This is to appear as a gesture of goodwill, but it is not. Abandonment of the test sight is forced upon North Korea by geology.
After the last test, other tunnels in the same area suffered cave-ins, with substantial loss of life. (Reuters) Tunnel collapse may have killed 200 after North Korea nuclear test: Japanese broadcaster.
The cause was fracturing of the rock, from a combination of susceptibility of the strata with too many tests in one area. The mountain has, in effect, been converted into a huge pile of rubble. Rubble has no bearing strength for voids. A new tunnel would collapse around the time the tunnel head enters a fracture zone. While tunneling through weak rock is possible, it requires techniques that are completely unfeasible for a single use tunnel.
If’ you’d like an official citation, refer to (USGS) OPEN FILE REPORT 01-312, The Containment of Soviet Underground Nuclear .Explosions. Quoting from page 37,
These changes in properties of rocks and rock masses in the vicinity of previously conducted underground explosions impose certain requirements on selection of a location for conducting underground tests. The location of a new test must be chosen such that zones of increased fracturing of the rock mass from previously conducted explosions do not intersect with the planned explosion. However, even with such a site selection, the new test is not always fully contained. For example, in the case of emplacement in Degelen tunnel 113 (16 Feb 73), leakage of gaseous products was observed through ground zero of tunnel 510, where a test had been conducted previously (28 Jun 70). A similar phenomenon was observed at the Novaya Zemlya test site, where the gaseous products of an explosion in tunnel A-3 (7 Nov. 68) escaped into the atmosphere through an adjacent tunnel (A-8) in the same rock mass. In the subsequent test in tunnel A-8, on 27 Sep 71, allowances were made for the nearby location of tunnel A-3.
This means that even if you succeed in digging the tunnel, the explosion may not be contained. Hence the fears reported by (Independent) North Korea tunnel collapse at nuclear test site could cause serious radiation leak.
So the North Koreans are giving up what they are forced to give up, and hoping we will see it as a generous gesture.
Will the North Koreans work around it? Inside a North Korean gold mine. More later.