Internet Reactor Laboratory
Approach to criticality / Reactor start up
Ind.1
Reactivity effects
05/12/16
TRAINING COURSE –ISIS REACTOR
TP n° 1
LAB 2&3
 Approach to criticality
 Reactor start up
 Reactivity effects around criticality
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Internet Reactor Laboratory
Approach to criticality / Reactor start up
Reactivity effects
LAB 2&3
Ind.1
 Reactivity effect of devices placed in the core
1. OBJECTIVES
The first objective is to determine a configuration of the control rods that corresponds to the
critical state of the reactor by an approach to criticality (maintaining k < 1). Using this
configuration, the Nordheim curve and the calibration curve of the control rod n° 6, the
conditions for the reactor start up are found. The reactor being brought to a critical state at
500 W, variation in core reactivity are studied as a function of the rod positions and of the
removal of devices placed in the core of the reactor.
2. APPROACH TO CRITICALITY

Record the counting rates measured on the two neutron detectors (BN1, BN2), used
to follow the neutron density at low power, for different positions of the B6 control rod
with k < 1, in order to determine the critical position of this rod.
B6 position
(mm)
0
BN1 counting
rate (c/s)
1 / BN1
BN2 counting
rate (c/s)
1 / BN2
100
200
300
400

Plot the curves 1 / BN as a function of the B6 rod position.

Using the last three data of each curve, determine the critical position of B6 rod.
3. REACTOR START UP

Using the critical position of B6 rod, the Nordheim curve and the rod calibration curve,
determine the position of B6 to be used to diverge with a given doubling time, in the
range of 30 to 60 s.
The reactor is started up by moving the B6 rod from the position 490 mm to the value
previously found.

Observe the evolution of the reactor power and doubling time as the function of time,
showing the prompt jump followed by the exponential increase of the power.
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Internet Reactor Laboratory
Approach to criticality / Reactor start up
Reactivity effects

LAB 2&3
Ind.1
From the measured value of the doubling time check if the critical position of B6 has
been accurately determined by the approach to criticality method. If necessary the
critical position can be recalculated from the measured doubling time.
3. POWER STABILISATION AT 500 W
The reactor power is increased up to 500 W.

Record the critical position of B6 and compare it to the values found previously.
4. MINOR CHANGES OF THE ROD POSITION AROUND CRITICALITY
The reactor being stable at 500 W (P0), the rod B6 is moved around its critical position
by ± 2 mm.

Observe the evolution of the reactor power P and P/P0.
5. AUTOMATIC OPERATION
The operator switches the reactor to the automatic operation system.

Observe the automatic adjustment of the rod position that maintains the power
constant.
6. REACTIVITY EFFET OF DEVICES PLACED IN THE CORE
In order to study the influence of experimental devices placed in the core on the reactivity,
four aluminium cylinders ( 20 mm) are successively extracted from an aluminium box that
replaces a fuel element in position n° 64 in the core. The reactor is then shut down and the
aluminium box is removed before the reactor is restarted and brought to the same stable
power.
When removing the cylinders, the reactor power is maintained stable by the automatic
operation system.

Report in the following table the critical position of B6 for each configuration of the
box.

Why is it necessary to shut down the reactor before removing the aluminium box ?

Using the B6 calibration curve determine the change in core reactivity associated with
the removal of the cylinders.
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Internet Reactor Laboratory
Approach to criticality / Reactor start up
Reactivity effects
States of the box
Zcriticality (mm)
LAB 2&3
Ind.1
 (pcm)
4 cylinder in place
1st cylinder removed
2nd cylinder removed
3rd cylinder removed
4th cylinder removed
Aluminium box removed
7. QUESTIONS
2.1
When and why is it necessary to carry out an approach to criticality before starting up
the reactor ?
2.2
Explain the principle of the approach to criticality and why the 1/N curves must cut the
x abscise for the critical position.
2.3
In practice, we observe that the curve 1/N do not vary linearly with the rod position,
except for the three last positions. Explain why.
2.4
Using the B6 rod calibration curve that give the efficiency of the control rod as a
function of the rod position, which curve could be drawn during the approach to
criticality ?
2.5
What is the influence of the neutron detector position on the shape of the curves ?
2.6
Why does the stabilisation of the counting rates takes more time when approaching
the criticality ?
2.7
Having found two different values for the critical position of B6 using 1 / BN1 and
1 / BN2, which value should be chosen to determine the conditions for reactor start
up ? Explain why.
3.1
Compare the values of the critical position obtained by the three methods (approach
to criticality, measurement of the doubling time, power stabilisation) and discuss
3.2
At low reactor power (< 1 kW), does the critical position depend on the power ?
3.3
If the reactor power was increase up to the nominal power (700 kW), do you expect to
have the same critical position ? Explain why ?
6.1
Explain the variations in rod position and core reactivity observed when removing
successively the three cylinders. Relate the observed variations to the elastic
diffusion and neutron capture occurring in water.
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