Massachusetts Institute of Technology Department of Nuclear Science and Engineering
22.0 6 Engineerin g o f Nuclea r Systems
Dynamic Behavior of PWR
PRISM Simulator Quick Reference Guide
Accessin g PRISM
On the Desktop o p en (do u b le click) ‘My Com puter’ Open (double click) the c:\ drive
Open (double click) the PRISM folder
You will see a num ber of f iles, the following of whic h are the m o st im portant: PRISM.EXE - PRISM executable file needed to run the code
PRISM.DAT - File o f inp u t d a ta o f in itial co nd itio n s b a sed on a fo ur loop Westing hou se PW R
PRISM.OUT - Output file of data of last run of PRIS M.EXE PGRAPH.EXE - Graphing program which reads PRISM.OUT
Initiate (double click) the PR ISM.EXE file to run PRI S M.
Initial PRISM Input
When entering PRISM y o u are presented with eight questions. For the first three questions, the default values are sufficient. For the nu m b er of loops , enter
<4>
for the reactor power enter
100%
Choose the units with whi c h you are most fam iliar. Finall y , use t h e plant default data. As you beco m e m o r e fam iliar with the program you m a y want to alter these inputs later.
Defi ni t i on s a n d Acr o ny m s
The PRISM code is littered with shortened ter m inology and reactor specific no m e nclature. To eli m inate con f usion, the f o l l owing list is s upplied from the Manual, with some ad ditions:
The NSSS View Screen
NSSS stands for “Nuclear Steam Supply S y stem”. The NSSS View screen will appear first after the TITLE screen (See Figure #2 i n Ap pendix A).
This screen provides a dy nam i c graphic mi m i c of the NSSS including the Reactor Coolant Sy stem (RCS ) and stea m g e nerators. In add ition, the f o llowing parameters are display e d:
FW Feedwater flo w rate in kg/s or lbm / s
FL RCS loop flo w rate in % of rated flow
HTR Pressurizer Control (P) and Backup Heat ers (B)
L Pressurizer l e vel in %
MWTH RCS therm a l power in MW
NL Steam generator narrow-range level in %
P Stea m gener a tor or pressurizer pressure in MPa or psig
RXPWR Reactor powe r (fission pow er plus decay heat) in % of full power
SI Flow Safety Injecti on flow in kg /s or lbm / s
SPRY Pressurizer spray flow rate in kg/s or lbm/s
ST Steam line flo w rate in kg/s or lbm / s
SUBCL RCS subcooling m a rgin in degrees C or F
SURG Pressurizer in -surge flow rate in kg/s or lbm / s
TAVG RCS average te m p erature i n degrees C or F
TC RCS cold leg te m p erature i n degrees C or F
TFW Feedwater t e m p erature in degrees C or F
TH RCS hot leg te m p erature in degrees C or F
TL Pressurizer li quid tem p erat ure in degrees C or F
TS Pressurizer sa turation tem p erature in degrees C or F
TV Pressurizer v a por tem p erat ure in degrees C or F
VL Reactor ves s e l level in %
WL Steam generator wide-range level in %
In the upper right-hand corner of the titl e m e nu ba r, the current sim u l a tion tim e is display e d i n “hours:m i nutes:seconds”. To the left of the si m u lation tim e, one of the followin g sim u lation statuses is dis p lay e d:
REAL This indicates that the sim u lation is runni ng in real time
FAST This indicates that the sim u lation is in fa st ti m e : the si m u l a tion will progress as fast a s the microprocess or can calcula te
FREEZE This indicates that the sim u lation is frozen
To the left of the si m u lation status the word ALARM will appear when an alarm condition exi s ts. The alar ms c a n be viewed on the ALARM PANEL s c reen discuss ed later in this chapter.
OPERATIONAL TRANSIENTS
Exercise 1: Approach to Criticality
Objectiv e
Use PRISM sim u lator to verif y the p r incip l es of subcritical re actor response and criticality ap proach:
Steady state subcritical multip licatio n count rate / Reactiv ity relationsh i p (count rate doubling)
Identificatio n of criticality
Theor y rev i ew
Show N (neutron population) vs. t for the following 5 cases:
1) Critical reactor, no neutron sources
2) Supercritical reac tor, no neutron sources
3) Subcritical reacto r , no n e utron sou r ces
4) Subcritical reacto r with neutron sou r ces
5) Critical reactor with neutron sources
Subcritica l multiplica tio n at steady-s t ate (asym p totic va lues ): N S/(1-K ef f )
N 1 /N 2 (1-K 2 )/(1 -K 1 ) (- 2 )/(- 1 )
S is the intensity of the ne utron sources (e.g., Pu-Be, Pu-R a). W hy do we need neutron sources to start up ?
This equation also shows that if the distance to cr iticality is halved, the cou n t ra te is doubled.
Show plot for K vs. t and N vs. t f o r approach to critica l ity.
Set up PRIS M as follows:
1. Start the pro g ram and enter initial an d contro l par a m e ters as f o llows: Total sim u lation tim e = 150 m i n. Accept (Y=retu r n / N=No)? <ENTER>
Tim e interval for hardco py data = 60 sec. Accept (Y=retu r n / N=No)? <E NTER>
Tim e interval for screen data displa y = 1 sec. Accept (Y=retu r n / N=No)?
<ENTER>
Enter num ber of RCS loops sim u lated (1 to 4): <1> Enter reacto r power (HFP=100;HZP=0): <0> Select display units (0=SI;1=British): <0>
Use default plant data. Accept (Y=return / N=No)? <ENTER> Are all input correct. Accept (Y=return / N=No)? <ENTER>
2. Silence PRISM alarm s for exercise <d>
3. Select prim ary indications screen <F3>
W e will f o cus on the f o llowing f our indica tors o n this pag e : “Source Ran g e (SR) Power”, “Step” (in the Control Rod Drive panel), “Startup R a te”, “Net Reactivity ”.
4. Record initial count rate from Source Range instrum e n t Record initial control rod hei ght (bank and steps)
Record rea c tivity v a lue
5. Shift to fast tim e <F9>
6. Pull control rods <UP cursor a rrow> until n e gativ e reac tiv ity is half of the value recorded in item 4. Record reactivity
7. W a it until steady-state is achieved, then record count rate from Source Range instrum e nt
Record change in count rate (Item 7 – Item 4)
Record control rod height (bank and steps)
Com p are count rates in Item s 4 and 7. Y ou should note that the count rate has doubled, in accordan ce with the theory.
8 Switch to Fa st Tim e . Again pull control rods <UP cursor a r r o w> until ne gative reac tivity is half of the value r eco rded in item 6. Record reactivity
9. W a it until steady-state is ach ieved, then switch to real time, then record count rate from Source Range instrum e nt
Record change in count rate (Item 9 – Item 7)
Record control rod height (bank and steps)
Com p are count rates in Item s 7 and 9. Y ou should note that the count rate has again doubled.
10. Final pu ll to, and recognition of, criticality.
a. Place a s m all piece o f opaque tap e (o r a Pos t -it note) over the reactivity indicator so it cannot be see n. This is done to prevent you from using the reactivity indica tor to identif y cr iticality.
b. Pull control rods until 120 steps on CR Bank D. Is the reactor critical? Criticality c a n be iden tif ied using s t a r t-up rate re m a ining positiv e and a c ontinuing increase in count rate.
c. The reactor should s t ill be sub c riti cal here. Wait un til you reach s t eady - state (i.e., zero start-up rate).
d. Pull Ban k D another 20 steps, an d then obser ve param e ters to se e if c r itica lity has been achieved. If not cr itical, continue to pull Bank D, 20 steps at a tim e. Call the reactor critical when you observe the proper indications (increasing count and steady positive Startup Rate). A pul l-and-wait m e thodology, moving rods about 20 steps should be used to ensure that criticality is not significantly overshoot.
e. Quickly record the following inform ation: Control rod height (bank and steps)
Count rate at criticality
f. Re m ove t h e tape from the reactiv ity indicator and r ecord its value
g. The actual rod height at which the react or went critical was 160 steps on Bank
D. Also, the value of reactivity shou ld be near 0. How close were you?
Optional Exercise (do it on your own at home)
11. Start ov er with th e sam e initial conditions: reset PRIS M <r>
Select prim ary indication screen <F 3> Shif t to f a st tim e <F9>
We want to dem onstrate now that criticalit y does not depend on the tim e it takes to pull the control rods out, but just on the control rods fi nal position. That is, th e contro l rod position at which criticality is achieved is independent on how fast or slow one has reached that position.
Pull directly to 160 steps on Bank D without stopping.
1. Pull rods to 160 steps on control bank D. W h en t h e rod pull begins, do not stop pulling until 160 steps on cont ro l rod bank D is reached.
2. At 160 steps on control rod bank D, verify reactivity is 0 and record the count rate
3. Com p are the critical count rate to count rate logged in Item 10. Since a pull and wait approach was used above, ther e was increased tim e for subcritical multiplicatio n to occur. T h erefore th e count rate sh ould have been m u ch bigger than in this case where rods were rapidly pulled to th e critical position.
Exercise 3: Change of Grid Power Demand
This exer cis e illustrate s the “load f o llowing” capa b ility of a PW R. This tr ansie n t is ch arac teriz e d by the following sequence of events:
1. The grid dem a nds more power
2. Higher steam flow rate is supplied to the turbine
3. As the outgo ing ste a m rate is g r ea ter than the inc o m i ng f eedwater r a te, the pressure and tem p erature in the s econ d ary side of the steam ge nerator (SG) decrease
4. Prim ary coolant co ld leg tem p erature (T c ) decreas es
5. Colder and denser water enters the core
more efficient neutron moderation
reactivity in creas es
core power increas es
6. Fuel tem p erature increas es
neutron capture (parasitic absorption) in creases (due to the D oppler effect)
reactivity decreas es
7. New steady-state condition at a high er power is reached
Start PRIS M with:
default values, where possible
4 loop
90% power
SI units
Go to the Pr im ary Plant Control Pan e l (F3)
Set Control Rod Drive m ode to “m anual”, th is will pr event an autom a tic inser tion o r rem oval of reactivity du ring the trans i ent and will enable a “cleaner” ob servation of the natura l load f o llowing ca pability of the reac tor.
Record the v a lues of:
o Reactor Po wer (%)
o Net Reactiv ity (pcm )
o Cold Leg Temperature ( C)
o Hot Let Temperature ( C)
o T rise in th e core ( C)
o Pressurizer (PRZ) Level (%) Go to the Secondary System Control Panel (F4)
Record the Secondary P r essure (MP a ) Freeze tim e (F10)
Following the instructions at the bottom of the screen, set the Turbine L o ad to 1065 M W e and the rate of change to 10% of full power per minute.
Restore real tim e (F8)
Observe the increasing S t eam Flow Rate s upplied to the turbin e and the slightly decreasing pressure in the secondary side of the S G .
Go to the Pr im ary Plant Control Pan e l (F3) Observe:
the decreas ing Cold Leg Tem p erature
the positive Net Reactiv ity
the increas in g Reactor P o wer
Switch to Fast Tim e (F9) and run until the tr ansient is over, a couple of m i nutes after the Net Reactiv ity re turns to 0 will do.
Record the v a lue of:
o Reactor Po wer (%)
o Net Reactiv ity (pcm )
o Cold Leg Temperature ( C)
o Hot Let Temperature ( C)
o T rise in th e core ( C)
o Pressurizer (PRZ) Level (%)
o Secondary P r essure (MP a )
Note that:
the Reacto r Power has increas ed
Cold Leg and Hot Leg Tem p eratures have decreased
However, the T rise in the core (which is proportional to Reactor Power) has increased
Pressurizer Level is lower becau se th e averag e tem p erature in the RCS is lower; norm a lly, the autom a tic Control Rod System and CVCS operate such that the average tem p erature of the prim ary coolant does not deviate too much and the PRZ Level is kept at a more or less constant value. If desired, this ex ercise could be ru n without setting the Co ntrol Rod Sy stem to “m anual” for com p arison
ACCIDENTS
Exercise 4: Reactivity Excursion
Set reactor at 100% power in PRISM Freeze tim e
In the “M alf unction” m e nu select “R eactiv ity Accident” with +100 pcm r eactivity step change and 5 s delay
Do you expect the reactor to trip? If so, upon what signal?
Run the simulation and observe what happens . G o to the AL ARM panel and identif y the f i rs t tr ip signal (high neutron flux)
Repeat the s i m u lation at 30% power. Does the re a c tor trip ?
Repeat the sim u lation at 30% power and +300 pcm. Do es the reactor trip now?
Exercise 5: Loss of Feedwater
Set reactor at 100% power in PRISM and freeze tim e
In the “Malf unction” m e nu select “M ain FW isolation” with 5 s delay Predict the reacto r behav i or upon is o l ation of the m a in FW lines.
Run the simulation and watch T and P increas e in the prim ary system until reacto r trip on high-P signal.
How did power change prior to scram ?
Af ter reac to r tr ip how is FW supplied to the SGs ?
Exercise 6: Steam Line Break
Set reactor at 100% power in PRISM and freeze tim e
In the “Malf unction” m e nu select “Steam Line Br eak bf. MSIV” in Loop 3 with break fraction 20% and 5 s delay
Run the simulation and watch reacto r power increase unt il re actor tr ip. W hy did the power increase followin g the steam line break ?
What happened in the P R Z following the steam line break?
Im portan t takeaway
Exceeding s p ecified set points for reactor p r essu re, neutron flux and neutron flux rate, pressurize r level, SG lev e l, m a in coolant flow, etc., will c a us e the reacto r to trip.
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22.06 Engineering of Nuclear Systems
Fall 20 10
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