Gas injection system of nuclear power plant and gas injection method therefor

A gas injection system of a nuclear power plant including a reactor and a gaseous waste disposal system, comprises a hydrogen injection unit for injecting hydrogen into a reactor of a nuclear power plant, an oxygen injection unit for injecting oxygen into a gaseous waste disposal system in which the oxygen is recombined with the hydrogen passed by way of the reactor, a densitometer for measuring a balance of concentration between the hydrogen and the oxygen in the gaseous waste disposal system, a flowmeter for measuring a flow rate of an exhaust gas of the gaseous waste disposal system, and a controlling unit for controlling the quantities of the hydrogen gas and the oxygen gas. According to the operation of the controlling unit, the injection quantities of the hydrogen and the oxygen from the hydrogen injection unit and the oxygen injection unit are controlled in accordance with the results of the concentration balance measurement and the flow rate measurement, a lag time between a time when the oxygen and the hydrogen are injected and the time when the oxygen and the hydrogen reach the gaseous waste disposal system is stored, respectively, and an injection timing from the hydrogen injection and the oxygen injection is controlled in response to the lag time.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a gas injection system of a nuclear power 
plant for injecting hydrogen and oxygen in a nuclear power plant and also 
relates to a gas injection method therefor. 
2. Description of the Related Prior Art 
Generally, there has been known a technique of reducing an oxygen 
concentration of water to be brought into contact with a stainless steel 
as a technique for controlling a stress corrosion cracking of the 
stainless steel. In a nuclear power plant, fixed quantities of hydrogen 
and oxygen are generated in a reactor by radiolysis of water. By the 
generation of hydrogen and oxygen in the reactor, a dissolved oxygen 
concentration of the reactor water is determined based on the 
temperature-pressure conditions, and the dissolved oxygen concentration is 
usually about 100-200 ppb. The radiolysis of water is controlled and the 
oxygen concentration in the reactor is lowered by the injection of the 
hydrogen into the reactor water. 
In the reactor, before the injection of the hydrogen, the hydrogen and 
oxygen in a main steam are sent in a volume ratio of 2:1, and after 
passing through a main condenser, they are recombined by a recombiner of a 
gaseous water disposal system and then become water to be recovered. 
However, in case of injecting the hydrogen into the reactor, the oxygen is 
reduced due to the reduction of the quantity of the radiolysis of water in 
the reactor, and the hydrogen is injected from the outside, so that the 
volume ratios of the hydrogen and the oxygen in the main steam is not 2:1 
and the quantity of the hydrogen extremely exceeds. Because of this 
reason, since an excess of the hydrogen is flowed into the gaseous waste 
disposal system, it is necessary to inject the oxygen so as to be 
recombined with the hydrogen. 
However, in such a case as described above, a problem of quantity of gases 
to be injected is caused. That is, the quantity of the hydrogen to be 
injected varies in accordance with an output power of the nuclear power 
plant and is usually in the quantity of about 30-120 Nm.sup.3 /Hr. Almost 
all quantity of the injected hydrogen is flowed into the gaseous waste 
disposal system. 
A flow rate of the exhaust gas which can be treated by the gaseous waste 
disposal system is about 20-40 Nm.sup.3 /Hr, which corresponds to the 
quantity for treating an air leaked from the main condenser. When an 
excess of hydrogen is flowed into the air, some portion thereof is 
recombined with the oxygen in the air, but the most portion thereof is 
flowed into the gaseous waste disposal system. The flow rate of the 
exhaust gas of the gaseous waste disposal system becomes 40-130Nm.sup.3 
/Hr, exceeding the allowable flow rate. 
Therefore, when the oxygen for the recombination with the hydrogen is 
injected into the gaseous waste disposal system, the hydrogen is 
recombined with the oxygen, and only the air leaked from the main 
condenser is treated in the gaseous waste disposal system. 
Since the quantities of the hydrogen and the oxygen to be injected are 
larger than the allowable flow rate in the gaseous waste disposal system, 
it is an important matter to maintain a balance of the quantities of the 
hydrogen and the oxygen at an inlet of the gaseous waste disposal system. 
Furthermore, since the hydrogen injected to a water system reaches the 
gaseous waste disposal system by way of the reactor, the main steam pipe 
and the main condenser of the nuclear power plant, there causes a time lag 
that elapses between the injection of the hydrogen and the reaching 
thereof to the gaseous waste disposal system, and it is difficult to 
define an injection timing of the oxygen to be injected into the gaseous 
waste disposal system, thus providing a problem. 
Still furthermore, the hydrogen becomes an explosion area when it exceeds 
4% in a volume ratio in a mixing condition with another gases. 
Accordingly, it is also important to avoid the condition where the 
hydrogen exceeds in its quantity. 
SUMMARY OF THE INVENTION 
An object of the present invention is to substantially eliminate defects or 
drawbacks encountered in the prior art described above and to provide a 
gas injection system of a nuclear power plant capable of controlling the 
quantity of the hydrogen so as not to exceed in the quantity to be 
injected and ensuring that the hydrogen and the oxygen reach the gaseous 
waste disposal system at the same time, and also provide a gas injection 
method therefor. 
This and other objects can be achieved according to the present invention 
by providing, in one aspect, a gas injection system of a nuclear power 
plant including a reactor and a gaseous waste disposal system, comprising: 
a hydrogen injection means for injecting hydrogen into a reactor of a 
nuclear power plant; 
an oxygen injection means for injecting oxygen into a gaseous waste 
disposal system in which the oxygen is recombined with the hydrogen passed 
by way of the reactor; 
a densitometer for measuring a balance of concentration between the 
hydrogen and the oxygen in the gaseous waste disposal system; 
a flowmeter for measuring a flow rate of an exhaust gas of the gaseous 
waste disposal system; and 
a control means for controlling injection quantities of the hydrogen and 
the oxygen from the hydrogen injection means and the oxygen injection 
means in accordance with the measurement results of the densitometer and 
the flowmeter, for storing a lag time between a time when the oxygen and 
the hydrogen are injected and a time when the oxygen and the hydrogen 
reach the gaseous waste disposal system, respectively, and for controlling 
injection timings from the hydrogen injection means and the oxygen 
injection means in response to the lag time. 
In another aspect, there is provided a gas injection method for a nuclear 
power plant including a reactor and a gaseous waste disposal system, 
comprising the steps of: 
injecting hydrogen into a reactor of a nuclear power plant; 
injecting oxygen into a gaseous waste disposal system in which the oxygen 
is recombined with the hydrogen passed by way of the reactor; 
measuring a balance of concentration between the hydrogen and the oxygen in 
the gaseous waste disposal system; 
measuring a flow rate of an exhaust gas of the gaseous waste disposal 
system; 
controlling injection quantities of the hydrogen and the oxygen in 
accordance with the results of the concentration balance measurement and 
the flow rate measurement; 
storing a lag time between a time when the oxygen and the hydrogen are 
injected and the time when the oxygen and the hydrogen reach the gaseous 
waste disposal system, respectively; and 
controlling injection timings from the hydrogen injection and the oxygen 
injection in response to the lag time. 
In both the aspects, in preferred embodiments, a quantity of the hydrogen 
to be injected in a steady state is controlled such that a volume ratio of 
hydrogen is less than twice a quantity of the oxygen to be injected. A 
volume of the hydrogen to be injected is controlled such that the volume 
of the hydrogen is less than twice the sum of a quantity of the oxygen 
contained in a leaked air of the gaseous waste disposal system and a 
quantity of the oxygen to be injected. An injection of the oxygen into the 
gaseous waste disposal system is controlled in a quantity to be less than 
that of an allowance flow rate, an injection of the hydrogen is controlled 
in a quantity to be twice the quantity of the injected oxygen, then a lag 
time that elapses between a moment of injecting the hydrogen and a time 
when a flow rate of an exhaust gas of the gaseous waste disposal system is 
lowered to a quantity before injecting the oxygen is stored, the quantity 
of the injected hydrogen is increased to a rated flow rate, and the 
quantity of the oxygen to be injected is controlled in a quantity to be a 
half of the hydrogen injected after the lag time into the gaseous waste 
disposal system. 
According to the structures and the characters of the present invention 
described above, by controlling the injection quantities from the hydrogen 
injection means and the oxygen injection means based on the measurement 
results of the densitometer and the flowmeter, a small quantity of the 
oxygen is firstly injected into the gaseous waste disposal system so as to 
prevent the hydrogen from exceeding 4% in volume ratio, and thereafter, 
the hydrogen twice in quantity of oxygen is injected, whereby all the 
quantities of the hydrogen and the oxygen can be recombined to become 
water in the gaseous waste disposal system. 
Furthermore, by measuring a period of time that elapses between the time 
when the injection of the hydrogen is started and the time when the flow 
rate of the gaseous waste disposal system is returned to that before 
injecting the oxygen or before injecting the hydrogen, there is obtained a 
lag time that elapses between the time when the injection of the hydrogen 
is started and the time when hydrogen reaches the gaseous waste disposal 
system. On the basis of this lag time, the quantities of the hydrogen and 
the oxygen to be injected are increased or decreased so that the flow rate 
in the gaseous waste disposal system does not exceed the allowable value 
and that the hydrogen does not exceed 4%, in volume ratio thereby 
continuously injecting necessary quantity of the hydrogen in a short 
period of time. 
The nature and further features of the present invention will be made more 
clear through the following descriptions with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention will be described hereunder 
with reference to the drawings. 
FIGS. 1 and 2 show an embodiment of a gas injection system of a nuclear 
power plant according to the present invention. Referring to FIG. 1, a 
hydrogen- oxygen generator 1 comprises an electrolysis unit of water, and 
hydrogen and oxygen are separated therefrom and stored in a hydrogen tank 
2 as a hydrogen injection means and in a oxygen tank 3 as a oxygen 
injection means, respectively. 
A hydrogen injection valve 4 is connected to the hydrogen tank 2 and an 
opening degree of the hydrogen injection valve 4 is controlled in response 
to control signals from a controller 6 as a storage (memory)-control 
means. When the hydrogen injection valve 4 is opened, the hydrogen stored 
in the hydrogen tank 2 is injected into a downstream side of a condensate 
demineralizer 7. The injected hydrogen is supplied to a reactor 8 such as 
BWR to reduce the oxygen concentration in the reactor 8 and, thereafter, 
treated in a gaseous waste disposal system 10 by way of a main condenser 9 
together with main steam. 
On the other hand, in order to treat the hydrogen in the gaseous waste 
disposal system 10, an oxygen injection valve 5 is connected to the oxygen 
tank 3 and an opening degree of the oxygen injection valve 5 is controlled 
in response to control signals form the controller 6. When the oxygen 
injection valve 5 is opened, the oxygen stored in the oxygen tank 3 is 
injected into an upstream side of a preheater 11 in the gaseous waste 
disposal system 10. A recombiner 12 is connected to the preheater 11, in 
which the oxygen is recombined with hydrogen to become water, which is 
then recovered to a condenser 13. 
A hydrogen concentration at an exit of the recombiner 12 is always 
monitored by a hydrogen densitometer 14 provided at the downstream side of 
the condenser 13 so as to monitor a balance of the quantities of the 
hydrogen and the oxygen. A quantity of an exhaust gas at a downstream side 
of the recombiner 12 is monitored by a flowmeter provided at the 
downstream side of the condenser 13, and the injection quantities of the 
hydrogen and the oxygen are controlled by controlling the degrees of 
openings of the hydrogen injection valve 4 and the oxygen injection valve 
5 through the operation of the controller 6 in response to the flow 
signals. 
An operation of the gas injection apparatus of the present embodiment will 
be described. 
Firstly, it is ensured by the flowmeter 15 that a flow rate of the gaseous 
waste disposal system 10 under a steady operating condition has an 
allowance against an allowance flow rate, and an injection of a small 
quantity of oxygen into an inlet of the preheater is then started. This 
injection quantity is set so that the flow rate of the exhaust gas in the 
gaseous waste disposal system 10 after the injection of the oxygen is 
allowable flow rate or less. 
Since the oxygen tank 3 is apart from the gaseous waste disposal system 10, 
a lag time to reach the gaseous waste disposal system is measured by a 
timer of the controller 6. The lag time is a period of time that elapses 
between the time when the injection of the oxygen is started and the time 
when the flow rate of the exhaust gas of the gaseous waste disposal system 
10 begins to increase, and is shown as t1 in FIG. 2. The flow rate of the 
exhaust gas of the gaseous waste disposal system 10 under this condition 
corresponds to the quantity of the injected oxygen in an increased 
quantity. 
Next, the hydrogen of the quantity twice the quantity of the previously 
injected oxygen is injected from the hydrogen tank 2. The injected 
hydrogen reaches the gaseous waste disposal system 10 by way of the 
reactor 8, the main steam pipe and the main condenser 9 with the lag time. 
Then, the hydrogen is recombined with the oxygen in the recombiner 12 with 
a ratio of 2:1, and all the quantity of the injected hydrogen and oxygen 
become water. Thus, the flow rate of the exhaust gas in the gaseous 
disposal system 10 is returned to that before the injection of the 
hydrogen and the oxygen. 
Accordingly, by measuring a period of time that elapses between the moment 
when an injection of hydrogen is started and the time when the flow rate 
of the exhaust gas begins to lower, a lag time that elapses before the 
hydrogen reaches the gaseous waste disposal system 10 will be obtained. 
This time is shown as t2 in FIG. 2. 
Thereafter, the injection rate of the hydrogen is increased to an initially 
required quantity. Then, the oxygen is increased to an initially required 
quantity after (t2-t1) hours. Thus, the hydrogen and oxygen reach the 
gaseous waste disposal system 10 at the same time in the volume ratio of 
2:1. Accordingly, since all the quantities of the injected hydrogen and 
oxygen are recombined to become water, the flow rate of the exhaust gas 
can be maintained in the same amount as that before the injection of the 
hydrogen and the oxygen. 
Furthermore, when the flow rate of the exhaust gas after the injection of 
the hydrogen and the oxygen is increased as Q3 shown in FIG. 2, the 
injection quantity of the hydrogen or oxygen is larger than a prescribed 
value, or the quantities of the hydrogen and the oxygen are in an 
unbalanced condition. 
This phenomenon occurs when the quantity of the injected hydrogen exceeds 
twice the total of the injected oxygen and the quantity of the oxygen in 
the leaked air, or when the quantity of the injected oxygen exceeds a half 
quantity of the injected hydrogen. 
In the former case, an excessive quantity of the hydrogen is injected, 
there is a danger of occurrence of hydrogen burning, providing an 
undesirable phenomenon. In this case, since the hydrogen concentration is 
detected by the hydrogen densitometer 14, an alarm is given when the 
hydrogen concentration exceeds the prescribed concentration. 
When the hydrogen concentration exceeds the prescribed value, the control 
signals are output from the controller 6 to the hydrogen injection valve 4 
in response to the signal of the hydrogen concentration and the signal of 
the excessive flow rate from the flowmeter 15, and the degree of opening 
of the hydrogen injection valve 4 is tightened in order to reduce the 
quantity of the injected hydrogen. Otherwise, the degree of opening of the 
oxygen injection valve 5 is widen in response to the same control signals 
as those described above to increase the quantity of the injected oxygen, 
thereby recovering the flow rate in the gaseous waste disposal system 10 
to the normal value. 
In the latter case, that is, when the quantity of the oxygen is excessive, 
there is no danger. However, in order to recover the flow rate in the 
gaseous waste disposal system 10 to the steady value, the signals are 
output from the controller 6 to the oxygen injection valve 5 in response 
to the signal from the hydrogen densitometer 14 announcing that the 
hydrogen concentration does not exceed the prescribed value and the signal 
from the flowmeter 15, and the degree of opening of the oxygen injection 
valve 5 is tightened to reduce the quantity of the injected oxygen. 
When the flow rate of the exhaust gas after the injection of the hydrogen 
and the oxygen is decreased as Q4 shown in FIG. 2, the quantities of the 
hydrogen and the oxygen are in an unbalanced condition and the quantity of 
the hydrogen is larger than that of the oxygen. That is, this phenomenon 
occurs when the quantity of the injected hydrogen exceeds twice the 
injected oxygen to such a degree that the quantity of the injected 
hydrogen does not exceed twice the total of the quantity of the injected 
oxygen and the quantity of the oxygen in the leaked air. 
In this case, since there are large quantity of the hydrogen, the oxygen in 
the leaked air is recombined with hydrogen, thereby reducing the flow rate 
in the gaseous waste disposal system 10. In order to eliminate such 
phenomenon, the quantity of the injected hydrogen may be reduced or the 
quantity of the injected oxygen may be increased. 
In case of reducing the quantity of the injected hydrogen, the control 
signals are output from the controller 6 to the hydrogen injection valve 4 
in response to the signal from the hydrogen densitometer 14 announcing 
that the hydrogen concentration does not exceed the prescribed value and 
the signal from the flowmeter 15 announcing that the flow rate is lower 
than the prescribed value, and the degree of opening of the hydrogen 
injection valve 4 is tightened to reduce the quantity of the injected 
hydrogen. On the other hand, in case of increasing the quantity of the 
injected oxygen, the degree of opening of the oxygen injection valve 5 may 
be tightened in response to the similar control signals to increase the 
quantity of the injected oxygen. 
According to the present embodiment, through the controlling of the 
injection quantities by the controller 6 based on the measurement results 
of the hydrogen densitometer 14 and the flowmeter 15, the oxygen of small 
quantity is firstly injected into the gaseous waste disposal system 10 so 
as to prevent the hydrogen from exceeding 4% in volume ratio, and 
thereafter, the hydrogen of a quantity twice oxygen is injected from the 
hydrogen tank 2, whereby all the quantities of the hydrogen and the oxygen 
may be recombined to become water in the gaseous waste disposal system 10 
Furthermore, by measuring a period of time that elapses between the time 
when injection of hydrogen is started and the time when the flow rate in 
the gaseous waste disposal system 10 is returned to that before injecting 
the oxygen or before injecting the hydrogen, there is obtained a lag time 
that elapses between the time when injection of hydrogen is started and 
the time when hydrogen reaches the gaseous waste disposal system 10. 0n 
the basis of this lag time, the quantities of the hydrogen and the oxygen 
to be injected may be increased or decreased so that the flow rate in the 
gaseous waste disposal system 10 does not exceed the allowable value and 
that hydrogen does not exceed 4% in quantity, thereby continuously 
injecting required quantity of the hydrogen in a short period of time. 
The gas injection method according to the present invention described above 
will be understood more easily with reference to FIG. 2, in which the axis 
of abscissa represents an elapsing time and the axis of ordinate 
represents the flow rate Q (Q1 to Q4). 
Referring to the time chart of FIG. 2, a point A is a starting time of 
injection of a small quantity of oxygen, a point B is a time when the 
injected oxygen reaches the gaseous waste disposal system, a point C is a 
starting time of injection of a small quantity of hydrogen, a point D is a 
time when the injected hydrogen reaches the gaseous waste disposal system, 
a point E is a time at which the injection of a prescribed quantity of 
hydrogen starts, and a point F is a time at which the injection of the 
prescribed quantity of the oxygen starts. 
As described above, according to a gas injection system of a nuclear power 
plant and the gas injection method therefor of the present invention, the 
injection quantities from the hydrogen injection means and the oxygen 
injection means are controlled on the basis of the measurement results of 
the densitometer and the flowmeter, the lag time that elapses between the 
time when the oxygen or hydrogen is injected and the time when the oxygen 
or hydrogen reaches the gaseous waste disposal system, and the injection 
timings from the hydrogen injection means and the oxygen injection means 
are controlled on the basis of the lag times, whereby the balance of the 
quantities of the hydrogen and the oxygen to be injected can be always and 
easily controlled to the ratio of 2:1. 
Furthermore, since the quantity of the injected hydrogen is prevented from 
being excessive, a danger of occurrence of hydrogen burning may be 
avoided. Further, the quantity of the injected hydrogen and the quantity 
of the injected oxygen can be controlled so as not to exceed the allowable 
flow rate of the gaseous waste disposal system, thereby enhancing the safe 
operation of the gas injection system of the nuclear power plant.