JPH0317116B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the subcritical measurement of uranium-containing materials during nuclear fuel production, and more particularly to the automatic measurement of subcritical or moderator concentrations in uranium dioxide powder by a subcritical measuring device. The apparatus includes cadmium thermal neutron shielding means to facilitate subcritical measurements and ensure their accuracy. Measuring moderator concentration is important. This is because the moderator acts to slow down or thermalize the fast neutrons produced by nuclear fission. Furthermore, fast neutrons are what make it possible to establish nuclear chain reactions. Uranium is typically stored in the form of uranium dioxide powder in five-gallon steel containers during the nuclear fuel manufacturing process. To ensure that the uranium material at any point cannot become critical, i.e., cannot sustain a nuclear chain reaction, the containers are stored separately and spaced apart at considerable administrative and economic cost. It will be done. One difficulty in ensuring the criticality safety of a large number of stored uranium containers is that the subcriticality of any given container cannot be known with certainty. If the subcriticality of each vessel can be conveniently and accurately determined, accurate calculations of the subcriticality of the entire vessel system of individual vessels can easily be made. Unfortunately, such a convenient and accurate measurement technique did not exist prior to the present invention. This hindered the development of efficient storage and accumulation technology for uranium. Individual uranium containers of the type utilized for nuclear fuel production are substantially subcritical under all moderator conditions. Therefore, it is useful to speak of subcritical rather than critical. For purposes of this disclosure, there is no need to state criticality. This is because the purpose of criticality safety planning is to ensure that all of the uranium that is to be used as reactor fuel does not develop any degree of criticality. Numerous variables are used in subcritical calculations, such as total volume, structure, uranium density, enrichment, moderator concentration, etc. Moderator concentration is of primary importance with respect to the present invention and includes the water present within the uranium. Uranium density, enrichment, and vessel volume or construction are generally relatively constant parameters. However, the most important critical parameter, ie the parameter that can undergo the greatest change, is the moderator concentration. The concentration of water in uranium material stored for nuclear fuel production can vary from zero to more than 100,000 ppm, regardless of its source. Regarding storage of multiple uranium containers,
It is understood that moderator concentrations greater than 20,000 ppm are not permitted for a given container and storage array configuration, such as an array containing uranium dioxide contained in multiple 5-gallon cans. It will be. Of course, a single vessel, independently provided and isolated from other similar vessels, is completely subcritical even when filled with water. For example, the single 5-gallon canister in which uranium is often stored is small enough to be subcritical even when submerged in water. It is impractical to perform destructive chemical analysis of samples to determine the subcriticality of each uranium container in a plant or storage area. This is because each container must then be opened. Also,
A certain amount of uranium is lost in the process. moreover,
Results of such chemical tests may not be available for hours or even days. Humidity detectors for subcritical measurements do not work effectively in the high moisture or moderator environments involved. Furthermore, while conventional humidity sensors are typically effective in measuring ambient humidity, they are not effective in measuring liquid water concentration. Additionally, many commercially available humidity sensors are unreasonably expensive. It is therefore an object of the present invention to provide a rapid, economical and convenient measurement technique and apparatus for determining the subcriticality of discrete quantities of uranium material. Another object of the present invention is to provide a relatively economical storage management system for storing uranium dioxide using the production of nuclear fuel rods for nuclear reactors. Other objects of the present invention are to facilitate cost-effective separation of highly subcritical and moderately subcritical uranium dioxide powders;
selectively permitting appropriate treatment of subcritical uranium; Another object of the present invention is to provide a non-destructive and non-chemical measurement of uranium moderator concentration without compromising the positive sealing of a sealed uranium dioxide container. The goal is to improve safety in laboratories and nuclear fuel processing facilities. Another object of the present invention is to provide a measuring device that can accurately measure the subcriticality of uranium dioxide in a sealed container by excluding or filtering out background thermal neutrons. According to the invention, by excluding the presence of background thermal neutrons, and as a result of the injection of epithermal neutrons into a uranium material consisting of a certain amount of uranium dioxide, the epithermal neutrons and thermal neutrons leaving the uranium material are The measurement of the ratio provides a device for accurately determining the subcriticality of uranium material. Thermal neutrons are blocked by shielding means surrounding the uranium. This shielding means absorbs thermal neutrons. A pulsed or continuous neutron source is utilized to emit fast neutrons, and these fast neutrons are slowed down by a support structure for the neutron source, thus providing a source of epithermal neutrons for injection into the uranium material. is ensured. A thermal/epithermal neutron detector and a separate epithermal neutron detector, which is provided separately and is shielded from thermal neutrons by a cadmium shield, are used to detect neutrons emitted from uranium material and, as a result, It is arranged to provide a combined thermal and epithermal neutron count and an epithermal neutron count. These counts are stored in a counter and used to determine the subcriticality of the uranium material according to experimentally derived relationships. If this measurement indicates that the subcriticality of the uranium material exceeds a predetermined level, the operator may isolate that particular uranium container from other containers;
This eliminates the risk of critical chain reactions. The preferred embodiment shown in FIG. 1 includes a laminate of polyethylene solids 10. The preferred embodiment shown in FIG. The solid bodies 10 support a fast neutron source 11 in recesses 12 provided therein.
A neutron shield 13 is provided on the polyethylene solid material 10.
is mounted, which includes side parts 14 and bottom part 15, and is preferably made of cadmium. Top 16 is shown as an open top. Inside the shielding body 13, there is a uranium substance to be tested for subcriticality,
For example, there is a container 17 of uranium dioxide powder. At least one epithermal neutron detector 18 and at least one combined thermal and epithermal neutron detector 19 are suitably installed within the cadmium shield 13 in the vicinity of the uranium dioxide container 17. For simplicity, the support structures of both neutron detectors are not shown. Neutron detectors 18, 19 are of the same type, but epithermal detector 18 is shielded with a suitable material such as cadmium to block thermal neutrons. This device converts fast neutrons from the neutron source 11 into epithermal neutrons, thereby causing the epithermal neutrons to enter the container 17. A suitable coaxial wire 20 is shown in FIG. 1, connecting each of the detectors 18 and 19 to a preamplifier 30 and
2, discriminator 33, counter 34, processor 35
and an indicator 36. Power supply 31 energizes the circuitry and detectors 18, 19 described above. Each detector is essentially an anode and a cathode that establish a high potential difference within a pressurized helium environment. When the incident neutrons ionize the helium atoms, a current avalanche or pulse is generated and the measurement detector 18
or 19 and the associated preamplifier. The operation of the device will be described below, but it may first be stated that the current pulses produced by the detector are amplified by a preamplifier 30 and an amplifier 32 and selectively passed by a corresponding discriminator 33. This means that it is counted by an adder or counter 34. The ratio of the counts of thermal neutrons and epithermal neutrons is the basis for subcritical calculations in the processor 35. The relational expression for subcritical calculation is derived as described below. The result of the subcriticality calculation by the processor 35 is shown on the indicator 36. This may cause an audible alarm to sound when the subcriticality is at a predetermined threshold level, for example when the moderator concentration is 20000 ppm. The best subcritical relational expression is established by the processor 35 shown in FIG. This relationship can be solved analogously, as shown in FIG. 2, or programmed in a suitable microprocessor. Various types of microprocessors are currently commercially available. The relational expression was derived experimentally and is expressed as follows. SI=(t/e)-b/p+n[%U-235](1) However, SI represents the subcritical value, t represents the thermal neutron count value, e represents the epithermal neutron count value, b is the ratio of thermal neutrons to epithermal neutrons when the moderator concentration is zero, and n is the rate of change of the ratio of thermal neutrons to epithermal neutrons with respect to the moderator with respect to the enrichment (mathematically, ∂ ² (t/e)/∂[%U-235]∂[%moderator]), where [%U-235] is the enrichment expressed as a percentage, and p is when [%U-235] is zero. is the rate of change of t/e with respect to moderator concentration in case of The constants b, n, and p in the above relational expression are a set of numerical values representing the specific shape to be used. Neutron counts do not include fission neutrons. Note that fission neutrons are in a relatively high energy range. As shown in FIG. 2, an analog circuit, which is equivalent to a microprocessor program, accepts the count at counter 34 through appropriate electrical leads. The constants mentioned above are entered by applying a constant voltage according to the table below. These values represent the specific shape of a single 5 gallon can.

TABLE The various components are connected by appropriate electrical leads 50. These components include a first divider 60, a first adder 61, a second divider 62, a comparator 63, a multiplier 64, and a second adder 65. Comparator 63 may, for example, be digitally included in software in a microprocessor, or it may be analogous, consisting of suitable commercially available discrete components. As you can see in Figure 2,
The divider 60 calculates the equation [(t+e)/
e], the adder 61 completes the numerator of the equation,
Multiplier 64 calculates the value of the product -0.062 [%U-235], adder 65 completes the denominator, and divider 6
2 completes the numerator and denominator, and the divider 62 calculates the value of the entire fraction consisting of the numerator and denominator. Comparator 63 then compares the output of the subcritical relation with a predetermined alarm threshold. This threshold is 2 in this case
% by weight, equivalent to a moderator concentration of 20000 ppm. If the calculated subcriticality index SI exceeds the threshold, the indicator 36 is
A trigger signal that reaches the alarm will sound the alarm. When this occurs, the operator manually removes the undesirable (too highly decelerated) container 17.
At this time, the container 17 is isolated from the remaining container of uranium dioxide with a lower moderator concentration, thus allowing differential and therefore more efficient safe storage of the uranium container. The neutron source 11 of the preferred embodiment is, for example, a 1 Kyrie americium beryllium source in which the americium emits alpha particles by spontaneous decay. In the case of americium-241, approximately 10 ⁶ neutrons are emitted per second as a result of alpha particles colliding with beryllium atoms in the source. Other suitable source materials are californium-252 and plutonium beryllium. Alternatively, a pulsed neutron source, such as the Kaman Sciences of Colorado Springs
An A620 neutron generator manufactured by Colorado Springs may be used. The spectrum of neutron energy from the neutron source 11 is high speed (0.5 MeV or more). In the case of americium beryllium, neutron source 11
is made, for example, as a thin disc or foil approximately 1/4 inch in diameter, suitably placed within a lead cask to protect personnel from undue radiation. The polyethylene solid 10 described above laterally surrounds and supports the lead cask and defines a recess 12. This recess is open at the top. Therefore, fast neutrons are emitted upward from the neutron source and directly enter the uranium dioxide container 17. Other fast neutrons proceed to the polyethylene solid 10 and are slowed down to thermal or extrathermal levels. A part of these neutrons escapes from the system, and the other part returns upward and heads towards the container 17. Thermal neutrons traveling upward are absorbed by the shield 13, and epithermal neutrons traveling with the thermal neutrons enter the uranium material. The shield 13 may be cylindrical, but other shapes and structures may be conveniently substituted. The door may be suitably attached to the side of the shield by hinges or the like.
The reason for providing a door in this manner is that uranium dioxide containers are heavy and installation of the container through the top may be inconvenient. For simplicity, this door is not shown. The shield 13 prevents external (including the neutron source and background) thermal neutrons from reaching the uranium container 17 and the detectors 18,19. The shielding body 13 is a reactor shielding corporation (Reactor Shielding Corporation).
It is preferably made of 30 to 40 mil thin cadmium plate, such as those commercially available from Biochemistry Corporation.
It is also preferred to cover the cadmium plate with a plastic or stainless steel coating to prevent contamination of the uranium during testing. The uranium dioxide powder 17 is stored in a standard commercial 5 gallon steel barrel. Other dimensions of containers may be utilized, for example silos as large as having a volume of up to 100 m ³ . 1/V type detectors 18, 19, for example helium-3 detectors, are used in this embodiment.
Such a detector includes a sealed gas-filled cylinder (not shown) and a suitably insulated center conductor that constitutes an electrode that applies a high voltage to the gas. Each incident neutron ionizes a portion of the gas, and the resulting collection of electrons on the electrodes generates a current pulse that is transmitted by the coaxial wire 20 to the preamplifier 30.
can be conveyed to. In this example, the gas may be helium at 10 atmospheres. proportional counter or detector,
For example, Reuter-Stokes
A helium-3 detector type RS-P4-410-30 can be used. Each epithermal detector 18 is shielded at both ends and sides with a suitable cadmium shield that absorbs thermal neutrons and prevents them from reacting within the detector 18. Detectors 18, 19 are suitably placed within a shielding structure 13 surrounding the uranium dioxide 17 under test, thus preventing exposure to external thermal neutrons. Epithermal detector 18 and composite detector 1
9, the various detectors are appropriately electrically connected to the corresponding preamplifiers 30. On average, the epithermal detector 18 and the combined detector 19
are placed in position near the uranium 17 to be measured and given an equal opportunity to receive neutrons passing through the uranium dioxide 17 of the sample. However, if equal opportunity is not provided, appropriate compensation will be made. In the present invention, various detectors 1
8 and 19 can be used in desired numbers. The power supply 31 energizing the detectors 18, 19 and the electrical circuitry for calculating the subcritical relationship may be of the Canberra 3002 type, for example. This can provide potentials of up to 3000V to the detectors 18,19. Various detectors 18 and 19, one each to the preamplifier 30
is connected to convert the input current pulses received from the detectors 18, 19 into low voltage pulses that are transmitted to the corresponding adjustable gain amplifiers 32. Preamplifier 30 may be, for example, a Canberra model 1706, and amplifier 32 may be, for example, a Canberra model 2011. Each discriminator 3 receiving the output of a corresponding amplifier 32
3 can be set to an appropriate level higher than the input noise. A suitable discriminator 33 is Canbella.
It is the 2032 type. The above-mentioned subcriticality relation (1) was derived experimentally by testing uranium samples of known moderator concentrations and enrichments in the apparatus of FIG. The thermal and epithermal counters calculated a composite "t+e" count, and the epithermal counter calculated an "e" count. The ratio of thermal to epithermal neutrons was calculated by dividing the former by the latter and subtracting 1 from it. This relationship can be expressed as follows. t+e/e=t/e+e/e=t/e+1 (2) Subtracting 1 from (t+e)/e, t+e/e-1=t/e+e/e-1=t/e (3) Therefore, The ratio of counts t/e can be compared to the known moderator concentration in FIG. In other words, the relationship between thermal neutrons and epithermal neutrons shown in Figure 3 is 0.71%U.
It was determined for various moderator concentrations at different enrichments including -235 and 3.80% U-235. Two assumptions were made to arrive at the information in FIG. That is, the ratio of thermal to epithermal neutrons (t/
e) is a linear function of the moderator concentration [M], and the relationship of the slope [M] of the ratio (t/e) is the concentration [%
U-235]. Following these assumptions, Figure 3 shows that the ratio (t/e) is a linear function of [M] and that m is [%U-235]
Show that it is a linear function of . That is, (t/e)=m[M]+b (4) m=n[%U-235]+p (5) However, t represents the thermal neutron count value, e represents the epithermal neutron count value, m is the slope of the curve of the ratio of thermal to epithermal neutrons versus moderator concentration, [M] is the moderator concentration in weight percent, and b is the thermal neutron pair when the moderator concentration is zero. is the ratio of epithermal neutrons, n is the slope of the relationship between m and [%U-235], and is the same as n in equation (1), and [%U-235] is the enrichment expressed as a percentage. and p is the rate of change of t/e with respect to the moderator concentration when [%U-235] is zero. In order to remove m as a variable, by substituting equation (5) into equation (4), the following equation is obtained. [M] = (t/e)-b/p+n [%U-235] (6) By definition, SI is the moderator concentration [M]. That is, SI=[M] (7) Therefore, SI=(t/e)-b/p+n[%U-235](8) From Figure 3, two equations in the form of relational expression (8) are obtained. and the following constants can be determined. b=0.53 (9) p=0.303 (10) n=-0.062 (11) Substituting (9), (10), and (11) into (8), SI=(t/e)-0.53/0.303- 0.062 [%U-235](
12) Since what is actually counted is the thermal/exothermal composite count value, equation (12) can be transformed as follows. SI=[(t/e)/e]-1.53/0.303-0.062[%U
-235] (13) This relationship establishes the value 1.53 at input number 43 and provides the basis for circuit 35 in FIG. A single measurement cycle according to the invention takes several minutes, which is much shorter than the hours required for conventional chemical tests. Although specific embodiments of the invention have been described above, it will be understood that many modifications can be made within the scope of the invention.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a subcriticality measuring device including a container for uranium dioxide powder and a shielding means for removing background thermal neutrons, and Fig. 2 is a schematic diagram of a subcriticality calculation circuit. The functional diagram, FIG. 3, is a graph showing the above-mentioned subcritical relationship. 10... Polyethylene solid, 11... Fast neutron source, 13... Neutron shield, 17... Uranium substance container, 18... Epithermal neutron detector, 19...
...Thermal neutron/epithermal neutron combined detector, 30... Preamplifier, 32... Amplifier, 33... Discriminator, 3
4...Counter, 35...Processor.

Claims (1)

[Claims] 1 Moderator concentration and isotopes U-235 and U-238
an apparatus for producing a subcritical value SI as a function of the enrichment of a uranium fuel material comprising The neutron is the U-
energy range above the range of thermal neutrons that cause substantial fission of U-235 and below the range of fast neutrons that cause fission of said U-238, thereby causing fission of said isotope and thus substantially eliminates unwanted neutrons that may result from nuclear fission;
a first neutron detector that produces a first detector signal representative of neutrons passing from the epithermal neutron source substantially undecelerated across the uranium material; neutrons that traverse the uranium material from the epithermal neutron source and are moderated to thermal energy by the moderator in the uranium material and the neutrons from the epithermal neutron source substantially unmoderated. a second neutron detector that produces a second detector signal representative of the sum of neutrons across the uranium material; divider means responsive to a signal for producing an output signal proportional to a ratio of said second detector signal to said first detector signal, wherein said ratio represents a moderator concentration in said uranium material; If the ratio of thermal neutrons to epithermal neutrons when the moderator concentration is zero is b, then the coefficient (1+b) is obtained from the ratio of the detector signals.
means to obtain a first coefficient by subtracting the coefficient n (slope of the relationship between the curve of the moderator concentration and the percentage of U-235 for the ratio of the sum of thermal and epithermal neutrons to the ratio of epithermal neutrons) and U means for producing a second coefficient by multiplying the enrichment of uranium material expressed as a percentage of -235 and the coefficient p (the derivative of the enrichment of the ratio of thermal to epithermal neutrons with respect to the moderator concentration); an apparatus for producing a subcritical value SI, comprising: means for subtracting from a second coefficient to produce a third coefficient; and means for dividing said first coefficient by a third coefficient to produce said subcritical value SI. . 2 comparing the sub-critical value SI with a predetermined threshold;
Apparatus according to claim 1, including means for generating an alarm when this threshold is exceeded. 3. The apparatus of claim 1, wherein the epithermal neutron source is a continuous epithermal neutron source. 4. The apparatus of claim 1, wherein the epithermal neutron source generates pulses of epithermal neutrons.