Radiation Protection in the Large Tokamak Device (JT-60) Facility

P-6a-314

Radiation protection in the large tokamak device（JT-60）facility

Isamu Akiyama*1，Takashi Umehara*1，Toru Tayama*1，Kozo Kodama*1，Naoyuki Miya*1 Kouzou Matsushita*1 ＊1 Naka Fusion Research Establishment, Japan Atomic Energy Research Institute

1. OUTLINE OF JT-60 JT-60 is the typical large tokamak device for fusion experiment in Japan. Discharge operation using deuterium gases began in July, 1991. JT-60 have made a new world record of the highest plasma temperature 5.2 8 ×10 Kelvin(45KeV) in 1996 and also achieved a highest equivalent fusion amplification factor of QDT =1.25 in June, 1998.

2. RADIATION DOSE MANAGEMENT IN JT-60 Main objectives of the radiation protection in JT-60 facility are as follow： (1) Measurement of radiation exposure rate for X(γ) and neutron rays generated in the JT-60 high temperature and high density plasmas during the deuterium operation，(2) Measurement of leakage radiation dose outside of controlled area， (3) Management of radiation dose caused by radionuclides during the in-vessel maintenance works in machines shutdown times. Radiation sources produced in the D-D nuclear fusion reaction are shown in Figure 1.

JT-60 control room 41Ａｒ 13 neutron ray 13Ｎ ray γ 3 ＪＴ－６０ facilty γ Ｈ

JT-60 hall 41ＡｒＴＬＤ stack monitor 13Ｎ γray Ｎ 3Ｈ

neutron area monitor JT-60 radio activation D D n n γarea ＴＬＤ environment monitoring room monitor monitoring room D Ｔ D Ｔ monitor(γ，ｎ)

ＨＦＣＭＴＬＤ

monitor panel ＨＦＣＭ contamination checking room

monitoring post

vaccum pump ＤＴ

Fig.1 Radiation sources produced in the D-D nuclear fusion For radiation monitoring in the radiation controlled area，some area monitors are installed around the JT-60 building hall. X(γ) ray area monitor can detect the burst radiation radiated from the plasma discharges in real-time. γ(X) ray area monitor (and neutron area monitor) are used for radiation working area monitoring during the machines shutdown period. For environment monitoring，dust monitor and gas monitors are installed in the stack area of the JT-60 experimental building. Both of Environment γ ray monitor and Environment neutron monitor are also located at two points on the site boundary. In addition to the above monitors， accumulated dose measurement by the thermo luminescent dosimeter (TLD) method is used for γ(X) rays， thermal neutrons and fast neutrons around the radiation controlled area and the NAKA-Establishment site boundary.

In the JT-60 hall，the data of maximum accumulated dose (TLD) for γ(X) rays and thermal neutrons were 58 mSv/month and 27 mSv/month，respectively, in the operation of FY 1996. Neutron production in this

1 P-6a-314 period (one month) was 2.7×1018(n). The accumulated dose (TLD) around the radiation controlled area， however，still remained at the background (BG) level (less than 0.3μSv/h).

In the radiation controlled area where radiological workers can access the points close to the JT-60 device for maintenance workers the machines shutdown period，dose rate of inside the vacuum vessel and outside the vessel port surface were 30-40μSv/h and 20μSv/h，respectively. Data of any other place in the hall indicated BG level. The in-vessel maintenance works characterizes the main radiation dose in the Naka-Establishment site. This is caused by radio activation of the vessel materials by D-D neutrons. Main radio activated products are

58Co，60Co. Time evolution of the γ ray dose rate on the vessel surface is shown in Figure 2

1992 1993 1994 1995 1996 1997 1998

80 Maintenance works in

Machine shutdown Total dose rate

times

60 定期点検

ｈ）ｈ）ｈ）ｈ）

/ / / /

μＳｖ μＳｖ μＳｖ μＳｖ

Dose rate ( Dose rate ( Dose rate ( Dose rate (

60Co dose rate

0 0 50 100 150 200 250 300 350 400

Machine operation cycles (week) Fig.2 Time evolution of the γ ray dose rate on the vessel surface

To reduce the external dose during the in-vessel maintenance works，radiation control has been conducted from FY 1991. After FY 1995，next two followings schemes were introduced in to the annual experimental planning stage； (a) Deuterium experiments with high neutron rate and long pulse discharge were arranged to be concentrated at early phase of experimental period because the vessel radio-activation levels after such discharges were intensive and it was required to take a long cooling time before the start of the in-vessel works. (b) The planned dose was decided in advance of the works，based on the evaluation of the annual neutron production，dose rate during the works and total time required for the works.

Concerning the practical stage of the in-vessel maintenance works，we placed thin plates of lead on to the working stage floor as shielding material. Due to above efforts the dose rate inside the vessel decreased by 20-30％ and reduction in the external dose was sufficiently obtained for the radiation workers.

The maximum individual dose and the maximum population dose of the radiation workers were 3.5mSv(FY 1995) and 78.5 person・mSv(FY 1997)，respectively. To reduce the radiation dose in future，some improvements on the working process are required in JT- 60，since it is expected that the dose rate inside the vessel will increase gradually as the experiments continued because of a build-up by the long half-life radio activated products (60Co，etc).

The radiation control during the in-vessel maintenance works will be minimized through further

2 P-6a-314 improvement on the JT-60 facility.

SPECIAL MENTION The nuclear accident to reach criticality broke out at the uranium fuel processing company JCO on 30 September,1999.The Naka-Establishment site is adjacent to the JCO，so that our environment radiation monitor could catch the neutron and γ ray signals emitted from JCO and could show the valuable data to make clear the phenomena during the time nuclear fission chain reaction accident continued.

REFERENCES 1. Ｋ.Ｙamane. The radiation control in the large tokamak device (JT-60)，Journal of Health physics, 32(2)1997