Experimental Nuclear reactor kinetic Flashcards
Why is Triga inherently safe?
Control
* - high and negative reactivity feedback
* - solid fuel mixed with moderator
Thermohydraulics
* - natural circulation
* - pool design
Containment
* - ambient pressure
How is triga fuel composed and why?
UZrH
H because it’s a moderator (Be is toxic, graphite is worse than H)
Zr because it’s good for neutron economy, has good thermal conductivity and is chemically stable
50/80% of fuel feedback coefficent is due to ZrH, the other part to doppler effect
what does the prompt fuel temperature feedback depends on?
pag.2
T_max fuel in triga?
1150°C
if water goes away from triga?
natural circulation air
Triga Mark 1 and 2 differences and similarities?
both pool type, water moderated, natural circulation, same fuel
triga 1 was below ground while triga 2 is above ground for easier irradiation of samples
What is core excess?
the amount of reactivity that I would get if I removed the control rods
I want it below total reactivity of rods over 2 in triga while i wnat it below 1 dollar in commercial to avoid prompt criticality
Triga fuel element composition?
8%w is U, enriched at 20%
cladding in Al or SS
graphite as reflector above and below
Now fuel has more H to be more stable
Triga cooling system?
pag 7
Triga CR?
SHIMis 3-3.5, B4C, for coarse tuning, close to center
REG is 1-1.5, B4C, for fine tuning, outside
TRANS is 2-2.5, borated graphite, for safety, middle
what’s the easiest way to control the multiplication factor?
Absorption rather than production and leakage
What are CR used for?
- Reactivity change
- start up and shut down
- neutron population control
- burnup compensation
What are CR made of usually?
- Boron
- Silver
- Indium
- Cadmium
Why in a PWR the reactivity of CR is higher than FR?
becuase FR have no temperature effect, no poisons and less burnup
Reactivity unit of measures?
pag 10
CR model for reactivity?
HP:
- homogeneous reactor
- 1 rod
- weak absorber
- 1 energy group
- no scattering
- flux is a sine
what’s the limit on core excess?
Drho_ce < Drho_tot/2
Why in commercial reactor each rod has a reacitivity worth less than a dollar?
toavoid prompt criticality
What is shutdown margin?
pag 14
what’s the limit on sutdown margin?
Drho_sm > 0.5 $
In which condition do you calibrate CR?
Criticality
zero power (no T effect)
no poison
write PK !
pag 15
Write PK in 1 group approx with solution
s1= lambda* rho0 / (beta-rho0)
s2 = -(beta-rho0) / LAMBDA
Reactor period method? passagges and math
pag 16 e 17
Uncertainty propagation
pag 18
Subcritical method passagges?
pag 19
what is M in subcritical method?
pag 20
why and how do we use PK in subcritical method?
pag 21 e 22
Prompt Jump with spurce?
pag 23
correla n1 e n0 alle due sources.
Mi sembra accessorio
Subcritical method - calculation on data
pag 24
advantages and disadvantages of subcritical moltiplication method
- PRO
- simplicity of measure
- short measuring time
- always subritical
- allow shim calibration
- Poisson distributed values
- CONTRO
- low accuracy
- dependence from source charateristics
- calibration of alfa * phi_s
subcritical method for core loading
pag 26
Rod drop method
A control rod is dropped in the reacotr and we measure the negative prompt jump effect.
A fast acquisition system is needed
Inverse method
Startingform PK I write the reactivity as a function of the power P. It’s very precise and needs precise instrumentation to measure it. Can be used to describe transient
What are FUEL reactivity coefficients due to?
Moderatorand Doppler
what are reactivty coefficient due to in water?
- Void (less moderator)
- Temperature (spectral hardening)
Why we want the reactivity coefficents to be negative?
Safety
scheme of neutronics, thermohydraulics and reactivity
pag 28
reference values for alfaT,f; alfaV,c; alfaT,mod, alfaEXP?
pag 29
How does triga fuel moderate?
- ZrH is good moderator for E_n > 0.137 rV
- ZrH makes upscattering for E_n < 0.137 eV
- ZrH is better than H for E >0.137 eV
- DUE TO:
- no H recoil
- lattice vibration
How can we quantify moderation and what are the main charateristics of a moderator?
Low mass number and high scattering crossection
mod = ln(E_in/E_out) * sigma_scat/sigma_abs
under vs over moderation?
pag 32
what is coeherent scattering
It’s when a neutron interact with the lattice and the scattering crossection increases.
what is the doppler effect?
pag 34
what causes spectral hardening and what are the effects?
pag 34
How is the temperature coefficent calculated?
pag 35
which effects does coolant temperature have on neutronics?
- positive due to scatter with H
- negative due to thermal voiding
In any case is very small
What is the void feedback coefficent?
If inside core is negative due to more moderation, if outside core (before reflector) is positive due to less absorption.
With void:
- f increases
- p decreases
- P_nl decreases
It’s void coefficent positive or negative?
pag 36
which parameter of 6 factor formula depends on voiding?
pag 37
void coefficent measurements steps?
pag 37
What is montecarlo used for?
To solve transport eqaution
Difference between MC solution and determinstic solution?
Montecarlo is:
* - more accurate
* - continuous energy
* - good for complex geometry
* - slow
* - difficult adjoint calculation
* - statistical uncertainties
Deterministic solution is:
* - faster
* - easy adjoint calculation
* - less accurate
* - multigroup
* - complicated to compute
Which are the principles of MC
-Transport
- Sampling
- Collection
criticality in montecarlo?
pag 40
What is serpent used for?
- reactor simulation
- burnup calculation
- validation
- full core modelling
- coupled multiphysics
- training
serpent steps?
- define materials
- define zones
- define cell to replicate
- define neutron population history
- define detectors
analog vs implicit estimation
pag 42
zerodimensional reactor model for triga
pag 45
What are the carateristics of 1D and 3D model of triga
pag 46
heat transfer coefficent?
Estimating beta_eff with MC?
pag 43 e 44\
