Method for calculating poison nucleon density of external circulation fuel balls of pebble-bed high-temperature gas cooled reactor

Abstract

The invention discloses a method for calculating poison nucleon density of external circulation fuel spheres of a pebble-bed high-temperature gas cooled reactor. Firstly, in-pile multi-group neutron flux distribution is obtained; simplifying a burnup chain of the important poison, and establishing and solving a burnup equation of the simplified burnup chain to obtain the equilibrium concentration of batches of all areas of the bottommost layer of the pebble bed at the current stage, which can return to a reactor core, namely the initial fuel ball poison concentration; establishing a burnup equation after the fuel balls are unloaded out of the reactor, and deducing and solving the nuclear density of a batch of poison unloaded from a runner before the poison stays out of the reactor for a period of time and returns to the reactor core; until the variation of the poison nucleon density of each batch of each runner which will return to the reactor core is solved; performing volume weighted averaging on the same batch of poison nucleon densities of different flow channels decayed for a period of time outside the reactor according to the area ratio of the flow channels; and finally obtaining the poison nucleon density of the batch returned to the reactor core after out-of-reactor decay and uniform mixing. According to the method disclosed by the invention, the poison concentration is more reasonable after the fuel balls subjected to out-of-reactor circulation return to the reactor core again.

Description

Technical Field

[0001] The invention relates to the technical field of nuclear reactor physics calculation, and in particular to a method for calculating the nucleon density of poisons in an external circulation fuel ball of a pebble bed type high temperature gas-cooled reactor. Background Art

[0002] The pebble bed type high temperature gas cooled reactor uses spherical fuel elements with a diameter of about 6 cm, and the number of fuel balls loaded in the core is about 420,000. The matrix of the fuel ball is graphite, in which TRISO fuel particles are dispersed. The core pebble bed is constructed by a graphite reflective layer, with a cylindrical structure on the upper part and a funnel-shaped structure connected to the discharge pipe on the lower part. Fuel balls are continuously loaded into the core from the top of the reactor, and spent fuel balls are continuously discharged from the discharge pipe at the bottom of the core. After the fuel balls are discharged, the fuel balls that have not reached the predetermined burnup depth can be sent back to the reactor for use. The fuel balls are circulated in the reactor more than 10 times on average. After being discharged from the core, the fuel balls go through the above process until they return to the core. During this period of time, the poison in the fuel ball decays and the nuclear density changes. If this effect is not taken into account, it will bring certain errors to the subsequent full-core diffusion calculation. Therefore, it is necessary to correct the poison concentration when the fuel ball returns to the core.

[0003] The existing technology is to compare the density of the same batch of poison nuclei in different flow channels at the bottom of the core that need to return to the core according to the radial cross-sectional area of ​​each flow channel as a volume weighted average. This technology takes into account the mixing effect after the fuel balls are unloaded, but does not take into account the decay of the poison in the fuel, which will introduce a certain error to the homogenized cross section of the uppermost area after the material is unloaded, thereby affecting the accuracy of the diffusion calculation. Summary of the invention

[0004] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a method for calculating the nucleon density of poison in the fuel ball of a pebble bed high temperature gas-cooled reactor. The present invention takes into account the decay of poison in the fuel ball of the pebble bed. The simplified burnup equation is solved to obtain the decayed poison nucleon density and use it to replace the poison concentration of the fuel ball when it is unloaded from the core, so that the poison concentration of the fuel ball after the fuel ball returns to the core after the pebble bed high temperature gas-cooled reactor has been returned to the core is more reasonable.

[0005] In order to achieve the above objectives, the present invention is implemented by the following technical solutions:

[0006] A method for calculating the nucleon density of poisons in a fuel ball of a pebble bed high temperature gas-cooled reactor comprises the following steps:

[0007] Step 1: Use the high temperature gas-cooled reactor core physics calculation program to calculate the multi-group neutron flux distribution in the reactor;

[0008] Step 2: Simplify the burnup chain of important poisons, establish and solve the burnup equation of the simplified burnup chain to obtain the equilibrium concentration of poisons in each batch of fuel balls in the bottom area of ​​each flow channel at a certain pouring stage;

[0009]

[0010] Where:

[0011] N I,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 135 I equilibrium concentration

[0012] N Xe,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 135 Xe equilibrium concentration

[0013] N Pm,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 149 Equilibrium concentration of Pm

[0014] N Sm,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 149 Sm equilibrium concentration

[0015] γ I,h ——The hth energy group 135 The fission yield of I

[0016] γ Xe,h ——The hth energy group 135 Xe fission yield

[0017] γ Pm,h ——The hth energy group 149 The fission yield of Pm

[0018] Σ f,h,j,i ——The macroscopic fission cross section of the hth energy group in the i-th batch in the lowest region of the j-th flow channel

[0019] ——The neutron flux density of the hth energy group in the i-th batch of balls in the lowest region of the j-th flow channel

[0020] λ I —— 135 The decay constant of I

[0021] λ Xe —— 135 The decay constant of Xe

[0022] λ Pm —— 149 Decay constant of Pm

[0023] λ Sm —— 149 The decay constant of Sm

[0024] —— 135 Microscopic absorption cross section of the h-th energy group of Xe —— 149 The microscopic absorption cross section of the hth energy group of Sm;

[0025] Step 3: Repeat step 2 until the equilibrium concentration of fuel ball poisons in all areas of the bottom layer of the pebble bed that will return to the core is obtained. This concentration is the initial concentration of each batch of fuel ball poisons in each bottom layer when they are unloaded from the core;

[0026] Step 4: Establish the burnup equation after the fuel balls are unloaded from the reactor, derive and calculate the nuclear density of a batch of fuel ball poisons unloaded from a flow channel before returning to the core after staying outside the reactor for a period of time;

[0027]

[0028]

[0029] Where:

[0030] N I,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 135 I Nucleon Density

[0031] N Xe,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 135 Xe nucleon density

[0032] N Pm,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 149 Pm nucleon density

[0033] N Sm,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 149 Sm nucleon density;

[0034] Step 5: Repeat step 4 until the density of poison nuclei in each batch of fuel balls that will return to the core in each flow channel is obtained;

[0035] Step 6: The density of poison nuclei of the same batch of different flow channels that have decayed outside the reactor for a period of time is calculated as a volume-weighted average according to the area ratio of each flow channel;

[0036]

[0037] Where:

[0038] N x,i (t)——nuclear density of the i-th batch of x poison nuclides before they finally return to the core

[0039] N x,i,j (t)——Nuclear density of the x-toxic nuclide in the i-th batch of the j-th flow channel

[0040] S j ——radial cross-sectional area of ​​the jth flow channel;

[0041] Until the density of poison nuclei is obtained after each batch that will return to the core has decayed outside the core and been evenly mixed.

[0042] Compared with the prior art, the present invention has the following advantages: the decay of the poison in the fuel ball in the off-core circulation is taken into account. The simplified burnup equation is solved to obtain and use the decayed poison nucleus density to replace the poison concentration of the fuel ball when it is unloaded from the core, so that the poison concentration of the fuel ball after the off-core circulation returns to the core is more reasonable. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Simplified burnup chain for xenon.

[0044] Figure 2 Simplify the burn chain for samarium.

[0045] Figure 3 The present invention is an overall flow chart of the calculation method of the nucleon density of poison in the fuel spheres of the pebble bed high temperature gas-cooled reactor. DETAILED DESCRIPTION

[0046] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0047] The method for calculating the nucleon density of poisons in the fuel ball of the pebble bed type high temperature gas-cooled reactor outside the reactor comprises the following steps: firstly, using a high temperature gas-cooled reactor core physics calculation program to calculate the distribution of multiple groups of neutron fluxes in the reactor; simplifying the burnup chain of important poisons, establishing and solving the burnup equation of the simplified burnup chain to obtain the equilibrium concentration of each batch of fuel ball poisons in the bottom region of each flow channel at a certain unloading stage; until the equilibrium concentration of the batch of fuel ball poisons that will return to the core in all regions of the bottom layer of the pebble bed is obtained, and the concentration is the initial concentration of each batch of fuel ball poisons in each region when unloaded from the core; establishing the burnup equation after the fuel balls are unloaded from the reactor outside the reactor, deriving and obtaining the nucleon density of a batch of fuel ball poisons unloaded from a flow channel before staying outside the reactor for a period of time and returning to the core; until the change of the nucleon density of each batch of fuel ball poisons that will return to the core in each flow channel is obtained; and the nucleon density of the same batch of poisons in different flow channels that decay outside the reactor for a period of time is calculated as a volume-weighted average according to the area ratio of each flow channel. Finally, the density of poison nuclei in the fuel balls returned to the core is obtained after decay outside the core and uniform mixing.

[0048] The following describes the application of the calculation of the nucleon density of poisons in the outer circulating fuel balls by taking the transition core from the full reactor loaded with 4.2% enrichment fuel balls to the full reactor loaded with 8.5% enrichment fuel balls as an example. Figure 3 As shown, the specific steps include:

[0049] Step 1: Use the high temperature gas-cooled reactor core physics calculation program NECP-Panda to calculate the multi-group neutron flux distribution in the reactor at the current stage;

[0050] Step 2: Simplify the burnup chain of important poisons such as iodine 135, xenon 135, promethium 149, and samarium 149. Figure 1 The figure shows a simplified xenon burnup chain. The production of xenon 135 consists of two parts, namely the decay of iodine 135 and the fission of nuclear fuel. Its disappearance is caused by its own decay and neutron capture reaction, as shown in Figure 2 The simplified burnup chain of samarium is shown. The source of samarium 149 is the beta - Decay, and its disappearance is due to the neutron capture reaction that occurs by itself. The equilibrium concentration of poisons in each batch of fuel balls in the bottom area of ​​each flow channel at the current pouring stage is obtained by solving the simplified burnup equation of the burnup chain.

[0051]

[0052] Where:

[0053] N I,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 135 I equilibrium concentration

[0054] N Xe,i,j(∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 135 Xe equilibrium concentration

[0055] N Pm,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 149 Equilibrium concentration of Pm

[0056] N Sm,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 149 Sm equilibrium concentration

[0057] γ I,h ——The hth energy group 135 The fission yield of I

[0058] γ Xe,h ——The hth energy group 135 Xe fission yield

[0059] γ Pm,h ——The hth energy group 149 The fission yield of Pm

[0060] Σ f,h,j,i ——The macroscopic fission cross section of the hth energy group in the i-th batch in the lowest region of the j-th flow channel

[0061] ——The neutron flux density of the hth energy group in the i-th batch of balls in the lowest region of the j-th flow channel

[0062] λ I —— 135 The decay constant of I

[0063] λ Xe —— 135 The decay constant of Xe

[0064] λ Pm —— 149 Decay constant of Pm

[0065] λ Sm —— 149 The decay constant of Sm

[0066] —— 135 Microscopic absorption cross section of the h-th energy group of Xe —— 149 The microscopic absorption cross section of the hth energy group of Sm;

[0067] Step 3: Repeat step 2 until the equilibrium concentration of fuel ball poisons in all areas of the bottom layer of the pebble bed that will return to the core is obtained. This concentration is the initial concentration of each batch of fuel ball poisons in each bottom layer when they are unloaded from the core;

[0068] Step 4: Establish the burnup equation after the fuel balls are unloaded outside the reactor, derive and calculate the nuclear density of a batch of fuel ball poisons unloaded from a flow channel after they stay outside the reactor for a period of time before returning to the core.

[0069]

[0070]

[0071] Where:

[0072] N I,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 135 I Nucleon Density

[0073] N Xe,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 135 Xe nucleon density

[0074] N Pm,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 149 Pm nucleon density

[0075] N Sm,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 149 Sm nucleon density;

[0076] Step 5: Repeat step 4 until the density of poison nuclei in each batch of fuel balls that will return to the core in each flow channel is obtained;

[0077] Step 6: The density of poison nuclei of the same batch in different flow channels that have decayed outside the reactor for a period of time is calculated as a volume-weighted average based on the area of ​​each flow channel.

[0078]

[0079] Where:

[0080] N x,i (t)——nuclear density of the i-th batch of x poison nuclides before they finally return to the core

[0081] N x,i,j (t)——Nuclear density of the x-toxic nuclide in the i-th batch of the j-th flow channel

[0082] S j ——radial cross-sectional area of ​​the jth flow channel;

[0083] Until the density of poison nuclei is obtained after each batch that will return to the core has decayed outside the core and been evenly mixed.

[0084] The cleverness of the method of the present invention lies in that it is approximately assumed that the fuel balls in the bottom area of ​​the core are in a shutdown state after being unloaded from the core, and the neutron flux density is zero. The burnup equation is solved to obtain the density of poison nuclei after extra-core decay, which effectively takes into account the decay effect of poison nuclides in the fuel balls participating in the extra-core circulation.

Claims

1. A method for calculating the nucleon density of poisons in the fuel spheres of a pebble bed high temperature gas-cooled reactor, characterized in that: The following steps are involved: Step 1: Use the high temperature gas-cooled reactor core physics calculation program to calculate the multi-group neutron flux distribution in the reactor; Step 2: Simplify the burnup chain of important poisons, establish and solve the burnup equation of the simplified burnup chain to obtain the equilibrium concentration of poisons in each batch of fuel balls in the bottom area of ​​each flow channel at a certain pouring stage; Where: N I,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 135 I equilibrium concentration N Xe,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 135 Xe equilibrium concentration N Pm,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 149 Equilibrium concentration of Pm N Sm,i,j (∞)——the number of balls in the i-th batch in the lowest area of ​​the j-th flow channel 149 Sm equilibrium concentration γ I,h ——The hth energy group 135 The fission yield of I γ Xe,h ——The hth energy group 135 Xe fission yield γ Pm,h ——The hth energy group 149 The fission yield of Pm Σ f,h,j,i ——The macroscopic fission cross section of the hth energy group in the i-th batch in the lowest region of the j-th flow channel ——The neutron flux density of the hth energy group in the i-th batch of balls in the lowest region of the j-th flow channel λ I —— 135 The decay constant of I λ Xe —— 135 The decay constant of Xe λ Pm —— 149 Decay constant of Pm λ Sm —— 149 The decay constant of Sm —— 135 Microscopic absorption cross section of the h-th energy group of Xe —— 149 The microscopic absorption cross section of the hth energy group of Sm; Step 3: Repeat step 2 until the equilibrium concentration of fuel ball poisons in all areas of the bottom layer of the pebble bed that will return to the core is obtained. This concentration is the initial concentration of each batch of fuel ball poisons in each bottom layer when they are unloaded from the core; Step 4: Establish the burnup equation after the fuel balls are unloaded from the reactor, derive and calculate the nuclear density of a batch of fuel ball poisons unloaded from a flow channel before returning to the core after staying outside the reactor for a period of time; Where: N I,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 135 I Nucleon Density N Xe,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 135 Xe nucleon density N Pm,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 149 Pm nucleon density N Sm,i,j (t)——The number of balls in the i-th batch in the lowest area of ​​the j-th flow channel that stay outside the reactor for a period of time t before returning to the core 149 Sm nucleon density; Step 5: Repeat step 4 until the density of poison nuclei in each batch of fuel balls that will return to the core in each flow channel is obtained; Step 6: The density of poison nuclei of the same batch of different flow channels that have decayed outside the reactor for a period of time is calculated as a volume-weighted average according to the area ratio of each flow channel; Where: N x,i (t)——Nucleon density N of the i-th batch of x poison nuclides before they finally return to the core x,i,j (t)——Nuclear density of the x-toxic nuclide in the i-th batch of the j-th flow channel S j ——radial cross-sectional area of ​​the jth flow channel; The final result is the density of poison nuclei that will be returned to the core after decaying outside the reactor and being evenly mixed.

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