Systems Exam 2 - N-16 & RCS Temperature

Question 1

PZR Instrument number designations

Answer 1

→they all begin with 4
→second number = which loop
→if 3rd number is 3 = wide range
→A = T-hot
→B = T-cold
→F = RSP instrument

Question 2

Cold Leg Power Shadowing

Answer 2

Nuclear Engineering has estimated that the effect on PR NIS is approximately 0.8 % per degree at 100% power.

The effect is decreased proportionally with power. For example at 50% power, the effect is halved or 0.4% per degree.

Question 3

Auctioneered Low T-ave is used to…

Answer 3

→generate C-16 at 553°F
OR
→generate C-16 when auctioneer low T-ave is < T-ref by 20°F

Question 4

Ave T-ave provides input to…

Answer 4

1. Pressurizer Level - used to develop program level
2. Steam Dump Control
3. T-ave Deviation Alarm
4. Rod Control
5. Reactivity Computer
6. Rod Insertion Limit (wires hooked up but system set such that it has no effect)
7. High T-avg alarm 587.2°F (591°F)
8. Tavg - Tref Deviation Alarm ± 2.5°F

Question 5

What signals does T-ave input to?

Answer 5

1. FW Isolation Signal (T-ave = 564°F AND P-4) requires 2 of 4 channels (Lo T-ave)
2. P-12 Lo-Lo T-ave permissive at 553°F causes Steam Dumps to isolate, requires 2 of 4 channels (can be bypassed for cooldown for 3 of 12 dumps)

Question 6

T-ave Defeat Switch Operation

Answer 6

T-ave defeat switch is different in that when a channel is taken to defeat it is replaced by the previous channel (4 replaces 1, 1 replaces 2 etc.) such that 1 channel provides 2 inputs.

Note: if the channel that replacement channel reads significantly higher/lower than the failed channel, then it will affect both the T-ave and Ave T-ave indications and could cause changes to the plant accordingly.

Question 7

Describe N-16 and how it varies with temperature

Answer 7

Although the half life of N-16 is short (7.11 sec) the loop travel time is ~10 secs therefore the N-16 detector not only detects the newly formed N-16 gammas but also those from the previous cycle (99% decayed away after 47 secs) therefore the detector is calibrated to compensate for this.

Additionally, the amount of N-16 gammas corresponds to the density of the water since the denser the water the more N16 therefore more gammas detected. To compensate for those times when the water temperature is not on program T-cold is input into the system to provide the density compensation.

Question 8

N-16 Failure

Answer 8

If T-cold increases then gammas should decrease, therefore compensation increases N-16 indication since T-cold increasing corresponds to an increase in power (N-16 indication follows failure mode of T-cold)

Question 9

OP N-16 Rx Trip Setpoint

Answer 9

112%
(kW/ft)

Question 10

C-4 Setpoint & Actions

Answer 10

Setpoint:
N-16 within 3% of OP N-16 trip setpoint on 2/4 channels.

Automatic Actions:
1. Rod Stop, all outward motion stops
2. turbine runback at ~200% per minute for 1.5 seconds, with a 28.5 second pause (equivalent to 10%/min power reduction)
3. runback continues until either the condition clears or power is <15%.

Question 11

Inputs to OT N-16 Setpoint Circuitry

Answer 11

1. PZR Pressure
2. PR Detectors (looks at ΔI for associated channel)
3. T-cold narrow range

(setpoint is 115% ± penalties)

Question 12

C-3 Setpoint & Actions

Answer 12

Setpoint:
N-16 within 3% of OT N-16 trip setpoint 2 of 4 channels

Automatic Actions:
1. Rod Stop: all outward motion stops
2. turbine runback at ~200% per minute for 1.5 seconds, with a 28.5 second pause (equivalent to 10%/min power reduction)
3. runback continues until either the condition clears or power is <15%.

Question 13

N16/Tcold Fails Low

Answer 13

→affected T-ave channel fails low (510°F)
→may cause a “Stop Turbine Loading” (C-16) to be actuated. (low auctioneered T-ave less than 553°F or Tref-Tave mismatch greater than 20°F).
→channel failure low will cause rods to withdraw if rod control is in AUTO, stops at C11
→PZR reference level decrease with charging flow decrease when in auto

Question 14

N16/T-cold Fails High

Answer 14

→affected T-ave channel fails high (630°F)
→rapid control rod insertion due to Tave-Tref mismatch if in AUTO.
→steam dumps will open if armed with a C-7 (loss of load).
→PZR reference level increase (to a maximum of 60%) with charging flow increase when in auto

Question 15

How does an RTD fail on an open circuit?

How does it fail on a short circuit?

Answer 15

→open circuit = fails high, infinite resistance

→short circuit = fails low, zero resistance

Question 16

What is the purpose of OT N-16?

How do changes in inputs correspond to changes in OT N-16 setpoint?

Answer 16

Purpose:
to prevent exceeding DNB limits

Setpoint Changes:
→as pressure ↓, setpoint ↓ because closer to DNB
→as pressure ↑, setpoint ↑ because farther from DNB
→as temperature ↑, setpoint ↓ because closer to DNB
→, as temperature ↓, setpoint ↑ because farther from DNB
→as long as ΔI remains “inside the doghouse,” then no penalties/changes; if not, then setpoint lowers

Question 17

What is the purpose of the OP N-16 Trip?

Answer 17

→provides assurance of fuel integrity (kW/foot) under all possible overpower conditions, e.g. no fuel pellet melting and less than 1% cladding strain
→limits the required range for an over temperature trip
→provides a backup to the High Neutron Flux trip

Question 18

What’s the difference between the OP N-16 setpoint and the OT N-16 setpoint?

Answer 18

The OT N-16 setpoint changes based on inputs. The OP N-16 setpoint does NOT change.