TEPCO began constant monitoring of noble gas in the PCV of Reactor 2 on February 19th, as they were decreasing the amount of water from the core spray system. Even though a nuclear reactor is not in a critical state, smaller amounts of nuclear fission continually “spark” in various locations in the melted fuel, and while this does continually release radiation, it is spontaneous, erratic, combated by the constant stream of water into the containment vessel, and not sustained like during normal operations of a nuclear reactor. The amount of spontaneous fission occurring is steady, which allows noble gases released from nuclear fission to be detected and provide operators an indication of the state of the melted materials.
TEPCO has scheduled tests to monitor the amounts of noble gas to ensure that sustained criticality does not occur in the crippled reactor, and uses borate acid to control the “spontaneous nuclear fission”. The boric materials do not quench the spontaneous fission from occurring, and some noble gases are constantly emitted from the melted fuels, forcing TEPCO to make difficult calculations to ensure that the density of noble gases is not over the threshold required for a state of sustained criticality.
On Monday February 20th, TEPCO found that an error message was displayed on the monitor for the noble gas (B system) of the gas control system of the Unit 2 Primary Containment Vessel and then could not check some of the parameters in the B system in the Main Anti-Earthquake Building.
The system has 2 systems of monitoring and TEPCO is currently restricted to only using the A system and able to monitor continuously. The nature of the error message has not been disclosed, and it is unknown why the workers were unable to check the parameters in the Main Anti-Earthquake Building.
TEPCO is still investigating the cause of the malfunction of the B system, and no root cause has been identified yet.
Reactor 2 Spent Fuel Pool Problems
At around 5:00 am on February 8, an alarm indicating “abnormally low suction pressure of the high pressure RO “pump” in the desalination apparatus for the Unit 2 spent fuel pool went off, and stopped working. The interlock arrangement closed all the isolation valves in the system.
In the evening of February 18, an alarm sounded in the backup Unit 2 spent fuel pool cooling system indicating that there was a large water flow difference at the gateway of primary system pump; the desalting facility and the system shutdown. The desalination equipment has been restarted and work continues to remove radioactive materials from the spent fuel pool, but there is no word on when the spent fuel pool issues will be resolved.