Anti-Radioactivity of Robots (1)

Remark on radiation-proof performance of Japanese anti-disaster robot.

Kazuya Yoshida, Professor, Tohoku University
Keiji Nagatani, Associate Professor, Tohoku University

Radiation is known to influence to the performance of electric/electronic equipments. There are two major ways the influence to appear:

TID (Total Ionizing Dose): Deterioration of electric/electronic equipments because of temporally accumulated amount of radioactive exposure.

SEE (Single Event Effect): Malfunction or destruction of a circuit element by the effect of one high energy particle.

The TID of some of anti-disaster robots was quantitatively evaluated based on the known data. It is said that the probability of occurrence of SEE is low in the nuclear power plant (SEE could not be neglected in the space environment. Here remark on TID is provided.

QUINCE and KENAF of International Rescue System Institute (IRS) consist of commercial off-the-shelf components. They are assumed to be resistible to around 50 [Gy] of TID without extra shield. This assumption is based on the experimental result (see Commentary.1). Another result says a representative PC is resistible to 20 [Gy] of TID (see Commentary.2). As for the unit conversion, 1 [Sv] equals to 1 [Gy], that is, 50 [Gy] implies to 50 [Sv] in gamma ray. The limit of TID for workers is legally defined as 100-250 [mSv]. If the safe factor is set 0.1, 50 [Gy] and 20 [Gy] (the admissible limit of a robot) are 20-50 times and 8-20 times larger the admissible limit of human workers respectively.

Of course, the assumption is based on the data for major but some of elements of QUINCE and KENAF. It does not assure the total robotic system to operate without problems. As it now stands, a robot needs to be used at the disaster site in the trial and error manner, including the evaluation of its SEE. Even in such a situation, it would be worth using a robot to reduce anxiety about the exposure of human workers at the site.

Commentary.1: Calculation ground for the TID limit of Quince and KENAF (50 [Gy])

In the case that commercial off-the-shelf components are applied to an artificial satellite, those resistance to radiation are tested and evaluated. The laboratory of Yoshida and Nagai in Tohoku University used public cobalt 60 irradiation facilities and conducted radioactive exposure test on the major components of IRS’s QUINCE and KENAF  before the earthquake disaster. According to the test, the motor drivers resulted in no failure with 20 [krad] exposure. The imaging function of the monochrome high-sensitive CCD cameras was alive before 6.7 [krad] exposure in spite of white-lined (or white-dotted) lesions on the captured images. The gyro sensors and the CAN drivers failed after 5-10 [krad] exposure. These results says the minimum TID to fail is 5 [krad], which is equals to 50 [Gy].

Commentary.2: Resistance to radiation of commercial-off-the-shelf components including PC

The literature [1] reports the experimental results of direct exposure of 4 off-the-shelf notebook PCs (mobile Pentium III 600MHz). According this report, a test program running on the PCs stopped after 23 [Gy] of TID at the minimum; and 3 of 4 PCs fails after 50 [Gy]. This report says also TID to fail of a commercial-off-the-shelf CCD camera is 119 [Gy] and that of an infrared camera is 110 [Gy].

[1] “Report of the subsidy business for nuclear energy disaster prevention aid system development. 1999 FY. (Ministry of Economy and Industry S).,” Manufacturing Science and Technology Center, II-95-99, August 2001.

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