Pump drive system for recirculation of coolant flow in a nuclear reactor vessel

The invention relates to a drive system for internal recirculation of coolant in a nuclear reactor (1), wherein the drive system for a main recirculation pump (10) for the coolant recirculation comprises a pump motor (11) connected to the main recirculation pump (10) and a static frequency converter (F) for connection to an alternating voltage network (16) and for feeding the pump motor (11). The frequency converter (F) comprises a rectifier (12) for connection between the alternating voltage network (16) and a d.c. intermediate link (15) and a first inverter (13) for connection between the d.c. intermediate link (15) and the pump motor (11). The invention is characterized in that the energy storage magazine, for example a flywheel (18), is connected to the d.c. intermediate link (15) via a second inverter (14). The invention is further characterized in that the second inverter (14) is adapted, in the event of loss of voltage from the alternating voltage network (16), to feed back energy from the energy storage magazine (18) to the d.c. intermediate link (15) for controlled direct or delayed reduction via the first inverter (13) of the speed of the main recirculation pump (10).

TECHNICAL FIELD 
The present invention relates to a drive system for a nuclear reactor, more 
particularly a drive system intended to drive a main recirculation pump 
for recirculation of coolant in a vessel arranged in the reactor. 
BACKGROUND ART, PROBLEMS 
A boiling water reactor normally comprises an external, substantially 
cylindrical, vertical container referred to as a reactor vessel, in the 
lower part of which a substantially cylindrical vertical moderator vessel 
is arranged. The moderator tank comprises a core of fuel rods. Between the 
outer wall of the moderator tank and the inner wall of the reactor vessel, 
there is an annular space referred to as a downcomer. The reactor vessel 
is partially filled with a coolant (water) for cooling a core of fuel rods 
arranged in the moderator tank. 
During operation of the reactor, that is, during nuclear fission, the water 
starts boiling when it has reached to approximately one-fourth of the 
core. The steam thus formed is separated from the water at the upper part 
of the reactor vessel, partly in steam separators and partly in steam 
dryers arranged to separate the last moisture residues in the steam before 
it flows out of the reactor vessel. The separated water flows down into 
the downcomer. To replace the water which is taken out of the reactor 
vessel in the form of steam, the reactor vessel is supplied with water via 
a feedwater inlet. Thus, the downcomer contains a mixture of incoming 
feedwater and water which is separated from the steam in the steam 
separators and the steam dryers. 
In the downcomer at the bottom of the reactor vessel, main recirculation 
pumps of plug-in type are arranged for recirculation of water from the 
downcomer and up through the core for continuous cooling of the fuel rods. 
The main recirculation pumps normally consist of vertical wet asynchronous 
machines operating in water under pressure. 
The adjustable recirculation flow of the coolant is utilized for 
controlling the output power from the reactor in that an increased coolant 
flow, in addition to an increased cooling of the fuel rods, also results 
in an increased power production (increased neutron generation) in the 
fuel rods. As a result of the thermal inertia in the fuel rods, the time 
constant for increased power production in the fuel rods differs from the 
time constant for the corresponding increase of the cooling requirement. 
If the supply of energy to one or more main recirculation pumps is 
disturbed, the cooling and the power production are interrupted 
instantaneously whereas the surface temperature of the fuel rods rises 
since the fuel rods contain a decay power in the form of thermal energy 
which is not yet exhausted. 
During normal operation of the reactor, the fuel rods are surrounded by a 
coolant film. At too rapid a reduction of the coolant flow, the decay 
power which is stored in the fuel rods will result in a brief overheating 
thereof. In those cases where this overheating leads to the heat flux from 
a fuel rods becoming very great in relation to the coolant flow, there may 
be a risk of dryout occurring, that is, the coolant film becomes so thin 
that it is unable to hold together. The coolant film is broken up and dry 
wall portions are formed, which locally leads to a considerably 
deteriorated thermal transmittance between the fuel rod and the coolant 
with an ensuing greatly increased surface temperature of the fuel rod. The 
increased surface temperature may lead to damage with serious consequences 
arising on the fuel rods, or to a shortening of the service life thereof. 
To secure against dryout, the power output from the fuel rod is limited 
such that a margin with respect to dryout in case of transients in the 
coolant flow is obtained. This margin, referred to as dryout margin, means 
that the fuel cannot be utilized as efficiently as would otherwise be 
possible. Therefore, from the point of view of fuel economy, it is 
desirable to minimize the dryout margin. One of the dimensioning factors 
for the dryout margin is disturbance of the coolant flow as a result of 
line power loss for shorter or longer periods. 
The dryout margin in the case of disturbances of the energy supply to the 
main recirculation pumps is dependent on how fast the main recirculation 
pumps unroll, that is, on the time rate of change of the pump speed. The 
time rate of change is determined by the kinetic energy of the pump, that 
is, the inertia in the drive system of the main recirculation pump. The 
unroll time is thus dimensioning for the power output from the fuel rods. 
To increase the unroll time it is desirable to increase the inertia in the 
drive system of the main recirculation pumps. Because of the design of the 
reactor, the space for pumps and motors is limited, which gives a 
pump/motor design which is relatively long and narrow with a limited 
moment of inertia. The limited space in the reactor vessel means that it 
is not possible to increase the dimension of the main recirculation pump 
to obtain increased inertia. The space in the reactor vessel only allows 
increased inertia by replacing material in certain parts of the main 
recirculation pump by heavier material. However, this is not sufficient to 
obtain the desired inertia. 
For driving a main recirculation pump, it is known to use a drive system 
comprising a pump motor for driving the pump and a rotating frequency 
converter for feeding the pump, the frequency converter comprising a motor 
which via a hydraulic coupling is connected to a generator. The rotating 
frequency converter is electrically connected between a supply alternating 
voltage network and the pump motor. The rotating frequency converter may 
be provided with a flywheel for increasing the inertia in the drive system 
and hence limiting the unroll speed of the main recirculation pump. The 
disadvantage of the rotating frequency converter is that it limits the 
speed of action when controlling the power of the reactor. The rotating 
frequency converter cannot maintain the speed of the main recirculation 
pump in case of a voltage loss but only extend the unroll time thereof. A 
poor efficiency is a general disadvantage for hydraulic couplings. 
Another way of extending the unroll time is to connect in the drive system, 
in series with the rotating motor-generator equipment (without hydraulic 
coupling), a static frequency converter, the static frequency converter 
comprising power electronics components in the form of a rectifier 
connected to a d.c. intermediate link which, in turn, is connected to an 
inverter for feeding the pump motor. The static frequency converter is fed 
from the rotating motor-generator equipment which, in turn, is fed from an 
alternating voltage network. In this way, the drive system is supplied 
with inertia via the rotating motor-generator equipment whereas the 
control speed is made possible by the static frequency converter. The 
disadvantage of this method of extending the unroll time is that modern 
conventional static frequency converters of pulse-width modulated type are 
sensitive to voltage deviations in the supply to the rectifier. In those 
cases where the rectifier is supplied from the generator of the rotating 
frequency converter, it is difficult to maintain the voltage from the 
generator when this unrolls. Older static frequency converters are less 
sensitive to voltage deviations, so the method described above is adapted 
to these older models. Further, the method requires two machines, namely a 
motor and a generator. The generator must be of a synchronous machine 
type, which is more expensive and less reliable than a simpler and more 
robust asynchronous machine. The network must be able to manage direct 
start of the flywheel, which limits the maximally possible inertia in the 
system. In addition, the motor and the generator must be dimensioned for 
the rated power of the main recirculation pump. Further, the rotating 
motor-generator equipment has a relatively large service requirement 
compared with the static frequency converter. 
The introduction of inertia in the drive system for a main recirculation 
pump is of advantage in the event of loss of line power but of 
disadvantage as regards the speed of action for control of the power 
production of the reactor. The requirement for control speed in the 
reactor has increased and, therefore, drive systems comprising only 
rotating frequency converters are no longer used in new designs. 
In a boiling water reactor, normally between four and ten main 
recirculation pumps are arranged. Only events which lead to unroll of the 
majority of these pumps give rise to dryout. 
One object of the invention is to achieve, for a given, total main 
recirculation pump dimension, an increased inertia in the drive system 
thereof for maintaining the speed of the main recirculation pump in the 
event of loss of supply voltage from the network. 
SUMMARY OF THE INVENTION, ADVANTAGES 
The invention relates to a drive system for a main recirculation pump for 
internal coolant recirculation in a nuclear reactor of boiling water type 
in which the drive system in case of voltage loss from a supply 
alternating voltage network, directly or with a delay, in a controlled 
manner reduces the speed of the pump to a predetermined minimum speed. In 
case of delayed reduction, energy supply to the main recirculation pump, 
and hence the speed thereof, are maintained for a predetermined period of 
time, whereupon, if the voltage does not return within the predetermined 
period, a controlled reduction of the speed of the pump is achieved. 
The maintenance of the energy supply to the main recirculation pump and the 
controlled reduction, respectively, are achieved by introducing inertia in 
the drive system of the main recirculation pump. The drive system 
comprises a pump motor for operation of the pump and a static frequency 
converter connected between the network and the pump motor. The static 
frequency converter comprises a rectifier connected between the network 
and a d.c. intermediate link and a first inverter connected between the 
d.c. intermediate link and the pump motor. The inertia is supplied to the 
drive system by connecting to the d.c. intermediate link, via a second 
static inverter, an energy storage magazine, which, for example, consists 
of a flywheel which is driven by an asynchronous machine. The second 
inverter is adapted to operate in two operating modes, rectifier and 
inverter mode, respectively. 
During normal operation, both the main recirculation pump and the flywheel 
are fed with energy via the common rectifier and the common d.c. 
intermediate link. During normal operation, the second inverter operates 
in the inverter mode for driving the asynchronous machine which functions 
as a motor for driving the flywheel. 
In case of loss of the line voltage to the static frequency converter, the 
second inverter is controlled such that energy is fed back from the 
flywheel via the asynchronous machine to the d.c. intermediate link such 
that the direct voltage level is maintained. Thus, in case of loss of the 
line voltage, the asynchronous machine serves as a generator and the 
second inverter operates in the rectifier mode. The energy stored in the 
flywheel can then be used for controlled reduction of the speed of the 
main recirculation pump or for maintaining the speed of the main 
recirculation pump for a predetermined period of time and then possibly 
reducing the speed. 
Thus, the invention entails a possibility of either reducing the speed of 
the pump directly or reducing the speed of the pump with a delay, that is, 
reducing the speed only when a certain predetermined period of time has 
elapsed and the line voltage has not yet returned. 
In those cases where a direct reduction is chosen, the dimensions of the 
second inverter and the asynchronous machine are adapted to the power 
which is required to reach sufficient inertia in the unroll of the main 
recirculation pump for direct reduction in the case of voltage loss. The 
second inverter and the asynchronous machine are dimensioned to provide at 
least the additional power required to limit the unroll from the inherent 
unroll of the pump to the desired unroll. 
In those cases where a delayed reduction is chosen, the flywheel is 
dimensioned such that the speed is maintained for the predetermined period 
of time such that the core is kept uninfluenced in case of loss of line 
voltage during this period of time. In those cases where the line voltage 
returns within the predetermined period of time, the main recirculation 
pumps remain in operation completely uneffected by the disturbance. The 
flywheel is reaccelerated to the original speed whereupon the drive system 
again manages a line voltage loss. In those cases where the voltage does 
not return, the speed of the main recirculation pump is reduced to a 
minimum speed whereupon it is tripped. 
It is known per se to feed several motors via separate inverters from a 
common d.c. intermediate link. It is also known to feed back energy, 
during braking, from an asynchronous machine to the d.c. intermediate link 
where either a braking resistor burns away the braking energy or the 
energy is fed back to the network via a rectifier adapted for this 
purpose. Using the braking energy from an asynchronous motor for 
accelerating another asynchronous motor connected to the same common d.c. 
intermediate link is also known in applications which include braking. 
However, the invention relates to feedback of energy from the flywheel to 
the frequency converter in those cases where the line voltage is cut off 
for further supply of energy from the frequency converter to the main 
recirculation pump for controlled direct or delayed reduction of the speed 
of the pump to a minimum speed. There is no possibility of storing energy 
in the frequency converter. If this were possible, the stored energy could 
be used for controlled reduction. The energy which is fed back to the 
static frequency converter must either be burnt in a resistor or be 
forwarded to a consumer. 
When the flywheel is connected via a separate second inverter, the speed of 
the main recirculation pump can be controlled independently of the speed 
of the flywheel. The flywheel can then constantly be driven by the 
asynchronous machine at full speed during normal operation. This allows 
maximum energy to be stored in the flywheel independently of the speed of 
the main recirculation pump. Only directly after a line voltage loss is 
the stored energy limited since the flywheel must then be reaccelerated. 
Since the speed of the flywheel is independent of the speed of the pump, 
the inertia of the flywheel does not limit a rapid change of speed in 
those cases where such change is necessary. A relatively rapid reduction 
is, for example, necessary during a reactor scram. Nor is the control 
speed of the drive system limited by the inertia of the flywheel. The 
invention permits the flywheel, if desired, to be oversized so as to 
provide an ample addition of inertia without negatively influencing the 
reduction in case of a reactor scram or the control speed, which would be 
the case if a rotating frequency converter were used. 
An important advantage of feeding back energy from the flywheel via the 
second inverter to the d.c. intermediate link is that an asynchronous 
machine can be used instead of a synchronous machine for operation of the 
flywheel. The asynchronous machine is considerably more robust and simple 
than the synchronous machine which has to be used if energy is to be 
transmitted electrically from the flywheel to the pump motor. Further, the 
asynchronous machine is both less expensive and more reliable than the 
synchronous machine. 
The invention permits the selection of both direct and delayed reduction of 
the speed of the pump when dimensioning the drive system. 
When selecting direct reduction of the speed of the pump, the dimensioning 
of the converter and the asynchronous machine may be adapted to the power 
which is required to reach sufficient inertia in the unroll of the main 
recirculation pump. The converter and the asynchronous machine are thus 
dimensioned to provide at least the additional power which is required for 
limiting the unroll from the inherent unroll of the pump to the desired 
unroll. 
The extended unroll time and the increased margin with respect to dryout 
connected therewith can be utilized for greater power output in the 
reactor or more cost-effective utilization of the fuel. 
The drive system described above may, of course, also be arranged in a 
reactor of a different type, for example a pressurized-water reactor or a 
graphite-moderated reactor. The description refers to internal main 
recirculation pumps but the drive system may, of course, be arranged also 
with external main recirculation pumps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a nuclear reactor 1 which comprises an external, substantially 
cylindrical, vertical container referred to as reactor vessel 2. Inside 
the reactor vessel 2 at the lower part thereof, a substantially 
cylindrical vertical moderator tank 3 is arranged. The moderator tank 3 
comprises a core of fuel rods (not shown). Between the outer wall of the 
moderator tank 3 and the inner wall of the reactor vessel 2, there is an 
annular space referred to as downcomer 4. The reactor vessel 2 is 
partially filled with a coolant 5 (water) for cooling the core. 
During operation of the reactor 1, that is, during nuclear fission, the 
water starts boiling when it has reached a level where it covers 
approximately one-fourth of the core. The steam thus formed is separated 
from the water at the upper part of the reactor vessel 2 by means of steam 
separators 6 and steam dryers 7 before the steam flows out of the reactor 
tank 2 via a steam outlet 8. The separated water flows down into the 
downcomer 4. To replace the water which is taken out of the reactor vessel 
2 in the form of steam via the steam outlet 8, the reactor vessel 2 is 
supplied with water via a feedwater inlet 9. The downcomer 4 thus contains 
a mixture of incoming feedwater and water which is separated from the 
steam in the steam separators 6 and the steam dryers 7. 
In the downcomer 4, at the bottom of the reactor vessel 2, main 
recirculation pumps 10 of plug-in type are arranged for recirculation of 
water from the downcomer 4 and up through the core for continuous cooling 
of the fuel rods therein. The main recirculation pumps 10 are usually 
driven by vertical wet asynchronous machines 11 operating in water under 
pressure. The arrows in FIG. 1 symbolically show the flow paths of the 
water and the steam, respectively, through the reactor 1. 
FIG. 2 shows the principles of a drive system for a main recirculation pump 
10 for internal recirculation of coolant in the nuclear reactor 1. The 
drive system comprises a pump motor 11 connected to the main recirculation 
pump 10 and a static frequency converter F for connection to an 
alternating voltage network 16 and for feeding the pump motor 11. The 
frequency converter F comprises a rectifier 12 for connection between the 
alternating voltage network 16 and a d.c. intermediate link 15 and a first 
inverter 13 for connection between the d.c. intermediate link 15 and the 
pump motor 11, The drive system further comprises a flywheel 18 connected 
to an asynchronous machine 17, which is turn is connected to the d.c. 
intermediate link 15 via a second static inverter 14. The second inverter 
14 operates in two operating modes, rectifier and inverter mode, 
respectively. The d.c. intermediate link 15 comprises an inductive reactor 
19 and a capacitor 20. The asynchronous machine 17 is adapted, in case of 
loss of voltage from the alternating voltage network 16, to feed back 
energy from the flywheel 18 to the d.c. intermediate link 15 via the 
second inverter 14 for controlled direct or delayed reduction of the speed 
of the main recirculation pump 10. 
The speed of the main recirculation pump 10 is controlled by control of the 
frequency fed to the pump 10 by means of the static frequency converter F. 
The speed of the pump 10 depends on the frequency of the alternating 
voltage. The frequency is controllable by the frequency converter F in 
which the rectifier 12 transforms the line voltage with constant frequency 
to direct voltage, whereupon the first inverter 13 transforms the direct 
voltage back to alternating voltage with controllable voltage and 
amplitude, and feeds it to the pump motor 11. 
During normal operation, the pump motor 11 and the asynchronous machine 17 
are fed with energy from the network 16 via the d.c. intermediate link 15. 
In this case, the second inverter 14 operates in the inverter mode for 
feeding the asynchronous machine 17 which serves as a motor for operation 
of the flywheel 18 and storage of energy therein. In case of loss of line 
voltage, energy from the flywheel 18 is fed back via the second inverter 
14, which then operates in the rectifier mode, back to the d.c. 
intermediate link 15 which, in turn, feeds energy via the first inverter 
13 and the pump motor 11 to the main recirculation pump 10 for direct or 
delayed reduction of the speed thereof to a predetermined minimum speed. 
In case of delayed reduction of the speed of the main recirculation pump 
10, the second inverter 14 is adapted, in the absence of voltage from the 
supply alternating voltage network 16, for a predetermined period of time 
to maintain the speed of the main recirculation pump 10 and then, unless 
the voltage returns after a time longer than the predetermined time, to 
reduce the speed of the main recirculation pump 10 to the predetermined 
minimum speed. 
The first inverter 13 and the second inverter 14 are of a pulse-width 
modulated type, that is, the alternating voltage is built up from pulses 
whose amplitude is equal to the direct voltage supplied by the rectifier 
12. The width, frequency and polarity of the pulses are controlled such 
that the alternating voltage is given the desired frequency and magnitude. 
The rectifier 12 is of a conventional diode design. Since there is no need 
for simultaneous acceleration of the flywheel 18 and the main 
recirculation pump 10, the rectifier 12 is dimensioned to manage the 
operation of the main recirculation pump 10. 
The first inverter 13 is dimensioned for the operation of the main 
recirculation pump 10. The magnitude of the second inverter 14 is 
determined by the extent to which the unroll is to be extended and by the 
losses in the drive system. 
The flywheel 18 which is driven by the asynchronous machine 17 is 
dimensioned for maintenance of or controlled reduction of the speed of the 
main recirculation pump 10 in the case of line voltage loss. The size of 
the flywheel 18 is determined by how long time the pump operation is to be 
maintained in case of voltage loss and by how rapidly reduction of the 
pump 10 is thereafter desirable. When dimensioning the flywheel 18 it is 
suitable that a line voltage loss of, for example, 0.5-1.5 seconds is 
managed without speed reduction of the pump 10 whereupon a controlled 
linear reduction of the pump speed to a minimum speed should be capable of 
being carried out in, for example, 5-10 seconds. 
The speed of the asynchronous machine 17 is independent of the speed of the 
main recirculation pump 10. To optimize the ability of the flywheel 18 to 
store energy, an asynchronous machine 17 with high-speed operation is 
preferably chosen. As the second inverter 14, the asynchronous machine 17 
is chosen in dependence on the extent to which the unroll time is to be 
extended and on the losses in the drive system. 
All the main recirculation pumps 10 arranged in a reactor 1 are preferably 
provided with the drive system according to the invention, but this is no 
necessity to obtain a well functioning recirculation system. The primary 
point is to provide a sufficient number of main recirculation pumps 10 
with the drive system according to the invention in order to attain the 
desired reduction and the cooling reduction. 
FIG. 3 shows a diagram with curves for different unroll times in the main 
recirculation pump 10. The y-axis of the diagram denotes the speed and the 
x-axis thereof denotes the time. The unbroken curve 21 shows the inherent 
unroll of the main recirculation pump 10 in case of line voltage loss, 
that is, the unroll time which is limited by the inherent inertia of the 
pump 10 and by inherent losses. The broken line 22 shows, in the case of 
line voltage loss, the speed profile for controlled reduction of the pump 
10 by means of the drive system according to the invention. The 
dash-dotted line 23 shows, in case of line voltage loss, the speed profile 
when the speed is maintained in the main recirculation pump 10 for a 
predetermined period of time whereafter the speed is reduced in a 
controlled manner. By means of the drive system according to the 
invention, it is clear from FIG. 3 that the critical speed profile 21 in 
case of line voltage loss is avoided. The vertical distance between the 
respective speed profiles is a measure of the extra power of the drive 
system which is required for extending the unroll time of the main 
recirculation pump 10 from one speed profile, for example 21, to the 
other, for example 22.