A summary of the accident at the Hadron Experimental Facility on May
23 2013(This summary was prepared based on the Japanese documents published at theJ-PARC website on May 25 and May 29. Some of the numbers are preliminary.)
Outline of the accident:
In the Hadron Experimental Facility experiments were conducted with theelementary particles that had been generated by irradiating gold target withproton beam. Due to the malfunctioning of the beam extraction system of the 50
GeV synchrotron, 2x10^13 particles were extracted for a very short period (5milliseconds whereas a normal period was 2 seconds). As the result, the goldtarget was overheated, causing possible damage to the gold target. As a result
radioactive materials leaked out into the experimental hall, and researchers wereexposed to the radiation. The dose levels have been measured for all the people
in the area.

The Hadron Experimental Facility has been closed since May 23 2013.
The contamination level in the Hadron Experimental Facility is estimated roughly
30 Bq/cm 2 .
34 out of the 102 people had the internal exposure up to 1.7mSv dose(preliminary figure) for two while the others were lower or not detectable level.
The radioactive materials were also leaked to the outside of the radiation
controlled area of the Hadron facility.
The radiation monitors around the Hadron hall showed a slight increase, butmonitoring posts at the site boundary did not show any meaningful increase. Oneof the two monitoring posts in the neighboring laboratory observed an increase ofradiation level for 30 min (6 nGy/h increase whereas the normal averaged level is
70 to 130 nGy/h). The amount of emission of the radioactive materials by
integrating the number was estimated to be 3nGy (or 0.003 µSv).
The cause(s) of the malfunction of the beam extraction system and the delay inevacuating researchers from the Hadron Experimental Facility is underinvestigation.

Time sequence of the accident
May 23rd
11:55 2 x 10 1 3 protons were extracted for a 5 milliseconds period, which isnormally done for 2 seconds. As a result, the intensity was 400 times
higher than normal. The machine protection system (MPS) worked andbeam stopped.
12:08

MPS was reset. The number of the secondary particles decreased.
Therefore, the beam trajectory was adjusted.

12:30

User operation restarted after tuning. The K1.8BR group noticed anincrease in background rate of the detector for their experiments. Started
to investigate the cause.

13:30γ-ray monitor in the hall indicated 10 times higher than normal level of
0.4 µSv/h.

14:26

Stopped beam. The value of γ-ray monitor decreased.

15:15

Turned on wall mounted fans of the hall to check if the monitor worked
properly. The value of γ-ray monitor decreased.

15:32

Restarted providing beam after γ-ray level became sufficiently
low. At the same time fans were turned off.

16:00

Ambient radiation level in the hall was found to be high (4 - 6 µSv/h).
γ-ray monitor value started to rise again.

16:15

Stopped beam.

16:30

Radiation safety section staff arrived at the hall and measured theradiation level.

17:00

Confirmed contamination by radio active materials.

17:30

People in the hall exited after the body radiation level was confirmed to be
below 4 Bq/sq.cm.

19:00~20:00 Detailed check was carried out working with radiation safety section.
23:30

All the people exited from the radiation management and controlled area
and the controlled area was sealed off.

May 24th
9:00
The head of the Safety Division reported the situation to J-PARC Director
and Deputy Director.
10:00

Meeting was held to discuss the situation and the discontinuation of theHadron experiments was decided.

17:30

Safety Division was notified by the Nuclear Fuel Cycle Engineering
Laboratories a monitoring post in their site recorded a small increase ofradiation level after 15:00 on the 23rd.

18:00

Record of the γ-ray monitor placed adjacent to the radiation controlledarea was analyzed closely. It appeared that the timings of the radiationlevel increase were coincident with the timings the fans were turned on.

19:00

Recognized the strong possibility of the emission of radioactive materials
to outside of the Hadron radiation controlled area and reported to theemergency contact at the JAEA Tokai site at 21:10.

21:11

Special task force was organized to take care of various issues of thisaccident.

22:15

Recognized that this corresponded the case required an official report bylaw.

22:40

The accident was reported to relevant authorities.

May 25th
01:00 Measurements with a whole body counter for four researchers wereconducted, showing that the total internal exposure dose reached to 1.7
milli Sv for one of four.

Measurements of dose equivalent rate and surface contamination density(the experimental hall of the Hadron Experimental Facility)
Location: observation deck and crane area (the upper part of the facilitybuilding)
Measurement time and date: 19:20-20:00 on May 23, 2013 (after operationshutdown)
Measurement methods
Dose equivalent rate (γ): NaI survey meter
Surface density (β): smear method
Measurement unit
Dose equivalent rate (γ): µSv/h (1cm dose equivalent rate)
Surface density (β): Bq/cm 2

Measurements of dose equivalent rate and surface contamination density(the experimental hall of the Hadron Experimental Facility)
Location: Experimental hall (Ground floor of the facility building)
Measurement Time and Date: 19:20-20:00 on May 23, 2013 (after operationshutdown)
Measurement methods
Dose equivalent rate (γ): NaI survey meter
Surface density (β): smear method
Measurement unit
Dose equivalent rate (γ): µSv/h (1cm dose equivalent rate)
Surface density (β): Bq/cm 2

Measurements of dose equivalent rate (Hadron Experimental Facility)
Location: Outside of the facility building in the radiation management area ofthe facility.
Measurement Time and Date: 19:20-20:00 on May 23, 2013 (after operationshutdown)
Measurement method
Dose equivalent rate (γ): NaI survey meter
Measurement unit
Dose equivalent rate (γ): µSv/h (1cm dose equivalent rate)

Ha dro nExpe rim en tal
Ha ll

Data included the effectof the Fukushima
accident: exhaust vent

Hadron Experimental Facility ・ distribution of γ -ray nuclide
Location: Experimental hallMeasurement time and date: 21:41 on May 23, 2013 (after operationshutdown)
Measurement method: HP・ Ge detector (smeared sample of the floor)

Count /channel
distribution
measurement

Radioisotopes in red circles are considered to be responsible for causing theinternal exposure.

channelChanges in γ -ray dose with time: measured by an area monitor installed in theHadron facility building~3 hours

Changes in γ -ray dose with time: measured by area monitors installed at anedge of the radiation controlled area of the Hadron Experimental Facility

Monitor
201

Monitor202

Posi ti ons of sm allspi kes in red ci rcl es
were very close t ot he ti mi ngs wh enf ans w ere t urned on.

Monitor203
Changes in radiation level with time byarea monitors at an edge of the radiation
controlled area of the Hadron ExperimentalFacility