Operation Hardtack II In September 1961, the Soviet Union resumed nuclear testing — including the test of the most powerful nuclear device ever designed, the Tsar Bomba, in October — and the United States followed suit with Operation Nougat.

Nuclear tests

Tamalpais

The Tamalpais test was conducted in the U12b.02 adit of Rainier Mesa by Lawrence Radiation Laboratory on October 8, 1958. Additional experiments included measuring permanent tunnel displacement, seismic ground shock measurement, gas sampling by Oak Ridge National Laboratory, and electromagnetic pulse measurement by the Department of Defense Effects Test Group. The detonation of the .072 kiloton device released significant radioactive contamination and damage to the U12 tunnel complex, with radiation levels of 50R/h at the main portal and in the 10,000 R/h range in the tunnel immediately following detonation. The following day, workers reentered the tunnel and triggered a significant hydrogen gas explosion that further damaged the tunnel and injured three workers. Following the tunnel's evacuation, a second hydrogen explosion occurred later the same day.

A 31 October 1958 meeting of the Atomic Energy Commission following the cancellation resulted in the following note:

During earlier tests in Hardtack II, University of California Radiation Laboratory (UCRL, now Lawrence Livermore National Laboratory) discovered that the planned primary stage for their Polaris nuclear warhead was not one-point safe. The primary was hurriedly redesigned to remedy this issue and was to be tested in Adams.

List of nuclear tests

{| class="wikitable sortable"

|+ United States' Hardtack II series tests and detonations

|-

!style="background:#ffdead;" | Name

!style="background:#efefef;" | Date time (UT)

!style="background:#ffdead;" | Local time zone

!style="background:#efefef;" | Location

!style="background:#ffdead;" | Elevation + height

!style="background:#efefef;" | Delivery <br />Purpose

!style="background:#efefef;" | Device

!style="background:#ffdead;" | Yield

!style="background:#efefef;" class="unsortable" | Fallout

!style="background:#ffdead;" class="unsortable" | References

!style="background:#efefef;" class="unsortable" | Notes

|-

! Otero

| 20:00:00.2

| style="text-align:center;" | PST (–8&nbsp;hrs)|| NTS Area U3q

| –

| underground shaft,<br />safety experiment

| XW-54 ?

| style="text-align:center;" |

| Venting detected,

|