TEPCO is carrying out the maintenance and management of the equipment for cooling fuel debris as well as continuous monitoring of the parameters including the temperatures of the reactors. The buffer tanks which are the water source of the circulating injection cooling facility that cools down the fuel debris was replaced with condensate storage tank (hereinafter referred to as "CST") on July 2013 and the operation of the CST reactor coolant injection system has been started. Thereby,improvement of seismic resistance and increase in capacity are being achieved for the tank, reducing the risk of losing water injection function due to reduction of a reactor water discharge line length. In addition, the desalination system (RO), which is one of the circulating injection cooling facilities,was installed and the construction for smaller circulation loop in the T/B of Unit 4 is being conducted. This construction will reduce the circulation loop (outdoor transfer pipes) from approx. 3km to approx. 0.8km (approx. 2.1km if transfer line for stagnant water included).
As described above, the parameters including the temperatures of the reactors indicated stable value which was lower than that immediately after the accident. Also the temperatures in the RPV and PCV have been declined due to the continuous cooling and decrease in decay heat, and it can be estimated to be in a stable cold shutdown state.
In the implementation plan, the reactor core re-damage frequency is approx. 5.9 x 10-5 / year according to the risk assessment of the reactor coolant injection system by probabilistic risk assessment. It can be confirmed that the risk is reduced compared to the reactor core re-damage frequency of approx. 2.2 x 10-4 / year evaluated in the 窶彝eport (Part 1) on facility operation plan (Revision 2) (December 2011). In addition, according to the evaluation of the reactor coolant injection system in the event of abnormality, even in the case where the amount of radioactive materials of three plants are assumed to be released due to an event equivalent to a severe accident greatly exceeding an expected level (water injection shutdown for 12 hours), the effective doses are approx. 6.3 x 10-5 mSv / year at the site boundary, approx. 1.1 x 10-5 mSv / year at a 5 km point from the Specified Nuclear Facility, and approx. 3.6 x 10-6 mSv / year at a 10 km point from the Specified Nuclear Facility and therefore, it is considered that there will not be a significant risk of exposure to the general public in the vicinity.