Implications of the draft ICRP-2005 recommendations on the radiation field of a CASTOR-Flask

The International Commission on Radiological Protection (ICRP) from time to time updates its recommendations on dosimetric quantities, needed to assess the health effects of ionising radiation. A main issue are the coefficients, used to convert the fluence of a specific type of radiation, e.g. of neutrons, to radiation dose. At present, the coefficients of ICRP-60 (1991) and ICRP-74 (1995) are widely used. With draft ICRP-2005, new coefficients were published for public discussion, which account for new aspects on health effects of radiation. Figure 1 compares the coefficients used to convert neutron fluences to doses of the new recommendation ICRP- 2005 and ICRP-74. The maximum value of the coefficients has moved to higher energies, while being lower in the thermal neutron energy range by about 50%. The consequence of using the new coefficients will depend on the specific radiation situation, but on the whole, neutron radiation dose may be reduced up to a factor of 2. - Influence of the new ICRP-2005 conversion coefficients on calculated doses of the radiation field of a CASTOR The neutron doses of a CASTOR with three different loads of 19 UO2-FAs, of 15 UO2- + 4 MOX-FAs and of 28 HAW-Canisters was calculated with the 3D Monte-Carlo radiation transport code MCNP-4C and for verification with DORTABLE, a 2D SN-code of GRS. The activity inventory of the load was calculated with the three GRS-codes ORIGEN-X, OREST, and NGSRC, taking into account the contribution of (α,n)-neutrons. Figure 2 compares the neutron source spectrum of the vitrified HAW and the FAs. Result of comparison: From the fluences, calculated by the radiation transport codes, doses were derived, using the neutron coefficients of ICRP-74 and ICRP-2005. The doses are reduced by about 16% to 17% for all loads throughout, also for the HAW-load. The case of the HAW-load is surprising, as the neutron source spectrum is much harder, and the source differs reasonably from that of the FA-loads. Though, it can be understood considering that neutrons from the HAW are moderated not only by the flask material but also by the glass matrix inside the HAW-canisters. The dose reduction of less than 50% is caused by coefficients, being reduced less than 50 %, especially for higher energies. Even the dose contribution of the 24% epithermal and thermal neutrons below 20 keV of the UO2-FAs (Fig. 3) is reduced only by 22% to 50%. - Conclusion: The reduction of neutron dose by draft ICRP-2005 increases with the share of thermal neutrons, depending on the system investigated. The reduction varies from 50% for a fully thermal neutron spectrum to zero for a completely fast neutron spectrum. In the case of a CASTOR-flask, a reduction of the neutron doses below 20% is expected. (authors)