Firstly, without know the specifics of your text, I tried to locate a general type document describing what a "reference leg" is and what purpose it serves. My understanding from the first link is that the "reference leg" is a reserve tank. Does this make sense.

http://www.globalresearch.ca/how-accurate-are-the-instrument...
LH: Good afternoon. I’m Lucas Hixson and I’m here today with Dave Lochbaum with the Union of Concerned Scientists, and we’re going to be speaking about what is called the Reactor Water Level Monitoring System at nuclear power plants. Water is used in the nuclear reactor as a critical neutron moderator and coolant. ****Water levels in a nuclear reactor are not monitored directly, but rather through an indirect monitoring system, which incorporates a reserve tank which is termed a reference leg.**** There have been some reported flaws with this cooling system throughout the years, some of the most notable being brought forth by Paul Blanche in the early 1990’s. Dave, can you explain to us some of the nature of his findings?


Other refs. to "reference leg" but I am not sure of their applicability to your text.
http://www.cas.go.jp/jp/seisaku/icanps/eng/02Attachment1.pdf
This type of indicator sometimes shows a reactor pressure that was lower than the actual one by around several dozen kPa gage due to a decrease in water level in the **reference leg.**

The reactor water level is then measured by a pressure differential transmitter from the value
obtained by subtracting the water pressure transmittedthrough said transmitter (hereinafter
referred to as the “reactor-side piping pressure”) from the water pressure transmitted through the
reactor-side piping (hereinafter referred to as the ***“reference leg ***-side piping pressure”).


Another link: http://www-pub.iaea.org/MTCD/publications/PDF/te_1044_prn.pd...
Inaccurate readings were observed in reactor vessel water level instrumentation at several BWRs during
controlled depressurization while entering plant outages or following reactor trips. These false readings
consisted of "spiking" or "notching" of level indication. In one instance, a sustained error in level
indication occurred. The root cause of these errors is the effect of non-condensible gas dissolved in the
*** reference leg*** of "cold reference leg" type water level instruments. Under rapid depressurization
conditions, non-condensible gases can cause significant errors in the level indication. Significant
spiking may automatically actuate such systems as the primary containment isolation system. After
spiking, which is of short duration, the indicated water level returns to actual level.

Others (I haven't looked)
http://webcache.googleusercontent.com/search?q=cache:mH_TAUE...
installation of safety-related instrument sensing lines in nuclear power plants. .... BWR licensees decided to install a **reference leg** backfill system to supply a ...