A passive decay-heat removal system for an ABWR based on air cooling

- A passive decay heat removal system for an ABWR is discussed using combined system of the reactor and an air cooler.
- Effect of number of pass of the finned heat transfer tubes on heat removal is investigated.
- The decay heat can be removed by air coolers with natural convection.
- Two types of air cooler are evaluated, i.e., steam condensing and water cooling types.
- Measures how to improve the heat removal rate and to make compact air cooler are discussed.

This paper describes the capability of an air cooling system (ACS) operated under natural convection conditions to remove decay heat from the core of an Advanced Boiling Water Reactor (ABWR). The motivation of the present research is the Fukushima Severe Accident (SA). The plant suffered damages due to the tsunami and entered a state of Station Blackout (SBO) during which seawater cooling was not available. To prevent this kind of situation, we proposed a passive decay heat removal system (DHRS) in the previous study. The plant behavior during the SBO was calculated using the system code NETFLOW++ assuming an ABWR with the ACS. However, decay heat removal under an air natural convection was difficult. In the present study, a countermeasure to increase heat removal rate is proposed and plant transients with the ACS are calculated under natural convection conditions. The key issue is decreasing pressure drop over the tube banks in order to increase air flow rate. The results of the calculations indicate that the decay heat can be removed by the air natural convection after safety relief valves are actuated many times during a day. Duct height and heat transfer tube arrangement of the AC are discussed in order to design a compact and efficient AC for the natural convection mode. As a result, a 4-pass heat transfer tubes with 2-row staggered arrangement is the candidate of the AC for the DHRS under the air natural convection conditions. The heat removal rate is re-evaluated as a function of air mass velocity at the inlet of heat transfer tube banks.