​Sellafield is located across the Irish Sea on the Cumbrian coast and is approximately 32 miles from the Isle of Man, on a clear day you can just about see it. The main activities at the plant include reprocessing of spent fuel from nuclear power reactors and storage of nuclear waste. There are no longer any nuclear power plants in operation at the Sellafield site. It was built in the late 1940s to manufacture plutonium for atomic bombs and Sellafield is one of the most radioactive places on earth. In its prime the plant was releasing eight million litres of contaminated waste into the sea every day. In 1957 the plant became the site of the worst nuclear accident in Great Britain's history, The Windscale Fire. This was a blaze that raged for three days, releasing radioactive gases into the air. The discharge of low level liquid wastes from the Sellafield site in the north west of England is the most significant source of artificial radioactivity in the Irish marine environment.
Now the site is mainly used for nuclear fuel reprocessing, and this and other activities gives rise to the discharge of low level radioactive materials in the form of liquids and gases into the environment. These discharges are regulated by the UK authorities and limits for releases are set by the Environment Agency of England and Wales (EA). Liquid radioactive waste is discharged from the plant into the Irish Sea via a pipeline, about 3 km from land. Gases are released from the plant via a number of chimneys (referred to as ‘stacks’). Discharges into the Irish Sea peaked in the mid-1970s and have dropped significantly in recent years. This is as a result of improved waste treatment facilities at Sellafield, which convert much of this radioactive waste into a solid for long-term storage.
As a result of the discharges from Sellafield, low levels of artificial radioactivity can be detected in sediments, seawater, seaweeds, fish and shellfish taken from the Irish Sea. A wide range of marine samples are collected and analysed on a regular basis by both the EA and the Manx Government. This monitoring can show where the radioactive particles become concentrated.  As expected many particles end up in sea bed sediment, so there are sometimes slight increases when the winter storms have been especially ferocious and stirred up the seabed. Generally, levels are falling from their peak in 1998. 
There are several radioactive isotopes that are monitored, Technetium-99, Caesium-137 and 134 and Cobalt-60. Of these, Tc-99 is regularly tested for by catching lobsters. Tc-99 concentrations in our local lobsters have declined from a peak of around 400Bqkg-1 in February 1998 to average 10 Bqkg-1 during 2015. These Tc-99 concentrations are lower than the levels found in lobsters caught off the Cumbrian coast. The EC recommended maximum permitted level for Tc-99 in seafood which is 1250 Bqkg-1, so these lobsters are safe to eat and regularly eating seafish will only make a minor contribution to your overall radiation exposure.
Now it’s not true to say that lobsters are immortal, but once they reach adulthood they don’t have many predators except humans. Good lobster fishery management sets minimum landing sizes for lobsters, ensuring that they are at least able to breed once before being caught. Small lobsters can get out of pots through the escape hatch or they are returned to the sea anyway.
Just as lobster pots discriminate against small lobsters, they also prevent very large lobsters from getting in.  Consequently, larger lobsters do tend to live a very long time.  The lifespan of European lobsters has been estimated at between 30 and 50 years.  Large lobsters have lived through the peak discharges from Sellafield, unlike their smaller 3-4 year old counterparts who got caught in lobster pots and tested. Lobsters have a fairly high affinity for Tc99 and they accumulate the radioactive particles in their bodies. But the only real predator for the large lobsters is, you’ve guessed it, divers. 
Something to think about the next time you wrestle a monster lobbie from under a rock

​Somewhere back in your very first diving course there will have been reference to air being compressible because it’s a gas, as opposed to water being non-compressible. When we start thinking about how molecules interact, it’s helpful to go back to school science. In solids the molecules are packed tightly together, they can vibrate but they aren’t free to move around much. In liquids the molecules can move but stay interacting with each other and in gases the molecules fly around freely bouncing off the walls of whatever contains them.
Thinking about gas molecules as random whizzing around bumping into other molecules and their container, it’s easy to understand why if we push more molecules into the container the number of collisions with the container wall will increase and the pressure goes up. This kinetic model of gases assumes that the gas particles themselves are very small (they are) and there’s a lot of space between these gas particles. That leaves lots of extra space for us to jam more molecules in there and compress the gas together. Hence gases are compressible.  This nicely explains Boyle’s Law (remember that?) but makes a number of assumptions which create a concept of an ‘Ideal gas’. Sadly, Ideal gases don’t exist and we have to deal with real gases. But at low pressures Boyle’s Law is fairly useful.
Liquids don’t behave in the same way because the molecules are already close together and interacting to a limited extent. Each molecule forms temporary associations with the other liquid molecules. If you heat up a solid to melt it into a liquid, you can measure the temperature increasing. As you get to the point where the solid it melts, the temperature will stabilise (even though you are still heating it). This is the point where the molecules are absorbing heat energy to give them the kinetic (movement) energy to move around. Once all the solid molecules have absorbed enough energy to melt into liquid, then you can see the temperature start to rise again as you carry on heating it. During this phase the molecules will move faster, but they still interact with each other. Keep heating and give them enough energy and they will manage to escape the interactions and form a gas.
Water is a bit of a strange liquid, because it’s molecules interact more than other liquids, and this gives it some strange properties. Water molecules form hydrogen bonds to other water molecules, and then they break these bonds and reform them with other nearby water molecules. Although these bonds are quite weak, there are lots of them. If it weren’t for hydrogen bonds then water wouldn’t be a liquid at all. When the other elements in the same family of the periodic table, like Sulfur and Tellurium, bind to hydrogen they form gases not liquids.
Water is most dense (the molecules are packed tighter together) at 4 degrees. At this temperature the hydrogen bonds are quite structured and pull the molecules tighter. As the temperature rises the bonds start to make and break more frequently and allow the water molecules to move a little further away from each other, so water becomes less dense. At temperatures below 4 degrees, hydrogen bonds don’t form so well and so as water cools to become a solid, it also becomes less dense. This explains why ice floats on liquid water (good news if you’re a polar bear) and why cold water sinks into the ocean (that’s  thermoclines).
So, these little fairly weak hydrogen bonds have a lot of influence and it’s them we are fighting against when we try to compress water. They have already done the job of pulling the oxygen dihydride molecules far closer together than we’d expect from the other elements in their group. It’s because of them that water isn’t a gas at room temperature.  And once we’ve got as far as forming a liquid, there isn’t much compression left to achieve. At 4km down in the ocean, water has a measurable compressibility of just 1.8%. It’s not quite true to say water isn’t compressible, it’s just not very compressible and for the depths that we will visit we can probably ignore the marginally increased density.
Resistance to compression for any substance can be described by the bulk modulus value. This is a measure of how much pressure must be applied to reduce the volume by 1%. For solids, these values are predictably very high, eg diamond is 443,000,000,000 Pascals and steel is 160,000,000,000 Pa. For ice I would need 2,000,000,000 Pa to compress it by 1%.  For liquids we would expect the values are lower and generally they are. Water bucks the trend though and needs 2,200,000,000 Pa. So, I actually need more pressure to compress water than I do to compress ice. Blooming pesky things those hydrogen bonds!

​As divers we all know about the difference between diving in fresh and seawater in terms of needing an extra bit of lead because seawater is just a little bit denser than freshwater. Density is affected by 3 things, salinity, temperature and pressure. At extreme depths the density of seawater increases due to the high pressures. Given that we are recreational divers, the increase in density due to depth isn’t something we need to worry about. There are two common ways of dealing with measuring the density, we could use the density figures in grammes per cm3 or specific gravity.  Specific gravity is a comparative scale, but it’s actually fairly easy to calculate actual density using the temperature and salinity for where we are diving (and there’s some great online software that allows you to plug in the relevant figures).
Salinity (how much salt in g is dissolved in 1kg of water) varies quite a bit around the world. To understand the factors affecting salinity you would need to look at how much freshwater run off enters the area. Freshwater is less salty and will have the effect of partially diluting the salt concentration. Local climate will influence how much water evaporates from the sea; in hotter conditions the concentration of salt rises. Lastly, we need to consider how much current circulates the water. One of the most saline seas is the Red Sea, with little freshwater, high temperature and confined circulation. Salinity levels at the northern end can be as high as 4.0% (much higher than the world average of 3.5%). By contrast, in the Baltic Sea, especially around the Gulf of Finland and the Gulf of Bothnia, salinity can be as low as 0.8%. There’s lots of freshwater running in, limited circulation of saltier water through the narrow channel to the North Sea and the lower temperatures reduce evaporation.  This sea water is so not salty that it could be safely drunk in a survival situation. In the UK we actually see a little variation between the west coast (3.5%) and the east coast (3.4%). Don’t try drinking this stuff for survival!
So salt levels can vary, and as they do, so will the density of the water that you need to displace. How often have you heard someone in a dive resort tell you that you might need a bit more lead here because the seawater is a bit more salty? This got me thinking, how much more salty would it need to be for me to notice,or are there other explanations?
Let’s start with the UK. The density of seawater on the Atlantic side at average temperature (assume 12oC) is 1.0267 g cm-3 whereas on the North Sea side, the density will be 1.0258 g cm-3. I know from experience that I haven’t noticed the difference. I do know that wearing twin 7’s in fresh water (1.000 g cm-3) means I don’t need any additional lead, but exactly the same setup in Manx seawater takes 4kg of extra lead. So, for each extra 0.0067g cm-3 I need another 1kg of lead to get my head under water. Taking this logic forward means that in the salty Northern Red Sea at the coldest part of the year (24o), when the density is 1.0275 g cm-3 I’ll be needing another 1kg to sink, which seems reasonable. But, and it’s a really big but, I don’t wear the same neoprene drysuit, Fourth Element undersuit and base layers in the Red Sea as I do in the UK. If I did, I’d need the extra lead and treatment for heatstroke!
The Red Sea is a bit of a salinity extreme; for most of the world the average salinity applies, which makes it pretty much the same as the UK.  I think it is much more likely that other factors come into play when we head off to other waters. Every time I drag my 5mm wetsuit out I struggle to remember just how much weight I had last time, and of course my suit will be compressing with use and losing buoyancy from new anyway. I err on the side of caution and take an extra lump of lead for the checkout dive. Let’s face it we would all rather be under the water having to control a little extra air in our BCDs than bobbing around like a cork on the surface while the dive master forces a smile and swims over to us with another block.
Most of us will use cylinders from the dive centre and there can be a huge variation in the weight of steel cylinders. A 12l cylinder can weight between 13 and 15 kg, and a 15l can vary from 16 to 19kg. Unless your dive centre has cylinders of different weights, they may not even appreciate that they have ‘lighter’ 12s. The situation is worse still if the cylinders are aluminium and are neutral in the water…then you’re really going to need some lead round your waist! Your confusion at needing more lead is easily explained by blaming it on the ‘salty water’ but perhaps there are more factors at play?