Transcript Residual heat generation

Residual heat generation
• Reduction of heat generation rate in the nuclear
reactor after the insertion of a negative reactivity
(shutdown of the reactor) is determined by the
following processes:
• - fission of fuel by prompt neutrons;
• - thermal inertia of the core material and the amount
of heat accumulated in it;
• - fission of fuel by delayed neutrons and
photoneutrons;
• - slowing-down of  and -radiation of fission products
accumulated during the operation of a nuclear reactor.
1
• Change in the power after the nuclear reactor
shutdown
2
• Power caused by fission by prompt neutrons is reduced
in a fraction of a second.
• Accordingly, Wosk,n,  decreases i.e. power caused by
slowing-down of fission fragments (Wfrag), deceleration
and capture of neutrons (Wn), absorption of
instantaneous -radiation (W).
• In fact, heat power is reduced more slowly due to the
inertia of heat decline accumulated in the nuclear
reactor materials.
• Thermal inertia depends on the materials of the core
and heat takeoff conditions.
• It can be neglected within a few seconds after the
power reduction.
3
• Thermal power caused by fission of delayed
neutrons, can be neglected in 35min.
• The main component of thermal power in any
nuclear reactor, in a few minutes after
shutdown there will be heat release W, for a
long period of time due to slowing-down , radiation of fission fragments and their decay
products, which is, in fact, called residual
heat.
• To calculate the residual heat power, one uses
formulas proposed by different authors.
4
• The most widespread is Wei and Wigner
formula:
• W, / W0 = 6,510-2[ст-0.2  (ст + Т)-0.2];
• W, / W0 = 6,510-3[ст-0.2  (ст + Т)-0.2];
where:
W,
W0
 power of residual heat of the nuclear reactor in time st (station
time) after a shutdown;
 power of a nuclear reactor before shutdown, in which it operated
per time T.
• In formula (1) st and Т are given in seconds,
• in (2)  in days,
5
• Graph for the approximate evaluation W, 
after NR shutdown at Т>> st.
6
• In this fig., this dependence is represented as a
graph, with the help of which we can solve
operational problems associated with residual
heat without cumbersome calculations.
• Graphical dependence enables the operator to
perform the following practical tasks:
• - define W, at any moment st after reactor
shutdown, if it operated per time Т at power W0;
• - evaluate station time st, at the end of which,
after reactor shutdown, W, decreases up to the
necessary level to move on to the autonomous
system of reactor shutdown cooling.
7
Ionization Сhambers
for Neutron Detection
• To register ionizing radiation one uses various
methods based on the measurement of the result
of the interaction of radiation with material.
• For example,
in the material detector quanta form fast electrons by the photoelectric
effect, the Compton effect or the effect of the
formation of electron-positron pairs.
• These electrons are recorded due to ionization
caused by them.
8
• Neutrons are detected by the two-stage
process,
• since the interaction between neutrons and
electrons, due to their magnetic moments, is
very small and cannot cause ionization of
atoms when neutrons pass through the
substance.
9
• Radiation detectors register only charged
particles.
• Therefore, they can register neutrons and quanta by the emission of charged particles
produced in the interaction of neutrons and quanta with material chamber.
• Neutrons can be registered, for example, as a
result of a nuclear reaction, followed by the
emission of charged particles that are
captured by the detector.
10
• Due to the fact that in the operating reactor
neutron radiation and, in some cases,
-radiation should be recorded, number of
methods found practical application is limited.
• As the "detection", reactions
10
5
Bn,   Li
7
3
• and fission reaction 235U are most frequently
used.
11
• Neutrons can be detected in other exothermic
reactions, for example,
• or ,
6
3
3
2
Lin,  H
3
1
He n, p  H
3
1
• however, they have not found practical
application in reactor technology.
• For the detection, the process of 235U fission by
neutrons is used, in which the charged fission
fragments of high energies (fission chambers) are
formed.
12
• For registration of secondary particles
(α-particles, fission fragments or electrons)
generated by the interaction of neutrons
• or -quanta with material, the ability of
charged particles to ionize gases is usually
used.
13
• By putting into volume, gas-containing,
electrodes in the form of two plates or
cylinders and attaching an electrical potential
difference to them, one obtains a current
proportional to the number of charged
particles and ionizing ability.
• Such particle detection device is called an
ionization chamber.
14
• To register neutrons, ionization chambers (IC) are
used, which are divided into pulse and current.
• In the pulse-type chamber, each detectable
particle produces pulse of current (counters).
• Current chambers are devices registering the
average emission level. In this case current of the
ionization chamber is determined by the intensity
of radiation.
• Ionization chambers and counters are used to
measure neutron flux outside the core.
• Therefore, they measure neutron leakage from
the core.
15
• The operation of ionization chambers is based
on collecting ions produced when passing
through the ionizing radiation chamber.
• With the passage of charged particles through
the material its electric field interacts with the
electron shell of atoms.
• As a result, some of the electrons are released
from the atoms and positive ions are formed
on the path of the particle.
16
• When passing through the material,
electromagnetic radiation (-quanta) is absorbed
as a result of Compton scattering and electronpositron pairs.
• In each of these processes charged particles
(electrons, positrons) occur which are capable to
ionize medium atoms.
• The operating principle of the neutron ionization
chambers is based on ionizing action of the
particles resulting from the reactions of the type
(n, a), (n, p), (n, f).
• The materials with which the neutrons are
detected - 10В, 3Не, 235U.
17
• Based on 10В, 10В(n,α)7Li reaction occurs.
Microscopic neutron absorption cross section
in the reaction - 3940 barn
• Based on 3Не, 3Не(n,p)3Н reaction occurs.
Microscopic neutron absorption cross section
in the reaction – 5300 barn.
• Based on 235U, 235U(n,f) fission reaction
occurs. Microscopic fission cross section – 582
barn.
18
• Chambers with the use of (n, a) reactions boric chambers.
• with the use of (п,р) reactions – helium
chambers,
• with the use of (n,f) reactions – fission
chambers.
• The detectable material (sensitive) to
neutrons can be in the ionization chamber in
the gaseous state or in a solid coating on one
of the electrodes.
19
• Scheme of current formation in the ionization
chamber:
1
-
2
In+I
-
+
7
n,
3
6
4
• 1 – power supply; 2 – current meter;
3 – insulator; 4 – ions; 5 – a radiator of the neutronsensitive layer;
6 – camera casing; 7 – electrode.
20
• For neutron registration, substance (radiator) is
introduced into the IC.
• As such radiator, for example, amorphous boron
layer or fissionable material coated on one or both
electrodes can be used.
• Conventionally, such layer is illustrated in Figure 5 on
the negative electrode of IC.
• This figure also shows the direction of the current
caused by the electrons and -quanta, measured by
galvanometer.
• IC with a layer of fissile material is usually called
fission chamber (FC).
21
• The current in the chamber circuit depends
on the voltage applied to the electrodes of the
chamber.
• In the small voltage applied to the electrodes
of the chamber, partial recombination of ions
will occur.
22
• Volt-ampere characteristics of the ionization
chamber for two values of neutron flus
density (2 1)
2(2)
Iн2
Iн1
1(1)
0
Uкр1
Uкр2
Uраб1
Uраб2
23
• As follows from the graph, after reaching a
certain critical voltage Ucr , further increase in
voltage does not result in the growth of
current through the chamber at a constant
intensity of radiation field in which it is
located.
• This voltage corresponds to almost complete
separation of all resulting ion pairs in the
operating volume of the chamber.
• The maximum current that can be obtained by
U  Ucr, called the saturation current Is.
24
• When increasing radiation intensity, saturation
current also increases.
• Ucr increases simultaneously due to the greater
probability of recombination of the ions because
of their greater density.
• Chambers should operate in saturation mode.
• In this case, current chamber is proportional to
neutron flux density.
• If the voltage on the chamber is insufficient to
obtain saturation current, measurements lose
accuracy.
25
• Saturation current value is inversely
proportional to the gas pressure in the
chamber and is inversely proportional to the
square of the distance between the
electrodes.
• A significant excess of Ucr can lead to the
striking of self-maintained discharge in gas or
a breakdown of the insulator of the ionization
chamber.
26
• -quanta falling into ionization chamber, form
electrons in gas and electrode material.
• Secondary electrons ionize gas and create
electric current in the circuit.
• Since neutron radiation in the reactor is
always accompanied -radiation, it is
necessary to take special measures to
separate the signal from neutrons and
compensation of -background.
27
• Properties of gas filling IC, are very important.
• In addition to the average energy expended in
gas for ion pair formation, mean free path of
the charged particles in the gas is of great
importance, which is defined by the type of
particles, the properties of gas and its
pressure.
• Ionization energy for most gases under normal
conditions is about 30 eV, and the average
free path -particle - some centimeters.
28
• If the gas pressure is different from atmospheric
pressure, average free path of the particle can be
considered to be inversely proportional to
pressure.
• If the distance between the electrodes is less
than the path of the particles, the number of ion
pairs formed in IC can be considered to be
proportional to the ratio of this distance to the
length of the particle path.
• For neutron registration, in the presence of background one uses special compensation IC
(neutron compensation chamber - NCC).
29
• Compensaton IC and the scheme of its turning on
1
n,
2
+n
I -
5
+
6
6
3
In+I
5
4
•
1 – casing; 2 - radiator; 3 - current meter; 4 insulator; 5 - power supply;
6–
electrode.
30
• Unlike conventional IC, compensation
chambers has two equal volume placed
together. In one of the volumes ions are
produced by neutrons and -quanta.
• In this volume there is a radiator of neutrons
in the form of boron layer or fissile material on
the electrodes. The volume may also be filled
with BF3 gas. In the other volume of the
compensation chamber, there is no neutron
radiator. Therefore, ions are formed only by
the interaction of -quanta with the material.
31
• When meter is turned on, it will register only
current In proportional to neutron flux density.
• The full compensation of -background cannot be
reached. Modern compensation chambers can
reduce component I about 100 times as
compared to conventional IC.
32
In-core detectors
• Inside the VVER vessel, temperature and
energy release control detectors are placed.
• Despite the difficult working conditions (high
temperature, pressure, intense gamma and
neutron radiation) detectors must be of
sufficiently high metrological and reliability
characteristics, have small dimensions and
structurally integrate with internal units.
33
• Energy release control detectors
• To measure power distribution over the
volume of the core, in the in-core monitoring
system detectors are used, called
beta-emission detectors of neutrons
• Compared with other types of neutronsensitive detectors, they have the following
advantages:
34
• small dimensions allow to place a large number
of the detectors in the reactor required to obtain
a detailed picture of energy release distribution
over the core volume;
• they do not require external power source, have
a high reliability, service lifetime at least one
reactor campaign, their sensitivity varies little
during operation, and these changes can be
corrected by calculations;
• they are simple in design, have good
reproducibility of the parameters (sensitivity
variation is not more than ± 1%) and low cost.
35
• The construction of these detectors consists of a
collector and emitter between which there is an
insulator.
• When irradiated by neutrons, emitter emits
electrons, which come through the insulator to
the collector and form electric current in the
external circuit.
• In the cable, output signal of beta-emission
neutron detector moves outside the reactor
vessel.
• In detectors used in the VVER reactors, emitter is
a rhodium wire 0.5 in diameter and 200 mm in
length.
36
• The scheme of radioactive transformations of
rhodium nuclei in neutron capture is shown in
the figure below.
37
Modes of operation
of the nuclear reactor
• Consider three basic modes of operation of
the nuclear reactor:
• start-up,
• operation at power on
• and shut-down,
• at the same time we assume that all
equipment is fully prepared for the operation.
38
• So, when rising the energy block to power, it is
necessary to warm up all the equipment, set the
appropriate pressure in the circuits and,
depending on the type of the reactor and nuclear
power plants to carry out a number of other
preparatory operations.
• The time of rising the unit to power is usually
determined by the permissible rate of the
equipment warmup and estimated by hours.
• All this will be considered as already performed,
and consider only the specifics of the reactor
itself.
39
• In the output of the nuclear reactor from the
subcritical state to the operating level of
power, it is necessary to increase the density
of neutrons by many orders from the density
corresponding to a subcritical reactor state to
densities corresponding to the level of
operating power.
40
• In the reactor operating at power, generation
of neutrons by fission processes is
immeasurably greater than that from a
permanent source.
• Therefore, when considering reactor kinetics
at the level of operating power, the value S
can be neglected.
• However, in the subcritical state it ultimately
determines the level of the neutron density
and it cannot be neglected.
41
• In steady subcritical state where dn/dt = 0
and dCi/dt=0, then:
•
• Hence, neutron density
•
• where Кeff<1 and represents the multiplication
factor in the sub-critical reactor.
42
• Neutron density in subcritical reactor depends
on the value S and depth of the subcriticality
1-Keff.
• It is directly proportional to power of the
neutron source and the smaller, the deeper
the subcritical reactor.
• In the limit for infinitely diluted reactor when
KEF = 0, neutron density is completely defined
by the value S.
43
• In the real reactor in the presence of fissile
material in the core, neutron density exceeds S,
as their generation is still due to fission
processes.
• According to the nuclear safety requirements,
output of the reactor from the subcritical to
critical state with subsequent output to power
must be carried out at a constant control of
neutron density change.
• This defines the limit of the neutron density in
the subcritical reactor, which should be at a level
of sensitivity of neutron detectors.
44
• In the reactor loaded with fresh fuel, sources
of neutrons in a subcritical state are
spontaneous fission of uranium nuclei and
cosmic γ -background.
• In this case reactor start-up, according to
safety requirements, is made of a relatively
deep subcriticality.
• Therefore, the initial neutron density, due to
these sources, is lower the sensitivity of
neutron detectors.
45
• To increase the neutron density to a manageable
level, neutron sources are placed inside the
reactor, such as polonium-beryllium (Po-Be).
• Radioactive nuclide Po emitting alpha particles,
in conjunction with Be, by which reaction
• Be(α, n)C is relatively efficient, is a very
common source of neutrons.
• When making neutron sources, other nuclides
are also used, which give not only the reactions
(α, n), and (γ, n), such as antimony - beryllium
source and other nuclear reactions with the
emission of neutrons.
46
• Thus, reactor start-up with a fresh (unirradiated) fuel is
performed in the presence of a neutron source located
directly in the core. Its power is determined by the
sensitivity threshold of starting neutron detectors.
• As a result of radioactive decay, neutron source power
decreases over time.
• Thus, the half-life of 210Po is 138 days, and
in 2 years the power of Ro-Be-source is reduced about
30 times.
• However, the reactor operated for a long time at
power, doesn’t need an independent neutron source,
as it accumulates radioactive fission products that emit
γ -quanta.
47
• In the presence of the materials in the core,
by which a reaction occurs (γ, n), additional
neutron sources are accumulated in the
reactor.
• The reaction (γ, n) with the formation of
the so-called photo-neutrons proceeds
relatively efficiently by beryllium and heavy
water.
48
• In the normal water mass content of
deuterium is about 0.016%.
• It is sufficient for light water reactor after its
shutdown to have a powerful source of
photoneutrons.
• The threshold energy at which the reaction
proceeds (γ, n) by deuterium or beryllium, is
respectively, 2.21MeV and 1.62 MeV,
• i.e. less energy of γ-quanta emitted by some
fission fragments.
49
• γ -background after reactor shutdown decreases
exponentially in accordance with the period of
the decay of radioactive fission products.
• Accordingly, photoneutron generation decreases.
• From this point of view, a long stop after reactor
shutdown is undesirable.
• However, the reactor, which is in operation for a
long time, is studied well, or its output from deep
subcriticality to a manageable level of neutron
density is allowed to be "blindly" according to a
pre-developed program.
50
• When turning off the reactor, it is necessary to
avoid deep subcriticality. The closer the
reactor to the critical state, the higher the
neutron density in all other things being
equal.
• Typically, in short-term shutdowns, not
associated with changes in the composition of
the core, subcriticality by the absolute value
|Δk| is not above 5%.
51
• However, one should always keep in mind that
the reactor start-up is a very important
operation, including at restart.
• In the shutdown period, repair and
maintenance work is usually held, in particular
in the control system.
• Therefore, no matter how the reactor was
mastered its start-up should be carried out
with all precautions.
52
• Extraction of control rods in the output from
political state to critical is carried out in small
steps, followed by aging and careful
measurement of reactor period.
• The figure shows a qualitative change in the
neutron density in time at the output of the
reactor from the subcritical state to critical or
supercritical.
53
• The dependence of the change in neutron density
on the time in the start-up mode at step
reactivity change
54
• Until time t0, reactor is in a subcritical state,
and neutron density in it corresponds to the
depth of subcriticality in this time interval.
• At the time t0, positive increment of reactivity
is given.
• Virtually in all power reactors this is done by
moving absorbing rods.
• By the purpose, they are usually divided into
control, shim and emergency protection.
55
• According to the of safety requirements, the
increment of reactivity per one step should be
smaller than the effective fraction of delayed
neutrons.
• Only in this condition in any step of increment
of reactivity in the subcritical state (including
critical), reactor misses the prompt criticality.
• This requirement is especially important in
reaching a critical state.
56
• After the first increment step of reactivity at
time t0 neutron density increases and reaches
a new constant level in accordance with a
smaller depth of subcriticality.
• It is important to make sure that on the
interval from t0 to t1 steady period is infinite that is, after the initial increment neutron
density remains at a constant level.
57
• At the time ti the next increment of reactivity
is produced and so on.
• Finally, after another increment of reactivity at
the time ti + 1 neutron density begins to
increase continuously - this means that the
reactor reached the supercritical state.
• Thus at time ti the reactor can be in a more
subcritical or in critical state already.
58
• If the reactor was still in the subcritical state,
its subcriticality was less than reactivity,
released per the next step.
• If the reactor was in a critical state already, its
runaway will begin with a valid period as the
step by reactivity is certainly less than the
value β.
59
• Reactor runaway is carried out until neutron
density reaches a level corresponding to the
operating power.
• At the time it reaches operating neutron
density, excess reactivity is removed, e.g. by
lowering control rod into the core, the reactor
becomes critical and from that moment on it
begins to operate at a constant power level.
60
• During runaway, when neutron density is close
to the operating, there is feedback by
reactivity due to temperature effect.
• According to the of safety requirements,
temperature coefficient of reactivity of
nuclear power reactors must be negative.
61
• This means that with growth of temperature
caused by increasing of power (neutron
density), reactivity decreases and the further
increase in power is terminated.
• Thus, reactors, having a negative coefficient of
reactivity, are self-regulating.
62
• In boiling water reactors, in which boiling
water acts at the same time as a coolant and
moderator, along with temperature coefficient
there is so-called void coefficient of reactivity.
• Its physical nature is due to the fact that a
change in vapor content leads to a change in
the number of nuclei of the moderator per
unit volume, which inevitably affects the
reactivity.
63
• In pressurized water reactors without boiling of
the coolant one should consider density
coefficient of reactivity associated with change in
water density depending on the temperature.
• Finally, there is power coefficient of reactivity,
due to changes in material temperature in the
core, and above all of the fuel due to the Doppler
effect, as well as changes in the density and void
fraction of the moderator and other effects when
switching from one power level to another, that is
taking into account the total effect of all the
factors.
64
• When designing nuclear reactors one tends to
ensure all the coefficients of reactivity thermal, steam, density and power to be
negative, although not at all power levels it is
possible to easily achieve.
• Nevertheless, at operating power levels it is
almost always implemented, and the reactors
are typically self-regulating.
65
• However, this does not mean that in the reactor
operation at the operating power levels the use
of regulatory units is not required.
• By itself, the transition from one power level to
another is associated with a change in neutron
density for which it is necessary to set a positive
or negative reactivity.
• The influence of the above mentioned reactivity
coefficients during the transition from one power
level to another leads to new arrangement of
shim and control rods.
66
• In the process of reactor start-up and rising to the
power fission products begin to form, which
inevitably affects the balance of neutrons and
reactivity.
• For thermal reactors it is important to keep in
mind xenon and samarium poisoning.
• The equilibrium xenon poisoning is achieved
within day 1.5-2 days, which is equivalent to
reducing the reactivity by about 3%.
• Samarium poisoning is much slower and is
calculated in weeks and reactivity reduction
when reaching the equilibrium samarium
poisoning is less than 1%.
67
• Further, during reactor operation there is also
the so-called slow change in reactivity due to
burnup of nuclear fuel and slagging, which
requires compensation.
• In pressurized water reactors, the role of the
compensator performs mainly dissolved boric
acid in the circulating coolant (but as fuel burn
its concentration decreases).
68
• In most other types of reactors this role is
performed by the movable shim rods,
when fuel burns up they are gradually
extracted from the core.
• As additional shim facilities, burnable
absorbers are used in some cases. In heavy
water reactors, slow change in reactivity can
be compensated by changing the level of
heavy water moderator in the calandria tank.
69
• The corresponding excess of the fuel mass, which
burns up in the reactor operation, is supported by
the replacement of the burnt fuel with fresh,
combined with the use of control and shim units.
• In the refueling reactors in the reactor operation,
the excess reactivity is relatively small and
depends on the rate of nuclear refueling.
• The primary excess of the fuel mass over the
critical is determined in these reactors by
temperature effect in the output from a cold
state to operating power level.
70
• In the reactors in which refueling is performed
only in the complete shutdown, fuel excess of
weight over the critical is loaded for the duration
of work from one refueling to another.
• The maximum reactivity of these reactors, which
must be compensated, occurs immediately after
a regular full or partial refueling.
• As fuel burnup, excess of reactivity decreases,
and before the next refueling it is practically zero.
71
• Reactor shutting down is carried out relatively
simple.
• To do this, the negative reactivity is given, i.e.
reactor with control rods is in subcritical state,
and neutron density is relatively quickly
reduced by many orders of magnitude.
• When short-term shutdowns, it should not be
below the sensitivity of neutron detectors. For
this purpose, the reactor is supported by
relatively small subcriticality.
72
• In the reactor shutdown the temperature of the
core materials gradually decreases, and due to
temperature effect reactivity is released.
• Therefore, for the reactor not to reach the critical
and supercritical state, released reactivity due to
temperature effect must be properly
compensated by the control units.
• In the reactor shutdown, neutron density
decreases relatively quickly, and by many orders.
Accordingly, energy release decreases due to the
interaction of neutrons with nuclear fuel.
73
• However, the residual heat release associated
with the radioactive decay of fission products
decreases significantly more slowly, and its
fraction at the time of reactor shutdown is
about 7% of the nominal power.
• Therefore, the reactor after shutdown still
needs long-term cooling.
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