THE NUCLEAR COMMUNICATIONS NETWORK
Japan Update / Brief No. 75 / 20 March 2011
Figures Released On Fukushima-Daiichi Seismic Design Reference Values
20 Mar (NucNet): The maximum ground acceleration near unit 3 of the Fukushima-Daiichi nuclear plant from the earthquake that struck northern Japan on 11 March 2011 was 507 gal – or 507 centimetres per second squared – which is above the plant’s design reference values of 449 gal, the Japan Atomic Industrial Forum (JAIF) said today.
It was also announced that the University of Tokyo has re-evaluated the maximum height of the tsunami wave and it had a peak height of at least 23 metres when it hit Miyagi and Fukushima prefectures. The plant design reference value was 10 metres.
Meanwhile, external power is available onsite at the plant and teams from the Tokyo fire service and national Self-Defence Force are continuing to pump seawater into the reactor pressure vessels (RPV) and primary containment vessels (PCV) at units 1, 2 and 3 in an effort to cool the nuclear fuel.
The International Atomic Energy Agency said workers were continuing efforts to restore plant systems at units 1, 2 and 3.
At unit 2, external power has been established to the control room meaning essential surveillance and control systems are again operational, Japan’s Nuclear and Industrial Safety Agency (NISA) said.
NISA said workers were preparing to connect an external power supply to the residual heat removal systems of unit 2 and then unit 3. If the systems have not been damaged, this will allow cooling and feeding of the reactors and the spent fuel pools (SFPs).
Seawater spraying into the SFP at unit 3 has begun and while the pressure in the PCV went up early today, it was not sufficiently high to justify venting. The local dose rate close to this unit is decreasing At unit 4, where there is no fuel in the reactor, seawater spraying of the SFP started today at 09:40 Japan time (01:40 central European time). An external power supply connection is being prepared, NISA said.
At units 5 and 6, diesel generators were repaired yesterday and there is now enough power available to each unit for the residual heat removal system pumps. NISA said the temperature has fallen in both SFPs and the units were now in cold shutdown. Workers were today preparing to connect an external power supply to both these units.
Cold shutdown means the reactors are in a safe mode, with cooling systems stable and under control, and with low temperature and pressure within the reactor.
Air Sample Analysis
Plant operator Tokyo Electric Power Company (Tepco) said today it made the first spectral analysis of air samples taken on the Fukushima-Daiichi site at noon on Saturday local time and found volatile iodine and particulate caesium isotopes as follows:
Iodine-131: 5900 becquerel per cubic metre (Bq/cubicm), above the limit of 1000 Bq/cubicm;
Iodine-132: 220 Bq/cubicm, below the limit of 70,000 Bq/cubicm;
Cesium-134: 22 Bq/cubicm, below the limit of 2000 Bq/cubicm;
Cesium-137: 24 Bq/cubicm, below the limit of 3000 Bq/cubicm.
The limits are for air being breathed by workers under surveillance, but Tepco said workers were wearing charcoal masks and no internal contamination is expected.
Environmental Radiation Monitoring
The Ministry of Education, Culture, Sports, Science and Technology (MEXT) has published updated data from its surveillance network of automatic measurement posts (MPs) at a distance of more than 20 kilometres from the plant site. Inside the 20 km zone, Tepco is responsible for the network, but under NISA’s surveillance.
The downward trend of all measurements has continued since yesterday.
Today (Sunday), the highest dose rates were measured at the north-eastern Fukushima Mountains (summit 864 m, 30 km from the plant site), with values between 45 and 110 microsieverts per hour (microSv/hr) at 12:00 noon Japan time today in comparison with 60 to 170 microSv/hr on Thursday.
In the valleys, where most people live, the readings range from 1 to 6 microSv/hr in a distance of 35 to 60 km from the plant site. This is 25 percent less than on Thursday.
All these readings refer to an area north-west of the plant. This was the direction in which prevailing winds were blowing during the first days of the accident when the main releases occurred.
In the Tokyo area, the dose rate stayed very low at 0.05 microSv/hr. Yesterday the government’s chief cabinet secretary Yukio Edano said radiation exceeding the government-set radiation level was detected in sampled milk in Fukushima Prefecture and in six samples of spinach in neighbouring Ibaraki Prefecture.
He said the radiation measured from the samples posed “no immediate threat” to health.