Research and Development Relating to the Accident at the TEPCO's Fukushima Daiichi NPS

1-24 Development of Special Measuring Equipment Using JMTR Irradiation Techniques=A New Water-Level Indicator Applicable under Severe Conditions=MIcable

A schematic of thesensor unit
Ǿ4.8 mm

Movement of positionof the water level indicator

Ǿ1 mm

100 m

Ǿ4 mm

350

Water temperatureǾ4.8 mm+㧙

Water

Temperature (͠)

HeaterAir

AlumelSheath (SUS316)

Measurement Chromel
point

200 mmMeasurementpoint

K-type Thermocouple

Sensor unit

300

Distancefromwatersurface‫ع‬㧦Measurement point

Fig.1-50 Schematic diagram of the water levelindicator
A heater and a thermocouple are installed in thesensor unit, which is connected to a radiationresistant MI cable. The sensor unit is heated with an
installed heater, and the water level is detected on
the basis of the temperature difference in air or inwater.

The water level in a light water reactor is monitored using
water-level indicators, such as float-electromagnetic-inductionand electrode-type indicators. During the accident at theTEPCO s Fukushima Daiichi NPS, measurement of the waterlevel in the spent fuel pools and in the pressure vessels was
not available owing to the station blackout, which caused adelay in information gathering. Thus, it is necessary todevelop a new, reliable water-level indicator that is available
under station blackout conditions.
The target indicator is small, portable, has high radiation
resistance, and operates on a battery-level power supply. To
achieve downsizing and high radiation resistance, a method
for measuring the water level using an installed heater andthermocouple was adopted.
A heater wire and a K-type thermocouple are installed in astainless sheath. This structure enables achieving a small
diameter. An MI cable is used as the lead wire and can beused in a high-radiation environment.
The water level is measured using the indicator, whichmeasures the temperature difference in the water or in the aircaused by the different heat transfer coefficients of the waterand air. To measure the water level with high accuracy, thetemperature difference is increased by the heater in theindicator using a suitable impressed voltage (Fig.1-50).
A characterization test was performed. The impressed27 ͠90 ͠

250
20015010050
0-150-100-50050100

150

Distance from water surface (mm)Fig.1-51 Results of water-level measurements using thedeveloped water-level indicator
The developed water-level indicator shows a large temperature
difference in water and air, regardless of the water temperature. The
measurement results for the change in the distance between thetemperature measurement point and the water surface indicate thehigh accuracy of the water-level indicator.

voltage was changed from 1 to 5 V, and the temperaturechange was measured in the water and in the air by moving
the detector in the vertical direction. From the test, it wasconfirmed that the water level can be detected using thesesmall voltages. Fig.1-51 shows the results of water-levelmeasurements performed with water at 27 ℃ and 90 ℃. A
voltage value of 5 V was impressed to the heater. The
developed water-level indicator showed a large temperature
difference in water and air, and the accuracy of themeasurement of the water level was approximately 20 mm. If
the water-level indicator is used in an actual reactor facility,
the measuring person and the apparatus should be kept awayfrom the sensor unit in order to avoid severe conditions.
Therefore, a water-level indicator with a 100 m MI cable wasalso manufactured, and a demonstration test was performed.
The results of this test indicated that the developed waterlevel sensor is sufficiently enough to be portable and that it ispossible to measure the water level using a small battery
power supply.
Importantly, the developed sensor unit functions in hightemperature and high-radiation severe environments, where
no one can access the measurement apparatus, because thesheath-type thermocouple and heater have high radiation
resistance.

Reference
Miura, K., Shibata, A. et al., Development of Reactor Water Level Sensor for Extreme Conditions, Proceedings of the 4th International Symposium on
Material Testing Reactors, Oarai, Japan, JAEA-Conf 2011-003, 2011, p.193-195.
JAEA R&D Review 201235