Radiation Dose in a Reactor Service Area of the AP-1000 Based on the Fukushima Accident

Cooling water as a radiation shield from a reactor core originates from the reactor core. In the case of an accident, when the cooling water level inside a pressure vessel decreases, the function of water as a radiation shield decreases as well. After the time passage of the reactor shutdown, the strength of gamma (γ) radiation source from the reactor core decreases. The combination of the decrease of the water that reduces the function of radiation shielding and the decrease of the source strength will affect the pattern of dose rate in the reactor service area (RSA) after an accident takes place. This paper discusses γ dose rate in an RSA of an advanced pressurized water reactor (PWR) (AP1000) nuclear power plant (NPP) based on the accident of Fukushima No. 1 reactor. The referred accident is the decrease in the level of cooling water until the it reached half of the height of the reactor core. The reactor core was not damaged since the γ source is still trapped in the matrix of the reactor core. The highest dose rate occurs when the water level decreases to the upper level of the reactor core. The dose rate above the pressure vessel at the RSA prior to the decrease of the water level was 0.7 rad/h, while the dose rate reached 740 rads/h when the water level reaches to the middle of the reactor core. Indeed, the dose rate at a point of 17.5 m horizontally from the center of the core tank was 0.057 rad/h. This condition requires highly selective control to enable entry to the area of the accident. Indeed, a large γ radiation rate due to a decline in the surface horizontal distance is around 6,404 times greater than those of cosmic and natural γ rays. Philippine Journal of Science 149 (3-a): 791-799, October 2020 ISSN 0031 7683 Date Received: 28 Jan 2020