According to Wikipedia on the 1966 Palomares B-52 Crash:
The first weapon to be discovered was found nearly intact. However, the conventional explosives from the other two bombs that fell on land detonated without setting off a nuclear explosion (akin to a dirty bomb explosion).
Are there any authors who said that those nuclear weapons could have detonated and destroyed a good part of Spain? Are there any relevant nuclear expert who support this thesis?
It seems unlikely that the 1966 incident could have resulted in the nuclear explosion of one or more of the four hydrogen bombs that fell at Palomares, and I am not aware of any serious claims to the contrary.
The 1975 summary report on the Palomares Incident has been declassified and is available as a pdf file on Archive.org. It notes that:
Small as it is, the probability of a nuclear yield in an accident makes nuclear weapon safety the first concern at all levels of military command, including that of the Commander-in-Chief. In response to our national policy with regard to nuclear safety, weapons designers employ a number of means to insure against an unplanned nuclear detonation. In general, weapons are designed so that a positive event or sequence of events peculiar to its planed mode of delivery or attack must occur before a weapon will produce a significant nuclear yield. It is reassuring that the safety engineering that was employed in the weapons was successful in preventing a nuclear explosion at Palomares and it is important to note that there has never been· an accidental nuclear explosion involving United States weapons.
Now, the US State Department also carried out an investigation into the so-called Broken Arrow incidents at Palomares and Thule. Their report is also declassified and available to download as a pdf file.
There is a 2009 report into the Palomares incident by the Danish Institute for International Studies (DIIS), titled The Marshal's Baton.
Finally, you might find a 2009 Master's Degree dissertation titled Dropping Nuclear Bombs on Spain - The Palomares Accident of 1966 and the U.S. Airborne Alert by John Megara interesting.
The four hydrogen bombs that fell on Palomares were not armed. It is actually not an easy matter to detonate an atomic weapon (for obvious reasons), and it is standard practice to ensure that the weapons are not armed until they are intended to be used. In this instance,
... the bombs could not be armed without the bomber crew flipping two different switches inside the plane that would cause the necessary circuits to be closed inside the bomb.
Some of the high explosive contained in two of the devices however did explode. This distributed the fissile material contained in the weapons over a large area. The state of the weapons is described in some detail in the DIIS report listed above.
When the weapon is deliberately deployed - as intended - these explosives detonate in a very precise order to create the initial nuclear explosion (which itself then acts as a trigger in the case of a hydrogen bomb). Although the details are classified (again, for very obvious reasons), the simplified sequence of steps required to cause a thermonuclear explosion are set out on the Wikipedia page on Thermonuclear Weapons. A simple detonation of part or all of the high-explosive, resulting from the impact of an unarmed device initiating the Mild Detonating Fuse (MDF), cannot do this.
destroyed a good part of SpainI don't see how that could've been possible at the yields in question.