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Nuclear Training
Measurement on a Subcritical Assembly
The objective of the experiment is the determination of a number of reactor physical parameters of a reactor core and the comparison with
theoretically calculated values.
A subcritical assembly is a reactor core with such dimensions or composition that the effective multiplication factor is below 1.
The assembly used for this experiment consists of fuel rods of slightly enriched uranium in a tank with light water. Because the
effective multiplication factor is appreciably smaller than 1, it is impossible to form a critical core, hence no safety control
system is needed.
In such a system no self-sustaining fission chain reaction can be maintained. Absorption and leakage of neutrons is larger than
the production by fission, hence a possibly present number of neutrons will die out very rapidly. By introducing an external neutron
source, however, a balance between production of neutrons from fission and the source and loss of neutrons due to absorption and
leakage will be reached. Therefore, a stationary neutron level in the assembly will result.
The Reactor Simulator
The purpose of this experiment is to gain insight into the kinetic behaviour of a nuclear reactor and to obtain some experience in
the control of a reactor.
The reactor simulator RESIDEL consists of a control panel, which is a strongly simplified copy of the control desk of the HOR
(Hoger Onderwijs Reactor), connected to a computer. Using the joysticks of the panel, one is able to move the control rod (CR),
the shim rod (SR) and the neutron source; in this manner changing the operating settings. The computer programme is continuously
calculating the actual value of the reactor power and of other variables; an analogue signal is sent to the meters and displays
of the control panel. For more information, visit the RESIDEL homepage at the
Section of PNR.
Calibration of a Control Rod
The purpose of this experiment is to determine the differential and integral reactivity curves of a control rod of the HOR
(Hoger Onderwijs Reactor).
Predominantly as a result of fission, the reactivity of a reactor core decreases slowly during the core cycle. At the end
of the cycle, when the reactivity is reduced to zero, a number of new fuel elements has to be installed. This produces
again over-reactivity. This over-reactivity has to be compensated by the control rods and in pressurised water reactors
by the boron, which is dissolved in the coolant (sometimes burnable poisons are added to the fuel as well). Because of
safety reasons, the negative reactivity that can be introduced by inserting a limited number of control rods has to be
sufficient to fully compensate the over-reactivity. Therefore, it is of importance to know the reactivity worth of every control rod.
Control Rod Calibration by the Rod-Drop Method
The objective of the experiment is to determine the total reactivity value of a control rod by the rod drop method.
In the previous experiment the reactivity effect of parts of a control rod (i.e. the differential reactivity) was determined
by means of the time constant method. The total reactivity effect can be determined by adding the reactivity effects of all
parts of the control rod (i.e. by integrating the differential reactivity). This total effect cannot be determined directly
by the time constant method because the (positive) reactivity effect is too large for a safe procedure.
In this experiment the total reactivity effect is determined by dropping the control rod into the core and analysing the
power transient caused by the negative reactivity insertion. The behaviour of reactor power as a function of time can be
found by solving the reactor kinetic equations. In this case a complete solution of the kinetic equations is needed for
two reasons: it takes too long time to reach the stable reactor period and the latter is not strongly dependent on reactivity
for strong negative reactivities.
Startup of the Delft University Research Reactor
The purpose of the experiment is familiarisation with the procedures for the start up of a research reactor.
HOR is a pool-type research reactor being used as a neutron source for various physical, chemical and biological experiments
and reactor physics research. Its nominal power is 2 MW and the average thermal neutron flux amounts to 2.1017
n.m-2s-1.
The MTR-type fuel elements contain 19 fuel plates with an "active" length of 60 cm. One fuel plate of a fresh fuel element
contains about 80 grams of uranium enriched to 20% in 235U; the uranium is in the form of a uranium-silicate alloy in aluminium
with an aluminium layer.
Reactor control is accomplished by four control rods containing boron carbide; these are positioned in special fuel elements
with only 10 fuel plates each, thereby leaving space for a control rod.
With the 20% enriched uranium fuel a rather compact core can be assembled. The fuel elements are positioned in a grid plate.
The core consists of about 20 fuel elements and a number of beryllium reflector elements and is positioned at a depth of about
6,5 meters in a water pool. The water between the fuel plates acts as a moderator and coolant. Water outside the core acts both
as neutron reflector and as neutron and gamma shielding. The concrete walls of the pool act as radial radiation shields. Coolant
is pumped downwards through the core and the heat is transferred to a secondary cooling circuit in a heat exchanger. The heat is
finally dissipated to the environment via cooling towers.
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