A classic demonstration explained, and misadventures with mercury

Sodium explosion

In reference to the article ‘Alkali metal explosion explained’, much work was carried out on this interaction in developing fast breeder nuclear power plants. Sodium·water heat exchangers are an essential link between the released nuclear energy and the steam circuit, and the fundamental mechanism of sodium·water explosions was investigated in the 1970s at the UK’s Berkeley Nuclear Laboratories.

Our experiments were carried out under argon and under water (that is, oxygen free), while recording the temperature and pressure, as well as capturing audio and high speed film (~3000 frames per second). This demonstrated clearly that the accepted text book wisdom of a hydrogen detonation as the source of the explosion was incorrect and that the initial reaction to form sodium hydroxide drives the explosion · the study reported in the article has the same conclusion. The explosion energy measured in the shock waves was estimated to be up to 17% of an equivalent mass of trinitrotoluene.

The total potential energy release per unit mass of sodium is split approximately 50:50 between the initial reaction to form the hydroxide and hydrogen, and the secondary oxidation of the hydrogen in the presence of oxygen. To explode, the sodium and water must be intimately mixed.

The sodium reacts with water to form a hydroxide layer, with the temperature rising to 600°C. The bulk liquid sodium and water are kept apart by a gaseous layer of steam and hydrogen. As the hydroxide layer grows, the reaction rate declines resulting in a fall in temperature and less hydrogen in the gas blanket. At around 300°C, the vapour blanket collapses allowing the water to come into contact with liquid sodium causing a violent mixing of the two liquids and a catastrophic increase in the overall reaction rate. The high speed film showed a cycle of growth and collapse finally resulting in a light emitting region 200mm across and temperatures up to ~900°C.

Obviously this is a different mechanism to that proposed in the recent article. If the process were carried out in contact with air, the secondary combustion of hydrogen would augment the energy released.

As to the matter of the classroom demonstration, burning hydrogen is frequently observed in the period before the explosion, which means that it is not accumulating and could not contribute to the initial event. Even if the hydrogen did not burn off, it would convect or diffuse away rapidly from the sodium sample as it moves randomly across the water’s surface.

R Newman CChem MRS
Dursley, UK

Dash for gas

John Mottram’s letter is partially correct: the US (and western Canada too) is benefiting hugely, but not uniformly, from fracking. Pennsylvania, Texas and North Dakota have embraced the technology, but other states (New York for example), have opted out and several Canadian provinces have bans or moratoria in place.

Why? Because in addition to requiring ca five million litres of fresh water per well, ca 1000 tonnes of silica sand and tens of thousands of litres of chemicals, some rather toxic, the process wreaks havoc on societies. Leaky wells or poorly handled wastewater lead to air and groundwater pollution, for example, and earthquakes, erosion and floods have also been linked to the process. Nearby communities also suffer noise and light pollution. It’s sad, but true.

F R Smith MRSC
Memorial University of Canada

Hg and me

Like Tony Kelland, I also worked on several mercury alkyls for my PhD, beginning in 1949. Dimethyl mercury and its halides, MeHgCl etc, have a strong odour, but with increasing carbon content the odour changes slightly and disappears by diphenyl or dibenzyl mercury. As Kelland did, all syntheses and purifications were performed in a fume cupboard or in vacuo, but the solid halides were handled on the end of a nine-inch metal spatula.

At home, each evening, I could smell these halides on my hands with each mouthful of food, but by some good fortune, I am still here. In those days, nobody wore rubber gloves (which could have been fatal, as Kelland notes), we only had asbestos gloves to guard against liquid oxygen spills.

Many years later, I learned that this work was supported by the Defence Research Establishment at Porton Down, to understand and improve on mercury fulminate detonators: so Kelland and I were probably, without knowing it, collaborators in this endeavour.

Interestingly, the terms of my ‘scholarship’ were that if I was absent without permission for more than two weeks, I would be liable for National Service, which at that time meant working in a coal mine.

Huw Pritchard, MRSC
Toronto, Canada

Miner correction

In the second paragraph of the article ‘Shiny and new’, David Jones states that the lung disease silicosis occurs in coal miners because nascent silica is released from the rock.

Lung disease among coal miners is not silicosis, but a type of pneumoconiosis caused by coal dust, mainly from near the coal face. Silicosis is a particular form of pneumoconiosis that affects quarry men and others who work with stone. The only type of stone dust found in any quantity in a coal mine is finely ground limestone that is deliberately spread on the floors to stop coal dust being blown into the air where it might contribute to an explosion.

R Close FRSC
Norwich, UK

Second mouse gets the cheese

The feature ‘India’s chemistry challenges’ was an excellent summary and I look forward to more reports from developing economies.

One of the longer term decisions India has taken is to proceed with a thorium advanced heavy water reactor. Thorium reactors have been ‘hyped’ in the popular press but they do have the advantage of producing very little long-lived transuranic waste. Global reserves of thorium are 2Tg; half of it in India!

The molten salt design (although not India’s present goal) is considered by some to be so inherently safe it could be installed in cities where waste heat can be diverted directly into homes. This is politically inconceivable in Europe, but times may change: extreme weather events and suppression of thermohaline convection could mean the UK climate approaches the latitudinal average. Neither the UK housing stock nor the NHS is prepared for prolonged cold winters.

Our nuclear industry seemed to have turned its back on thorium, primarily because the UK’s 70 years of world-class R&D focused on the U–Pu cycle and we lack expertise in Th. My guess is that India also lacked expertise at some point. However there is now an ‘All-party parliamentary group on thorium energy’ and the UK’s National Nuclear Laboratory is becoming involved in both Norwegian and Indian projects.

Is thorium the acceptable face of nuclear power? What is certain is that implementing nuclear energy in many European countries must now satisfy three conditions: technical feasibility (including safety), economic feasibility (including negative externalities) and public acceptability.

At the Climate Engineering Conference in Berlin last year, I joked that in a decade or two India will be selling thorium reactors to Germany. It got a laugh, but several people said afterwards that I’m probably right. Will India also be selling to the UK? An aphorism sometimes used in product development circles is: ‘The second mouse gets the cheese’.

J Evans MRSC
University College London, UK

Big is beautiful

The recent Organic matter column ‘Enthralled by evaporation’ brought back happy memories of my student work placement in the early 1990s at Eli Lilly’s manufacturing site in Speke, near Liverpool, UK.

I worked in the synthetic chemistry group that produced kilogram quantities of pharmaceuticals for clinical trials. I have never forgotten the novelty of using equipment that was basically identical to that which I had used in the undergraduate labs, only much larger: heating mantles wide enough to sit in, Büchner funnels the size of dinner plates and round-bottomed flasks that were nerve-wracking to carry whether empty or full.

Setting up the reactions required some care, especially where stirring was needed, to make sure that there wouldn’t be any problems over the many hours required for the large quantities involved to react. This also led to my first experience with night shifts as we had to make sure the reactions were supervised.

So I can also relate to Chemjobber’s mention of the hypnotic qualities of watching stirring and rotating equipment, especially in the small hours. There were occasional moments of excitement, however, such as the time my supervisor and I prepared a solution of sodium in liquid ammonia prior to carrying out a Birch reduction: the total volume of several litres changed from colourless to deep blue in the space of a second or two · a genuine ‘wow’ moment!

R Pallant MRSC
Slough, UK

Term time

In response to Philip Ball’s article on neologisms, in the early 1960s we tentatively introduced the terms bay and peninsular protons, to distinguish between the hydrogens on polycyclic hydrocarbons · bay was more widely accepted.

Around the same time, John Smith and I had been gathering the solid state NMR spectrum of CaHPO4‚2H2O, known as brushite (after George Brush). When we went on to CaHPO4‚H2O, we thought it sensible to describe it as ‘scrubite’, but the Chemical Society editor was less sympathetic (J. Chem. Soc., 1962, 1414).

D W Jones FRSC
Bingley, UK