104 Sv/h. This is based on an activity of 37 TBq (1 MCi) of Co-60 as a point source (modelling as an area source might be more accurate but the widget method above presents worse case which is fine). Therefore the highest dose rate shown by the widget is 113.5 Sv/h at 1m through 2 TVT of water (so a 0.01 reduction is applied to the unshielded dose rate at 1 m as expected).
The widget allows you to analyse the effect of shielding alone, or a combination of shielding and distance. Both are your friends in the scenario presented.
The widget does not consider the incremental rise in dose rate as you enter the pool / pond (or fall as you return to the surface), or the mode of entry. This is a deliberate simplification. One mode of entry into the water is that you trip and fall in (assuming there is no guarding which would be extremely unlikely). Another mode of entry, even more unlikely, is that you dive in (!). Falling in might imply a more general decent to perhaps 2m depth, and then assuming you can swim, buoyancy will bring you to the surface (in conjunction with swimming upwards). Diving would imply a more forced entry, perhaps reaching a greater depth in a shorter time. Regardless, assuming a maximum depth for a specified time will present the highest dose rate / dose accumulation possible - which is what you need if you were considering the scenario professionally. At the dose rates that exist under the water you are essentially proving if an accumulated dose from falling in is in the micro Sv, mSv or Sv range.
The radiation unit of choice is the Sv. It is true that if you were sinking vertical downwards the dose rate at your feet will be different to the dose rate at your head. For the widget our Ionactive man is horizontal to negate this issue (i.e. represents whole body effective dose). As you play with the widget you will see that very close to the source, longer exposures will produce significant accumulated dose, within the deterministic radiation range - you could then express exposures in Gray (Gy), but there is negligible numerical difference between Sv and Gy in our scenario as presented.