# Method for adjusting digital reactimeters to current state of reactor by composition of fuel fissionable elements

**FIELD: monitoring safety characteristics and variables of nuclear power units at nuclear power stations.**

**SUBSTANCE: proposed method that can be used, for instance, to adjust reactimeters of nuclear power stations with type RBMK-1000 and incorporates provision for reducing systematic measurement error by two orders of magnitude includes introduction of sets of variables characterizing correlation between fissionable elements of fuel and desired burn-up step in charge-coupled device module of reactimeter; determination of current value of mean fuel burn-up using data of centralized checkup system of reactor unit; and selection of respective mentioned variables by varying position of switch on rear panel of reactimeter. Use is made of sets of variables α _{i} (fractions of i group delayed neutrons in delayed neutron generation responsible for total composition of fissionable isotopes of desired burn-up fuel) and sets of variables λ^{eff} (effective constants of i group delayed neutrons). Reactimeter adjustment scale is calibrated in terms of mean fuel burn-up in reactor core.**

**EFFECT: enhanced measurement accuracy, and nuclear safety, facilitated adjustment to current state of reactor by composition of fissionable elements of fuel.**

**2 cl 3 tbl**

The invention relates to the control characteristics and parameters of nuclear safety of reactor units (RU) of nuclear power plants (NPP) and, in particular, can be used to configure reectometer applicable to NPPs with RBMK reactors (reactor high power channel), on the current state of the reactor on the composition of the fissile fuel elements.

As the combustion fuel, and transition to operation of the fuel assemblies (FA) a new type of changes the ratio of fissile materials in the fuel. This in turn causes a change in the ratio of delayed neutrons (Z.N.), generated by various fissile isotopes in the fuel. Since for each fissile isotope (U^{235}U^{238}Pu^{239}Pu^{241}) the distribution generated by Z.N. by the parameter generation λ_{ik}(i - group number Z.N., k is the number of isotope) has its own characteristics, and these distributions affect the behavior of the reactivity of the reactor, when determining the reactivity required setting digital reectometry aimed at the actual ratio of fissile elements in the reactor fuel. This setup allows you to minimize systematic measurement error due to the neglect of the detailed correlation of various groups Z.N. generated by the fuel loading is scoi reactor.

Known, taken as a prototype, the way to configure reectometer used in digital reectometry CTA-9 ("Technical description and operating instructions. A", inv. No. 13-177 389, 1996, SSC RF IPPE). Model reectometry CTA-9 from the full set of isotopes (U^{235}U^{238}, R^{239}, R^{241})characterizing reactor fuel, account for only 2 of the isotope - U^{235}and Ri^{239}. Consideration of a larger number of fissile isotopes is limited hardware capabilities reectometry. Scale settings reectometry CTA-9 calibrated in units of the magnitude of the contribution of Ri^{239}in the generation of..

How to configure reectometry the prototype is as follows:

- in the module persistent storage device (ROM) to introduce discrete sets of source data. A separate set is a set of 12 values α_{ik}(i=1-6 - group number Z.N., k=1-2 - room isotope), which characterizes the ratio of different groups Z.N. in the fuel loading of the reactor is described by the composition of fissile fuel elements - U^{235}+Ri^{239}(parameter values : α_{ik}- share Z.N. i-th group generated by dividing the k-th isotope, the total number Z.N., generated by all fissile isotopes). Sets generated depending on the magnitude of the contribution of Ri^{239}in the generation of n;

directly before performing physical measurements to PN of the data system of centralized control (cbfv) determine the average fuel burn-up;

- for a given value of average fuel burn estimate of the magnitude of the contribution of Ri^{239}in the generation Z.N. γ_{k}=β_{k}q_{k}/β where β_{k}constants Z.N. (tabular data), the corresponding share Z.N. in the total number of neutrons produced in the fission of the k-th isotope;, q_{k}- the proportion of neutrons produced in the fission of the k-th isotope in the total number of neutrons generated by all fissile isotopes (the values of q_{k}listed in table 1 of "a Comprehensive methodology to determine physical and dynamic characteristics of the RBMK reactors", RD EA 0137-98);

- set the switch accounting Ri^{239}(with discrete scale in units of the parameter γ_{k}in the position closest to the real value of this parameter, thus choose a set of values it α_{ik}corresponding to the current composition of the fissile isotopes of fuel in the reactor core.

This method of setting reectometry on the current state of the reactor core composition of fissile fuel elements has a number of disadvantages. The disadvantages of the method setup reectometry-the prototype of which are:

1. Low accuracy of the control parameters of the nuclear safety RU (due to a significant systematic error of measurement reactivity)that does not guarantee limits and conditions of safe operation ROUX.

2. The effect of increasing the systematic error of measurement reactivity with increasing burn-up fuel.

3. Nonconservative nature of the measurement result (revaluation) critical from the point of view of nuclear safety parameters of the reactor (for example, subcriticality).

4. The complexity of the instrument settings required pre-procedure assessment of the contribution of Ri^{239}in the generation Z.N. for the current state of the reactor core.

5. The scale (γ (Pu^{239})=0, 10, 20, 30 and 40%) of the selected fail - most of it corresponds unreal today, the average burnup of more than 15 MW· d/THD.

The invention solves the problem of increasing the accuracy of the control parameters of the nuclear safety ROUX with RBMK-1000 reactors by reducing (two orders of magnitude compared with the reference values of the systematic errors of measurements of reactivity and simplify the procedures for configuring reectometry on the current state of the reactor on the composition of the fissile fuel elements.

To obtain this result comprises the reception method of configuring a digital reectometry on the current state of the reactor on the composition of the fissile fuel elements in the module ROM reectometry introduce the sets of parameter values, characterizing the ratio of fissile materials in the fuel with a specified step in burnout; determine the current value of the average burnup of data from the CCS RU and choose the appropriate sets of values of these parameters by changing the switch position on the back of reectometry.

Distinctive features of the proposed method lies in the fact that the use of sets of parameter values α_{i}- share Z.N. i-th group in the generation Z.N., meet the full composition of fissile isotopes fuel specified burnout, and sets parameter values λ

eff |

i |

In addition, the feature is that the scale settings reectometry calibrated in units of the average burnup of the fuel in the reactor core.

In the proposed method uses different compared to the prototype approach for implementing the configuration reectometry on the current state of the reactor on the composition of the fissile fuel elements with limited hardware capabilities of reectometry. In the prototype the approximate description of the full composition of fissile elements is implemented by taking into account only part of the fissile isotopes of fuel - most will present the selected (largest contribution to the generation of neutrons) or the most typical from the point of view of the consideration of the peculiarities of distribution Z.N.
parameter generation λ_{ik}. In the proposed method, the same problem is solved by the introduction of effective parameters of the problem, corresponding to the complete description of the composition of fissile elements of one equivalent element, with which compliance is range Z.N., approximately describing the real range Z.N. while maintaining split Z.N. groups:

The ratio between the contributions of the different fissile isotopes in the neutron emission is determined primarily by the burning of fuel. This ratio affects the behavior of reactivity in transition processes, which is reflected in the behaviour of the neutron signals of the sensors. Is determined by the proportion specified by the values of the above parameter q_{k}uniquely associated with the parameter γ_{k}characterizing the contribution of individual isotopes in the generation Z.N. Because the settings reectometry - α_{ik}associated with γ_{k}ratio α_{ik}=and_{ik}γ_{}k, where a_{ik}constants Z.N. (tabular data), the corresponding share Z.N. i-th group in the total number Z.N. generated by dividing the k-th isotope, and α_{ik}and consequentlyanddepend only on fuel burn. Type of fuel (enrichment, the presence of vigorous the first absorber) amends the second order of smallness,
that is particularly true in respect of fuel with an enrichment of 2.4% and erbium FA (ATVs) with an enrichment of 2.6 and 2.8% (see table 1 below).

The observed fact is of fundamental importance for the technical implementation of settings reectometer on the current state of the active zone on the composition of the fissile fuel elements - this setting is to configure the current value of the average fuel burn-up.

Table 1 The magnitude of the proportional contribution of individual isotopes in the generation of fission neutrons (q _{k}, %) for the cell of the RBMK reactor at different Energoservice fuel with an enrichment of 2% and 2.4%, ATVs enrichment of 2.6%and 2.8%. | ||||||||||

The fuel type | k | Isotopes | Energoservice fuel, MW-day/kHz | |||||||

0 | 5 | 10 | 15 | 20 | 25 | 30 | ||||

FA, 2.0% | 1 | U-235 | 95.48 | 70.71 | 55.94 | 43.79 | 33.98 | 20.04 | 4.25 | |

2 | Pu-239 | 0 | At 23.85 | 35.66 | 43.60 | 49.72 | 57.03 | At 65.88 | ||

3 | U-238 | 4.52 | 4.65 | 4.99 | 5.49 | 6.08 | 6.83 | 7.81 | ||

4 | Pu-241 | 0 | 0.79 | 3.43 | 7.18 | 11.32 | 16.10 | 22.06 | ||

FA, 2.4% | 1 | U-235 | 95.92 | 75.49 | 62.51 | 51.73 | 41.59 | 29.59 | 15.19 | |

2 | Pu-239 | 0 | 19.77 | 30.53 | 38.01 | 44.11 | 51.26 | 59.77 | ||

3 | U-238 | 4.08 | 4.21 | 4.50 | 4.90 | 5.44 | 6.14 | 7.09 | ||

4 | Pu-241 | 0 | 0.53 | 2.46 | 5.36 | 8.87 | 13.01 | 17.95 | ||

ATVs, | 1 | U-235 | 95.76 | At 75.44 | 62.51 | 51.75 | 41.62 | 29.62 | 15.22 | |

2.6% | 2 | Pu-239 | 0 | 30.53 | 38.03 | 44.16 | 51.34 | 59.87 | ||

3 | U-238 | 4.24 | 4.28 | 4.50 | 4.86 | 5.35 | 6.02 | 6.94 | ||

4 | Pu-241 | 0 | 0.53 | 2.46 | 5.36 | 8.87 | 13.02 | 17.97 | ||

ATVs, | 1 | U-235 | 95.68 | 75.40 | 62.48 | At 51.74 | 41.62 | 29.63 | 15.24 | |

2.8% | 2 | Pu-239 | 0 | 19.72 | At 30.51 | 38.03 | 44.18 | 51.38 | 59.95 | |

3 | U-238 | 4.32 | 4.35 | 4.55 | 4.87 | 5.33 | 5.96 | 6.82 | ||

4 | Pu-241 | 0 | 0.53 | 2.46 | 5.36 | 8.87 | 13.03 | 17.99 |

In the practical application of the proposed method increases the safety of operation of RU due to the increased accuracy of control and ensure the conservativeness of the OC the NOC parameters and characteristics of the reactor, significantly simplifies the process of setting reectometry on the current state of the reactor of the fuel burn.

So, for example (see table 2):

1. Reduced by two orders of magnitude compared to the prototype of the systematic error of the measurements (compare columns 5 and 7, table 2).

2. Achieved conservative estimate (see column 6 of table 2) parameters of nuclear safety (measurement results)that guarantees the limits and conditions of safe operation ROUX.

The data of table 2 are the result of experimental-computational simulation of measurements, including modeling of the neutron signal sensors by introducing into the reactor a "jump" reactivity 5.0β_{eff}. As the "reference" model for modeling measurements of reactivity was considered a model taking into account the full set of fissile fuel elements (U^{235}U^{238}, R^{239}, R^{241}). The difference between the simulation results of measurements when using other models from the reference model (δ ) is interpreted as an estimate of systematic measurement errors.

Table 2 The dependence of the results of measurements of reactivity (β _{eff}from the average energy-producing reactor fuel for RBMK prototype and the proposed method | ||||||

Reference "reference" model, the full composition of fissile isotopes (U^{5}+U^{8}+Pu^{9}+Pu^{41}) | The prototype, limited composition of fissile isotopes (U^{5}+Pu^{9}) | The proposed method (α_{i}and λ_{i}^{eff}) | ||||

The result of the sec; sec; , β_{eff} | δ , % | The result of the sec; sec; , β_{eff} | δ , % | The result of the sec; sec; , β_{eff} | δ , % | |

1 | 2 | 3 | 4 | 5 | 6 | 7 |

about | 5,000 | 0.0 | 5,233 | of 4.66 | 4,998 | -0,04 |

5 | 5,000 | 0.0 | 5,281 | 5,62 | 4,997 | -0,06 |

10 | 5,000 | 0.0 | 5,339 | of 6.78 | 4,996 | -0,08 |

15 | 5,000 | 0.0 | 5,413 | compared to 8.26 | 4,995 | -0,10 |

20 | 5,000 | 0.0 | 5,506 | 10,12 | 4,994 | -0,12 |

the recommended way to configure digital reectometer on the current state of the reactor is as follows:

- in the module ROM reectometry introduce discrete sets of source data.

A separate set is a set of 6 values α_{i}characterizing the ratio of different groups Z.N. in the fuel loading of the reactor described in full composition of fissile fuel elements, and a set of 6 values λ

eff |

i |

directly before performing physical measurements at the RU data from the CCS to determine the average fuel burn-up;

- set the switch settings reectometry (with discrete scale in units of the average fuel burn-up) in the position most close to the real value of this parameter, to thereby select one of five sets of values α_{i}and λ^{eff}corresponding to the current composition of the fissile isotopes of fuel in the reactor core.

0.0308

0.1202

0.3196

1.2752

3.3114

Table 3 The sets of parameter values α _{i}and λ_{i}^{eff}in the proposed method of configuring digital reectometer on the current state of the reactor on the composition of the fissile fuel elements | ||

The position of the switch settings reectometry on the current status of reactor | α_{i} | λ_{i}^{eff} |

“0” (P=0 Muts/kg) | 0.033 0.219 0.196 0.395 0.115 | 0.0124 0.0305 0.111 0.301 1.14 3.01 |

“1” (P=5 MVTS/kg) | 0.0309 0.2139 0.1934 0.3894 0.1272 | 0.0125 0.0307 0,1149 0.3086 1.1959 3.1749 |

“2” (P=10 Muts/kg) | 0.0304 0.2155 0.1933 0.3860 0.1299 | 0.0125 0.0307 0.1165 0.3116 1.2169 3.2125 |

“3” (P=15 Muts/kg) | 0.0294 0.2164 0.1926 0.3832 0.1338 | 0.0126 0.0307 0.1183 0.3153 1.2435 3.2565 |

“4” (P=20 Muts/kg) | 0.0281 0.2168 0.1915 0.3805 0.1389 | |

When P=0 MW-day/kg (switch position “0”) adopted q^{(8)}=0 and not (q^{(8)}=4.08% (see table 1). The reason is that when this switch standard is a standardization of reectometry with the simulator reactor kinetics, the model which is implemented 6 group a set of constants Z.N., appropriate accounting only U^{235}. |

Currently, the proposed method is prepared for introduction into commercial operation at Smolensk NPP.

1. The way to configure digital reectometer on the current state of the reactor on the composition of the fissile fuel elements, including the input module persistent storage device (ROM) reectometry discrete sets of values of parameters characterizing the generation of neutrons in the fuel loading of the reactor with a known average fuel burnup, the definition of the current value of the average burnup of data from the system of centralized control (CCS) reactor unit (RU) and the selection of appropriate sets of values of these parameters by changing the switch position on the back of reectometry, characterized in that use sets of parameter values α_{i}- delayed neutron fraction (Z.N.) of the i-th group in the generation Z.N., from ecause full composition of fissile isotopes fuel specified burnout,
and the corresponding sets of parameter values λ

eff |

i |

2. The method according to claim 1, characterized in that the scale settings reectometry calibrated in units of the average burnup of the fuel in the reactor.

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**SUBSTANCE: proposed method that can be used, for instance, to adjust reactimeters of nuclear power stations with type RBMK-1000 and incorporates provision for reducing systematic measurement error by two orders of magnitude includes introduction of sets of variables characterizing correlation between fissionable elements of fuel and desired burn-up step in charge-coupled device module of reactimeter; determination of current value of mean fuel burn-up using data of centralized checkup system of reactor unit; and selection of respective mentioned variables by varying position of switch on rear panel of reactimeter. Use is made of sets of variables α _{i} (fractions of i group delayed neutrons in delayed neutron generation responsible for total composition of fissionable isotopes of desired burn-up fuel) and sets of variables λ^{eff} (effective constants of i group delayed neutrons). Reactimeter adjustment scale is calibrated in terms of mean fuel burn-up in reactor core.**

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