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!
At the end of 2016, diver training agencies including BSAC launched Sea Survival training courses developed in conjunction with the RNLI. Despite the common view of the RNLI as being the provision of lifeboats and crew, there is much more that they are involved in from a safety at sea perspective, with particular foci on fishing industry accidents, Swim Safe training courses and safety advice for all water users. It’s astounding how many fishermen don’t wear lifejackets, especially local pot-boat skippers who often work alone.
To try to educate fishermen the RNLI brought a dozen of them from around the UK down to their training base in Poole. All the skippers had previously attended the mandatory Personal Survival Techniques course (and it’s predecessors) which are run in swimming pools around the country. The RNLI trainers asked about lifejackets and got the usual story, the fishermen had them but rarely wore them. The general feedback was that as strong swimmers they were confident that should they fall in the sea, they would be able to swim back to their boat, climb up the tyres on the side and self-rescue. Interestingly, qualified divers and anyone who swam in the sea was excluded from the test group. Repeated attendance at sea survival training had led each fisherman to conclude that their lifejackets weren’t necessary. The RNLI sought to challenge that belief.
The night before the training course, the trainers opened the doors around the training pool to let out the heat. Overnight the water temperature dropped to 15 degrees. If you are a diver around the British Isle I am sure there are days where you dream of 15 degree water! At the first attempt the fishermen were asked to wear their normal deck attire and jump in to deep water to simulate falling off their boat. With no life jackets on, the impact of cold water shock was immediate. None of the 12 fishermen lasted longer than 5 minutes before a rescuer intervened. Post dip interviews revealed their shock and surprise at how debilitating the cold water was, definitely nothing like their sea survival training course.
Cold water shock is an immediate short-lived response to immersion in water less than 15 degrees. Blood vessels at the skin contract rapidly, increasing blood pressure and the heart rate. An initial gasp for air can be followed by a breathing rate that is 6-10x higher than normal. It is likely that cold water shock accounts for most deaths when people have unexpectedly entered the water. If you are not wearing flotation during this phase, keeping your head above water becomes the biggest problem. Over the next 10 minutes, cold incapacitation reduces blood supply to the muscles, making it difficult to swim or self-rescue. A crew member throwing a life ring to you during this time will be frustrated that you can’t actually hold onto it or kick towards the safety of the vessel.
The following day the exercise was repeated but this time with lifejackets being worn. The same cold water shock reaction was initiated, but the fishermen didn’t have to work so hard to keep their airway out of the water, the cold incapacitation stage took longer therefore improving their chances of getting back to the ladder on their boat. You can see the videos from this exercise on the RNLI website.
This started me thinking about why divers were excluded from the test group. I’ve realised I still brace myself for the cold water after decades of diving. OK, I’m wearing a drysuit and the cold water shock reaction is pretty much limited to my head and hands. But how many of us drop beneath the surface in anticipation of that brain freeze moment? As the blood vessels rapidly contract they stimulate the trigeminal nerve sending pain signals to your brain. It hurts for a few moments until you become acclimatised. The fishermen in the RNLI training exercise couldn’t get past that brain freeze feeling.
I think we sometimes underestimate the impact that cold water immersion has on new divers. I can recognise it enough now, but when I think back to learning in a wetsuit I can remember the feeling of panic, rising heart rate and accelerated breathing rate as I used to get into the water. Although we will all recognise increased air consumption by trainee divers, perhaps part of this is their reaction to cold water immersion? I’m sure that with experience comes the anticipation, the forced control of breathing rate for the first few seconds, but until our new divers have developed their response, maybe we should keep a close eye on them for those first couple of minutes? If your trainees are hoofing through their air and their buoyancy is being disrupted by their rapid breathing rate, maybe it’s something to consider?
Michelle has been scuba diving for over 20 years. Drawing on her science background she tackles some bits of marine science. and sometimes has a sideways glance at the people and events that she encounters in the diving world.