THERMAL ANALYSES

2.3.1. Reference to DPC design for thermal analysis

The design of the DPC and the description of the contents need to be referenced according to Section 2.1.1.

2.3.2. Assumptions for thermal analysis

The thermal behavior has to be demonstrated for normal operation, off-normal operation, and accidents. Reference [1] lists off-site transport conditions that have to be analyzed. According to Ref. [1], analysis is required for RCT, NCT, and ACT, each defined by testing conditions. For on-site transport and storage, the conditions to be addressed have to be taken from the analysis of the operational conditions, incidents, and accidents that have to be considered for these activities as described in Section 1.5.2.

Most of the acceptance criteria for transport are derived from Ref. [1]. In addition, the criteria are also derived from national regulations or as agreed upon by the national regulators.

In general, components of the DPC and its contents important to safety need sufficient heat removal capability to ensure the following acceptance criteria are met:

(a) External surface temperature of the DPC package for transport purposes have to meet national and international transport regulations requirements.

(b) Criticality.

(c) Release of radioactive materials to the environment.

(d) Direct radiation doses and dose rates to the public or workers.

2.3.3. Description and validation of methods for thermal analysis

The following are some of the points that need to be considered in a thermal analysis.

(1) An assessment needs to be made of the thermal behaviour under RCT, NCT, and ACT each before and after storage, and under normal, off-normal, and accident conditions of storage for:

(a) The containment system;

(c) Components providing criticality control;

(d) Components for which their performance will have a consequential effect upon (a), (b), and (c).

This assessment includes the fissile material thermal behaviour and any structure used to maintain the geometry of the fissile material for the criticality safety assessment.

(2) Evaluate the effects of insolation for a period of 12 hours according to Ref. [1], para. 657.

Averaging insolation over 24 hours is not acceptable. For outside storage, the same effects need to be considered.

(3) Consider the presence of protective systems liable to impede heat dissipation in RCT (e.g. tarpaulins, canopies, additional screens, outer packaging [containers, boxes, etc.]), if applicable.

(4) For storage, as well as 3), consider adjacent DPC packages.

(5) Justify simplifying assumptions used for calculation (for example the absence of trunnions).

(6) The DPC package in accident conditions needs to be analysed in the position (horizontal or vertical) which is most thermally challenging to the DPC and contents.

(7) The solar insolation before and after the fire test needs to be considered as defined in Ref. [1], para. 728. For storage, a different solar insolation may be applicable.

(8) The absorptivity of the external surface of the DPC according to Ref. [1], para. 728 will not be lower than 0.8 without additional justification during and after the fire test to account for deposits upon the DPC surface. The absorptivity will also not be lower than the possible maximum value of the emissivity.

(9) The evaluation of the minimum and maximum temperatures of the various components of the DPC has to take account of all the possible positions for the contents.

(10) The time dependent temperatures of fuel and components have to be determined during the total storage period. This information can be used for the evaluation of ageing effects (Section 1.7).

(11) The profile of burnup distribution and decay in irradiated fuels has to be taken into account in the thermal analyses.

(12) The influence of combustible materials that generate additional heat input and affect the fire duration has to be taken into account for safety analyses.

(13) Analysis of the influence of the devices specified in Section 1.4(e) in fire conditions on the performance of the DPC package needs to be performed, if applicable.

(15) Operational conditions during DPC loading as well as storage and transport have to be described. The heat removal capability in combination with operational conditions (which can be very important during the loading procedure, for example if using vacuum drying) has to be such that the temperature of the spent fuel assembly, including that of the cladding, does not exceed the maximum allowable value. In addition, other safety related components of the DPC also have to not exceed their maximum allowable temperatures. This can be demonstrated by measurements or calculations.

(16) Analysis has to consider processes foreseen to degrade or impair the system over time.

Changes to thermal properties related to ageing (Section 1.7), for example changes to the internal gas composition, need to be considered.

(17) All thermal loads and processes resulting from the spent fuel decay heat have to be given appropriate consideration in the design. Typical items for consideration include:

(a) Thermally induced stresses;

(b) Internally and externally generated pressures;

(c) Heat transfer requirements;

(d) Effect of temperature on subcriticality.

2.3.4. Thermal assessment

The thermal assessment has to be conducted for all operational scenarios planned for the DPC package (Section 1.5.2). Sections 2.3.1 to 2.3.3 describe conditions to be considered for the thermal analysis.

2.3.4.1. Experimental thermal test

Experimental thermal tests for transport have to be carried out in accordance with guidance provided in Ref. [4], para. 728. Analogous methods may be applied to storage situations.

(1) When thermal analysis is based on test results, show that the temperature measurements were performed at thermal equilibrium.

(2) When the thermal test is made in a furnace and where some DPC components burn, the concentration of oxygen present in the environment of the furnace has to be controlled to be the same as that obtained in a hydrocarbon fire. In addition, control of heat input has to be considered thoroughly.

2.3.4.2 Thermal analysis calculations

(1) Calculations using computational models are to be verified and validated. It must be demonstrated that input parameters (material properties, characteristic values, boundary conditions, etc.) describe sufficiently and precisely the bounding conditions that the DPC

(2) The safety margins on temperature results derived using numerical modelling need to be commensurate with the uncertainty of the numerical model.

(3) If uncertainties exist regarding important input parameters (e.g. material properties), conservative design calculations including the possible range of material properties need to be performed to assess limiting values.

(4) All data used (material properties, boundary conditions, etc.) and calculation results are to be documented in detail.