Heated junction thermocouple cable arrangement

An improved heated junction thermocouple cable arrangement for use with a nuclear reactor heated junction thermocouple having sensors with an unheated thermocouple, a heated thermocouple, and a heater coil, shares power conductors between multiple heater coils. The thermocouple conductors are separated from the power conductors to eliminate the possibility of interference with the thermocouple conductor signals. The heater coils are ganged in parallel and if one heater coil fails, power is still supplied to the remaining heater coils connected to a pair of power conductors.

FIELD OF THE INVENTION
 The present invention relates to an improved heated junction thermocouple
 cable arrangement for use with a nuclear reactor heated junction
 thermocouple level measurement system.
 BACKGROUND OF THE INVENTION
 When a pressurized water nuclear reactor is first activated after a
 shutdown, the reactor vessel is completely filled with a fluid coolant
 such as water. During nuclear reactor operation, the fluid coolant is
 circulated through the core to remove the generated heat. During normal
 reactor operation the coolant remains in a liquid state as it passes
 through the core. During abnormal operation, such as when there is a leak
 in the coolant system, the fluid coolant within the reactor vessel may
 change state to become a two-phase mixture of liquid and gas.
 A heated junction thermocouple level measurement system with a plurality of
 sensors is placed within the reactor vessel and located in the upper guide
 structure. Each sensor includes a heated thermocouple, an unheated
 thermocouple, and a heater coil. The system is used to determine if liquid
 coolant is present above the reactor core at the level of each sensor.
 Typically, a heated junction thermocouple level measurement system has
 eight completely separate sensors. The sensors are typically vertically
 spaced by equal increments in the reactor vessel between the fuel
 alignment plate and the reactor vessel head to give a coolant level
 indication over the entire height above the reactor core.
 Five separate wires must be connected to each sensor to provide the
 measurements from the thermocouples and to provide electricity to the coil
 heater.
 The forty wires making up the eight sensors engage the reactor vessel wall
 by means of a penetration connector. The five wires associated with each
 of the eight sensors are typically combined into a single cable.
 Preferably, the cable is mineral insulated, meaning that the cable is
 manufactured from completely inorganic material. Typically, the cable
 includes a copper-lined stainless steel sheath and conductors insulated
 with a mineral oxide such as silicon dioxide. Such components help ensure
 that the cable is able to withstand the effects of extremely high
 temperatures such as those associated with nuclear combustion and is still
 fully usable afterwards.
 The eight sensors are then connected to a heated junction thermocouple
 probe by means of one or more containment, bridge, and head lift rig
 cables. A containment conduit containing containment cable engages the
 penetration connector. A bridge conduit engages the containment conduit. A
 head lift rig conduit engages the bridge conduit. Finally, the probe
 engages the head lift rig conduit. The cables provide the necessary power
 for the eight heaters and connect the other wires to signal processing and
 display equipment.
 In one prior art embodiment, eight separate containment, bridge and head
 lift rig cables are used for each of the sensors. In a second prior art
 embodiment only one each of a containment, bridge, and head lift conduit
 are used.
 Each free end of the conduits includes a connector adapted to engage a
 mating connector. A total of 40 pins or sockets are required at each
 conduit end to connect the forty different wires from each of the sensors
 with the heated junction thermocouple probe.
 There are a number of significant drawbacks with the current heated
 junction thermocouple cable arrangement. The requirement of having forty
 separate wires requires either the use of individual very large and
 unmanageable containment, bridge, and head lift conduits or a significant
 number of smaller conduits, each of which each must be separately
 manipulated and located. Problems with cable labeling and clutter are
 greatly increased when multiple conduits are used. Yet, the ability to be
 able to easily manipulate and locate the heated junction thermocouple
 cable arrangement is of critical importance during reactor refueling when
 time spent in the so-called "hot" region of the reactor vessel must be
 minimized.
 There are also significant drawbacks associated with requiring forty pins
 and sockets to mate the various wires with corresponding conduits. There
 is a constant trade off between connector size and the number of pins. As
 connectors become smaller, so do problems with pin and socket reliability.
 For example, it becomes easier to bend or distort the pins when handling
 the connectors or when mating or separating the connectors. Also, it is
 not often possible to easily replace a connector with damaged pins. This
 is particularly true if the number of circuits passing through the
 connector is large or if the shell is hermetically sealed to the
 interfacing instrument or conduit. However, if connectors are too big, the
 ability to move the associated conduits is compromised. Both damaged pins
 and bulky conduits also undesirably increase time spent in the "hot" zone
 of the reactor vessel.
 There are also issues raised by having eight independent cables, each cable
 having the five wires discussed above. The connectors are very small and
 are easily damaged during the disconnection and connection process.
 Combining power wires with sensor wires in the same cable increases the
 potential for electrical interference with the sensor signals carried by
 the remaining three wires in the cable. Further, the need for additional
 insulation in each cable for increased insulation resistance adds to the
 bulk of the cable and the difficulty in handling the cables.
 SUMMARY OF THE INVENTION
 The present invention is directed to a nuclear reactor heated junction
 thermocouple level measurement system having a plurality of sensors. Each
 sensor includes both an unheated thermocouple and a heated thermocouple. A
 heater coil is placed adjacent the heated thermocouple. There is a
 distinct thermocouple conductor of a first polarity associated with each
 thermocouple and a thermocouple conductor of the opposite polarity shared
 between the thermocouples. Two power conductors are used to supply
 electricity to the heater coil.
 There is a plurality of thermocouple sensor cables, each of the sensor
 cables associated with the thermocouples of a single sensor. There is also
 a plurality of power cables. A power cable provides electricity to more
 than one heater coil, the heater coils associated with a power cable
 ganged in parallel such that if one heater coil fails, the rest still
 receive power.
 By having a power cable separate from a sensor cable, prior art problems
 associated with the power conductors potentially affecting the sensor
 signals carried from the thermocouples are eliminated.
 In a preferred embodiment, all of the power cables and sensor cables are
 received in a single flexible metal conduit. For the same number of
 sensors, the reduction in the number of wires providing power to the
 heater coils provides a number of significant advantages. A smaller
 conduit is more manageable than either eight conduits of five wires each
 or a single conduit with the eight five-wire cables contained in the
 flexible sheath known in the prior art. Opposite ends of a conduit include
 a connector having either pins or sockets. Pins may be easily bent or
 distorted. The likelihood of damage to the pins is reduced if the number
 of pins is reduced.
 In a preferred embodiment of the invention, there are eight sensors
 associated with a heated junction thermocouple sensor. Two pairs of power
 wires are associated with two sets of four heater coils and three sensor
 wires are associated with each of the sensors. Thus, penetration
 connectors to containment cable and to the bridge and head rig cables have
 either 28 sockets or pins. A containment conduit having connectors at
 either end engages the penetration connector. A bridge conduit having
 connectors at either end engages the containment conduit. A head lift rig
 conduit having connectors at either end engages both the bridge conduit
 and an existing heated junction thermocouple probe. The head lift rig
 conduit acts as a transition conduit having 28 pins or sockets in the
 connector engaging the bridge conduit and 40 pins or sockets in the
 connector engaging the probe. Jumpers connect select pins or sockets of
 the 40 pin or socket connector such that power to the probe is
 transitioned from four wires to sixteen wires.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
 As illustrated in FIG. 1, a heated junction thermocouple level measurement
 system 10 comprises a heated junction thermocouple cable arrangement 12
 and a plurality of sensors in the probe 42.
 As best shown in prior art FIGS. 2 and 3, five separate wires A, B, C, D
 and E must be connected to each sensor assembly 14. Two of the wires, A
 and C, comprise thermocouple conductors of a first polarity, usually
 positive. A positive thermocouple conductor is typically formed from a
 Ni.sub.90 /Cr.sub.10 thermocouple alloy sold under the tradename
 Chromel.RTM.. A negative thermocouple conductor is typically formed from a
 Ni.sub.95 Al+Mn+Si.sub.5 thermocouple alloy sold under the tradename
 Alumel.RTM.. The first thermocouple conductor A of the first polarity
 forms a component of heated thermocouple 16 while the second conductor C
 of the same polarity forms a component of the unheated thermocouple 18. A
 third wire B comprises a thermocouple conductor of the opposite polarity.
 The conductor B is typically shared between both the heated and unheated
 thermocouples 16, 18. The remaining two wires D and E are typically nickel
 clad copper and act as positive and negative power conductors providing
 electricity to heater coil 20.
 To prevent inaccurate readings, the heated and unheated thermocouples 16,
 18 of each sensor 14 are physically displaced from one another so that
 heat from the heater coil 20 positioned next to the heated thermocouple 16
 does not affect the voltage generated by the unheated thermocouple 18.
 The heated and unheated thermocouples 16, 18 are monitored for both the
 absolute temperatures of the thermocouples as well as the differential
 temperature between the two thermocouples making up a sensor 14. The net
 voltage generated by each of the thermocouples 16, 18 are a function of
 the temperature difference between them. The heated thermocouple 16 will
 generate a voltage representative of its temperature. The unheated
 thermocouple 18 will also generate a voltage representative of its
 temperature.
 When liquid coolant surrounds both the thermocouples 16, 18 the heat
 generated by the heater coil 20 will be transferred to the surrounding
 coolant. Therefore, the temperature of both thermocouples will remain
 essentially identical. Since the voltage produced by the heated
 thermocouple opposes the voltage produced by the unheated thermocouple,
 the net voltage should be small.
 When there is an absence of liquid coolant surrounding both the
 thermocouples 16, 18 the heat produced by the heater coil 20 does not
 transfer as well to the surrounding gaseous coolant. As a result, the
 heated thermocouple temperature will rise above the unheated thermocouple
 temperature and a much larger net voltage results between the two
 thermocouples 16, 18.
 The thermocouple wires A,B and C have an outer diameter in the range of
 approximately 0.01 to 0.02 inches and more specifically an outer diameter
 of 0.015 inches. The prior art heater coil wires D and E have an
 approximate diameter of 0.040 inches.
 The five wires A through E associated with each of the eight sensors 20 are
 typically combined into a single cable C.sub.1 through C.sub.8 as best
 shown in prior art FIG. 2. The outer diameter of cables C.sub.1 through
 C.sub.8 is approximately 0.25 inches. In turn one or more cables C.sub.1
 through C.sub.8 are secured within a conduit 30. Typically, each conduit
 30 is formed from a flexible metal hose or sheath sold under the tradename
 Penflex.RTM.. If all eight cables are secured within a single conduit 30
 the outer diameter of the resulting conduit is more than 0.75 inches.
 In direct contrast to the teachings of the prior art, the present invention
 significantly reduces the number of wires from forty to twenty-eight, as
 shown in FIG. 3. The sixteen separate heater coil conductors D and E
 traditionally associated with cables C.sub.1 through C.sub.8 of the prior
 art are reduced to a total of four wires D' and E'. The wires representing
 power conductors D' and E' have a diameter in the range of approximately
 0.06 to 0.09 inches and more specifically approximately 0.08 inches. The
 thickness of wires D' and E' permit greater power to be transmitted
 through the wires. One set of wires D' and E' are isolated within a
 separate cable C.sub.9 while the other set of wires D' and E' are isolated
 within a separate cable C.sub.10. An advantage of having the wires D' and
 E' separated from the sensor wires A, B, and C is that the wires D' and E'
 have a tendency to interfere with the sensor signals if the five wires are
 shared within a single cable.
 Cables C.sub.9 and C.sub.10 have an approximate diameter of approximately
 0.31 inches. The remaining eight cables C.sub.1 ' through C.sub.8 ' only
 have three wires, namely the thermocouple conductors represented by wires
 A, B and C. As a result, the corresponding cables C.sub.1 ' through
 C.sub.8 ' have an outer diameter of approximately 0.11 inches.
 It is advantageous to have cables C.sub.1 ' through C.sub.8 ' with a
 smaller diameter, namely 0.11 inches as compared to approximately 0.25
 inches. Cables C.sub.1 ' through C.sub.8 ' are more easily bent and
 manipulated than the prior art five wire cables C.sub.1 through C.sub.8.
 If all ten cables are secured within a single conduit 30' the outer
 diameter of the resulting conduit is approximately 0.55 inches,
 significantly less than the greater than 0.75 inches required in the prior
 art. The mass is reduced by approximately 30%. Having a smaller and
 lighter conduit promotes installation and handling particularly in the
 "hot" area of a nuclear reactor. The decrease in conduit size and mass is
 achieved even though the outer diameter of wires D' and E' is at least
 three and preferably four times greater than that of wires A, B, and C.
 Further, unlike the prior art, the eight heater coils 20 are broken into
 two sets of heater coils, one set provided electricity by wires D' and E'
 of cable C.sub.9 and the other set provided electricity by wires D' and E'
 of cable C.sub.10.
 As best shown in FIG. 5, each set of the heater coils 20 is ganged in
 parallel. The wires D' and E' are very strong and able to resist potential
 damage while also providing adequate power to the heater coils 20.
 Even if one of the heater coils fails, the use of wires D' and E' still
 permits the remaining heaters to function. The total resistance in the
 circuit R(T) is typically equal to the resistance of each heater divided
 by the number of heaters or R(H)/4. Thus, if one of the heaters coils 20
 fail and becomes an open circuit, the total resistance increases to
 R(H)/3. The power to the heaters is only reduced by 25% with current
 decrease according the formula I=V/R and can be compensated for if
 necessary by increasing voltage.
 As best shown in FIG. 1, having fewer wires also promotes the ability to
 connect various cables together between containment penetration connector
 40 and heated junction thermocouple probe 42. A containment conduit 44
 includes a 28 pin or socket connector 46 and 48 at opposite ends,
 connector 46 mating with penetration connector 40. In turn, containment
 conduit 44 mates with a bridge conduit 50, the bridge conduit having a 28
 pin or socket connector 52 and 54 at opposite ends, connector 52 mating
 with connector 48. Bridge conduit 50 also mates with a head lift rig
 conduit 56, the lift rig conduit having a 28 pin or socket 58 and a 40 pin
 or socket 60. In a preferred embodiment, the lift rig conduit 56 acts as a
 transition conduit, allowing a prior art probe 42 with a 40 pin or socket
 62 to mate with a corresponding 40 pin or socket 60. Socket 60 has jumpers
 64 between select pins 66 in the backshell of the connector to provide
 sixteen pins or sockets for the power conductors which are reduced to four
 in the cable conduit 56. Alternatively, the mating socket may have the
 jumpers.
 The transition between a 40 pin or socket connector to a 28 pin or socket
 connector preferably takes place as close to the "hot" zone of the reactor
 vessel penetration by probe 42 as possible. By having fewer conductors in
 the cable near the reactor vessel, the time necessary and problems
 associated with completing the connection or disconnection operation are
 significantly decreased. There is less likelihood of bending or breaking
 pins when handling bridge conduit 50 and head lift rig conduit 56.
 The disclosed embodiments and examples are given to illustrate the present
 invention. However, they are not meant to limit the scope and spirit of
 the present invention. Therefore, the present invention should be limited
 only by the appended claims.