Secondary Startup Neutron Sources as a Source of Tritium in a Pressurized Water Reactor (PWR) Reactor Coolant System (RCS)

Abstract

SummaryCurrently there are large uncertainties in the attribution of tritium in a Pressurized Water Reactor (PWR) Reactor Coolant System (RCS). The measured amount of tritium in the coolant cannot be separated out empirically into its individual sources. Therefore, to quantify individual contributors, all sources of tritium in the RCS of a PWR must be understood theoretically and verified by the sum of the individual components equaling the measured values. The traditional accounting for tritium in the RCS assumes there are two main contributors. The first is soluble species in the RCS, and the second is release from fuel. This stems from the historical knowledge that when fuel had stainless steel cladding it was a major contributor. When the fuel cladding was changed to zirconium alloys in the 1970s, it was assumed that fuel was still the remainder of the source not attributable to the soluble species. Current investigation indicates that this may not be the case. The hypothesis of this paper is that the Zircaloy clad fuel source is minimal and that secondary startup neutron sources are the significant contributors of the tritium in the RCS that was previously assigned to release from fuel.Secondary startup neutrons sources are made of cold-pressed antimony-beryllium pellets in stainless steel cladding. Beryllium is a source of tritium when irradiated with neutrons. Because the secondary source pellets have high porosity and stainless steel has a high permeability for tritium, the tritium that is made in the secondary sources permeates to the coolant. This secondary source contribution of tritium in the coolant is calculated computationally and statistically shown to account for all tritium in the coolant previously assigned to the fuel. Nine reactor cycles from three reactors were analyzed, and the results support the hypothesis of the paper.v

Recommendations for Tritium Science and Technology Research and Development in Support of the Tritium Readiness Campaign, TTP-7-084

“…The reviewer noted that many improvements to WBN1 coolant analysis have been made over the years and much insight into TPBAR (and reactor) performance has been gained as a result. It was analyses of this type that led to the discovery that secondary source rods, rather than fuel, contribute significantly to tritium concentration in the WBN1 coolant (Shaver and Lanning 2010). This realization ran counter to conventional wisdom in the nuclear industry, but had important consequences for attributing tritium in the coolant to TPBARs.…”