1. Explosives Items of Interest
From Recent MLRC Sub Committee discussion

2. A MSHA incident 8 months ago
On May 28, 2010, James S. Bennett, supervisor, age 62, died when an unplanned delayed initiation of explosives occurred. The victim and Michael Engelke, miner, entered a blast area to check it. When they were within 25 feet of the face, the misfire detonated and both miners were struck by fly rock from the blast. Engelke was injured, hospitalized, and later released.
This was an 18 hole blast of 1.5 inch holes loaded with Anfo, fired with Long Period Nonels, and a Safety Fuse as a starter

3. Details, Details, Details of the Delayed Blast
ANFO had an expired shelf life and sat for a long period of time in moist conditions. ---it would not have been as sensitive to initiation from the primer as it was
Another problem resulting from this extended storage is "Fuel Migration“ (fuel oil separating from ANFO)--- ANFO becomes under-fueled, less sensitive to detonation.
Two holes were wet and muddy.
Primer used in these two blastholes was small 10 gm instead of the recommended 1 ½ X 8 inch cartridge

4. Why the Delayed Blast?
Citation was issued October 2010: Mine management engaged in aggravated conduct constituting more than ordinary negligence in that they knew the requirements of burning explosive material, were aware of multiple recent failures at this mine of charges to explode when expected, were aware of an obvious and improper audible report from this blast, and had reason to suspect burning explosives at the blast site, yet failed to ensure that miners waited the proper period of time before returning to the area.
30 CFR § Burning explosive material. If explosive material is suspected of burning at the blast site, persons shall be evacuated from the endangered area and shall not return for at least one hour after the burning or suspected burning has stopped.

5. Misfires and Wait Times
Some mines use half hour wait times after a blast while others follow a 10 minute wait time.
Some sites do all re-blasting of misfires at the end of the shift.
Nonels are most common in development headings and Cut & Fill breasting where misholes are more noticeable and need re-blasting, but they are considered quite reliable
Electronic detonators (due to their cost) are more common in longhole / blasthole blasts where misholes are less noticeable (are fired at the end of the shift) and less likely to require re-blasting (unfired explosives are rare in this muck).
Key factor in re-entering after a blast was the ventilation aspect (dilution and removal of blast gas contaminants), --- this takes longer than a specified wait time.

6. Misfire Good Practice
ALWAYS wait at least 30 minutes with fuse detonator misfires and at least 15 minutes with electric and other non-electric detonator misfires, unless the manufacturer recommends otherwise, before returning to the blast area..- IME
ALWAYS wait a minimum of 30 minutes with electronic detonator misfires unless the manufacturer recommends additional time before returning to the blast area.- IME
The sound of the new primer firing is not a dependable indication that the entire charge of misfired explosive material has detonated. The primer may have caused some of the charge to burn, creating hangfire which could eventually detonate from a build up of heat and pressure. It is recommended that no-one return to the blast area for at least an hour following the re-firing of a misfired charge.- ISEE