Wednesday, March 16, 2011

Spent fuel pools at Unit 4

Water levels a concern for Unit 4 spent fuel pools

One of the big issues for those following the news right now has been the status of the spent fuel cooling ponds at Fukushima Daiichi Unit 4. Unit 4 was undergoing regular maintenance prior to the earthquake; no fuel was in the reactor. Thus, until now it has not been as serious of a concern as Units 1, 2, and 3.

Reports from the Japan Industrial Atomic Forum (JAIF) indicate low water levels at the spent fuel pool, with damage suspected to the fuel rods. An explosion and fire have also been reported at Unit 4. While some suspect the explosion may have been hydrogen-related, the spent fuel pool was significantly cooler than the reactors at Units 1 and 3, with temperatures reported to be around 85 C (185 F) over the past two days - thus, the explosion remains difficult to understand. Several fires have also broken out over the last two days in the spent fuel storage area - sources indicate that these have been machinery oil fires, which have since been contained. Reports of low water levels at the Unit 4 spent fuel storage pool appear to have begun at least 24 hours ago (as of 19:00 hours on March 15).

It is difficult to determine the current status of the fuel pool, given conflicting accounts. U.S. NRC chairman Gregory Jazcko has indicated that it is his staff's belief that, "there is no water in the spent fuel pool." This was immediately contradicted by a TEPCO spokesperson who indicated "the condition is stable", however no further updates are available at the TEPCO website. However, water levels at the spent fuel pools have been a reported concern for some time, according to both World Nuclear News and NEI.

The Nuclear Industrial Safety Association - Japan's version of the NRC (and part of the Ministry of Economy, Trade, and Industry - METI) - has most recently reported the following:
<Unit 4>
  • It was confirmed that a part of wall in the operation area of Unit 4 was damaged. (06:14 March 15th) 
  • The fire at Unit 4 occurred. (09:38 March 15th) TEPCO reported that the fire was extinguished spontaneously (11:00 March 15th) 
  • The temperature of water in the Spent Fuel Storage Pool at Unit 4 had increased. (84 ℃ at 04:08 March 14th)
  • The fire occurred at Unit 4. (5:45 March 15th) TEPCO reported that no fire could be confirmed on the ground.(06:15 March 16th)
  • The water injection was stopped. (14:00 March 16th)

CNN and NHK are currently reporting that JSDF helicopters are currently attempting to drop water onto the Unit 4 building in order to add coolant, as well as water cannons (essentially, fire engines) on standby.

Spent fuel pool basics

Unlike the fuel immediately after shutdown like in Units 1 and 3, spent fuel sitting in the cooling pool is considerably cooler, although it still generates enough heat to require water cooling. Spent fuel is generally about 10-13 feet in height; the water covering this fuel is generally at least another 20 feet in height above the fuel itself. This water serves a dual purpose - it both keeps the fuel cool and provides a good measure of radiological shielding; exposure rates immediately above the spent fuel pool are such that workers can generally be in the area without special protective equipment - generally less than 2 millirem per hour (about a fifth of the dose of a chest x-ray).

A spent fuel storage pool (Image courtesy of IEEE spectrum)

The spent fuel storage pool is kept at atmospheric pressure, unlike the reactor. Under normal conditions, pumps will circulate cooling water in order to keep the rods cool, although under emergency conditions, natural circulation is expected to take over (i.e., where warmer water expands and grows less dense, thus rising to the top, while cool water, which is denser, sinks - thus providing a natural "circulation" for heat removal around the rod.) These spent fuel pools tend to be quite robust - a simple failure of pumps or piping would not be able to drain the water level from these pools, which are made of thick concrete and steel.
Generally, the only way significant water level changes could be expected in the spent fuel pool are from evaporation of the water or if a large crack were to develop in the pool itself. Readings over the last two days indicate that the temperatures of the pools were elevated, at around 85 C (185 F) - while certainly warm, well below boiling. (However, one can expect an increased rate of evaporation at this temperature). The IAEA confirms these temperatures for the prior two days, however no data was available for today. If accounts of diminished water levels are to be believed, this would imply that there may be structural damage to the building; this much is unknown at this time.

Reports indicate that there may be fuel damage due to dropping water levels - this would likely be in the form of cladding failures, which have released radioactive fission gases similar to the process in Units 1, 2, and 3. The major concern for workers right now is in personal safety - without the protective layer of water, there is significantly less radiological shielding, making it far more difficult to operate around the spent fuel pool. This would appear to be the reason for using water cannons and helicopter drops in order to supply emergency water, in order to bring radiation levels back down (by providing an adequate layer of water for shielding). 

For those keeping score at home, the radiological source from these spent fuel rods would be deep-penetrating gamma radiation; the danger to workers is that the exposed would be that in the absence of the several feet of water shielding, the spent fuel rods would essentially act as a "gamma flashlight" pointing out of the fuel pool.

Depending upon the age of the fuel rods themselves (which determines the level of decay heat), it seems less likely that any kind of actual fuel melting would occur itself. Rather, the chief issue appears to be in the radiological release from ruptured fuel cladding as well as the higher levels of radioactivity, which make it extremely difficult for workers to get close to the pool in order to refill it with coolant - hence the use of helicopters and water cannons from the ground. 

I will continue to update as I learn more - unfortunately, the news on this appears to be very scarce, and mixed at best.

Update: JAIF confirms helicopters dropping seawater on Units 3 and 4 due to low coolant levels. Private sources have also begun to confirm that water levels are very low at Unit 4.