Effects of a change in the design of cooling water injection on risk reduction in a beam tube rupture

Abstract

In this paper, the difference between the current design and a newly proposed method for the injection of supplementary cooling water into a nuclear research reactor in response to the rupture of a beam tube is investigated. Here, the newly proposed method or design is to maintain the cooling water in the chimney at a certain level solely by gravitational coolant feeding from the emergency water supply system (EWSS) tank to prevent nuclear fuel damage, without the sump recirculation operations described in the current procedure for the beam tube accident. The reduction of risk to the reactor core, quantified in terms of core damage frequency (CDF), as a result of the proposed changes to the design of the emergency water supply in the event of a beam tube rupture is as follows. 1) The overall CDF for the reactor with the newly proposed design for emergency cooling water injection is about 4.16E-06/yr, approximately 8.6% lower than the CDF of the existing design (4.55E-06/yr). 2) The CDF specifically due to a beam tube rupture is 6.33E-08/yr, representing an 85.9% decrease compared to that with the existing design (4.49E-07/yr). In addition to these risk-reduction effects, it is also worth noting that changing the emergency water injection design allows pure coolant to be supplied to the EWSS tank, eliminating the need to decontaminate the reactor after an accident.