1 Scope
This document specifies guidelines for the analysis of radiation source terms for cores, spent fuel assemblies and fuel-related assemblies in pressurised water reactor nuclear power plants.
This document applies to the analysis of the radiation source term of the core, spent fuel assemblies and fuel-related assemblies of pressurised water reactor nuclear power plants and is intended to be used as a reference for other reactor types (e.g. heavy reactors, high temperature gas-cooled reactors, fast breeder reactors, etc.).
This document does not apply to the storage, transport, reprocessing and disposal of spent fuel away from the reactor.
2 Normative references
There are no normative references in this document.
3 Terminology and definitions
The following terms and definitions apply to this document.
3.1
Fuel assembly
A group of fuel elements that are assembled together and are not disassembled during core loading and unloading. [Source: GB/T 4960.2-1996, 3.10]
3.2
fuel associated assembly
A collective term for control rod assemblies, neutron source assemblies, combustible poison assemblies and resistance plug assemblies. [Source:GB/T 4960.2-1996,3.13]
3.3
activation product
The radioactive product resulting from the bombardment of a material with neutrons, protons or other nuclear particles. [Source: GB/T 4960.1-2010, 7.58]
3.4
fission product
Fission fragments and their decay products resulting from nuclear fission. [Source: GB/T 4960.1-2010, 6.13]
4 Basic requirements
4.1 The core radiation source shall be analysed for neutron source intensity, Y source intensity, nuclide accumulation and, if necessary, elemental mass. The source term for spent fuel assemblies shall be analysed for neutron source intensity, Y source intensity and nuclide inventory. The radiation source term of the fuel-related assembly shall be analysed for nuclide activity, Y-source intensity, and neutron source intensity of the neutron source material.
4.2 The analysis of the radiation source term shall take into account a certain degree of conservatism depending on the object and purpose of the analysis.
4.3 The analysis of radiation source terms shall be performed using validated software and using evaluated nuclear databases.
4.4 Radionuclide calculations.
a) The nuclide production term shall include:
1) neutron reaction generation pathways;
2) decay production pathways;
3) other pathways, such as (a,n), (Y,n) and other possible nuclide generating reactions, external supplementary nuclides, etc.
b) Disappearance of nuclides should include:
1) decay disappearance pathways;
2) disappearance pathways for neutron reactions;
3) other pathways, e.g. other reactions that may lead to the disappearance of nuclides, nuclide removal, etc.
c) Calculation parameters should include:
1) Fuel design parameters including initial fuel enrichment, fuel loading, fuel toxicity, etc;
2) Power operation history parameters including operating power, operating time and stoppage cooling time;
3) The selection of the cross-sectional database should be compatible with the fuel design parameters and the power operation history parameters.
4.5 The factors considered in the neutron and Y source strength analysis should include differences in the neutron and Y energy released.
4.6 For nuclear fuel that has undergone multiple cycles or where variable power exists in reactor operation, calculations may be based on specific power operation histories.
5 Core radiation source term
5.1 Neutron source intensity and Y-source intensity of the core during power operation
5.1.1 The main sources of radiation in the core during power operation are neutrons and Y-rays produced during the fission process.
5.2 Post-deactivation neutron and Y source strengths in the core
5.2.1 Post-deactivation core neutron sources shall include:
a) neutrons from spontaneous fission of steel-based nuclides;
b) neutrons from the reaction of alpha rays from the decay of copper-based nuclides with light nuclei in the fuel; c) neutrons from neutron source assemblies.
5.2.2 The sources of post-deactivation cores should include:
a) Activation product decay;
b) decay of fission products; c) decay of steel-based nuclides.
5.2.3 Neutron source strength and source strength calculations should be based on fuel partitioning according to parameters such as fuel enrichment and fuel irradiation history. 5.2.4 Neutron source strength and source strength calculations should encompass all phases of the life cycle and generally use values at the end of the life cycle.
5.2.5 The neutron source intensity and source intensity can normally be calculated using the full stack average source intensity and, if necessary, the source intensity results by fuel partition.
5.2.6 When the core part facing the dose analysis location has a higher fuel consumption or power than the core average, it is advisable to calculate the radiation source intensity based on the actual fuel consumption or power or to adopt other conservative methods for the analysis.
5.3 Accumulation of nuclides in the core
5.3.1 The core nuclide inventory analysis should be based on fuel enrichment, fuel irradiation history and other parameters for fuel practice, and the inventory of fuel assemblies should be calculated separately for each zone.
5.3.2 Core nuclide inventory calculations should be carried out using conservative fuel consumption or power, but best estimates of core nuclide inventories for severe accident analysis are preferred.
5.3.3 The selection of nuclide species for core nuclide inventory analysis should be based on the principle of greater contribution to the final dose consequences. Three factors should be considered:
(a) Larger nuclide production. Longer half-lives and higher activity;
b) the higher migration capacity of the nuclide, e.g. to the fuel rod air gap coolant, containment and the environment;
c) The nuclides are more hazardous to humans and the environment.
5.3.4 For source item analysis during normal operation, the types of nuclides that should be included in the core nuclide inventory are listed in Table A.1 in Appendix A.
5.3.5 For design basis accident safety analysis, the core nuclide inventory shall include isotopes of nitrogen, argon, iodine and absolute elements, as shown in Table A.2.
5.3.6 For severe accident analysis, the core nuclide inventory should include the nuclide species shown in Table A.3. If the elemental mass of the core is required, the elemental species should be selected as shown in Table A.4.
6 Source of radiation in spent fuel assemblies
6.1 Sources of neutrons in spent fuel assemblies should include:
a) neutrons from the spontaneous fission of steel-based nuclides;
b) neutrons from the reaction of alpha-rays from the decay of steel-based nuclides with light nuclei in the fuel;
c) neutrons from neutron source assemblies.
6.2 The Y source intensity in spent fuel assemblies shall include:
a) the fission products of the fuel itself and the decay source strength of copper-based nuclides;
b) the Y source intensity of the decay of activation products of structural materials other than fuel, within the time frame of unloading, storage and transfer of the spent fuel assembly, if the source intensity is comparable to that of the fuel itself, e.g. the upper and lower ends of spent fuel assemblies in conventional pressurised water reactor nuclear power plants.
6.3 The radiation source term for spent fuel assemblies should give the results of the source term after multiple cooling-off times, with the cooling-off time range covering the maximum storage time of the spent fuel assembly.
6.4 It is desirable that the spent fuel assemblies in the different partitions of the spent fuel pool are given their respective nuclear fuel neutron source strengths and Y source strengths, and that the source strength results for each partition are conservative enough to encompass the source strength results for any of the fuel assemblies in that partition.
7 Radiation source term for fuel-related assemblies
7.1 If the fuel-related assembly will be withdrawn from the fuel assembly for separate operation or for waste disposal, the activation product nuclide activity and Y source intensity of the fuel-related assembly shall be provided.
7.2 For reactors with an external neutron source, the neutron source intensity, Y-source intensity and nuclide activity of the neutron source core material shall be provided.
7.3 The activation products that contribute significantly to the radiation source term should be selected according to the material composition of the fuel-related assemblies.
7.4 The nuclide reaction chain considered in the material activation calculations may be simplified to take account of the chemical composition of the structural material, the amount of nuclide produced, the half-life and the radioactivity.
7.5 The mass shares of the different elements in the structural material shall be selected conservatively.
7.6 The calculation of the neutron energy spectrum used for neutron activation products may be obtained by means of numerical calculations related to neutron transport. In the calculation of thermal neutrons, considerations shall also include the effect of changes in fuel consumption and changes in boron concentration on the calculation results.
7.7 The factors to be considered in the analysis of the radiation source term for fuel-related assemblies shall include the influence of the radiation exposure history.
7.8 The radiation source term for fuel-related assemblies shall give the results of the source term after multiple cooling down times and the cooling down times shall cover the storage, transfer and handling of the fuel-related assemblies.
7.9 The radiation source term for fuel-related assemblies shall be conservative and able to encompass the radiation source term results for the same type of fuel-related assembly.
Appendix A (informative) Selection of nuclides and element types for core accumulation analysis
Appendix B (informative) Types of nuclides used in the analysis of radiation source terms for common typical materials in fuel-related assemblies
Bibliography
1 Scope
2 Normative references
3 Terminology and definitions
4 Basic requirements
5 Core radiation source term
6 Source of radiation in spent fuel assemblies
7 Radiation source term for fuel-related assemblies
Bibliography
