Failure Knowledge Database / 100 Selected Cases

Radiation Leaks from Nuclear Power Ship “Mutsu”
September 1, 1974 on the Pacific Ocean near Aomori (800 km east of the Cape Shiriya)
Masayuki Nakao (Institute of Engineering Innovation, School of Engineering, The University of Tokyo)
When the first Japanese nuclear power ship “Mutsu” was in its experimental voyage in 1974. At 800 km east ofthe cape Shiriya in Aomori, it began leaking radiation when its crew brought the reactor power up to 1.4% offull capacity. The alarm went off as it detected radiation leaks; fast neutrons made their ways out of the reactor
shielding (streaming). This incident made national headlines. Concerned about the dangers posed by the ship tothe community and th e fishing industry, the locals at Ohminato harbor, where Muts u was built, refused to let
the ship return to the harbor.
1. Event
The first Japanese nuclear powered merchant ship “Mutsu” was put to sea for the first experimental voyage
in 1974. At 800 km east of cape Shiriya in Aomori, the alarm went off as it detected radiation leak when itscrew brought the pressurized-water r eactor up t o 1 .4% of fu ll capac ity. This in cident made na tional
headlines. Concerned about the dangers posed by the ship to t he community and the fishing industry, thelocals at Ohminato harbor, where Mutsu was built, refused to let the ship return to the harbor.
2. Course
Based on th e “Basic plan o n the studies ne cessary for the research a nd development of nu clear power ed
ship by Japan Atomic Energy Research Institute” stipulated by the government, the nuc lear powered ship
“Mutsu” was built as a prototype commercial ship for transporting special cargos and training crew.
Authorities expected the leading 7 shipbuilding companies to enter the bid for constructing the ship, but thebids fell short of th e ex pectation due to the l ow budget. They evaluated o utsourcing t o an o verseas
company such as Babcock & Wilcox Co . ( U.S.A.), ho wever, that would have m ade n o difference in th e
construction costs, and they decided to commission Japanese companies to build the reactor. Through theShipbuilders’ Association of Japan, the h ull was commissioned to Ishikawajima-Harima Heavy Industries
Co., Ltd. and the reactor to Mitsubishi Atomic Power Industries, Inc.
Figure 1 illustrates the nuclear powered ship “Mutsu”.

1Failure Knowledge Database / 100 Selected Cases

Purpose: Prototype

Gross tonnage: 8240 t

Reactor: Pressurized water reactor

Length overall: 130 m

Output of main engine: 10000 horsepower

Thermal output: 36 MW

Molded breadth: 19 m

Maximum speed: 32 km/h

Nuclear power: 145000

Molded depth: 13.2 m

Normal speed: 30 km/h

Draft: 6.9 m

Auxiliary power: 18 km/h

Cruising distance: NM (Under contemplation)
Crew capacity: 80

Figure 1. Nuclear Powered Ship “Mutsu” [4]Ishikawajima-Harima H eavy Industries s tarted c onstruction of t he h ull at i ts S econd Tokyo Factory on
November 17, 1968. The hull of “Mutsu” was launched on June 12, and delivered to the registered port ofOhminato at Mutsu Bay on July 13, 1970.
The reacto r was co mpleted on August 25 , 19 72, and the nu clear fuel w as load ed on S eptember 4. T heofficials announced the test run of the ship whe re the reactor was to be operated at lo w output. The local
fishermen and inhabitants protested against this experiment, unexpectedly to the officials, and the test run
at the mooring facility a nd offshore within th e bay wa s po stponed. After several neg otiations, t hegovernment, the Japan Nuclear Ship Development Agency and the local community agreed to test the ship
away in the outer sea.
On Augus t 2 6, 1974, while t here w ere some protest activities, t he sh ip left the po rt of Oh minato. OnAugust 28, the ship’s reactor attained cr iticality for the fir st time in the t esting area 800 km east of ca pe
Shiriya in Aomori.
At around 17:00 on September 1, the alarm went off as it detected an increase in radiation when its cr ew
brought the reacto r u p to 1.4% o f full capacity. The mass media re ported this in cident sa ying, “Nuc lear
powered sh ip Mut su leaked radi oactivity”. Concer ned ab out th e d angers po sed by the sh ip to thecommunity and the fi shing industry , th e co mmunity (the town Mu tsu and Aomori P refecture) an d th efishing i ndustry, refused to all ow t he s hip to r eturn to the har bor, alt hough th ey once h ad accepted t hereactor test.
On October 14, a compromise was reached and the government, the local government of Aomori, the townof Mutsu and the Aomori Prefectural Fishery Cooperation signed an agreement on Mutsu’s entrance into its
home harbor and withdrawal from its current home harbor. The ship returned to the port of Ohminato on2

Failure Knowledge Database / 100 Selected Cases
October 15.
3. Cause(1) Cause of the radiation leakRadiation leaked from the shielding ring. The alarm went off as it detected fast neutrons leaking out of
the reactor shielding (streaming). The faulty design of the reactor shield was due to lack of experience.
Only few models of reacto r shields had been designed in Japan at tha t time, and there wer e f ew
experienced reactor shield designers. The engineers made poor judgments about the capacity shielding
with h ard to calculate complex shapes. Al though W estinghouse Electric Co mpany (U.S.A.) ha dreviewed the d esign o f the re actor shie ld a s requested and hadwarned abou t the possibility of“streaming”, the designer made no correction to the original design.
(2) Lack of comprehensive examination on radiation shielding efficiency
Because the s hip a nd i ts r eactor w ere commissioned to d ifferent in dependent companies, r adiation
shielding was not designed in a cohesive and integrated way. The ef ficiency of the shielding was not
examined comprehensively.
(3) Report of “radiation” leak as “radioactivity” leak
The mas s media reported r adioactivity leak, inst ead of radiation leak. This made the co mmunity andthe whole Japan fear radiation contamination of seafood (scallop) and pr oduced an image of Mu tsu to
be “the nuke-leaking ship”. Of ficials and investigators i nherited this ter minology “ radioactivity”,
fueling bitter protests and general public distrust. It caused a significant delay in the project while thegovernment tried to persuade harbor authorities to allow the ship to berth.
4. Immediate Action
The government created the Mutsu Radiation Leak Inve stigation Commission to investigate the details on
the accident , an d the Co mmission submitted a rep ort on May 13 , 19 75. W hile r eporting issues on t heproject, organization, technology an d co ntract, th e Com mission evaluated positively that Muts u satisfied
the relatively high standards in the technological aspect. It then concluded that Mutsu would accomplish its
objectives wi th ap propriate repairs and modific ations, and reco mmended 6 improvements i n its
development plan.
On March 18, the Atomic Energy Commission established the Nuclear Powered Vessels Council to discuss
the future development of n uclear powered ships, as we ll as to review the Mutsu’s development plan andthe role of the Jap an Nu clear S hip Res earch and Dev elopment Agen cy. On June 10, the Atomic Ener gy
Commission p ublicly stated its disappointment i n t he government agen cy’s inadequate judgment andinefficiency that inv ited the public’s distrust, granted that the reactor was shut down before contamination
became serious. It supported the Investigation Commission’s recommendations and promised to implement
them in the d evelopment plan as swiftly as possible. It also expressed support for the continuation of thenuclear po wered ship progr am, the efforts in elev ating the level of tech nology of Jap an Nuclear S hip3

Failure Knowledge Database / 100 Selected Cases
Research and Development Agency, and a thorough review and overhaul by a governmental agency.
On September 11, the Nuclear Powered Vessels Council submitted a report supporting Mutsu’s completion
based on th e original s chedule. It em phasized Mutsu ’s contribution to th e advancement of the in dustry’s
technology th rough domestic commissioning o f its cons truction, ex perimental voyages for stu dies o n its
safety and adaptability, benchmark e xperiments f or b uilding the nu clear sh ip da tabase and design
improvements. It also recommended fundamental research and development of reactor equipment for ships
while developing Mutsu.
On September 23, in r esponse to the above repo rt, the Atomic Ene rgy Comm ission s upported thedevelopment of Mu tsu t o be nefit shipb uilding and shipp ing, as well as a pa rt of the e nergy poli cy for
nuclear power ’s f uture a daptations to v essels. It a greed to upd ate t he exi sting “ Basic p lan o n t he studi es
necessary for the research and development of nuclear powered ship” in a ccordance with the research anddevelopment leading to the ship’s commercial use. It also agreed to extend the period of the Japan Nuclear
Ship Research and Development Agency Law, which was originally set to end on March 31, 1976.
The Science and T echnology Agency and the Mi nistry of T ransport of J apan jointly establi shed theOverhaul and Re pair Technology Com mittee c omprised of specialists fr om related fi elds ( established onAugust 12, 1975). The Committee, which was delegated responsibility in reviewing the overhaul and repair
plan of t he Japan N uclear Ship Research a nd Development Agen cy, submitt ed its first r eportacknowledging the validity of the plan and assuring the environmental safety.
On December 12, the government decided on the continuation of Mutsu’s development at the Ministerial
Conference on Nuclear Powered Vessels.
5. Countermeasure
Mutsu underwent lengthy repairs since 1978 until 1982 at the port of Sasebo.
The reactor shield had the following repairs and modifications (Figure 2).
(1) The new primary shield at the top is made of neutron-absorbing serpentinite concrete.
(2) The new a uxiliary shiel d i s ma de of heavy concr ete, whi ch was pr eviously made fr om le ad an dpolyethylene.
(3) Neutron-absorbing chrysotile hea t i nsulator wa s inst alled to the fl ange j oint of the r eactor pr essurevessel.
(4) Neutron-absorbing zirconium hydride was added to the cap of the reactor pressure vessel.
(5) Layers of pol yethylene shield were ad ded to the s urface of the doubl e bot tom of the r eactor
containment (exterior).
(6) A shield made of serpentinite concrete and silicon was added to the bottom of the reactor containment.

4Failure Knowledge Database / 100 Selected Cases

Figure 2. Reactor Shielding Improvements [5], [6]6. Summary
The faulty design of the reactor shield caused radiation leak during the f irst p ower output exper iment ofMutsu. While the engineers shut down th e reactor before contamination became serious, the mass media’s
wording of “radioactivity” leak u ndermined t he ima ge of Muts u. This undesirable image impa cted th edevelopment. Radiation h as various t ypes such a s alpha radiation composed of the nuclei of hel ium-4
atoms, beta r adiation co nsisting of energetic electrons o r po sitrons, and gamma ra diation t hat a rehigh-energy electromagnetic waves. Radioactivity refers to the particles that are emitted from nuclei as aresult of nuclear instability and they emanate some form of radiation. Radioactivity leak describes, for anexaggerated example, leak of uranium or primary coolant water. The first Japanese nuclear powered ship
“Mutsu” was completed in February 1991 af ter numbe r of experiments an d throu gh overhaul in theSekinehama Mooring P ort, it s mother p ort since 1 983. It was d esigned to travel 8 2,000 k m ( circles theearth twice) with the reactor using 42.g of uranium-235 as fuel (5000t if petro leum). After accomplishingits objective, Mutsu was decommissioned in 1992. By that time in the world, Otto Hahn of West Germanyand Savanna of the U.S.A had already completed their experiments and research voyages. There is no planto develop nuc lear po wered co mmercial s hips, bec ause con struction of nuclear powered s hips are
economically inefficient unless the ships’ size is larger than 100000t. People may wonder what significancethe lengthy project had, spending more than 120 billion yen in 25 years. At least, it generated investigation
reports on radia tion leak, which deter mined the d irect ca use, and offered a pa radigm for a n am bitious
development project.

7. Knowledge(1) Factually accurate report on an accident is required to take an appropriate action.
The officials originally reported the Mutsu accident to be “radiation leak”. However, it ended up being
reported as “radioactivity leak”, which neither legal entity nor governmental agency corrected.
5Failure Knowledge Database / 100 Selected Cases(2) It is always difficult to recover from negative publicity. The mass media has great influence on generalpublic. A good ex ample is the “Sayama dioxin report” broadcasted in 1999, in which th e network’s
false report on a serious dioxin contamination in vegetables led to a collapse in vegetable prices (actualcontamination was found in tea leaves).
(3) Commissioning a project to more than one company by dividing the tasks increases the likelihood offailure in the r esulting product. (The hull and the reactor of Mutsu wer e commis sioned to dif ferent
independent companies.)
8. Background
The trend in th e s hipbuilding and the s hipping indus tries at that time was in creases in size an d s peed toaccommodate the growing trade volume. It was found economically inefficient to infinitely increase output
with the existing propulsion engine. The problems of oil prices and supply spurred di scussion on nuclear
powered vessels on a global scale.
In order to operate nuclear powered vessels for commercial purposes, nuclear powered vessels must have acompetitive strength against the existing vessels. They must also be proven safe and reliable. Technologicaldevelopment and g lobal s afety st andard, as w ell as navigational regulatio ns w ere req uired to put su ch
vessels into practical use.
Considering the strength o f t he shipbuilding indu stry and thefuture nuclear po wer era, Japan ese

government em phasized the research an d dev elopment of nu clear po wered vessels. It a lso emp hasized
active participation in the s tandardizing safety of nuc lear power ed vessels to enable e arly realization of
their commercial applications.
The government started development of Mutsu based on the “Basic plan on the studies necessary for theresearch and development of n uclear powered s hip”, along w ith the es tablishment of the J apan Nucl ear
Ship Research and Development Agency in August 1963.
Looking b ack the histo ry, t he d evelopment of Mu tsu was understandable considering th e g lobal tren d atthat time, the government acted too slowly to drop the nuclear powered ship program. Some may even saythat Japan has a tendency to take no action even when it is necessary and we have no words to talk back tosuch criticism.
References[1] Mutsu Radioactivity L eak In vestigation Commission (1975) “M utsu Radioactivity L eak In vestigation
Report”, Atomic Energy Commission Monthly Report, May 1975, Vol. 20, No. 5, At omic Ene rgy
Commission.
[2] Development of the First Nuclear Powered Ship “Mutsu”,
http://aec.jst.go.jp/NC/about/hakusyo/wp1976/sb20601.htm[3] Genshiryoku Hyakka Jiten (Nuclear Energy Encyclopedia), http://mext-atm.jst.go.jp/atomica/[4] “Development (Seika) of Nuclear Powered Ship MUTSU” (1992) Japan Atomic Energy Agency.
6Failure Knowledge Database / 100 Selected Cases[5] “Profile (Gaiyoo) of Nuclear Powered Ship MUTSU” (1990) Japan Atomic Energy Agency.
[6] Japan Atomic Energy Agency (Editor), Nuclear Powered Ship Directorate (1992) “Trajectory (Kiseki) ofNuclear Powered Ship MUTSU – Current and Future R&D”, Genshiryoku Koogyoo (Nuclear Science andEngineering), Vol. 1, No. 4, 25.

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