This has been reported as "President Trump’s administration plans to loosen nuclear weapons constraints and develop more ‘usable’ warheads" in The South China Morning Post, January 10, 2018. Key parts of that article are:
"The Trump administration plans to loosen constraints on the use of nuclear weapons and develop a new low-yield nuclear warhead for US Trident [D5 SLBMs] according to a former official who has seen the most recent draft of a policy review....[This is aimed to deter] Russia from using tactical warheads in a conflict in Eastern Europe.
... Arms control advocates have voiced alarm at the new proposal to make smaller, more “usable” nuclear weapons, arguing it makes a nuclear war more likely, especially in view of what they see as Donald Trump’s volatility and readiness to brandish the US arsenal in showdowns with the nation’s adversaries.
Problems with this new policy include:
1. making nuclear war against a non-nuclear enemy more acceptable? That is US nuclear weapons
can be used in a conventional war, not just as nuclear deterrents?
2. The intended targets of these "low-yield" nuclear weapons won't trust them to be "limited to
low-yield" or targeted countries won't limit their nuclear response anyway. China, Russia, North
Korea, or Pakistan could hit the US or US allies (intentionally or in panic) with high yield nuclear
3. Use them or Lose Them: China, Russia but mainly North Korea, or Pakistan could feel they have
to use all available nuclear weapons (of any yield) due to the fear they may lose them to further
US low-yield strikes.
4. Could another mass casualty 9/11 justify the US responding with low-yield weapons that kill
vastly more people, cause unintended health effects to US allies and upset the whole global
5. Would, as yet, only conventionally armed countries be encouraged to develop nuclear arsenals to
deter Trump's latest low-yield policy? and
6. Firing even low-yield missiles exposes a submarine to enemy satellite or radar detection and rapid
destruction. Would the destruction of a US Ohio SSBN, its 23 remaining SLBMs (with say 100
nuclear warheads total) be an acceptable trade-off if the SSBN was ordered to fire just one "low-
yield" warhead on one of its Trident D5 SLBMs?