Discoloration of initially white (a) HMWPE and (b) UHMWPE by gamma-radiation, from left to right: 0, 50, 100, 200, 400 and 600 kGy
Source: BAM, Division Safety of Storage Containers
Polyethylene (PE) with high (HMWPE) and ultra high (UHMWPE) molecular weight is used i.a. as neutron shielding material in casks for radioactive waste due to its high specific hydrogen content. The radioactive radiation that the material absorbs during its service consists of neutrons as well as gamma radiation and can lead to changes in the polymeric structure. As casks for radioactive waste are currently licensed for up to 40 years of interim storage, the safety-related functionality of the used PE materials has to be assured during this period according to the specifications despite the influence of radiation and elevated temperatures. The corresponding safety evaluations are conducted at BAM's Safety of Storage Containers division in the context of license procedures under nuclear legislation. In addition, experiments such as the presented irradiation and characterization of PE shielding materials are performed. A HWMPE and a UHWMPE were each irradiated with doses of 50, 100, 200, 400 and 600 kGy and subsequently analysed with polymer characterisation methods such as differential scanning calorimetry, infrared spectroscopy and dynamic-mechanical analysis. The results indicated that irradiation led to oxidation, chain scission and additional crosslinking. Furthermore, a slight density increase was detected. The higher the irradiation dose was, the more pronounced were the observed material changes that showed the same trends for both PE materials. The chemical changes such as oxidation and crosslinking involve a loss of hydrogen. However, as the observed effects were small, the present results indicate that the safety-relevant function for the relevant operational service is ensured.
The results were published in the journal „Radiation Physics and Chemistry“.
Effect of high-dose gamma irradiation on (U)HMWPE neutron shielding materials
Anja Kömmling, Kerstin von der Ehe, Dietmar Wolff, Matthias Jaunich
Radiation Physics and Chemistry, Volume 142, January 2018, Pages 29-33
BAM Department Containment Systems for Dangerous Goodsn, Division Safety of Storage Containers