June 17, 2016

Japanese Nuclear Propulsion 1 - The Mutsu Episode


Mutsu Diagram (Courtesy

S has located a chapter in Japan's history that is surprising. That Japan developed and utilised "dual-use" nuclear reactor propulsion to power the ship Mutsu in the 1970s to 1992.

In Comments in mid June 2016 S identified Japan’s one public example of a nuclear propulsion for a ship.

Japan's Atomic Energy Agency (JAEA) built a nuclear propelled ship, the Mutsu, nearly 50 years (1968-1970). Mutsu's had a 36 MW Mitsubishi pressurised water reactor (PWR). The reactor was completed in August 1972. Low enriched (less than 20% U235) Uranium was loaded into the reactor in September 1972.

Minor radiation (neutrons and gamma rays) emanating from Mutsu's reactor during workup assessments, at sea, in September 1974. This was misinterpreted by the Japanese press as a leak of radioactive substances. The ray "leakage" was only equivalent to twice as much as the usual "radiation" from an old-style television picture tube.

With this press coverage the Mutsu's future became a high profile anti-nuclar issue. Local fisherman blocked Mutsu's return to her home port for more than 50 days.

The Japanese Government finally came to an agreement with the local government and fishermen. The Mutsu was allowed back to port on condition that it was to find a new home port, and the ship returned to Ōminato on 15 October.

In Sasebo, between 1978 and 1982, various modifications were made to the reactor shield of the Mutsu, and its home port was moved to Sekinehama in 1983. Following an overhaul, the Mutsu was completed in February 1991. She then completed her original design objective of travelling 82,000 kilometres in testing, and was decommissioned in 1992. Over 25 years the programme had cost (about US$ 1.2 billion).” [quite a modest sum for reactor development.]

But Mutsu had a second life - after its reactor was removed in 1995 it was renamed the (non-nuclear) Mirai.

During and after the Mutsu experience Japan's Atomic Energy Agency has not planned, built or purchased a nuclear propelled ship.

S Comments

Building nuclear submarines is technically possible for Japan and many in the Japanese Navy (JMSDF) would be happy having Japanese nuclear propelled submarines. But public and political anti-nuclear sensitivities in general and memories of the Mutsu, have blocked any serious thought of Japanese nuclear propelled submarines. Even if the Japanese Navy built nuclear submarines, no local governor of a Japanese prefecture would permit such a submarine to enter a port. 

Pete's Comment

This is also noting that Japan's major submarine bases are Kure (in Hiroshima Prefecture) and Yokosuka (near extremely populated Yokohama-Tokyo). It is odd that US nuclear propelled submarines and the US Seventh Fleet supercarrier share the Yokosuka base - but that is a nuclear political paradox.

An isolated naval base in Japan would probably need to be built to permanently base Japanese nuclear submarines. Australia has a similar problem in that Fleet Base East (Sydney Harbour) could not serve as the number two base (including emergency porting) for any future Australian nuclear submarines.

The military value of a Japanese civilian ship reactor is that it represents a dual-use technological testbed for future miniaturisation and increase in power output (100 MW would be a submarine minimum). This is a necessary preliminary for a mature submarine reactor. Though the French Rubis class SSN had a 48 MW reactor - that was for an unusually small 2,400 tonne SSN with only 14 x heavyweight weapon shots.

Next week S and Pete will complete Japanese Nuclear Propulsion 2 - further down the technological track towards a Japanese submarine reactor capability.

S and Pete


Anonymous said...

The strategic factors facing Japan that require strategic thinking and solutions are not nuclear propulsion for some ships. They are birth rate, aging and current account deficit if Japan is to maintain its status in the world and its ability to face a rising China. None of those problems can be delayed forever and they are already delayed far too long.

Peter Coates said...


Yes I agree no specific change in weapon mix would help Japan.

Japan is unfortunate enough to have 3 major opponents in its region (China, Russia and N Korea) who have nuclear forces and large conventional forces.

Along the lines you indicate a healthy increase in population of military age and growth in GDP would help.
All that would help boost Japan's main problem, which I think is low defence spending GDP per capita.

See the Stockholm International Peace Research Institute Table at https://en.wikipedia.org/wiki/List_of_countries_by_military_expenditures where Japan's defence spending GDP per capita is just 1%. This is quite small given strategic threats against Japan.



MHalblaub said...

Dear Pete,
Even Germany tried nuclear propulsion for a ship, the "Otto Hahn".
38 MW reactor and a core height of 830 mm. On German Wikipedia is a drawing to scale the reactor size of 35 cubic meters.

The "Otto Hahn" nuked (sailed) 3 years before the "Mutsu".

Peter Coates said...

Hi MHalblaub [at 21/6/16 7:23 AM]

Just by coincidence - I made up a Table - which I'll publish today - where I note the year, 1968, when Otto Hahn was launched.

Interesting is https://en.wikipedia.org/wiki/Germany_and_weapons_of_mass_destruction#Cold_War_and_beyond

"The United States provides about 60 tactical B61 nuclear bombs for use by Germany under a NATO nuclear weapons sharing agreement. The bombs are stored at Büchel and Ramstein Air Bases, and in time of war would be delivered by Luftwaffe Panavia Tornado warplanes."