Japan, particularly in this era of Trump uncertainty, has available the three essentials of a nuclear weapon capability:
A. can be assumed to have the technical know-how to build a nuclear device (easily the gun-type
for which plans have apparently been circulated on the Internet over the years),
B. has maintained its weapon delivery capability via Epsilon missile, and
C. the essential materials that go into a nuclear device including:
= Uranium 235 that has been stockpiled for Japan's dormant nuclear power reactors
= Has a reprocessing plant that can be modified to produce weapon's grade Plutonium.
= Has Hitachi and SILEX knowledge (point 2. below).
= Had about 47 tons of Plutonium at the end of 2016.
To put A, B and C together there may well be a Japanese nuclear weapons working group (under a much blander name) drawn from personnel in Japanese nuclear regulatory bodies, the Japanese Ministry of Defense and (for rockets/missiles) the Japan Aerospace Exploration Agency (JAXA).
The main takeaway from the Submarine Matters article of March 23, 2018, that I have is:
"Given France's dual-use military-civilian nuclear industry structure any entity, like US General Electric-Hitachi involved in France's nuclear power stations will also have some knowledge and influence over France's military nuclear effort."
1. In particular Japan/Hitachi may gain some knowledge of nuclear weapon requirements from the takeover of France's Alstom Power [including nuclear] Systems.
2. Also Hitachi could gain military nuclear knowledge from the corporate memory of its partner General Electric (GE). GE, up to 1993, has been involved in nuclear reactors with military applications. See https://www.brookings.edu/u-s-nuclear-weapons-production-and-naval-nuclear-propulsion-complex/ in part points to an early (pre-electricity use) nuclear reactor:
"Knolls Atomic Power Laboratory [KAPL]
(Niskayuna and West Milton, New York; Windsor, Connecticut)
SIZE: 170 acres (0.3 square miles) at Niskayuna; 3,900 acres (6.1 square miles) at West Milton; 10.8 acres at Windsor
BUDGET: $274.2 million [including DOE’s Schenectady Naval Reactors Office] (100 percent defense-related) (2001)
EMPLOYEES: 65 [federal]; 2,700 [contractor] (as of 9/30/97)
FUNCTION: Designs, builds and tests prototype NAVAL NUCLEAR REACTOR and trains U.S. NAVY PERSONNEL [naval references capitalized/bolded by Pete] in their operation and maintenance. Maintains two operational and two inactive (defueled) test reactors at Niskayuna, NY, and an inactive (defueled) reactor at Windsor, CT (shut down in March 1993).
RADIOACTIVE MATERIALS ON-SITE: 1.6 metric tons of uranium-235 and 171.7 grams of plutonium
CONTRACTOR: KAPL, Inc. (formerly Lockheed Martin-KAPL Company, Inc., a subsidiary of Lockheed Martin Corporation)
FORMER CONTRACTOR: General Electric Company, 1947-1993"
3. Also GE-Hitachi took over the Australian developed (in the 1990s) "SILEX Laser Uranium Enrichment Technology" : Silex invented and initially developed the ‘SILEX’ laser-based uranium enrichment technology in Sydney during the 1990’s. The technology was licensed exclusively in 2006 to GE-Hitachi Global Laser Enrichment LLC (‘GLE’), a business venture currently comprising GE (51%), Hitachi (25%) and Cameco (24%). Silex and GLE jointly continue to commercialise the technology for potential deployment in the USA. The target markets are the global nuclear fuel markets for natural and enriched uranium, worth several billions of dollars annually.