The EU's nuclear links with Russia
Following the loss of the Malaysian airliner last week, European leaders are once again wrestling with the question of how to respond to Russia over its role in the Ukraine crisis.
They are reluctant to get tough, much more so than the United States.
The EU could easily end up doing itself a lot of economic harm, most obviously if Russia were to respond by turning down the gas.
Russia is a very important oil exporter too, though that is a more liquid market - to coin a phrase - where it is not so hard to find alternatives if you fall out with one major supplier.
But there is also a significant role in Europe's energy sector for Russian nuclear supplies and the potential for significant disruption in the EU.
Nuclear energy is an important source of electricity in the EU.
Some countries are planning to phase it out, notably Germany. But even so, projections last year from the European Union see more than a fifth of EU electricity coming from nuclear power plants up to the middle of the century.
About half of EU states have some nuclear power - though there is a marked variation between countries.
In France, which is one of the world's biggest producers and users of nuclear power, 75% of total electricity generation is nuclear. In the UK, the figure is 18%, while Italy is the largest EU economy to have none.
Russia comes into this picture at several points.
First, it is an important supplier of the raw material for nuclear fuel, uranium.
There is an international market for uranium, so there are alternative sources, but Russia accounts for 18% of EU supplies (behind Kazakhstan and Canada), so switching is not that simple.
Second, there is the business of enrichment to make the uranium suitable for power generation - and 30% of this work is done by Russian companies.
There is another potential source of vulnerability, too. The EU has a significant number of older, Russian-designed nuclear reactors - 18 in all.
This is a reflection of past political relations with the Soviet Union.
Finland, which was never formally part of the Soviet bloc but did have a close relationship, has two - and all the reactors in Bulgaria, the Czech Republic, Slovakia and Hungary are Russian-designed.
Hungary also has an agreement for two more to be built.
These reactors account for a significant part of many of these countries' electricity needs - more than 50% for Slovakia and over 45% for Hungary.
The fuel for a reactor also has to be supplied in a form - called a fuel assembly - that meets the specifications of the particular reactor, and for Russian-designed reactors the fuel comes from a Russian company, TVEL.
So anything that disrupted the supply of the fuel assemblies needed for these countries' reactors would be a serious problem for them.
A recent document produced by the European Commission put it like this: "There is no diversification, nor back-up in case of supply problems (whether for technical or political reasons)."
The Commission went on to argue: "Ideally, diversification of fuel assembly manufacturing should also take place, but this would require some technological efforts because of the different reactor designs."
Other EU countries, including the UK, are not exposed to this specific risk.
There are some technical features of nuclear power that do ease any supply worries. A nuclear plant is only re-fuelled occasionally - typically every 12 to 24 months for modern designs. And if a scheduled re-fuelling is missed, the plant can continue to operate with declining output.
Compared with coal or gas it is also easier to store large amounts of fuel needs, as the volume of material is much smaller.
There are plenty of reasons for EU countries to be wary about tighter sanctions against Russia. The need for nuclear supplies is only one of them.
Hawks and doves
The British group Open Europe has done some analysis of official statements about the issue, attempting to rank countries according to how hawkish or dovish they are on sanctions.
Although it certainly does not prove anything about the respective governments' motives, it is striking that none of those with Russian-designed reactors are strongly hawkish.
Apart from the Czech Republic, they are all at least slightly to the dovish end of the scale and one of them, Bulgaria, is the most dovish of all, according to Open Europe's measure.
It is worth repeating one point. Gas is the big issue for commercial relations between Russia and the EU - but nuclear energy matters too.