MA CHANGWEN, BO JINHAI,

JA HAIJUN, GAO ZUYING

Institute of Nuclear Energy and Technology,

Tsinghua University,

Beijing, China

Abstract

A Series of tests has been done for the three cases ( breaking pipe opened in the gas plenum, near the liquid level and submerged in the water) in the Test Heating Reactor.

Experiments show that the three cases of LOCA ( Loss of Coolant Accident ) have different patterns. In the case of a breaking pipe connected into the gas plenum the quantity of lost water is indepondent of the diameter of the breaking pipe. In the case of a breaking pipe located near the liquid level, the quantity of lost water depends on the location of the pipe. In the case of breaking pipe submerged in the water, all water above the break will be discharged.

The patterns of the discharge for the three cases are given in the paper.

Key words: Loss of coolant; Nuclear heat reactor

1. Introduction

District heating reactors must be built near the heating load ( heat grid ) . That means they must be built near cities. So the safety requirements for DHR are very strict.

For integral reactors protection against loss of coolant is one of the most important considerations.

On the pressure vessel of a integral reactor, there is no penetration pipe with big size, LOCA is possible only through pipes with small sizes. The possible ways of LOCA are as follows:

Ф Water discharge through the safety valve.

CD Water discharge caused by a pipe breaking ( The pipe is opening in the steam plenum ) .

® Water discharge caused by a breaking pipe (the pipe is opening under the water level.

For checking the design safety of the 5MW Test Heating Reactor ( NHR-5 ) and investigating the characteristics of LOCA during the discharging process, a series of tests have been done on the model thermohydraulic loop in INET.

The test model loop was built on the base of similarity theory for the NHR-5.

The loop consisted of two circuits. The first circuit included two heating test sections (to model fuel assemblies ) , a rising pipe (to model the chimney) , steam-water separator, steam condenser, cooler, down pipe, valves ( to model resistances ) and measurement apparatus.

The Condenser and cooler are cooled by the water of the secondary circuit. Secondary circuit water transfer heat to the environmental air by an aircooler.

In order to study the effects of drain position on the drain process, three positions of the drain orifices have been selected. The first drain orifice is located at the top of the loop. It is used to model the case of “ safety valve opened and can’t be reset” . The second orifice is located in the steam plenum of the separator. It is used to model the case of a breaking pipe opened in the steam plenum. In the NHR-5, most pipes penetrating the pressure vessel are connected to the steam plenum. The third drain orifice is located under the water level. It is a pipe, opening under the water level at about 3m. It is used to model the case of a boron-injection pipe breaking.

The diameter of the heating rods, heating length, height of the chimney, resistance coefficient at the inlet of the heating section, total resistance of the loop, density of the heating power and parameters are the same as those in the 5MW Test Heating Reactor, in order to keep the geometric and hydraulic similarity conditions.

In the water discharge test through the safety valve, according to the geometric similarity, the diameter of the drain orifice should be selected at 4. 3mm. To study the relationship between the orifice

diameter and quantity of drain water, besides 4.3, orifice diameters 6.0 and 8.5 m m are also selected in the tests.

During the test, the pressure of the steam plenum, water temperatures at the inlet and outlet of the heating section, drain water flow rate, and surface temperature of the heating elements are measured. Pressure and pressure defferences are measured through transducer type 1151. Drain water quantity is measured by weighting the condensate and measuring the water level.