Choices 1) and 2) requires plant shutdown, while 3) can easily be implemented without shutdown and without much expense (Figure 3).
Figure 3 shows how the actual weight of the (remaining) condensate in the wet leg (AWL) is measured and how that is subtracted from the normal reference wet leg (RWL). The calculated difference (ΔL) is the height of the lost condensate in the wet leg. Under emergency conditions, by adding this amount (ΔL) to the level reported by the d/p cell (NL), the corrected level (CL) is obtained. It is recommended that both signals (NL and CL) be sent to the control room to provide the operators with the needed information concerning the conditions in the reactor. Any number of d/p cells (LT) can be added to the reactor, and the closer they are vertically, the more accurate their readings will be. In addition to reporting the level, they can also measure trends and other variables, such as the steam-to-water ratio, etc.
In-Core Level Measurement
The ex-core level measurement will approximate the in-core level only so long as the fuel rods are covered by water, but once the ex-core level drops below the suction of the jet diffusers, it will not. Therefore, direct in-core measurement is also needed. In many cases, such as Fukushima, they were not provided.
One method of in-core level measurement is to correlate it with the gamma radiation distribution inside and outside the reactor. The vertical gamma radiation distribution is related to water level, because water is more of a moderator than steam. On the other hand, because gamma radiation is also a function of the neutron flux and of the speed of water recirculation, special correction models and algorithms are needed to obtain the water level from gamma radiation distribution.
Other possible ways to detect in-core level (or steam/water ratio) are based on the thermal or electric conductivity, or neutron modulation, etc. differences between water and steam.
Dr. David Nyce designed such a thermal conductivity-based, in-core level sensor for the Knolls Atomic Power Laboratory. In that design, a number of different length metal probes are inserted, each equipped with two vertically separated thermocouples (TC). The one located at the tip is heated, while the second, unheated reference thermocouple is a few inches above the tip. In the case of this sensor, if water covers both TCs, the temperature difference (ΔTw) will be lower than the temperature difference (ΔTs) when both are covered by steam.
If all nuclear power plants used the correct level measurement design shown in Figure 3, their safety would be much improved. In the next article in this series, I will describe other ways automation could have prevented the Fukushima accident.