Xe-135 concentration, which is a short half-life FP, is continuously monitored by the gas radiation line monitor installed in the PCV gas control system of each Unit. While the criticality criterion has been set to 1Bq/cm3, no sign of criticality has been shown. In addition, evaluations have been carried out by using various conditions, such as compositions and shapes of fuel debris, deposition shape, composition and mixed amount of structural materials. The results indicated that the possibility of reaching criticality was low.
A boric acid water injection system has been installed in order to make it subcritical or prevent criticality for the case when the fuel debris reaches re-criticality or there is a possibility of re-criticality. Two tanks of boric acid water has been installed (one for spare), and alkalescent sodium pentaborate solution which has little effect on structures will be injected through the reactor coolant injection system. This system can achieve the boron concentration of 510ppm, which is equivalent to the reactivity of more than 5%.k. In addition, when boric acid is exhausted up, the sea water having the reactivity reduction effect of approx. 3%.k will be injected. The time required from the occurrence of criticality to the completion of injection takes usually 6 hours and 22 hours at the longest even assuming the equipment is damaged.
The assessment of the impact during re-criticality was performed conservatively to evaluate the radiation dose at the boundary of the site assuming that the critical state with the output level,equivalent to 100 times the criticality criterion for Xe-135 concentration continues for one day. It indicated that the evaluated exposure dose at the site boundary was 2.4 x 10-2 mSv and it does not have significant impact.