Mark, thanks for posting this; sorry to have been so long in replying. The Guardian article you link to is misleading about the Chernobyl disaster, even more so than the World Nuclear Association article it references. Contrary to both articles, the second, larger, explosion was indeed a (small) nuclear explosion. From the Guardian article:
The net result of this errant test was a massive steam explosion, replete with enough kick to blow the 2,000 ton reactor casting clean through the roof of the reactor building.
Despite the sheer explosive force of the eruption, what ensued was not a nuclear blast. The spectre of the cold war has left an unfortunate conflation between nuclear weapons and nuclear power, but it is important to note that they operate on very different principles. <b>The Chernobyl explosion was instead a conventional high-pressure failure due to excess steam [Clark’s note: this is blatantly misleading]</b>. Seconds later, the remaining coolant flashed to steam and a second <b>even greater explosion</b> occurred, dispersing the shattered nuclear core and effectively terminating the chain reaction.
No mention that the greater explosion was a nuclear explosion. The World Nuclear Association present only one of several theories, known by 2009 at the latest to be false:
Two explosions were reported, the first being the initial steam explosion, followed two or three seconds later by a second explosion, <b>possibly</b> from the build-up of hydrogen due to zirconium-steam reactions.
Note how WNA cover themselves with the word “possibly”. However, if we check Wikipedia:
However, the sheer force of the second explosion, and the ratio of xenon radioisotopes released during the event, indicate that the second explosion could have been a <b>nuclear power transient</b>; the result of the melting core material, in the absence of its cladding, water coolant and moderator, undergoing runaway prompt criticality <b>similar to the explosion of a fizzled nuclear weapon.</b> This nuclear excursion released 40 billion joules of energy, the equivalent of about ten tons of TNT. The analysis indicates that the nuclear excursion was limited to a small portion of the core.
Of course with Wikipedia we must always check references, and the reference for this is an academic paper:
Estimation of Explosion Energy Yield at Chernobyl NPP Accident, Sergey A. Pakhomov, Yuri V. Dubasov, Pure and Applied Geophysics, May 2010, Volume 167, Issue 4, pp 575-580
The value of the 133Xe/133mXe isometric activity ratio for the stationary regime of reactor work is about 35, and that for an instant fission (explosion) is about 11, which allowed estimation of the nuclear component of the instant (explosion) energy release during the NPP accident. Atmospheric xenon samples were taken at the trajectory of accident product transfers (in the Cherepovetz area); these samples were measured by a gamma spectrometer, and the 133Xe/133mXe ratio was determined as an average value of 22.4. For estimations a mathematic model was elaborated considering both the value of instant released energy and the schedule of reactor power change before the accident, as well as different fractionation conditions on the isobaric chain. Comparison of estimated results with the experimental data showed the value of the instant specific energy release in the Chernobyl NPP accident to be 2·105–2·106 J/Wt or 6·1014–6·1015 J (100–1,000 kt). This result is matched up to a total reactor power of 3,200 MWt. However this estimate is not comparable with the actual explosion scale estimated as 10t TNT. This suggests a local character of the instant nuclear energy release and makes it possible to estimate the mass of fuel involved in this explosion process to be from 0.01 to 0.1% of total quantity.
Ironically, this “nuclear power excursion”, otherwise known as prompt criticality, blew the damaged core to pieces, dispersing the fuel and terminating the nuclear reaction. Although this released further radioactive pollutants into the atmosphere it prevented ongoing meltdown such as the three that apparently continue at Fukushima, five years after that disaster.