Hydronuclear tests can be used to develop new types of low-yield nuclear weapons. The 2002 study by the National Academy of Sciences on CTBT verification reported:
At the lower end of the very-low-yield category, Russia could develop and test new very-low-yield tactical weapons in the range of 10 to 100 tons. Regarding seismic detection, the 10-ton weapon could confidently be adequately tested under decoupling conditions even at Novaya Zemlya [Russia's nuclear test site], and might even be tested in a steel or composite containment so that it would give no ground shock at all. Indeed, with its experience in testing and weapons design, Russia could develop a 10-ton nuclear weapon using only hydronuclear tests in the kilogram-yield range, and be reasonably confident of its performance.
In January 2016, Dr. John Foster, probably the greatest living U.S. nuclear weapons designer, stated that hydronuclear tests "of less than one ton" yield could provide high confidence in the "performance [of nuclear weapons] at low yield."  This is very important because, as noted above, the internal Russian definition of a nuclear test is apparently one metric ton of TNT.Hence, the range of hydronuclear testing appears to be up to a ton of TNT yield in Russian testing policy, although this does not exclude the possibility they are really conducting low-yield tests.
Senior Russian officials and Russian press reports during the Bush administration reported that Russia was introducing new and improved nuclear weapons. For example, in 2005, Russian Defense Minister Colonel General Sergei Ivanov asserted, "We will develop, improve and deploy new types of nuclear weapons."  According to Colonel General Vladimir Verkhovtsev, then-chief of the Defense Ministry's 12th Main Directorate, Russia's nuclear weapons organization, the newly developed and manufactured nuclear munitions will have "improved tactical and technical specifications...." 
A number of Russian press reports indicated that Russia had developed a new warhead with a weight 100-kg with a yield of about 100-kt. This warhead was apparently a new design. According to Russian expatriate Pavel Podvig, an expert on Russian strategic forces, the warhead for the new Bulava-30 SLBM is better than the best Soviet-era designs which he says we're in "the 110-130-kg range (this includes reentry vehicle body and electronics) and [had] yields of 50 and 75 kt. respectively."  However, Podvig's claim that this type of increase in yield-to-weight ratios is easy to do without testing is simply not true.
The Implications of Possible Russian Low-Yield Nuclear Testing
If Russia is testing at the sub-kiloton level or higher (i.e., at yields of several kilotons or even up to ten kilotons) which is technically possible without detection, or without conclusive detection because of the difficulty of detecting and proving a nuclear test seismically, if conducted in a manner to minimize the seismic signal, the military implications are much more significant than from hydronuclear testing. This, at a minimum, permits full yield tests of low-yield or low-collateral damage nuclear weapons, weapons effects tests and very important tests of new primaries (fission triggers) for new types of thermonuclear weapons. As Dr. Michaela Dodge, then with the Heritage Foundation, has written, "The United States would have a lot to gain from conducting very-low-yield nuclear-warhead experiments. For example, it could further validate computer codes it uses to make judgments about the impacts of aging on nuclear warheads. It could improve the proficiency of people in charge of the U.S. nuclear warhead stockpile and increase overall flexibility and resiliency of the U.S. nuclear posture." 
In a 2012 talk at the Heritage Foundation, Ambassador Dr. Paul Robinson who participated in the CTBT negotiation said, "At that time , we in the U.S. labs requested that the permitted test level should be set to a level which is, in fact, lower than a one-kiloton limit, which would have allowed us to carry out some very important experiments, in our view, to determine whether the first stage of multiple-stage devices was indeed operating, successfully."  The implication of this is that much of what the Clinton administration told the Senate about maintaining the U.S. nuclear stockpile without nuclear testing was a lie. The experts told the Clinton administration that low-yield nuclear testing was necessary and they ignored the advice and promulgated disinformation. Despite this, the CTBT was defeated by a majority vote in the U.S. Senate, a clear commentary on how weak the case for it was.
Russian cheating involving covert low-yield testing would have critical military implications. According to Siegfried Hecker, "[M]ost [new] designs could be adequately tested at yields between one and ten kilotons."  He is apparently talking about new thermonuclear weapons. Testing at 10-kilotons outside of known nuclear test sites is assessed to be possible with intentionally covert testing in salt mines without a serious risk of detection. If the Russian nuclear tests are sub-kiloton, they will have high confidence that their nuclear weapons work while we will have diminishing confidence in ours. Covert testing in the low-kiloton range up to 10-kilotons could have significant implications for the development of new high yield nuclear weapons, giving the Russians confidence that their weapons will work. General Ashley's statement linking Russian development of new types of nuclear weapons to their nuclear testing practices suggests he may be talking about low-yield testing rather than hydronuclear testing.
Before the political decision by the Clinton administration on a zero-yield CTBT, the only real debate in the U.S. nuclear weapons community was whether the U.S. could live under a 1-kiloton testing threshold. The Clinton administration not only ignored the technical advice it received from the national laboratories but it largely silenced them using threats of retaliation.
As Admiral Robert Monroe, former Director of the Defense Nuclear Agency has noted, "…two respected Los Alamos nuclear experts, raise serious questions about the reliability and performance of U.S. nuclear weapons!"  These individuals, John C. Hopkins and David H. Sharp concluded that "…the scientific foundation for assessments of the nuclear performance of U.S. weapons is eroding as a result of the moratorium on nuclear testing."  They pointed out that U.S. nuclear weapons are physically different from the versions that were actually tested and that "the current nuclear test moratorium precludes a decisive determination of whether these changes in physical state adversely affect performance."  This is not the first time nuclear weapons scientists and engineers have told our political leadership this. A New York Timesarticle by James Glanz in November 2000 noted the concerns of nuclear weapons scientist about the reliability of U.S. nuclear weapons without testing. Harold Agnew, former Director of the Los Alamos National Laboratory, bluntly said that if significant redesigns of stockpile weapons were needed, "…to consider putting those things in the stockpile without testing is nonsense." What we have now is almost 30 years of making changes to solve detected problems and in the life extension programs because of necessity without testing while the Russians have tested. Thomas Thomson, a weapons designer at the Lawrence Livermore Nation Laboratory, stated, "I think you just accept the fact that you're going to have a decline [in weapons reliability] …You try to make it as gradual as possible."  The Russians, on the other hand, are introducing improved nuclear weapons designs.
Secretary of Defense Robert Gates warned in October 2008 that, "To be blunt, there is absolutely no way we can maintain a credible deterrent and reduce the number of weapons in our stockpile without either resorting to testing our stockpile or pursuing a modernization program." [Emphasis in the original.] We have done neither. By 2003-2005, a consensus emerged in the national laboratories that it would "…be increasingly difficult and risky to attempt to replicate existing warheads without nuclear testing and that creating a reliable replacement warhead should be explored."  Unfortunately, the politicians did not want to hear this assessment. Indeed, even the Bush administration's proposal to develop inherently more robust nuclear warheads (without nuclear testing) was rejected by the Congress.
Today, we do not have "science-based stockpile stewardship," but more like "political science-based stockpile stewardship" while, conversely, Russia has science-based development of new and improved nuclear weapons. Our politicians have corrupted the very nature of the scientific process. Because of the liberal ideology that has dictated U.S. nuclear weapons policy for decades, we have much higher cost and much less reliability and certainly no enhancement in effectiveness. Within a decade or two, this is going to result in a national security disaster. There will be increasing uncertainty concerning the effectiveness of a declining U.S. nuclear deterrent while there will be little uncertainty about increasing Russian nuclear capabilities.
Dr. Mark B. Schneider is a Senior Analyst with the National Institute for Public Policy. Before his retirement from the Department of Defense Senior Executive Service, Dr. Schneider served in a number of senior positions within the Office of Secretary of Defense for Policy including Principal Director for Forces Policy, Principal Director for Strategic Defense, Space and Verification Policy, Director for Strategic Arms Control Policy and Representative of the Secretary of Defense to the Nuclear Arms Control Implementation Commissions. He also served in the senior Foreign Service as a Member of the State Department Policy Planning Staff.