Abstract
This report is responsive to 2017 Idaho National Laboratory (INL) Memorandum Purchase Order No. 00181805, statement of work 13687 “HTR Graphite R&D – FY ORNL 2017 MPO”, Deliverable 12A “issue final report on the HTV PIE to INL”, due June 15th 2017.
The HTV (High Temperature Vessel) was a HFIR (High Flux Isotope Reactor) high temperature target rod capsule in support of the DOE’s (Department of Energy) High Temperature Reactor (HTR) program. The HTV capsule contained eight sub-capsules, each with nine graphite specimens. The sub-capsules were designed to operate at ~900°C, ~1200°C or ~1600°C. The actual irradiation temperature was established by interrogating SiC and graphite TMs during PIE (post-irradiation examination. The capsule dose ranged from 1.49 dpa to 3.34 dpa, and varied with specimen position within the capsule the maximum dose occurring at the reactor mid-plane and the minimum dose occurring at the core periphery.
The irradiated graphite specimens were examined during PIE to establish their high temperature dimensional and elastic (Young’s) modulus changes. The PIE results are reported and analyzed here. The dimensional and modulus changes were shown to have a strong thermal dependence.
The dimensional changes were small, being of the order of a few percent or less, ranging from a maximum shrinkage of -3.1% (PCEA WG, Tirr=1500°C at 3.3 dpa) to +0.131% (2114 AG, Tirr = 869 at 3.2 dpa). The specimen volume shrinkages varied from –0.81 (2114, Tirr = 840 at 3.1 dpa) to -9.15% (PCEA, Tirr=1500°C at 3.3 dpa). The volume changes were larger at higher Irradiation temperatures. Of the six grades examined, only 2114 moved into the net positive dimensional swelling situation. Significantly, the volume change for 2114 remained negative at the doses and temperature tested here. Dimensional and volume changes were explained in terms of the temperature dependence of the crystal growth rate, the anisotropy of the crystal growth (crystallographic c-axis growth and crystallographic a-direction shrinkage) and the thermal closure of aligned micro-porosity
The modulus changes were much larger, being of the order of a few tens of percent. A trend of lesser fractional increases of Young’s Modulus with an increasing Tirr was noted. The minimum increase in Young’s Modulus was 20% (H-451 WG, Tirr=1500°C at 3.3 dpa) and the maximum 71% (IG-110, Tirr = 840(actual) at 3.3 dpa). The rise in Young’s modulus is attributed to dislocation pinning by lattice defects produced by neutron irradiation and the temperature dependency is related to the irradiation temperature influence on the number of defect available to pin dislocations.