JPS6138830B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method that can be used, for example, for power distribution monitoring in nuclear reactor core management (fuel assembly burnup measurement, output flattening work), and for reactor safety protection signal output monitoring. Conventionally, as detectors for monitoring the output or power distribution of a power reactor, there are a small nuclear fission ionization chamber and a self-powering type detector. The former is a fissile material ( ²³⁵ U,
²³⁹ Pu, ²⁴¹ Pu), a relatively large current can be obtained for measurement, but the applied fissile material absorbs thermal neutron flux and wears out quickly, so the thermal neutron sensitivity must be corrected frequently. Also, the period of use in the furnace is short (6 months to 1 year)
In 2010), the absorption of the fission ionization chamber itself is large, which distorts the thermal neutron flux distribution at the measurement location.
There are drawbacks such as the need for correction and the need for a high-voltage power source, which complicates the equipment. The latter method uses a material that induces short-lived beta radioactivity when irradiated with neutrons as an emitter, and measures the current generated by this emitting beta rays to detect neutrons. is not necessary, the measurement system is simple, and
It has advantages such as low production cost and robustness of the detector, small absorption of thermal neutron flux that does not disturb the distribution in the reactor, and long life in the reactor, but the response is slow ( ⁵¹ V: 5.4 minutes). , Rh: 1 minute, Co: 14 minutes) and is sensitive to both thermal neutron flux and gamma ray flux. Therefore, in a nuclear reactor where thermal neutron flux and gamma rays coexist, measurement values may vary, and The major drawback is that the gamma ray sensitivity is of opposite polarity, which causes a great deal of interference.
For these reasons, conventional self-powered detectors for measuring thermal neutron flux have very slow responses and cannot be used as control signals or signals to reactor protection systems, and cannot be used accurately due to gamma ray interference. It is difficult to measure thermal neutron flux, making it unsuitable for power distribution monitoring for combustion management of modern fuel assemblies. There are also methods that use a small fission ionization chamber and a self-powered detector in combination, but this not only complicates the measurement system but also does not eliminate the drawbacks of the small fission ionization chamber. The purpose of the present invention is to solve the above-mentioned drawbacks of the conventional technology, and to provide a core monitoring system that has low absorption of thermal neutrons and can measure thermal neutron flux with high responsiveness and accuracy without disturbing the reactor core. The purpose is to provide a method. In order to achieve such an object, the present invention actively combines a detection section that is sensitive to thermal neutron flux and a detection section that is sensitive to gamma rays. By separating each current component from the gamma ray sensitivity, it is configured to accurately grasp the state of the reactor core output. In other words, by using the present invention, it becomes possible to accurately measure the thermal neutron flux distribution within the reactor core, accurately manage the burnup of fuel assemblies, and monitor reactor output with good response characteristics and high reliability. It is. Hereinafter, the present invention will be explained in detail based on the drawings.
An example of a composite self-output type detector used in the present invention is shown in FIGS. 1 and 2. In this embodiment, the emitter 1 for thermal neutrons and the emitter 2 for gamma rays are
They are buried in an insulator 3 at appropriate intervals, and are surrounded by a collector 4, with a lead wire 5 extending from each emitter 1 and 2, and a lead wire 5 continuous from the collector 4 made of the same material. The lead portions 6 are each pulled out and guided to a connector 7. The emitter 1 for thermal neutrons is activated by irradiation with thermal neutron flux and emits beta rays, for example, ⁵¹ V or ¹⁰³ Rh is used, and the emitter 2 for gamma rays is used to emit beta rays. (Photoelectric effect, Compton scattering, electron pair creation)
For example, platinum (Pt) is used. The material of the collector 4 is, for example, Inconel or stainless steel. The detector used in the method of the present invention is not limited to such a configuration. As shown in FIG. 3, a thermal neutron self-output detection section 9 is spirally provided around the collector of the gamma ray self-output detection section 8 in the form of a rod, and lead wires from each of the detectors 8 and 9 are provided. Alternatively, as shown in FIG. 4, a thermal neutron self-output detection section 9 may be connected to a small guide tube 10 that absorbs thermal neutron flux, such as aluminum or zirconium alloy. and gamma ray self-output detection unit 8
They may be arranged alternately in a spiral pattern, and the lead wires from each of the detection sections 8 and 9 may be connected by a connector 7. Now, when the above-mentioned composite self-power detector is installed in a field where thermal neutrons and gamma rays coexist (a nuclear reactor core), the self-power for thermal neutron flux after a sufficient period of time has elapsed with the output constant. The output current I ₁ (n+γ) from the mold detection section is expressed by the following equation. I ₁ (n+γ)=I _o +I〓 …(1) I _o =S _vo φ _o …(2) I〓=S _v 〓φ〓 …(3) Here, I _o : Thermal neutron of the emitter Current due to flux only (A) I〓: Current due to emitter gamma rays only (A) S _vo : Pre-calibrated thermal neutron flux sensitivity (A/n/v) S _v〓 : Pre-calibrated gamma ray sensitivity (A/n/v)
R/h) φ _o : Thermal neutrons (n/v) at an arbitrary output of the reactor φ 〓 : Gamma dose rate (R/h) at an arbitrary output of the reactor. Next, the output current I ₂ (n+γ) from the gamma ray self-output type detector at the same location is expressed by the following equation. I ₂ (n + γ) = S _po φ _o + S _p 〓φ〓 ...(4) Here, S _po : Pre-calibrated thermal neutron sensitivity (A/n/v) S _p =: Pre-calibrated Gamma ray sensitivity (A/R/h) By the way, conventional self-output thermal neutron detectors are used under the assumption that I _o (=S _vo φ _o )≫I〓 (=S _v 〓φ〓). was. However, in an actual reactor core, the thermal neutron flux is 1×10 ¹⁴
n/v, the gamma ray dose rate generated by nuclear fission is more than 10 ⁸ R/h, so a thermal neutron self-output detector with each detection sensitivity shown in Table 1 is used independently. ^In _{the case of _} ^_ reach Furthermore, in the case of a vanadium emitter, the gamma ray sensitivity has a negative polarity, so if I〓 is ignored, the actual output will be underestimated. Therefore, the current caused by gamma rays in the vanadium emitter cannot be ignored. Furthermore, as the n/γ ratio in core measurements becomes smaller, errors included in thermal neutron flux measurement results become larger. Therefore, when the conventional self-powered thermal neutron detector is used alone, the measurement accuracy of thermal neutron flux deteriorates, and it cannot be used for core management.

[Table] In contrast, the present invention uses a detector that combines a self-output type detection section for thermal neutrons and a self-output type detection section for gamma rays. Using the above-mentioned equations (1) to (4), the current I _o due to the true thermal neutron flux from the thermal neutron self-output detection section can be determined by the following equation. Here, K ₁ : A constant determined from the thermal neutron flux sensitivity and gamma ray sensitivity of each of the two detection sections, K ₂ : A constant determined from the gamma ray sensitivity of the two detection sections. As can be seen from equation (5), the true current I _o due to thermal neutron flux is calculated from the current I ₁ (n+γ) from the self-output type detector for thermal neutron flux, and the current I ₂ from the self-output type detector for gamma rays. It can be obtained by subtracting the current obtained by multiplying (n+γ) by a constant K ₂ and further multiplying by a constant K ₁ . Therefore, if the true current I _o due to thermal neutron flux is determined by such a method, it is possible to accurately grasp the state of the reactor core power. In addition, if a positive reactivity disturbance is applied to the core from a constant output state and the output changes, the output current I ₂ of the instant-response gamma ray self-output detector
From the change in (n+γ), a change in the reactor core output can be detected immediately without any time delay, and a signal can also be sent to the reactor safety protection system. Next, an example of an output monitoring device using the present invention is shown in FIG. The reactor is of the heavy water moderated pressure tube type. Briefly, the structure is such that a fuel assembly 22 is put into a pressure pipe 23, and a number of them are further inserted into a briquette-like calandria tank 24, and the calandria tank 24 is filled with heavy water 25 as a moderator. The coolant light water flows from the bottom to the top in the pressure pipe 23, during which time heat is removed from the fuel assembly 22 and steam is generated. Now, a plurality of (four in FIG. 5) composite self-output type detectors 12a, .
It is inserted between the two. Each detector 12a,...
The output of ..., 12d is sent to the signal separator 13a, ...
..., 13d, the current is separated into a current I ₁ (n+γ) of the thermal neutron self-output type detector and a current I ₂ (n+γ) of the gamma ray self-output type detector. Each of these currents is transmitted through current amplifiers 14a, 14a', . . . , 14d,
It is amplified at 14d'. The current I ₂ (n+γ) is multiplied by a constant K ₂ suitable for the detection section by the gamma ray coefficient multipliers 15a, . . . , 15d, and sent to the local output amplifiers 16a, . The current I _o due to the true thermal neutron flux is calculated by multiplying the difference from I ₁ (n+γ) by a constant K ₁ . Each detector 12
The output of a, ......, 12d is the local output display meter 19
a, . Furthermore, if the average output is higher than a preset average output (determined from the linear output of the fuel assembly), a signal is transmitted to the reactor safety scram signal generator 18. The increase in current I ₂ (n+γ) due to the sudden increase in reactor power is sent directly to the reactor safety protection scram signal generator 18 from the current amplifiers 14a', . can generate a scram signal. This power monitoring device has the functions of accurate local power distribution monitoring of the reactor core and fast-response power monitoring functions, and can perform core management (linear power of fuel assemblies, burnup) with high accuracy. Since the present invention is configured as described above, the contribution of gamma rays that is 16% or more included in the measurement value of thermal neutron flux by the conventional self-output detector for thermal neutrons is reduced to 1% or less. This improves the technology for core performance evaluation, allowing accurate measurement of thermal neutron flux without disturbing the core, and therefore improving core management (fuel assembly linear power, burnup). It also has the effect of being able to respond quickly to changes in the reactor output by proactively using the fast-response gamma ray signal component, which is extremely effective from a safety perspective. Yes, even when turning it into a device, the electronic circuit for the conventional self-output type detector can be used as is, and by actively using the latest IC amplifiers, it can be made compact and inexpensive. It also has many advantages, including a longer lifespan and lower manufacturing costs than small fission chambers.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an example of the detector used in the present invention, Fig. 2 is a sectional view thereof, Figs. 3 and 4 are explanatory diagrams showing other examples of the detector, and Fig. 5 is an explanatory diagram showing an example of the detector used in the present invention. FIG. 2 is an explanatory diagram showing an example of an internal output monitoring device. 1... Emitter for thermal neutrons, 2... Emitter for gamma rays, 3... Insulator, 4... Collector, 8...
...Self-output type detection section for gamma rays, 9...Self-output type detection section for thermal neutrons.

Claims (1)

[Scope of Claims] 1. A composite self-power detector having a thermal neutron self-power detector and a gamma ray self-power detector is installed in a reactor core where thermal neutrons and gamma rays coexist, and the composite self-power detector is installed in a reactor core where thermal neutrons and gamma rays coexist. From the current I ₁ (n+γ) from the self-output type detector for thermal neutrons and the current I ₂ (n+γ) from the self-output type detector for gamma rays of the type detector, the following formula, I _o = K ₁ {I ₁ (n+γ)−K ₂ I ₂ (n+γ)} However, K ₁ : A constant determined from the thermal neutron flux sensitivity and gamma ray sensitivity of each of the two detection sections, K ₂ : A constant determined from the gamma ray sensitivity of the two detection sections, Therefore, find the true current I _o due to the thermal neutron flux,
A reactor core power monitoring method characterized in that the reactor core power state is grasped thereby.