An enriched natural uranium (mostly U238 with a few percent of U235) reactor cannot undergo a fission explosion of any size, and that is because U238's capture cross-section for fast neutrons is too large. Fast neutrons typically get captured by U238 atoms before they reach the few U235 atoms present. The U238 atoms are transmuted (Mostly to PU239) but do not release a cascade of fast neutrons, so the process is not self-sustaining.
A sustained reaction can only take place if there is a moderator present such as graphite or water. This slows the neutrons into the 'thermal' kinetic energy range. The slow neutrons don't greatly interact with U238, and hence continue until they reach a U235 atom. Where they cause fission, releasing 3 more fast neutrons. And, so on.
Remove the U238 and the moderator is no longer needed. Then, a fission explosion is possible. It still requires fast assembly though, otherwise the heat produced as the material becomes 'critical' and intensely radioactive will just result in a meltdown and disintegration.
The main explosion risk in commercial reactors, and the likely cause of the explosions at Chernobyl and Fukushima, is zirconium fuel pin cladding burning in steam or water, which releases hydrogen. Zirconium is chosen for its low neutron absorbtion combined with good physical strength. It is a metal, similar to magnesium. Like magnesium it is inflammable, but quite hard to ingite. Once ignited though, it burns furiously and is hard to extinguish. If the burning metal comes into contact with water it robs the oxygen, leaving hydrogen gas. Hydrogen is explosive over a very wide range of mixtures with air. It takes only a tiny spark to ignite a hydrogen/air mixture, and if in a strong confined vessel such as a containment building, the resulting explosion is powerful. In this case the strong building works against itself, leading to an even bigger bang.
The world's nuclear industry urgently needs to move away from designs using zirconium fuel cladding. This is by far the greatest safety risk in existing reactors. It turns a meltdown and local, manageable, contamination into a disaster area with pollution spread for miles around.
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